CHAPTER
4A
Basic Chemistry
Introduction
Chemistry is
the study of
matter…
Mud engineers deal with chemistry
every day. Chemistry is the study of
matter, including its composition, its
properties, and its transformation
into, or reaction with, other substances (chemical reactions). Matter
is something that has mass and
occupies space.
Mass is a measure of the quantity
of matter or the amount of material
something contains. Mass is one of
the fundamental quantities upon
which all physical measurements are
based. Mass causes matter to have
weight in a gravitational field and
inertia when in motion. The weight
of something is the force of gravity
acting on a given mass and is directly
proportional to the mass times the
gravitational force (acceleration).
Common units for mass are grams
(g) and pounds-mass.
Volume is a measure of the quantity
of space occupied by matter. Common
oilfield units of volume are gallons (gal),
barrels (bbl), cubic feet (ft3), liters (l) and
cubic meters (m3).
Density is defined as the ratio of mass
divided by volume. Common oilfield
units of density are pounds per gallon
(lb/gal), pounds per cubic foot (lb/ft3),
kilograms per cubic meter (kg/m3) and
grams per cubic centimeter (g/cm3).
Specific gravity is a special expression
of density often used for liquids and
solids. It is the ratio of a substance’s
density divided by the density of pure
water at a stated temperature, usually
4°C. Likewise, the density of gases is
often expressed as a “gas gravity,” or
the ratio of the density of a particular
gas divided by the density of pure air
at standard conditions.
Classification of Matter
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All substances fall into one of three
physical states:
• Solid
• Liquid
• Gas
Solids usually have higher density
than liquids and gases. They are substances which are not fluid and, therefore, do not flow when force is applied.
Solids do not readily conform to the
shape of their container.
Liquids usually have a density less
than solids, but greater than gases.
Liquids will readily conform to the
shape of their container. Both liquids
and gases are fluids which “flow”
when a force is applied.
Gases not only conform to, but
expand, to fill their container.
All substances can also be separated
into one of two categories:
Basic Chemistry
4A.1
• Homogeneous (pure substances)
• Heterogeneous (mixtures
of substances)
An example of a homogeneous
material would be table salt, wherein
each grain is identical in chemical
composition. An example of a heterogeneous (non-uniform) material would
be riverbed gravel; it is a mixture of
rocks — from a variety of sources —
having different chemical composition, appearance and properties.
Drilling fluids and most materials
found in nature are mixtures.
Homogeneous materials (pure substances) are occasionally found in
nature, but more often are manufactured by processing to separate dissimilar materials or to remove impurities.
Pure substances can be identified
because they are homogeneous and
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Elements
cannot be
broken
down or
subdivided
into simpler
substances…
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Basic Chemistry
have uniform composition, no matter
how they are subdivided or where they
are found.
Pure substances can be separated
into one of two distinct categories:
• Elements
• Compounds
Elements cannot be broken down
(decomposed) or subdivided into simpler substances by ordinary chemical
methods. Elements are the basic building blocks of all substances and have
unique properties. Compounds can be
reduced into two or more simpler substances (either elements or groups of
Element (Latin)
Aluminum
Arsenic
Barium
Boron
Bromine
Cadmium
Calcium
Carbon
Cesium
Chlorine
Chromium
Copper (cuprium)
Fluorine
Hydrogen
Iodine
Iron (ferrum)
Lead (plumbum)
Lithium
Magnesium
Manganese
Mercury (hydrargyrum)
Nickel
Nitrogen
Oxygen
Phosphorus
Potassium (kalium)
Silicon
Silver (argentum)
Sodium (natrium)
Sulfur
Tin (stannum)
Titanium
Zirconium
Zinc
Symbol
Al
As
Ba
B
Br
Cd
Ca
C
Cs
Cl
Cr
Cu
F
H
I
Fe
Pb
Li
Mg
Mn
Hg
Ni
N
O
P
K
Si
Ag
Na
S
Sn
Ti
Zr
Zn
elements). A pure substance is a compound if it can be subdivided into at
least two elements. All compounds are
formed by the combination of two or
more elements. If a pure substance
cannot be separated into two or more
elements, it must be an element.
It is convenient to refer to an element with an abbreviation called a
symbol rather than the full name.
Table 1 contains the chemical name,
symbol, atomic weight and common
valence (electrical charge) of the elements of most interest to the drilling
fluid industry.
Atomic Weight
26.98
74.92
137.34
10.81
79.90
112.40
40.08
12.01
132.91
35.45
52.00
63.55
19.00
1.01
126.90
55.85
207.19
6.94
24.31
54.94
200.59
58.71
14.00
16.00
30.97
39.10
28.09
107.87
22.99
32.06
118.69
47.90
91.22
65.37
Common Valence
3+
5+
2+
3+
12+
2+
4+
1+
16+
2+
11+
13+
2+
1+
2+
2+
2+
2+
5+
25+
1+
4+
1+
1+
22+
4+
4+
2+
Table 1: Common elements.
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Basic Chemistry
4A.2
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Atomic Structure
Diffuse cloud of light electrons (–) orbiting the nucleus in
structured shells.
Atoms are
the basic
building
blocks for
all matter…
The mass
of either a
proton or
a neutron
is roughly
1,837 times
greater than
the mass of
an electron.
All matter is composed of discrete
units called atoms. The atom is the
smallest unit into which an element
can be divided and still retain the element’s unique chemical properties.
Atoms are the basic building blocks for
all matter; they are the smallest units
of an element that can combine with
atoms from another element. Atoms of
different elements have different properties. Atoms are neither created nor
destroyed in chemical reactions.
Atoms contain three subatomic
particles:
• Protons
• Neutrons
• Electrons
The atom has two distinct zones: a
small, dense nucleus, which contains
the protons and neutrons, surrounded
by a diffuse cloud of electrons. The size
of an atom depends almost entirely on
the amount of volume occupied by
the electron cloud of the atom, while
virtually all the mass is located in the
nucleus (see Figure 1).
The nucleus is approximately spherical, 10 – 4 angstrom (Å) or 10–14 m in
diameter and contains protons and
neutrons only. A proton has a positive
charge while a neutron has no charge.
The electron cloud, or shell, also is
approximately spherical, 1 Å or 10 –10 m
in diameter and contains only electrons, which orbit the nucleus much
like a miniature solar system. An electron has a negative charge equal in
strength to the positive charge of a
proton. In neutrally charged atoms
Nucleus is compact and dense, containing heavy protons (+) and
neutrons (neutral).
Figure 1: Atomic structure.
(no valence), the number of electrons is
equal to the number of protons so that
the net charge of the atom is neutral.
Some atoms can gain or loose electrons so that a charged atom, called an
ion, is formed. When an electron is
lost, a positive charge results. A positively charged ion is called a cation.
Similarly, when an electron is gained,
a negatively charged “anion” is formed.
The mass of either a proton or a neutron is roughly 1,837 times greater than
the mass of an electron. Due to this
huge difference in mass, the mass of
the protons and neutrons in the nucleus
account for the approximate total
mass of the atom (see Table 2).
Particle
Proton
Neutron
Electron
Charge
Positive (1+)
None (neutral)
Negative (1-)
Mass (g)
1.6724 X 10 – 24
1.6757 X 10 – 24
0.000911 X 10 – 24
Table 2: Mass and charge of subatomic particles.
Basic Chemistry
4A.3
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
The lightest
and simplest
element is
hydrogen…
Each element
may have
several
atomic
structures
called
isotopes…
Basic Chemistry
The nucleus of an atom is very
dense; approximately 1,770 tons/in.3
(98,000 kg/cm3). The electron cloud
has a diameter 10,000 times larger
than the nucleus. The larger volume
of the low-density electron cloud balances the high density of the nucleus
to an average density of 2 to 20 g/cm3.
The lightest and simplest element is
hydrogen, which has only one proton
in each nucleus. Naturally occurring
atoms contain between 1 and 93 protons in their nucleus. Heavier atoms,
with even more protons, have been created in laboratories, but are unstable
and do not occur naturally. All atoms
that have the same number of protons
in their nuclei have identical chemical
properties and are called elements.
There are 92 naturally occurring elements which, in various combinations,
form the physical world.
The number of protons (p+) in the
nucleus is used to define each element
and is called the “atomic number” (z).
Hydrogen, with just one proton, has an
atomic number of 1. The sum of the
number of protons and neutrons (n) in
the nucleus of an atom is called the
“atomic mass number” (a), a = p+ + n.
Each element may have several
atomic structures called isotopes, each
with a different number of neutrons in
its nucleus, giving each a different
atomic weight. Although these isotopes
of an element will have different atomic
weights, they will have identical chemical properties, and form compounds
with the same properties. Isotopes are
written with the atomic number (z) as
a subscript before the chemical symbol
and the atomic mass number (a) as a
superscript ( zaX). Hydrogen has three
isotopes. The most common isotope of
hydrogen has no neutron in its nucleus
(11H), the second most common isotope
has one neutron (21H) and the third isotope contains two neutrons (31H). All
Basic Chemistry
4A.4
three of these isotopes contain only one
proton in the nucleus.
The atomic mass scale is a relative
mass scale based on the mass of the
carbon isotope, 126 C, which has a mass
of exactly 12.0 atomic mass units (amu).
This scale is used to simplify expressing
such small values of mass for each isotope of each element. The mass of one
neutron, or one proton, is roughly
equal to 1 atomic mass unit (amu).
The atomic weight of an element is
equal to the weighted average mass of
all the isotopes of the element on the
atomic weight scale. For example, the
three hydrogen isotopes, 11H, 21H and
3
1H, have masses of 1.0078, 2.0140 and
3.01605 atomic mass units, respectively.
The fraction of each isotope that occurs
in nature are 0.99985, 1.5 x 10 – 4 and
10 –11. The atomic weight of hydrogen,
therefore, is: (0.99985) 1.0078 + (1.5 x
10 –4) 2.0140 + 10 –11 (3.01605) = 1.0079.
Some combinations of neutrons and
protons are not stable in the nucleus
of an atom. These unstable nuclei will
break apart naturally, or “decay,” to
form atoms of entirely different elements. This “decay” is a nuclear (physical) reaction which involves neither a
chemical reaction with oxygen nor a
biological activity normally associated
with chemical decay. Isotopes that are
subject to nuclear decay are said to be
radioactive. When an atom decays, it
releases subatomic particles and energy
(radioactivity). Radioactivity is widely
used for laboratory analytical evaluations of chemicals and minerals. Some
wellbore evaluation logs use either a
radioactive source or natural background radiation to identify and evaluate formations and formation fluids.
Atoms of one element bond with
atoms of other elements to form compounds either by transferring electrons (ionic bonding) or by sharing
electrons (covalent bonding). Bonding
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Two or more
elements can
combine
to form a
compound.
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Basic Chemistry
is the combination of attractive forces
between atoms which make them act
as a compound or unit. The manner
in which bonding occurs has important implications for a compound’s
physical properties.
Two or more elements can combine
to form a compound. Elements in a
compound are bound together by
shared electrons. Compounds have
different chemical and physical properties than the elements from which
they are formed. For instance, both
hydrogen and oxygen are gases at
standard conditions, but when combined to form water, they exist as a
liquid. Table 3 lists the chemical
name, formula and common name
for the most common compounds of
interest to the drilling fluids industry.
The smallest unit into which a compound can be divided is a molecule, a
combination of tightly bound atoms.
Molecules are composed of two or more
chemically bonded atoms. Atoms in a
molecule always combine in particular
ratios and with specific orientations.
Molecules cannot be divided into any
smaller unit and still retain the compound’s unique chemical properties.
The composition of a compound
can be described by a simple chemical
formula using the atomic symbols and
subscript numbers which show how
many of each atom are in the simplest
molecule. For example, the smallest
particle of carbon dioxide is one molecule with one carbon atom bonded to
two oxygen atoms and can be represented by the chemical formula CO2.
Groups of atoms bonded together
also can be ions (polyatomic ions). For
example, the hydroxyl ion, OH–, is an
anion with a net 1- charge, or one extra
electron. The ammonium ion, NH4+, is a
polyatomic cation with a 1+ charge.
Name
Formula
Common Name
Silver nitrate
AgNO3
—
Aluminum oxide
Al2O3
Alumina
Barium sulfate
BaSO4
Barite
Barium carbonate
BaCO3
Mineral witherite
Barium hydroxide
Ba(OH)2
—
Calcium
hydroxide
Ca(OH)2
Hydrated lime
Calcium sulfate
CaSO4
Anhydrite
(anhydrous)
Calcium sulfate CaSO4 • 2H2O
Gypsum
(hydrous)
Calcium carbonate
CaCO3
Limestone,
marble, calcite
Calcium chloride
CaCl2
—
Calcium oxide
CaO
Quick lime,
hot lime
Hydrochloric acid
HCl
Muriatic acid
Hydrogen oxide
H2O
Water
Sulfuric acid
H2SO4
—
Hydrogen sulfide
H2S
—
Name
Formula
Common Name
Magnesium oxide
MgO
Mag ox
Magnesium
Mg(OH)2
—
hydroxide
Nitric acid
HNO3
Aqua fortis
Potassium chloride
KCl
Muriate of potash
Sodium hydroxide
NaOH
Caustic soda
Sodium
bicarbonate
NaHCO3
Baking soda
Sodium chloride
NaCl
Salt
Sodium carbonate
Na2CO3
Soda ash
Sodium sulfate
Na2SO4•10H2O
Salt cake,
Glauber’s salt
Sodium acid
Na2H2P2O7
SAPP
pyrophosphate
Sodium
Na6P4O13
Phos
tetraphosphate
Silicon dioxide
SiO2
Quartz, silica
Zinc carbonate
ZnCO3
—
Zinc sulfide
ZnS
—
Zinc oxide
ZnO
—
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Table 3: Common compounds.
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Basic Chemistry
4A.5
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Valence
Valence
determines
which
elements
or ions will
combine and
in what ratio
they will
combine.
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The valence of an element or ion is
the number of electrons it can gain,
lose or share in order to become a stable, neutrally charged compound. The
hydrogen atom is selected as the reference and has one positive bond, or a
valence of 1+. Valence determines
which elements or ions will combine
and in what ratio they will combine.
For example, one atom of chlorine
(Cl) combines with one atom of hydrogen (H, 1+ valence) to form hydrochloric acid (HCl), so chlorine must
have a 1- valence. One oxygen atom
(O) combines with two hydrogen atoms
to form water (H2O), so oxygen has
a 2- valence. One sodium atom (Na)
combines with one chlorine atom
(Cl, 1- valence) to form salt (NaCl), so
sodium must have a valence of 1+. One
atom of calcium (Ca) combines with
two atoms of chlorine to form calcium
chloride (CaCl2), so calcium must have
a 2+ valence. Following the same line
of reasoning, the valence of K in KCl
must also be 1+. If we consider the
compound H2SO4, we see that the
valence of the sulfate group or ion
(SO4) must be 2- since there are two
hydrogen atoms. The sulfate ion (SO42–)
is taken as a complete unit. In the case
of caustic soda (NaOH), since Na has a
valence of 1+, then the hydroxyl ion
(OH) must have a valence of 1-. For
calcium hydroxide (lime), since calcium has a valence of 2+ and the
hydroxyl ion has a valence of 1-, then
there must be two hydroxyl ions in the
compound Ca(OH)2. Many elements,
such as iron, chrome, nickel, chlorine
and sulfur, can have several valences.
Valence also is often referred to as the
“oxidation state” (as it is listed later in
Table 5). Table 4 is a list of common
elements and ions (groups) with their
respective symbols and valences.
Element
Hydrogen
Oxygen
Potassium
Sodium
Calcium
Magnesium
Aluminum
Zinc
Iron
Silver
Carbon
Phosphorus
Sulfur
Chlorine
Ion or Group
Hydroxide
Oxide
Carbonate
Bicarbonate
Sulfate
Sulfite
Sulfide
Nitrate
Nitrite
Phosphate
Ammonium
Acetate
Formate
Thiocyanate
Symbol
H
O
K
Na
Ca
Mg
Al
Zn
Fe
Ag
C
P
S
Cl
Symbol
OH
O
CO3
HCO3
SO4
SO3
S
NO3
NO2
PO4
NH4
C2H3O2
CHO2
SCN
Valence
1+
21+
1+
2+
2+
3+
2+
3+, 2+
1+
4+
5+
2+,4+,6+
1-,1+,3+,5+,7+
Valence
12212221131+
111-
Table 4: Common symbols and valence.
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Basic Chemistry
4A.6
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Electron Shell
The outermost orbit is
designated as
the valence
electron orbit
(valence
shell)…
Electrons orbit the nucleus of an atom
in orderly arrangements called electron
shells. Each shell can hold only a specific maximum number of electrons.
The first orbital or shell must not contain more than two electrons and, in
general, each succeeding shell cannot
contain more than eight electrons. Each
subsequent shell has an orbit of larger
diameter. Completely filled shells form
stable (less reactive) structures, i.e., they
tend not to accept or give up electrons.
Individual atoms generally begin by
having a balanced electrical charge
(with the same number of electrons as
protons), but can give up or accept electrons to fill shells. The outermost orbit is
designated as the valence electron orbit
(valence shell) because it determines the
valence an atom will have. The arrangement of elements called the “Periodic
Table” lines up elements with the same
number of electrons in the “valenceshell” into columns (see Table 5).
Ionic Bonding
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As shown in Figure 2, sodium and
chlorine form the sodium chloride compound by sodium losing and chlorine
gaining an electron (called ionic bonding) to form filled outer electron shells.
The sodium atom (atomic number 11)
has 11 protons and 11 electrons; therefore, the 11 electrons are arranged with
two in the first shell, eight in the second shell and only one electron in the
third shell. The one electron in the outermost “valence-shell” makes sodium
want to give up one electron when
combining with other atoms to form a
stable structure with its last shell filled.
If one electron is lost (1- charge), sodium
would become an ion with a 1+ charge,
or valence.
The chlorine atom (atomic number
17) has 17 protons and 17 electrons.
The electrons are arranged with two in
the first shell, eight in the second shell
and seven in the third shell. The seven
electrons in the outermost “valenceshell” make chlorine want to gain one
electron to fill its last shell. If one electron is gained, chlorine would become
an ion with a 1- charge or valence.
Thus, the combination of one atom of
sodium and one atom of chlorine forms
the stable sodium chloride compound,
Basic Chemistry
4A.7
Sodium atom
Chlorine atom
11+
17+
1 electron
outer shell
7 electrons
outer shell
Neutral atoms
11+
17+
Transfer of an electron
+
–
Ionic bond
11+
Transfered
electron
Na+ ion
17+
Cl– ion
Each has 8 electrons outer shell
Sodium chloride compound
Figure 2: Electron shells and ionic bonding.
Revision No: A-0 / Revision Date: 03·31·98
3b
4b
5b
6b
7b
8
1b
2b
3a
4a
5a
6a
7a
0
2
He
4.00260
1.008
3
Li
+1
4
Be
Key
+2
Atomic number
50
Sn
Symbol
6.94
+1
4A.8
+1
+1
Revision No: A-0 / Revision Date: 03·31·98
(223)
13
Al
Group 8
20
Ca
+2
38
Sr
56
Ba
88
Ra
(226)
+3
44.9559
+2
39
Y
57*
La
+3
89**
Ac
24
Cr
50.941
+2
+3
+4
+5
+4
41
Nb
+3
+5
42
Mo
+4
73
Ta
+2
+3
+4
40
Zr
+3
72
Hf
23
V
92.9064
91.22
+3
138.9055
+2
22
Ti
47.90
88.9059
+2
137.34
+1
21
Sc
178.49
180.947
104
105
+2
+3
+4
54.9380 +7
26
Fe
+6
43
Tc
44
Ru
+6
75
Re
+2
+3
+6
51.996
95.94
+5
74
W
6
C
+2
+4
-4
12.011
+3
26.9815
87.62
+1
132.9055
87
Fr
Transition elements
+2
40.08
85.467
55
Cs
12
Mg
+3
10.81
24.305
39.102
37
Rb
Oxidation states
9.01218
22.9898
19
K
+2
+4
5
B
118.69
Atomic weight
11
Na
0
25
Mn
+4
+6
+7
+2
+3
55.847
+4
+6
+7
76
Os
+2
+3
58.9332
+3
101.07
98.9062
27
Co
45
Rh
77
Ir
+2
+3
58.71
+3
46
Pd
+3
+4
78
Pt
29
Cu
+1
+2
63.546
+2
+4
106.4
102.9055
+3
+4
28
Ni
47
Ag
79
Au
+2
65.37
+1
107.868
+2
+4
30
Zn
48
Cd
+2
80
Hg
49
In
+3
81
Tl
32
Ge
+3
50
Sn
+1
+3
82
Pb
+1
+2
+3
+4
+5
-1
14.0067 -2
-3
8
O
15
P
16
S
+3
+5
-3
30.9738
+2
+4
72.59
33
As
+2
+4
51
Sb
+2
+4
83
Bi
15.9994
+3
+5
-3
34
Se
+3
+5
-3
52
Te
+3
+5
84
Po
121.75
+4
+6
-2
190.2
192.22
195.09
196.9665
200.59
204.37
207.2
208.9806
(209)
(227)
*Lanthanides
58
Ce
59
Pr
60
Nd
61
Pm
62
Sm
63
Eu
64
Gd
65
Tb
66
Dy
67
Ho
68
Er
69
Tm
70
Yb
71
Lu
**Actinides
90
Th
91
Pa
92
U
93
Np
94
Pu
95
Am
96
Cm
97
Bk
98
Cf
99
Es
100
Fm
101
Md
102
No
103
Lr
17
Cl
35.453
+4
+6
-2
-1
35
Br
10
Ne
0
20.17
+1
+5
+7
-1
18
Ar
+1
+5
-1
36
Kr
0
39.948
79.904
83.80
+4
+6
-2
53
I
+1
+5
+7
126.9045 -1
54
Xe
+2
+4
85
At
86
Rn
(210)
(222)
127.60
186.2
9
F
18.9984
78.96
183.85
Table 5: Periodic table.
-2
32.06
74.9216
118.69
114.82
+1
+2
+2
+4
-4
28.086
69.72
112.40
+1
+3
31
Ga
14
Si
7
N
0
0
131.30
0
Basic Chemistry
2a
+1
-1
CHAPTER
4A
Basic Chemistry
1a
1
H
CHAPTER
4A
Atoms with
one, two and
three valence
electrons find
it easier to
give away
electrons…
Basic Chemistry
NaCl. The one sodium valence electron
is transferred from the sodium atom to
the outer shell of the chlorine atom.
Why does sodium give up its electron and why does chlorine accept it?
The theory dealing with the behavior
of atoms assumes that every atom tries
to achieve a full outer electron shell
with eight electrons. Atoms with one,
two and three valence electrons find it
easier to give away electrons, whereas
those with four, five, six and seven
find it easier to accept them. In the
case of sodium and chlorine, sodium
gives one electron, resulting in a net
charge of +1, and chlorine takes on
one electron, resulting in a net charge
of 1. Neither atom is considered neutral with these changes. The sodium
atom has become an ion with a positive
charge (written as Na+) and the chlorine atom has become an ion with a
negative charge (written as Cl–).
The evidence to support electron
transfer and the formation of ions is
the phenomenon that melted NaCl
is a conductor of electricity, and if a
current is applied to the molten salt,
sodium metal collects at the negative
pole of the cell (cathode) and chlorine
gas collects at the positive pole (anode).
Thus, the sodium ion is called the
cation and the chloride ion is called
the anion. In writing the name of a
compound, the cation is usually written first. Since sodium gives up an electron, it is said to be electropositive, and
since it readily gives up this electron, it
is said to be strongly electropositive.
Covalent Bonding
…covalent
bonding
is the
simultaneous
sharing of
electrons.
The sharing of electron pairs to form
bonds between atoms is called covalent bonding. Unlike sodium chloride,
where there is a transfer of electrons
(ionic bonding), covalent bonding is
the simultaneous sharing of electrons.
Both water and hydrogen gas are good
examples of compounds with covalent
bonds (see Figure 3). In a water molecule, one electron from each of the two
hydrogen atoms is shared with the six
electrons in oxygens’ second electron
shell to fill it with eight electrons.
Likewise, each hydrogen atom in a
water molecule shares one of the six
electrons from the oxygen atom’s second electron shell to fill its first electron
shell with two electrons. Compounds
with a high degree of electron sharing
have strong interatomic forces with
weak intermolecular forces. The weak
intermolecular forces usually are not
sufficient to maintain a rigid structure.
Because of this, covalently bound
compounds are often liquids and gases.
Basic Chemistry
4A.9
Hydrogen bonding: Some covalent
compounds have incomplete sharing
of the electron in the bond. This results
in partial postive and negative charges
on the atoms arranged in a manner
which polarizes the molecule. In water
(H2O), for example, the two hydrogen
atoms remain partially positive and the
oxygen atom remains partially negative. The negative charges of oxygen
dominate one side of the molecule,
while the postive charges of the hydrogen atoms dominate the other side,
forming a polar molecule (see Figure 4).
The hydrogen atoms of water molecules are attracted to the oxygen atoms
of other nearby water molecules. This
attraction of the positive hydrogen
pole of one molecule to the negative
oxygen pole of another molecule is
referred to as hydrogen bonding. The
forces of hydrogen bonding are estimated to be only one-tenth to onethirtieth as strong as those of covalent
bonding. These weak bonds easily
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Water:
2 Hydrogen atoms
Oxygen atom
Water (H2O)
Combine to form
_______________________
_______________________
1+
1+
_______________________
_______________________
_______________________
Covalent bond
8+
_______________________
8+
Shared electrons
_______________________
_______________________
_______________________
1+
1+
_______________________
_______________________
_______________________
1 electron
outer shell
6 electrons outer shell
Each atom has full outer shell
Hydrogen gas:
2 Hydrogen atoms
Combine to form
Hydrogen gas (H2)
_______________________
_______________________
_______________________
1+
1+
Covalent bond
1+
Shared electrons
1+
Figure 3: Covalent bonding of water and hydrogen gas.
The polarity
of water
explains
some of the
phenomena
seen in
drilling
fluids.
alternate between molecules and change
association, i.e., making and breaking
bonds between nearby molecules.
Hydrogen sulfide (H2S) is a gas even
though its a heavier molecule than
water (H2O), because the charge distribution is not polar. The two hydrogen
atoms have only weak positive charge
and the sulfur is only weakly negative,
forming a balanced structure. The lack
of a strong polar structure allows the
individual molecules to diffuse into
a gas under standard conditions.
The polarity of water explains some
of the phenomena seen in drilling
fluids. Clays and shales are strongly
charged, complex structures. The attraction of the water molecule’s charges to
the charge sites of clay platelets leads to
clay hydration. Clays have a strong negative charge on their large planar surface
and positive charges along their thin
edges. The positive hydrogen side of the
water molecule is attracted to and will
hydrogen bond to the large negative
Basic Chemistry
4A.10
Water molecule (H2O)
H+
Hydrogen side
positive charge
O2–
Oxygen side
negative charge
H+
Polar charge orientation
H+
O2–
H+
H+
O2–
H+
H+
H+
O2–
O
2–
H+
H+
H+
Attraction of positive hydrogen
side to negative oxygen side
O2–
H+
Figure 4: Polar molecule and
hydrogen bonding in water.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
O
O
H
H
H
O
H
O
H
H
–
O
O
O
O
H
+
O
H
O
O
O
H
O
O
H
+
Si
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
O
Al
Al
Al
Al
Al
Al
OH
O
O
Si
+
Si
H
O
O
H
OH
O
O
Si
H
O
H
O
Si
+
O
O
H
O
H
O
O
O
Si
+
Si
H
O
H
OH
OH
OH
O
O
H
OH
O
H
O
Si
+
+
O
O
Si
OH
O
–
H
Silica layer
O
H
H
Hydrogen
bonded water
O
H
Silica layer
Alumina layer
Al
O
H
O
H
Hydrogen
bonded water
O
H
+
H
Si
OH
Clay
O
H
O
H
O
H
Si
Al
+
H
O
H
+
H
O
H
O
H
O
Figure 5: Hydration of clay by water through hydrogen bonding.
Ionic bonding
is as strong
as covalent
bonding and
both are much
stronger than
hydrogen
bonding.
clay surface. This water adsorption can
be many water layers thick, spreading
and swelling adjacent clay layers (see
Figure 5). Cation exchange (exchange
of ionic bonded cations) within a clay
can displace the water of hydration and
flocculate the clay particles, because
their bonds are stronger than water’s
weak hydrogen bonds.
Many compounds contain both
covalent and ionic bonds. Soda ash
(Na2CO3) is an example of a compound
that contains both covalent and ionic
bonds. The bonds between the carbon
and oxygen in the carbonate group
(CO32 –) are covalent (electron sharing),
while the bonds between the sodium
ion (Na+) and the carbonate group are
ionic (electron transfer). When soda ash
dissolves, the sodium dissociates from
the carbonate group, while the carbon
and oxygen of the carbonate group
continue to function as a single unit.
Ionic bonding is as strong as covalent
bonding and both are much stronger
than hydrogen bonding.
Compounds
A compound is a substance that is
composed of elements in definite
proportions. Common table salt is
an ionic compound; it can be broken
down into the elements sodium (Na)
and chlorine (Cl). The following
apply to all compounds:
Basic Chemistry
4A.11
• The composition of a compound
is always the same; it is definite
and exact.
• Elements lose their identity (their
characteristic properties) when they
combine to form a compound.
• A compound is homogeneous.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Formulas
A compound’s
formula
represents
one molecule
of the
compound.
The sum of
the atomic
weights of
the atoms in
a chemical
formula is
called the
formula
weight.
Since a specific compound always contains the same elements combined in
exactly the same ratio, it is possible to
represent its composition by means of a
formula. A compound’s formula represents one molecule of the compound.
Atoms and chemical compounds
do not react with or form just one
molecule at a time. Instead, millions of
molecules and atoms are reacting simultaneously. Due to their small size, it is
impossible to count the number of
atoms involved in chemical reactions.
Weight is used to measure the quantity
of chemicals involved in chemical reactions. Since one atom of sodium weighs
22.99 amu and one atom of chlorine
weighs 35.45 amu, then based on ratio,
the atoms in 22.99 g of sodium would
combine with the exact number of
atoms in 35.45 g of chlorine to form
salt. This principle of ratio works for
any unit of measure — grams, pounds,
kg, tons, etc. — but the gram is the unit
of measure most commonly used.
When expressed in grams, the atomic
weight corresponds to 6.023 x 1023
atoms. This amount is one “gram-atom”
molecular weight or “mole.”
A mole is a quantitative unit of
measure that contains the exact
number of atoms, molecules or formula units which have a mass in
grams equal to the atomic, molecular
or formula weight. A mole of an element contains the same number of
chemical units (atoms, molecules or
units) as exactly 12 g of carbon 12,
or Avogadro’s number; 6.023 x 1023
of chemical units. A common usage
of the mole is the formula weight
expressed in grams. For NaCl (salt),
the formula weight is 58.44, so one
mole of sodium chloride would be
58.44 g.
Basic Chemistry
4A.12
The number of atoms of an element
in a compound’s formula is equal to
the number of moles of that element
needed to make one mole of the compound. In water, two moles of hydrogen react with one mole of oxygen to
form one mole of water. Expressed on a
weight basis, hydrogen (atomic weight
1.01) combines with oxygen (atomic
weight 16.00) in the ratio of 2.02 g
of hydrogen to 16.00 g of oxygen;
i.e., a ratio of two moles of hydrogen
to one mole of oxygen. The formula,
therefore, is H2O.
Carbon (atomic weight 12.01)
combines with oxygen in the ratio
of 12.01 g of carbon to 32.00 g of
oxygen to form carbon dioxide. The
formula, therefore, is CO2. The subscript 2 refers only to the oxygen,
and means that there are two oxygen
atoms per molecule. Atoms in a formula which do not have a stated subscript, such as the carbon atom, are
understood to have a subscript of 1.
The sum of the atomic weights of the
atoms in a chemical formula is called
the formula weight. If the chemical formula of a substance is the molecular
formula, the formula weight is also the
molecular weight. Thus, the formula
weight of NaCl is 58.44. This value is
obtained by adding the atomic weight
of sodium (22.99) to the atomic
weight of chlorine (35.45).
A specific compound always contains
the same elements combined in exactly
the same ratio by weight and the composition is represented by the simplest
formula that describes the compound.
CaCl2, Fe2O3 and BaSO4 are examples
of formulas of compounds.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Stoichiometry - Stoichiometric Reactions
Stoichiometry
deals with
the quantities
and exact
ratios of
substances
which react.
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
The reason compounds contain fixed
ratios of elements is because atoms
react with other atoms according to
their valence. As discussed previously,
atoms react according to these ratios
based on the fixed weights of each
atom involved. Determination of the
weights is called “stoichiometry”.
Stoichiometry deals with the quantities
and exact ratios of substances which
react. Stoichiometric calculations allow
the exact weight and ratio of chemicals
which will react to be determined so
that a desired result can be achieved.
Equivalent Weight
In many cases, chemical testing and
reactions are done with unknown materials. Because we don’t know the exact
composition, it is often convenient to
express results in terms of “equivalents”
of a standard compound instead of
moles. For example, in mud engineering, we titrate water-base mud filtrate to
measure “total hardness” and express
the result as if it were all calcium. This
total hardness titration actually measures both magnesium and calcium, so
we are expressing the total hardness in
calcium “equivalents.”
The equivalent weight is defined as
the formula weight of an element, compound or ion divided by how many
times it takes part in a specific reaction.
As an example, for acids the number of
hydrogen atoms in the chemical formula determines the equivalent weight.
Acids react by donating protons (hydrogen ions). Suppose sulfuric acid (H2SO4,
formula weight 98) is used to reduce
pH. The reaction can be written as:
H2SO4 + 2OH– → 2H2O + SO42–
One mole of H2SO4 reacts with two
moles OH–. From this standpoint, 1⁄2
mole of sulfuric acid is equivalent to
one mole of hydroxyl. To remove only
one mole of OH–, then only 1⁄2 mole of
H2SO4 is needed. On a formula weight
basis this would be 98 ÷ 2 or 49 g.
Therefore, the equivalent weight of
H2SO4 is 49 g. To consume the same
amount of OH– using hydrochloric acid
(HCl, formula weight 36.5) which has
only one hydrogen atom, then the reaction can be written as:
HCl + OH– → H2O + Cl–
Since one mole of HCl reacts with
just one mole of OH–, then the equivalent weight of hydrochloric acid is its
formula weight, 36.5 g. By this equivalent weight method, 49 g of H2SO4 is
equivalent to 36.5 g of HCl. Using this
principal if pilot testing in the laboratory shows that it takes 36.5 lb/bbl of
HCl to neutralize a high pH fluid, and
only H2SO4 was available to use at the
rig, then 49 lb/bbl of H2SO4 would also
neutralize the fluid.
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
Basic Chemistry
4A.13
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Balancing an Equation
Chemical
equations
must always
have an equal
number of
each atom…
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
One of the first steps involved in
determining stoichiometric reactions
is to balance the chemical equation.
Chemical equations must always have
an equal number of each atom on both
sides of the equation. If all of the reactants and products are known, the best
approach is to single-out one element
of known valence and balance the
entire equation on the basis of that element. Many elements can have more
than one valence, which complicates
the process. If present, oxygen should
be used to balance the equation. The
arrow or arrows indicate chemical reactions or transformations and should be
considered to be like an equals sign (=)
in mathematics. Consider the following
unbalanced equation involving the
reaction between iron (Fe3+) and oxygen
(O2–) producing iron oxide:
Fe3+ + O22– → Fe23+O32–
This equation is not balanced with
respect to the number of atoms or
valence charges. Starting with oxygen,
the equation is not balanced since
there are two oxygen atoms on the
left side and three on the right. First,
balance the number of oxygen atoms,
then the iron atoms. The valence
charges are also not balanced, with
four negative charges (2 x 2-) on the
left and six negative charges (3 x 2-)
on the right. If a 2 is used in front of
the iron oxide and a 3 in front of the
oxygen, the equation then becomes:
Fe3+ + 3O22– → 2Fe23+O32–
The number of oxygen atoms and
negative charges are now balanced.
_______________________
Now, however, the iron must be balanced. As a result of balancing the oxygen atoms, there is now one iron atom
on the left and four iron atoms (2 x 2)
[with 12 positive charges, 2 x 2 x 3+]
on the right. In order to completely
balance the equation, there must be
four iron atoms on the left with 12
positive charges on the left. If a 4 is
placed before the iron, the equation
is balanced:
4Fe3+ + 3O22– → 2Fe23+O32–
Stoichiometrically, four moles of iron
combine with three moles of oxygen
to yield two moles of iron oxide.
Consider the following problem:
Using the reaction from above, how
many grams of oxygen would be
required to react with 140 g of iron
to produce iron oxide?
4Fe + 3O2 → 2Fe2O3
4 moles iron + 3 moles oxygen →
2 moles iron oxide
atomic weight Fe = 55.85,
so 4 moles Fe = 4 x 55.85 = 223.4 g
atomic weight O ≈ 16,
so 3 moles O2 = 3 x 2 x 16 = 96 g
Since only 140 g of iron is used (not
223.4), the ratio of 140 divided by 223.4
can be multiplied times the 96 g of oxygen to determine the amount of oxygen
needed to react with 140 g of iron.
Oxygen required =
140 g Fe
x 96 g O2 =
223.4 g Fe
60.2 g O2
Therefore, it takes 60.2 g of oxygen
to react with 140 g of iron to produce
iron oxide.
_______________________
_______________________
_______________________
_______________________
_______________________
Basic Chemistry
4A.14
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Solubility
SOLUTIONS
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
If sugar is added to water, it will dissolve, forming a solution of sugar in
water. The solution is homogeneous
when no particles of sugar can be seen.
The sugar is referred to as the solute; it
is the substance that is dissolved. The
water is referred to as the solvent; it is
the substance that does the dissolving.
Small additions of sugar will dissolve
until a point is reached at which the
solution is unable to dissolve additional
sugar. This will be indicated when the
crystals that are added drop to the bottom of the glass and will not dissolve,
even if the contents of the glass are well
stirred. A solution which has dissolved
all the solute that it is capable of dissolving at a given temperature is said to
be saturated and this quantity of solute
is referred to as its solubility.
EFFECTS
Many ionic
compounds
are soluble
in water.
H+
H+
O 2–
H+
H+
O 2–
O 2–
H+
Na+
H+
H+
O 2–
O 2–
H+
H+
O
2–
H+
H+
Sodium ion is compact with relatively strong bonds with water.
H+
O
2–
H+
O 2–
H+
H
+
H+
OF BONDING
The solubility of compounds in polar
solvents, like water, can generally be
explained by their bonding. Polar
covalent compounds, such as CO2,
usually are soluble in water. When
their attraction to water’s hydrogen
bonds is greater than their attraction
to the charges of other molecules of
the compound, the molecules of the
compound will disperse into solution.
Nonpolar covalent compounds, such
as methane (CH4), are usually insoluble in water and other polar solvents,
but are often dispersable in nonpolar
solvents, such as diesel oil. When nonionic compounds dissolve, they become
molecularly dispersed not ionized.
Many ionic compounds are soluble in
water. If the forces of attraction between
the water molecules and the ions are
greater than the forces holding the ions
in their crystals, the ions will attract a
“shell” of water molecules and be freed
from their crystalline lattice, as shown
for sodium chloride (see Figure 6).
Basic Chemistry
H+
4A.15
Cl–
O 2–
H+
O 2–
H+
H+
H+
O 2–
H+
Chlorine ion is larger with weaker bonds with water.
Figure 6: Ionization of sodium chloride in water.
Salts (compounds) with either monovalent cations (sodium (1+) in soda ash,
Na2CO3) or monovalent anions (chlorine (1-) in calcium chloride, CaCl2) are
usually soluble in water. The salts of
multivalent cations combined with multivalent anions (calcium (2+) and sulfate
(2-) [in gypsum CaSO4, for example]) are
usually insoluble or only sparingly soluble. The forces holding ions together in
the salts of multivalent cations and multivalent anions are much greater than
the forces holding the ions together in
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Ionic bonded
compounds
dissolve or
solubilize
into ions…
Basic Chemistry
the salts of either monovalent cations or
monovalent anions.
Ionic bonded compounds dissolve or
solubilize into ions, whereas covalent
bonded compounds are soluble as molecules. Brines are solutions of a high concentration of soluble salts and are more
viscous than freshwater because the dissolved salts reduce the free water by
attracting large shells of water around
themselves, restricting the free movement of the water. Ionic compounds are
generally insoluble in nonpolar solvents
such as diesel oil.
QUANTIFYING
SOLUBILITY
The quantity of solute that will dissolve in a quantity of solvent to give a
saturated solution is referred to as the
solubility of the solute in the solvent.
Solubility of a solid, liquid or gas in a
liquid is generally expressed in units
of grams of solute per 100 g of water.
Table 6 lists the degree of solubility of
several common compounds used in
drilling fluids.
Water-base
drilling fluids
are generally
maintained
in the 8 to
12 pH range…
Solubility
(g per
100 g
Compound
Common Name
water)
NaOH
Caustic soda
119
CaCl2
Calcium chloride
47.5
NaCl
Sodium chloride (table salt) 36
KCl
Potassium chloride
34.7
Na2CO3
Soda ash
21.5
NaHCO3
Sodium bicarbonate
9.6
CaSO4
Anhydrite
0.290
Ca(OH)2
Lime
0.185
MgCO3
Magnesium carbonate
0.129
CaCO3
Limestone
0.0014
Mg(OH)2
Milk of magnesia
0.0009
BaSO4
Barite
0.0002
ZnO
Zinc oxide
0.00016
Table 6: Solubility of common chemicals.
FACTORS
AFFECTING SOLUBILITY
• Temperature
• pH (acid or base)
• Ionic environment (salinity)
• Pressure
Basic Chemistry
4A.16
1. Temperature. Solubility increases
with increased temperature for most
solids and liquids. The solubility of
gases usually decreases with increasing
temperature.
2. pH. pH is a measure of the relative
acid or base character of a solution
(described in detail later). The solubility
of many chemicals is a function of pH.
Some chemicals, such as the multivalent salts of hydroxide and carbonate,
are more soluble in acidic conditions.
Other chemicals are soluble only over
a neutral pH range, and still others
(organic acids such as lignite and lignosulfonate) are more soluble as pH
increases to >9.5. Calcium and magnesium ions are soluble at acid to neutral
pH, but become less soluble at high pH,
as shown for calcium in Figure 7. As
the hydroxide ions increase with pH,
they react with the calcium and magnesium to precipitate calcium hydroxide
and magnesium hydroxide.
Other compounds, such as carbonate
and sulfide ions, change species with
increasing pH. For instance, CO2, a gas,
reacts with water to become carbonic
acid at low pH. It will react with hydroxide to form bicarbonate ions in a neutral
pH range and finally carbonate ions at
high pH (see Figure 8).
The solubility of many mud chemicals is a function of the pH of the solution. For instance, not only are lignite
and lignosulfonate more soluble above
pH 9.5, but products like DUO-VIST (xanthan gum) are more effective in the 7 to
11 pH range. Other additives are sensitive to high pH. Products like POLY-PLUST
(PHPA) hydrolyze and become less effective at high pH (>10.5). Most individual mud products have an optimum
pH range for maximum performance.
Water-base drilling fluids are generally
maintained in the 8 to 12 pH range for
improved chemical solubility and performance, as well as for anti-corrosion
and safety reasons. Mud engineers
should familiarize themselves with these
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
Basic Chemistry
…in solution,
the compound
with the
lowest
solubility will
precipitate
first.
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_______________________
_______________________
_______________________
_______________________
_______________________
1,000
900
800
700
600
500
400
300
200
100
0
100
Ca(OH)2 added to
NaOH solutions 68°F
the left
HCO3-
0
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Caustic soda (lb/bbl)
HCO3-
60 Increasing salinity
40 shifts curves to
20
0
CO32 -
CO2
80
Percent
Filtrate calcium (mg/l)
4A
2
4
HCO36
8
pH
10
12
14
Figure 7: Decreasing solubility of calcium
with increasing pH.
Figure 8: Carbonate-bicarbonate equilibrium.
ranges and maintain the drilling fluid
system within the optimum pH range.
3. Ionic environment (salinity). Of
particular importance to mud engineering is the chlorides concentration,
or salinity. An increase in salinity generally increases the solubility of other
salts and additives and will affect such
chemical reactions as precipitation. For
example, calcium sulfate (gypsum and
anhydrite) has its greatest solubility in
a 15% salt solution, wherein it is four
times as soluble as it is in freshwater.
This trend decreases as the salinity
approaches saturation. Lime (calcium
hydroxide) also is more soluble at moderate salinity. Even polymers, which are
sensitive to precipitation by divalent
cations and other conditions, are more
stable in saline environments. The
ionic environment of the solvent has
a great impact on the chemical reactions
that will take place and the stability of
various products.
4. Pressure. An increase in pressure
increases the solubility of a gas in a
liquid, but has practically no effect
on the solubility of liquids and solids.
This increased solubility of gas is particularly important when we consider
the downhole chemical environment,
where intruding or entrained gases
are subjected to high pressure and
may be solubilized.
The importance of the relative solubility of chemicals is that in solution, the
compound with the lowest solubility
will precipitate first. For example, if calcium chloride (high solubility) were
mixed into water, it would ionize into
calcium and chloride ions. Then, if soda
ash (moderately soluble) were added, it
would ionize into sodium and carbonate ions, and calcium carbonate (low
solubility) would precipitate immediately as the soluble calcium and soluble
carbonate ions react. Relative solubility
can be used to determine what chemical
to add to remove an undesirable chemical. The other ions which are present in
the solvent affect solubility.
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_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
Basic Chemistry
4A.17
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
pH and Alkalinity
pH
The pH
value is used
to describe
the acidity or
basicity of
solutions.
The (aq)
indicates
that the ions
are dissolved
in water,
forming an
aqueous
solution.
The pH value is used to describe the
acidity or basicity of solutions. The pH
value is defined as the negative log of
the hydrogen ion concentration. Low
pH values correspond to increasing acidity and high pH values correspond to
high basicity. A change of one pH unit
corresponds to a ten-fold increase in
hydrogen ion concentration.
Water is a weak electrolyte which
exists in nature as molecules of H2O. It
can ionize to form hydronium (H3O+)
and hydroxyl (OH–) ions. The ionization of water is statistically rare, with
only one molecule in 556 million
being ionized. Water is in equilibrium
with the above ions according to the
following equation:
H3O+ (aq) + OH– (aq)
2H2O
The (aq) indicates that the ions are
dissolved in water, forming an aqueous
solution. Since water ionizes itself, the
process is called autoionization. This
equilibrium with the hydronium and
hydroxide ions provides the basis for
the classification of acids and bases.
Acids are substances that add hydrogen
ions, (H+), when dissolved in water,
increasing the hydronium concentration [H3O+]. The concentration for different ions is indicated by the chemical
being shown in brackets, such as [H+]
for the concentration of hydrogen ion.
The equilibrium expression for the
autoionization process is:
Kw = [H+] [OH–]
This type of equilibrium expression is
used frequently to describe equilibrium
conditions of related chemical species.
The equilibrium constant is given the
symbol Kw, where the subscript (w)
refers to water. At 25°C, Kw = l.0 x 10–14,
Basic Chemistry
4A.18
Kw depends on temperature (increases
Kw) and ionic concentration of the
solution (salinity).
Kw, the product of [H+] and [OH–],
remains constant providing the temperature is constant. In a neutral solution, the concentration of hydrogen
[H+] is equal to the concentration of
hydroxide [OH–]; therefore, each would
have a concentration of 1.0 x 10–7, and
the solution would have a pH of 7.0.
If [H+] increases, the [OH–] decreases
and the solution becomes more acidic.
Likewise, if the [OH–] increases, then the
[H+] must decrease and the solution
becomes more basic.
H+ (aq) and OH– (aq) ions are always
present in aqueous solutions in equilibrium with the solvent. H+ (aq), and OH–
(aq) can react with other ions, influencing the concentrations of other ions in
the solution. For this reason, reference
to the concentrations of H+ (aq) and
OH– (aq) are made. To assist in this reference, the terms pH and pOH are
defined as:
pH = - log [H+]
pOH = - log [OH–]
A convenient relationship between
pH and pOH is found by taking the
negative logarithm (indicated by a p)
of Kw, which gives:
pKw = -log Kw = -log [H+] -log [OH–]
Using the definitions of pH and pOH,
given above, we find that at 25°C:
pKw = pH + pOH
since Kw = l.0 x 10 –14, then
pKw = - log Kw = 14, which gives
pH + pOH = 14
This relationship of acids and bases
with values for pH and [H+] plus pOH
and [OH–] is shown in Figure 9.
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CHAPTER
4A
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Alkalinity is
not the same
as pH…
Basic Chemistry
pH
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
[H+]
1
10–1
10–2
10–3
10–4
10–5
10–6
10–7
10–8
10–9
10–10
10–11
10–12
10–13
10–14
Acids
Neutral
Bases
[OH–]
10–14
10–13
10–12
10–11
10–10
10–9
10–8
10–7
10–6
10–5
10–4
10–3
10–2
10–1
1
pOH
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
removed from the solution, and the pH
remains neutral. Again, if the salt contains the cation of a strong base and
the anion of a strong acid, the solution
will remain neutral. However, if the
salt contains the cation of a strong base
and the anion of a weak acid, its solution will be basic (increasing pH), as
is the case with Na2CO3 (soda ash).
Conversely, if the salt contains the
cation of a weak base and the anion of
a strong acid, its solution will be acidic
(decreasing pH). Naturally, if an acid is
added, the pH would decrease, whereas
the pH would increase if a base were
added to a neutral solution.
Figure 9: pH scale, acids and bases.
Note that low pH values correspond
to acids and low pOH values correspond
to basic solutions. A change of one pH
or pOH unit corresponds to a change in
molar concentration by a factor of 10. A
solution with a pH of 2 is not twice as
acidic as a solution with a pH of 4; it is
100 times as acidic as a solution with a
pH of 4.
Remember, the value of Kw changes
with temperature and ionic concentration (salinity) so that pH values measured with an electronic pH probe may
not be valid unless the instrument
(meter or probe) and measurement
are compensated for the temperature
of the liquid and the salinity of the
fluid. High-salinity pH measurements
may require the use of a special “saltcompensated” pH probe.
As discussed previously in regard to
salts, when NaCl (a neutral salt formed
by the combination of a strong acid
and a strong base) is dissolved in water,
the Na+ ions do not combine with OH–
ions (to reduce pH), because NaOH is a
strong base. Likewise, the Cl– ions do
not combine with H+ ions (to increase
pH) because HCl is a strong acid. As a
result, neither H+ ions nor OH– ions is
Basic Chemistry
4A.19
ALKALINITY
Alkalinity titrations determine OH–,
HCO3– and CO32– concentrations by
measuring the amount of acid required
to reduce pH. Borates, silicates, phosphates, sulfates and organic acids (like
lignite) also can enter into the titration
and/or treatment calculations based on
alkalinity values. Alkalinity is the combining power of a base measured by the
quantity of acid which can react to form
a salt. In mud engineering, phenolphthalein alkalinity (P) is reported as the
number of milliliters of 0.02 N H2SO4
(water-base muds) required to titrate a
milliliter of filtrate (Pf) or mud (Pm),
reducing the pH to 8.3. The methyl
orange filtrate alkalinity (Mf) measures
the acid required to reduce pH to 4.3.
Alkalinity is not the same as pH,
although their values usually trend in
the same direction. A strong base, such
as CAUSTIC SODA, added to pure water
would show this correlation between
alkalinity titration values and pH, as
shown in Table 7; however, due to the
presence of HCO3–, CO32– — as well
as calcium and magnesium — in oilfield waters and drilling fluids, no
correlation should be made.
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4A
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Basic Chemistry
pH
7
8
9
10
11
12
13
14
NaOH
(lb/bbl)
0.0000014
0.000014
0.00014
0.0014
0.014
0.14
1.4
14
Pf
OH–
(cc 0.02N H2SO4) (ppm)
0.000005
0.0017
0.00005
0.017
0.0005
0.17
0.005
1.7
0.05
17
0.5
170
5
1,700
50
17,000
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Table 7: Relationship of pH and alkalinity for pure water.
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This table shows how small concentrations of caustic soda (NaOH) in pure
water cause relatively high pH values
and filtrate alkalinity. To observe the
effect of a more complex ionic environment, compare the higher amount
of caustic required to increase pH in
seawater as shown later in Figure 10.
Alkalinity measurements (Pf, Mf
and other values) are used to calculate
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hydroxyl, bicarbonate and carbonate
concentrations as described in API
13B-1, Section 8, Table 8.1, and in the
Testing chapter. Because the Mf value
can be an unreliable indication of bicarbonate contamination if organic acids
or organic salts (like lignite or acetate)
are used, an alternative procedure uses
a pH measurement and the Pf value to
calculate the carbonate and bicarbonate concentration. These calculations
help monitor and diagnose carbon
dioxide, bicarbonate and carbonate
contamination. In addition, these values give the mud engineer a more
thorough understanding of the ionic
and buffering environment of the
mud system, far beyond what can
be learned from a pH value alone.
Acids, Bases and Salts
All acids
contain
hydrogen.
Acids can be described as substances
which have a sour taste, cause effervescence in contact with carbonates, turn
blue litmus red and react with bases,
alkalis, and certain metals to form salts.
All acids contain hydrogen. Acids are
termed as “strong” or “weak,” according to the concentration of hydrogen
ion (H+) that results from ionization.
Bases can be described as having a
bitter taste, a “slippery” feeling in solution, an ability to turn red litmus paper
blue, and an ability to react with acids
to form salts. Bases do not cause effervescence in contact with carbonates.
Acids react with bases to form salts. A
base is termed strong or weak, depending on the quantity of the molecule
which disassociates into hydroxyl ions
(OH–) in solution.
Basic Chemistry
4A.20
Both acids and bases can be either
strong or weak, depending on the
elements in the compound and
their valence.
Salts are simply combinations of the
anion (negative ion) from an acid and
the cation (positive ion) from a base. A
salt may be neutral or have a tendency
toward the acid or base side, depending
on the relative strengths of the respective ions or groups. As discussed previously, the combination of a weak acid
and a strong base forms an alkaline
salt, while the combination of a strong
acid with a weak base forms an acidic
salt, and the combination of a strong
acid with a strong base forms a neutral
salt. Table 8 is a list of the most common acids, bases and salts that are
used in drilling fluids.
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CHAPTER
4A
Basic Chemistry
Chemical Name
Hydrochloric acid
Sulfuric acid
Nitric acid
Phosphoric acid
Carbonic acid
Citric acid
Sodium hydroxide
Potassium hydroxide
Magnesium hydroxide
Sodium carbonate
Calcium hydroxide
Calcium oxide
Sodium chloride
Potassium chloride
Calcium chloride
Calcium sulfate
Common Name
Muriatic acid
—
Aqua fortis
Orthophosphoric
Soda (sparkling)
—
Caustic soda
Caustic potash
Magnesium hydrate
Soda ash
Slaked lime
Quicklime
Rock salt
Potash (muriate of)
—
Anhydrite (gyp)
Formula
HCl
H2SO4
HNO3
H3PO4
H2CO3
H3C6H5O7
NaOH
KOH
Mg(OH)2
Na2CO3
Ca(OH)2
CaO
NaCl
KCl
CaCl2
CaSO4 ( • 2H2O )
Type
Acid (strong)
Acid (strong)
Acid (strong)
Acid (mod. weak)
Acid (weak)
Acid (weak)
Base (strong)
Base (strong)
Base
Base (weak)
Base (strong)
Base (strong)
Salt
Salt
Salt
Salt
Table 8: Common acids, bases and salts.
BUFFER
Many oilfield
liquids and
mud-treating
chemicals are
buffered
solutions.
SOLUTIONS
Certain solutions called buffer solutions resist large pH changes when
a base or acid is added to a solution.
Many oilfield liquids and mud-treating
chemicals are buffered solutions.
Buffer solutions generally consist of
either a weak acid and a salt that contain the same anion or a weak base and
a salt that contain the same cation. The
buffering action of a solution consisting of a weak acid plus a salt of the
acid comes about because: (1) added
base reacts with the weak acid to form
more of the common ion and (2)
added acid reacts with the common
ion to produce the weak acid. An
example of a weak acid is carbonic acid
(H2CO3). An example of a weak base is
ammonium hydroxide (NH4OH). If a
small amount of strong acid, such as
HCl, is added to pure water or to a
dilute solution of acid in water, the
hydrogen ion concentration (pH) of
the water or solution is noticeably
increased. If the same small amount
of acid is instead added to a buffered
Basic Chemistry
4A.21
solution of a weak acid and the soluble
salt of that acid, the increase in hydrogen ion concentration (pH) is so slight
that for all practical purposes it is negligible. The anions of the salt of the weak
acid have taken the H+ ions as fast as
they were added and have reacted with
them to form more of the weak acid.
The net result is that the hydrogen ion
concentration has changed only very
slightly; and the pH is little changed.
This phenomenon can take place
very easily when the mud engineer is
titrating (with an acid) the alkalinity
endpoints. In fluids with high carbonates, bicarbonates and hydroxides, as
soon as carbonates are converted to
bicarbonates, a buffer solution begins
to develop which resists changes in pH.
Triethanolamine, lime and magnesium oxide are all chemicals used
to buffer pH-sensitive mud systems.
Buffering can be highly beneficial to
maintain stable fluid properties and
to resist the detrimental effects of
various contaminants.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
ELECTROLYTE
All solutions
of ionic compounds are
electrolytes.
Substances whose water solutions conduct electricity are called electrolytes
and contain both positive and negative
ions. All solutions of ionic compounds
are electrolytes. All acids, bases and salts
are electrolytes. Electrovalent compounds are formed by transfer of electrons (ionic bonds); in the process,
positive and negative ions are produced.
The resulting solid compounds (salts
and hydroxides) all have an ionic crystal
lattice structure. This means that, in the
case of these electrovalent compounds,
the ions are formed when the compound is formed. They exist before the
compound is dissolved in water. When
such a compound is dissolved, the ionic
crystal lattice is broken down and the
ions disassociate in solution; the water
merely acts as a solvent. Typical equations to illustrate the disassociation of
these already existing ions are:
Salt: NaCl = Na+ + Cl–
(as shown in Figure 6)
Lime: Ca(OH)2 = Ca2+ + 2OH–
Since the ability of a solution to conduct electricity is dependent upon the
presence of ions, it can be concluded
that solutions that are excellent conductors contain high concentrations of
ions (completely ionized), while solutions that are poor conductors contain
low concentrations of ions (not completely ionized). Electrolytes that are
completely ionized are called strong
electrolytes, whereas those that are not
completely ionized are called weak electrolytes. With very few exceptions, salts
are strong electrolytes. Most hydroxides
(except Mg2+) are strong electrolytes and
accordingly classified as strong bases.
Acids such as HCl, H2SO4 and HNO3
are strong electrolytes and, therefore,
are classified as strong acids; most others are moderately weak and are classified as weak acids. Pure water is a weak
electrolyte, and not as conductive as
salt solutions.
Osmosis
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Osmosis is a phenomenon that takes
place when two solutions of greatly different solute concentration (salinity)
are separated by a semi-permeable
membrane. During osmosis, there is
a net movement of solvent (water)
through the membrane from the solution with a lower solute concentration
(lower salinity) to the solution with the
higher solute concentration (higher
salinity). Therefore, osmosis will tend
to transfer solvent until the two solutions have a similar solute concentration (salinity). The driving forces in
this process are the difference in solute
concentration and the character of the
semi-permeable membrane.
The “activity” of a solution is a measure of the vapor pressure or “relative
humidity” and is related to solute concentration (salinity). Water would have
an activity of 1.0; higher salinity reduces
activity. In drilling water-sensitive shales,
it is desirable for the drilling fluid and
formation to have a similar activity to
minimize the transfer of water from
the drilling fluid to the formation. Oiland synthetic-base fluids have the
potential to transfer water from their
emulsified water phase (usually calcium
chloride brine) if it’s activity is higher
than the activity of the formation
through osmosis.
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Basic Chemistry
4A.22
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Titrations
The chemical
tests made
in mud
analyses
are called
titrations.
Chemicals
used to
determine the
endpoint in
titrating
are called
indicators.
The chemical tests made in mud analyses are called titrations. Titrations are
procedures which use standard solutions of a known concentration (N1)
to determine the unknown concentration (N2) of a sample of known volume
(V2). The basic equation involving this
quantitative analysis is:
V2 x N2 = V1 x N1
Solving this equation for N2 gives
the following:
V
N2 = 1 x N1
V2
For a known sample volume (V2),
using an indicator and titrating with a
solution of known concentration (N1),
it is possible to determine the unknown
concentration (N2) of the sample by
measuring the volume (V1) required to
reach the endpoint. Proper procedures
are outlined in the Testing chapter for
quantitative measurements using standard solutions for determining the
Indicator
Original Color
Phenolphthalein
Pink/red: pH >8.3
Methyl orange/
Green: pH >4.3
brom cresol green
Methyl orange
Yellow/orange: pH >4.3
Brom cresol green
Blue: pH >3.8
Thymolphthalein
Colorless: pH <9.5
Methyl red
Yellow: pH >5.4
Calmagite or manver
Wine red:
or erio black T
Presence Ca2+ and Mg2+
CalVer II or Calcon
Wine red:
Presence Ca2+
Potassium chromate
Yellow
solution
important chemicals. Care should be
exercised to follow the exact procedure.
Formulas are also provided which will
enable the engineer to make the necessary calculations without having to
convert units.
INDICATORS
Chemicals used to determine the endpoint in titrating are called indicators.
Indicators are compounds that change
color with either a change in pH or a
change in chemical concentration. The
color changes of acid-base indicators do
so at a specific pH value. Different indicators change color in acidic, neutral or
basic pH conditions. Chemical indicators are used which change color in the
presence of calcium, magnesium chlorides and bromides. Table 9 lists the
most common indicators used in mud
analysis with the titrating chemical
used and the color change the indicator
undergoes at a specific condition.
Color Change
Colorless: pH <8.3
Yellow: pH <4.3
Titration
Pm Pf Pom
Mf
Titrating Chemical
Sulfuric acid
Sulfuric acid
Pink/red: pH <4.3
Mf
Sulfuric acid
Yellow: pH <3.8
—
Sulfuric acid
Blue: pH >9.5
—
Caustic solution
Pink/Red: pH <5.4
PHPA
Sulfuric acid
Blue/purple:
Total hardness Standard Versenate
Absence Ca2+ and Mg2+
(0.01m EDTA)
Blue/purple:
Calcium
Standard Versenate
Absence Ca2+
(0.01m EDTA)
Orange/red:
Chlorides
Silver nitrate solution
Excess AgNO3
Table 9: Common indicators.
Basic Chemistry
4A.23
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CHAPTER
4A
Basic Chemistry
Concentrations of Solutions
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If the net
positive
valence is 1,
normality
and molarity
will have
the same
numerical
value.
Molality (m). A molal solution is a
solution which contains one mole of
solute per kilogram of solvent. Thus, a
1 m (molal) solution of NaOH would
be 40 g of NaOH per 1,000 g of water.
Molarity (M). A solution which contains one mole of solute per liter of solution is called a molar solution. Thus,
0.1 M (molar) HCl is a hydrochloric acid
solution which contains 1/10 mole or
3.646 g of hydrogen chloride per liter
of solution.
If the normality (N) (see below) of a
solution is known, then the molarity
(M) can be calculated by dividing the
normality by its net positive valence.
Molarity =
Normality ÷ net positive valence
Normality (N). A 1.0 normal (N) solution is defined as a solution with a concentration that contains 1 g-equivalent
weight of a substance per liter of solution and is usually written as just 1.0 N.
For example, 1.0 N solution of HCl has
36.5 g solute per liter of solution.
Likewise, a 1.0 N solution of H2SO4
has 49 g solute per liter of solution.
If the Molarity (M) of a solution is
known, the Normality (N) can be calculated by multiplying the molarity of
the solute by its net positive valence.
Normality =
Molarity x net positive valence
If the net positive valence is 1, normality and molarity will have the
same numerical value.
Basic Chemistry
4A.24
Milligrams per liter (mg/l).
Milligrams per liter is a weight-volume
relationship. A 100-mg/l solution contains 100 mg of solute per liter of solution. Milligrams-per-liter concentrations
are often improperly reported as parts
per million, which is a weight-weight
relationship. Milligrams per liter can
be converted to parts per million if the
fluid density is known, by dividing
the mg/l value by the specific gravity
of the solution.
Parts per million (ppm). Parts per
million, abbreviated “ppm” is simply
the concentration by weight of one
chemical expressed in per million parts
of the total. It is normally used to
measure small concentrations. It is the
same as the weight fraction (decimal)
times one million (1,000,000) or the
weight percent times 10,000. Saturated
salt water, for example, is 26% salt by
weight; therefore, it would contain
260,000 ppm salt (26 x 10,000 =
260,000). Parts per million also can be
calculated directly from a titration
which yields a concentration in milligrams per liter by dividing the mg/l
value by the specific gravity of the
solution.
Equivalent Parts per Million (EPM).
EPM is the unit chemical weight of
solute per million unit weights of solution. The EPM of a solute in solution is
equal to the parts per million divided
by the equivalent weight.
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4A
Basic Chemistry
Concentration*
1 m (molal)
1 M (Molar)
1 N (Normal)
100,000 mg/l
100,000 ppm
Solute Weight
1 g mole wt
1 g mole wt
1 g equiv wt
100,000 mg
100,000 mg
Solvent Weight
1,000 g
—
—
—
900,000 mg
Solution Volume
—
1 liter
1 liter
1 liter
—
Solution Weight
1 g mole wt + 1,000 g
—
—
—
1,000 g
Table 10: Solution concentrations.
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* There is no standard starting solvent
volume for any of these solution concentrations. A specific weight of solute is
either added to a specific weight of solvent or solvent volume is added until a
final volume of solution is obtained.
Example: For a simple 1.148 g/cm3 (SG)
sodium chloride solution with a volume
of 1,000 cm3 which contains 230 g of
dissolved salt, calculate the following
compositions and concentrations:
a) Composition in weight %
b) Composition in volume %
c) Molality
d) Molarity
e) Normality
f) mg/l sodium chloride
g) ppm sodium chloride
h) EPM
i) Weight ratio of NaCl to H2O, (lb/lb)
Total solution weight =
1,000 cm3 x 1.148 g/cm3 = 1,148 g
Water weight = total wt - salt wt =
1,148 - 230 = 918 g
a) Composition in weight %:
Weight % NaCl =
(230 ÷ 1,148) x 100 = 20.0%
Weight % H2O =
(918 ÷ 1,148) x 100 = 80.0%
b) Composition in volume %:
SG pure water (20°C) = 0.998 g/cm3
Volume of pure water =
(918 ÷ 0.998) = 920 cm3
Volume % of water =
(920 ÷ 1,000) x 100 = 92%
Volume of sodium chloride =
1,000 - 920 = 80 cm3
Volume % sodium chloride =
(80 ÷ 1,000) x 100 = 8%
c) Molality — moles of solute per
kilogram of solvent:
Basic Chemistry
4A.25
Molecular weight sodium chloride:
58.44
Gram-moles NaCl =
(230 ÷ 58.44) = 3.94
Since there are only 918 g of water,
not 1 kg (1,000 g), then the number
of gram-moles must be adjusted for
the 1 kg value.
Molality = 3.94 x (1,000 ÷ 918) =
4.287 gram-moles NaCl per kg
d) Molarity — moles of solute per
1 liter of solvent:
From c) above, there are 3.94 grammoles NaCl in the original 1,000-cm3
(1-l) solution.
Molarity = 3.94 gram-moles-per-liter
e) Normality — gram equivalent
weight per liter solution:
Since sodium chloride has a net positive valence of 1, normality is the
same number as molarity.
Normality = 3.94 gram-equivalent
weights per liter
f) mg/l sodium chloride:
Since 230 g (230,000 mg) sodium
chloride are contained in the 1,000-cm3
(1-l) solution:
mg/l sodium chloride =
230,000 mg ÷ 1 l = 230,000 mg/l
g) ppm sodium chloride — weight
ratio x 1,000,000:
ppm sodium chloride = (230 ÷ 1,148)
x 1,000,000 = 200,350 ppm
h) EPM — ppm divided by
equivalent weight:
EPM sodium chloride =
(200,350 ÷ 58.44) = 3,428 EPM
i) Weight ratio NaCl to H2O:
Grams NaCl ÷ grams H2O =
(230 ÷ 918) = 0.2505
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
Mixtures, Solutions, Emulsions and Dispersions
A solution is
a homogenous mixture
of two
or more
substances.
All of these terms have significant
meanings to the mud engineer. The subtleties in meaning often are overlooked
and not appreciated.
A mixture is a combination of two
or more substances with no consistent
composition throughout. Mixtures of
different kinds of nuts or candies are
common. Each substance in a mixture
retains its unique properties.
A solution is a homogenous mixture
of two or more substances. A solution
has a consistent composition throughout. Solutions can be solids in solids
(metal alloys), solids in liquids (sugar
in water), liquids in liquids (alcohol in
water), gases in liquids (carbonated
beverages) or gases in gases (air).
Whenever two substances that can
react with each other are in solution,
they will usually do so.
A dispersion is a two-phase system
in which one phase consists of finely
ground solid particles distributed in
the second phase. Drilling fluids with
clays and solids are dispersions.
An emulsion is a stable mixture of
immiscible liquids held together by
emulsifiers. Diesel oil and calcium
chloride brine are not soluble in each
other; however, they can be combined
to form an emulsion with the brine
being emulsified in the diesel oil to
form oil-base mud systems.
Common Chemical Reactions in Mud Chemistry
This section concerns mud chemistry
problems commonly facing most mud
engineers. Reactions with various treatment chemicals are illustrated with resultant precipitants or results. For a more
detailed discussion of these reactions,
see the chapter on Contamination
and Treatment.
used with caution for this purpose,
because SAPP reduces pH and is not
stable at high temperatures as a mud
thinner. The reaction of SAPP with
gypsum is as follows:
Na2H2P207 + H2O → 2NaH2PO4;
2NaH2PO4 + 3CaSO4 →
Ca3(PO4)2↓ + 2Na+ + 3SO42 – + 4H+
_______________________
GYP
CEMENT
_______________________
The calcium ion, which can be derived
from gyp or when drilling anhydrite,
is a contaminant in most water-base
muds. One way to reduce calcium is to
add soda ash. The following equation
illustrates how calcium sulfate (or gyp)
can be removed using treatments of
sodium carbonate (soda ash) to form
calcium carbonate. The downward
arrow implies precipitation of an insoluble solid, whereas an upward arrow
would imply generation of a gas.
CaSO4 + Na2CO3 →
CaCO3 ↓ + 2Na+ + SO42 –
Sodium Acid Pyrophosphate (SAPP)
can also be used for treating anhydrite contamination. It should be
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
OR ANHYDRITE CONTAMINATION
Basic Chemistry
4A.26
CONTAMINATION
Cement contains lime, Ca(OH)2, a
source of calcium and a flocculant
which can be removed by adding
sodium bicarbonate (NaHCO3). The
reaction is as follows:
Ca(OH)2 + NaHCO3 →
CaCO3↓ + NaOH + H2O
This reaction forms sodium hydroxide and will result in elevated pH even
after the calcium from the cement is
removed. For severe cement contamination, an acid should be used in combination with sodium bicarbonate to
maintain an acceptable pH value.
Common acids used with sodium
bicarbonate are citric acid, acetic
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
acid, SAPP, and organic acids like
lignite or lignosulfonate.
SEAWATER
CARBON
DIOXIDE GAS CONTAMINATION
Carbon dioxide (CO2) is an acidic gas
which exists in many formations and is
a common mud contaminant. CO2 in
an aqueous solution will form carbonic
acid (H2CO3), which will convert to
bicarbonate groups (HCO3–) at medium
pH values, then carbonate groups (CO32–)
at higher pH, as illustrated previously
in Figure 7. Small influxes may be
treated with caustic soda:
CO2 + H2O → H2CO3 (carbonic acid);
2NaOH + H2CO3
2Na+ + CO32+ + 2H2O (pH > 11)
Massive influxes should be treated
with lime. Be advised that the calcium
carbonate precipitate may cause scale
to form on the surface of the drillstring and is difficult to remove from
the fluid with solids-control equipment
due to the ultra-small (submicron)
particle size.
H2O
Ca(OH)2 + CO2 → CaCO3 ↓ + H2O
14
13
Distilled water
12
Freshwater mud
Seawater
11
pH
Caustic soda
is used to
reduce the
magnesium
and calcium
in seawater…
Magnesium ions and calcium ions are
present in seawater. Both of these ions
are detrimental to water-base muds.
Since magnesium hydroxide (Mg(OH)2)
and calcium hydroxide (Ca(OH)2) are
relatively insoluble at higher pH, caustic should be used to remove magnesium and suppress the solubility of
calcium. The reactions are as follows:
Mg2+ + 2NaOH →
Mg(OH)2↓ + 2Na+ (pH >10)
Ca2+ + 2NaOH →
Ca(OH)2↓ + 2Na+ (pH >11)
Caustic soda is used to reduce the
magnesium and calcium in seawater
by first precipitating magnesium as
Mg(OH)2, and then increasing pH to
suppress the solubility of calcium and
precipitate lime. If lime is used in seawater it, too, will remove magnesium,
but the resulting calcium levels will be
very high and are undesirable. Gulf of
Mexico seawater requires 1.5 to 2 lb/bbl
caustic soda (4.3 to 5.7 kg/m3) to precipitate all magnesium, then convert the
calcium to lime, resulting in a pH >11.0
(see Figure 10). In seawater, the preferred
treatment for magnesium removal is
caustic, while the preferred treatment
for calcium removal is soda ash.
10
Mg removed — Ca
begins to convert to
Ca(OH)2
9
Magnesium begins
to precipitate as
Mg (OH)2
8
7
0
0.5
1.0
1.5
2.0
Caustic soda (lb/bbl)
Seawater: pH 8.0, 390 mg/l Ca, 1,300 mg/l Mg, 19,000 mg/l Cl
2.5
Figure 10: pH vs. caustic soda for seawater.
Basic Chemistry
4A.27
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4A
Basic Chemistry
CARBONATE
AND BICARBONATE
CONTAMINATION
…H2S should
always be
precipitated
with a source
of zinc…
Bicarbonate (HCO3–) and carbonate
(CO32–) contamination can occur
through the conversion of CO2 gas,
mentioned above, or from the thermal
degradation of organic additives such as
lignite and lignosulfonate and from biodegradation of starch and other additives, among other sources. These ions
can be removed with calcium. However,
since calcium bicarbonate Ca(HCO3)2 is
soluble, all bicarbonates must be converted into carbonates (above a pH of
about 11) before they can be completely
precipitated as calcium carbonate. The
removal of bicarbonates and carbonates
can be achieved with any source of soluble calcium under conditions of constant pH (if pH is high enough) or by
increasing the pH with caustic soda in
the presence of calcium.
Lime (Ca(OH)2) is preferred for converting HCO3– to CO32– and then precipitating the carbonates as CaCO3,
especially if the system pH will not be
elevated to >11.
2HCO3– + Ca(OH)2 →
CaCO3↓ + OH– + H2O
CO32– + Ca(OH)2 → CaCO3↓ + 2OH–
When a constant pH value must be
maintained, a combination of gyp and
lime treatments is required:
Ca(OH)2 + 2HCO3– →
CaCO3↓ + CO32– + 2H2O
CaSO4 • 2H2O + CO32– →
CaCO3↓ + SO42– + 2H2O
HYDROGEN SULFIDE
(H2S) CONTAMINATION
Hydrogen
sulfide is a
poisonous
and
dangerous
acidic gas…
Hydrogen sulfide is a poisonous and
dangerous acidic gas encountered in
many formations and produced fluids.
It can quickly deaden senses and can be
fatal even at low concentrations. H2S is
characterized by its typical “rotten egg”
smell. For safety reasons, it should be
neutralized immediately with caustic
Basic Chemistry
4A.28
soda or lime to increase the pH to >11.5
to form sulfide (S2–), then precipitated
with a source of zinc.
Neutralization with caustic soda:
2NaOH + H2S
2Na+ + S2– + 2H2O (at pH >11.5)
Neutralization with maintaining
excess lime:
H2S + Ca(OH)2 →
Ca2+ + S2– + 2H2O (pH >11.5)
Converting hydrogen sulfide into just
sulfide by increasing pH is not a permanent reaction. If the pH were to
drop into the acid region, sulfide will
convert back into the poisonous hydrogen sulfide form. For this reason, H2S
should always be precipitated with a
source of zinc, such as zinc oxide.
Removal by precipitation with
treatments of SULF-XT (ZnO):
H2S + ZnO → ZnS↓ + H2O
REMOVAL
OF OXYGEN WITH
OXYGEN SCAVENGER
Dissolved oxygen will cause increased
corrosion and can be removed with
treatments of a chemical containing
sulfite. Liquid ammonium bisulfite
solutions are the most common oxygen scavengers and react with oxygen
as follows:
NH4HSO3 + 1⁄2 O2 + OH– →
NH4 + SO3 + H2O;
SO3 + 1⁄2 O2 → SO4
Soluble calcium may react to form
calcium sulfite, which has a maximum
solubility of about 43 mg/l in cold
water and less in hot water. The sulfite
residual at the flow line should not be
expected to exceed this level if the mud
contains high levels of calcium.
ACID
TREATMENTS
The filter cakes containing acid-soluble
weight materials, such as calcium carbonate used for non-damaging drill-in
fluids, are often removed from the well
with hydrochloric acid treatments.
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CHAPTER
4A
_______________________
_______________________
Basic Chemistry
Calcium carbonate dissolution using
hydrochloric acid:
CaCO3 + 2HCl →
CaCl2 + CO2↑ + H2O
_______________________
PHOSPHATES
_______________________
_______________________
Sodium Acid Pyrophosphate (SAPP)
and other phosphates will react with
and precipitate calcium according to
the following reaction:
Na2H2P2O7 + H2O → 2NaH2PO4;
2NaH2PO4 + 3Ca(OH)2 →
Ca3 (PO4)2↓ + 4H2O +2NaOH
Although the reaction has caustic as
a byproduct, pH is still reduced, since
SAPP is a weak acid and the reaction
leaves only two hydroxyl groups for
every six original hydroxyl groups.
_______________________
EFFECT
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
OF LIGNITE ON CALCIUM
Lignite contains calcium (from 1.5 to
5%) as part of its chemical makeup.
Organic acids like lignite also have the
ability to tie up calcium chemically.
This complexed calcium exists as a solid
particulate (flock) and may not be large
Basic Chemistry
4A.29
enough to be removed by mechanical
means. The filter cake will filter out
most of the calcium complexed with
the lignite. However, some lignitecomplexed calcium will pass through
the filter cake and paper so that it will
be collected with the filtrate. During
the titration of filtrate calcium, the
readily available calcium is titrated
first. As the titration continues, the
complexed calcium solubilizes and
this, too, will be titrated. This complexed calcium is not available for
chemical reactions in mud systems.
One lb/bbl of solubilized lignite has
the potential to complex 200 mg/l calcium. Keep in mind that although the
calcium titration may show a sufficient
excess of calcium, none may be available to react with carbonate and bicarbonate groups. So, it may be possible
to be experiencing a carbonate problem even though some calcium is
titrated in the filtrate when using
lignite as an additive.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
Introduction
…it is
necessary to
understand
basic clay
chemistry
in order
to properly
control
water-base
muds.
…clay
minerals…
are used
to provide
viscosity, gel
structure and
fluid-loss
control.
A thorough understanding of clays can
be the mud engineer’s most valuable
tool. Clay may be added intentionally,
such as M-I GELT, or it may enter the
mud as a major contaminant through
dispersion of drill solids. In either case,
it becomes an active part of the system.
For this reason, it is necessary to understand basic clay chemistry in order
to properly control water-base muds.
Clay chemistry is also important with
regard to interactions between waterbase muds and shales which affect
wellbore stability.
Clay is a broad term commonly used
to describe sediments, soils or rocks
consisting of extremely fine-grained
mineral particles and organic matter.
A good example is the clays (or sometimes called gumbo clays) found in the
backyard or along riverbanks. These
clays are often soft and plastic when
wet, but become hard when dry. This
“soft when wet, hard when dry” physical property can be related to the presence of certain clay minerals. Clay is
also used as a group term for particles
with a size less than 2 microns in
diameter, which includes most of
the clay minerals.
Clay minerals are fine-grained
aluminum silicate minerals having
well-defined microstructures. In mineralogical classification, clay minerals
are classified as layered silicates because
the dominant structure consists of layers formed by sheets of silica and alumina. Each sheet is a thin, plate-like
structure and is called a unit layer. A
typical layered silicate mineral, for
example, is mica or vermiculite, which
can be split into thin layers along the
cleavage planes. Most clay minerals are
platy in morphology. Depending on
the repeating units of the structure,
Clay Chemistry
4B.1
clay minerals can be further classified
as to the ratio of silica to alumina layers such as 1:1, 2:1 and 2:2, as well as
according to whether they are layered
or needle-shaped clay minerals.
In the drilling fluids industry, certain clay minerals such as smectite, a
major component of bentonite, are
used to provide viscosity, gel structure
and fluid-loss control. Formation clays
are unavoidably incorporated into the
drilling fluid system during drilling
operations and they may cause various problems. Thus, clay minerals
can be beneficial or harmful to the
fluid system.
The term bentonite is used for
commercially-mined sodium montmorillonite (which is a form of smectite)
that is used as an additive for drilling
mud (i.e. M-I GEL or GEL SUPREMEE).
Geologically, bentonite is a bed of altered
volcanic ash. One of the biggest
deposits of this volcanic ash occurred
over 60 million years ago in areas of
North America now known as the
Blacks Hills of Wyoming and South
Dakota, and the Big Horn Mountains
of Wyoming. Bentonite clay mined in
Wyoming actually comes from this
volcanically deposited bentonite bed.
Bentonite clay mined in other areas of
the world may be from other types of
geological deposits.
Because of their small particle sizes,
clays and clay minerals are analyzed
with special techniques such as x-ray
diffraction, infrared absorption and
electron microscopy. Cation Exchange
Capacity (CEC), water adsorption and
surface area are some of the properties
of clay minerals that are often determined in order to better characterize
clay minerals as well as to minimize
drilling problems.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
Types of Clays
Clays exist
in nature
with a
stacked or
layered
structure…
Separating
these packs
into multiple
layers is
known as
dispersion.
There are a large number of clay minerals, but those with which we are
concerned in drilling fluids can be
categorized into three types.
The first type are needle-shaped,
non-swelling clays like attapulgite or
sepiolite. It is believed that the shape
of the particles is responsible for the
clay’s ability to build viscosity. The natural fine crystal size and needle shape
causes it to build a “brush heap” structure in suspension and thereby exhibit
high colloidal stability even in the
presence of high electrolyte concentration. Owing to their shape and
non-swelling characteristics, these clays
exhibit very poor filtration control. For
this reason, attapulgite is primarily used
as a viscosity builder in saltwater muds
and sepiolite is most often used as a
supplemental viscosifier for geothermal
and high-temperature fluids. These
clays are rarely, if ever, present in formation shales. M-I sells attapulgite
under the name SALT GELT and sepiolite
under the name DUROGELT.
The second type are the plate-like,
non-swelling (or slightly swelling)
clays: illite, chlorite and kaolinite,
discussed later.
The third type are the plate-like,
highly swelling montmorillonites.
The second and third types of clay
minerals are found in formation shales
in the following order in decreasing
amounts: (1) illite, (2) chlorite, (3)
montmorillonite and (4) kaolinite.
Because these clays are present in
drilled formations, they become dispersed in the drilling fluid system in
varying amounts. The montmorillonite
in shales is usually calcium montmorillonite since it is in equilibrium with
the formation water, which is normally
rich in calcium.
Clay Chemistry
4B.2
Sodium montmorillonite (Wyoming
bentonite, M-I GEL and GEL SUPREME)
is also normally added to a mud to
increase viscosity and reduce fluid loss.
The filtration and rheological properties of the mud become a function of
the amounts of various clays contained
in the mud. Since the montmorillonite
is intentionally added to a mud to control these properties, the other clay
types may be considered contaminants, as they are not as effective
as a commercial clay.
Clays exist in nature with a stacked or
layered structure, with each unit layer
roughly 10 angstroms (Å) thick. This
means there are about a million layers
of clay per millimeter of thickness. Each
clay layer is highly flexible, very thin
and has a huge surface area. An individual clay particle can be thought of
as being much like a sheet of paper or
a piece of cellophane. One gram of
sodium montmorillonite has a total
layer surface area of 8,073 ft2 (750 m2)!
In freshwater, the layers adsorb water
and swell to the point where the forces
holding them together become weakened and individual layers can be separated from the packs. Separating these
packs into multiple layers is known as
dispersion. This increase in number of
particles, with the resulting increase in
surface area, causes the suspension to
thicken. Figure 1 is an actual photomicrograph of a bentonite particle. Note
that it resembles a fanned-out deck of
cards. Several of the plate-like particles
can be seen overlapping each other. It
is this characteristic shape of the particles that produces the so-called “shingling” effect that is so important to
fluid-loss control.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
2µ
2µ
ca
Sili
a
i
mn
Alu
ca
Sili
Tetrahedral
Octahedral
Tetrahedral
One
unit
layer
10 Å
Figure 2: Idealized montmorillonite particle.
sheets. Three-layer clays are built of
unit layers composed of two tetrahedral sheets on either side of one octahedral sheet, somewhat like a sandwich
(see Figure 2). Two-layer clays are built
of unit layers consisting of only one
tetrahedral and one octrahedral sheet.
Clays can either be electrically neutral or negatively charged. For example, pyrophyllite [Al2Si4O10 – (OH)2],
a neutral clay, as shown in Figure 3,
is similar to the negatively charged
montmorillonite.
Figure 1: Photomicrograph of bentonite particles.
Clays can
either be
electrically
neutral or
negatively
charged.
Clays are usually either of the twolayer type like kaolin or three-layer
type such as montmorillonite, chlorite
or illite. Each plate-like clay particle
consists of a stack of parallel unit layers. Each unit layer is a combination
of tetrahedrical (pyramid) arranged
silica sheets and octahedrical (eightfaced) arranged alumina or magnesia
All surface charges balance
–
O
–
O
O
Si
O
+
–
O
O
Si
Si
O
O
OH
Al
+
Si
+
O
Al
OH
Si
O
–
+
O
O
Si
Si
O
O
OH
Al
O
O
O
O
Si
O
+
Al
OH
Si
O
–
+
O
O
O
O
Si
+
Si
O
O
Al
Al
O
O
OH
O
O
+
+
O
O
Silica
layer
O
Al
Si
O
+
Si
OH
Si
O
O
Si
OH
Al
O
O
Si
O
OH
O
–
Alumina
layer
Silica
layer
Figure 3: Electrically neutral pyrophyllite.
Clay Chemistry
4B.3
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
MONTMORILLONITE CLAYS
(THREE-LAYER CLAYS)
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
If just one atom of magnesium (Mg2+) is
substituted for one atom of aluminum
(Al3+) in the lattice structure (arrangement of atoms), it will then possess a
surplus electron or negative charge
(see Figure 4). The net negative charge
is compensated by the adsorption of
cations (positive ions) on the unit
layer surfaces, both on the interior and
on the exterior surfaces of the stack.
The cations that are adsorbed on the
unit-layer surfaces may be exchanged
for other cations and are called the
exchangeable cations of the clay. The
quantity of cations per unit weight of
the clay is measured and reported as
the CEC. The cation may be a singlecharge ion such as sodium (Na+) or a
double-charge ion such as calcium
(Ca2+) or magnesium (Mg2+). Thus, we
have sodium montmorillonite, calcium
montmorillonite and/or magnesium
montmorillonite. Although Wyoming
bentonite is generally described as
sodium montmorillonite, the exchangeable calcium and magnesium may constitute 35 to 67% of the total exchange
capacity. The most typical property of
montmorillonites is that of interlayer
swelling (hydrating) with water (see
Figures 5 and 6).
Na+
–
O
–
O
O
Si
O
+
O
Si
Si
O
O
OH
Al
+
Si
+
O
Al
OH
Si
O
O
+
–
O
O
Si
Si
O
O
O
O
O
Si
O
Na+
+
Al
OH
Si
O
O
+
–
O
Si
O
O
O
O
Si
+
O
O
O
+
+
O
O
Silica
layer
O
Al
Si
O
+
Si
Al
OH
Si
O
O
OH
Mg
O
O
Si
OH
Al
CLAYS)
Illites have the same basic structure
as montmorillonites, but they do not
show interlayer swelling. Instead of
the substitution of Mg2+ for Al3+ as in
montmorillonite, illite has a substitution of Al3+ for Si4+, still giving a negative charge. The compensating cations
are primarily the potassium ion (K+),
as shown in Figure 6. The net negative
lattice charge that results from these
substitutions, by compensating potassium ions, is usually larger than that
of montmorillonite by as much as one
and a half times.
OH
Mg
O
ILLITES (THREE-LAYER
Surface-bonded cations
Surplus negative charges
–
O
In addition to the substitution of
magnesium (Mg2+) for aluminum (Al3+)
in the montmorillonite lattice, many
other substitutions are possible. Thus,
the name montmorillonite often is used
as a group name including many specific mineral structures. However, in
recent years, the name smectite has
become widely accepted as the group
name, and the term montmorillonite
has been reserved for predominantly
aluminous members of the group. This
group of minerals includes montmorillonite, hectorite, saponite, nontronite
and a number of other specific minerals.
Si
O
OH
O
–
Alumina
layer
Silica
layer
Figure 4: Substitution of Mg 2+ for Al 3+ causing a negative-charged particle.
Clay Chemistry
4B.4
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
Tetrahedral
alumina
OH
Octahedral
silica
OH
Interlayer
distance
OH
Tetrahedral
alumina
Exchangeable cations nH2O (adsorbed water)
Swelling
Next unit
layer
tetrahedral
alumina
Oxygens
and
OH
Hydroxyls
Aluminum, iron, magnesium
Silicon, ocassionally aluminum
Figure 5: Structure of smectite.
Only the
potassium
ions on the
external
surfaces
can be
exchanged
for other
cations.
The spacing between unit layers is
2.8 Å. The ionic diameter of the K+ is
2.66 Å. This allows the K+ to fit snugly
between unit layers forming a bond
that prevents swelling in the presence
of water. Since the unit layers do not
swell and separate when exposed to
water, the potassium ions (K+) between
the unit layers are not available for
exchange. Only the potassium ions on
the external surfaces can be exchanged
for other cations.
Among the 2:1 clay minerals, smectite, illite, and mixed layers of illite and
smectite are encountered during drilling
Clay Chemistry
4B.5
shale formations and often cause various problems in borehole stability and
drilling fluid maintenance. The troublesome nature of these clay minerals can
be related to the weakly-bonded interlayer cations and weak layer charges
that lead to swelling and dispersion
upon contact with water. With increasing burial depths, the smectite gradually
converts into illite/smectite mixed-layer
clays and finally to illite and mica. As
a result, shale formations generally
become less swelling but more dispersive in water with increasing depth.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
CHLORITES (THREE-LAYER
Kaolinite
does not
contain
interlayer
cations or
surface
charges…
CLAYS)
Chlorites are structurally related to the
three-layer clays. In their pure form
they will not swell, but they can be
made to swell slightly with alteration.
In these clays, the charge-compensating cations between montmorillonitetype unit layers are replaced by a layer
of octahedral magnesium hydroxide,
or brucite (see Figure 6). This layer has
a net positive charge because of some
replacement of Mg2+ by Al3+ in the
brucite layer.
Chlorite is often found in old, deeply
buried marine sediments and normally
does not cause significant problems
unless present in large quantities. The
cation exchange capacity of chlorite
varies from 10 to 20 meq/100 g, primarily due to broken bonds. The interlayer
distance of chlorite is usually about
14 Å. Chlorite also may form mixedlayer clays with other clay minerals
such as smectite. The resultant mixedlayer clay would have the properties of
both types of clay minerals.
Group
Kaolinite
Talc
Smectite
Vermiculite
Illite
Mica
Chlorite
Sepiolite
Palygorskite
Structure
1:1 layer
2:1 layer
2:1 layer
2:1 layer
2:1 layer
2:1 layer
2:2 layer
2:1 chain
2:1 chain
Charge
Nil
Nil
0.3 - 0.6
1.0 - 4.0
1.3 - 2.0
2.0
Variable
Nil
Minor
KAOLINITES (TWO-LAYER
CLAYS)
Kaolinite is a non-swelling clay that
has its unit layers bound tightly
together by hydrogen bonding. This
prevents expansion of the particle
because water is unable to penetrate
the layers. Kaolinite does not contain
interlayer cations or surface charges
because there is little or no substitution in either tetrahedral or octahedral
sheets. However, some minor charges
can come from broken bonds or impurities. Therefore, kaolinite has a relatively low cation exchange capacity
(5 to 15 meq/100 g). Kaolinite is commonly found as a minor to moderate
constituent (5 to 20%) in sedimentary
rocks such as shales and sandstones.
A summary of clay minerals is shown
in Table 1 and a schematic comparison
of the various clay structures shown in
Figure 6.
Exchange
Cation
None
None
Na+, Ca2+, K+, Mg2+
K+, Mg2+
K+
K+
Brucite layer
None
None
Interatomic
Distance
(Å)
7.2
9.3
11 - 15
14 - 15
10
10
14
12
10.5
Swelling
None
None
Variable
Variable
Nil
None
Nil
Nil
Nil
Table 1: Commonly encountered clays.
Clay Chemistry
4B.6
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CHAPTER
4B
Clay Chemistry
H2O
H2O
H2O
H2O
Mg(OH)2
KK
H2O
H2O
KK
H2O
1 crystal
Mg(OH)2
1 crystal
H2O
H2O
Mg(OH)2
H2O
Sepiolite
Needle-shaped clay
Mg(OH)2
KK
1 crystal
KK
H2O
H2O
KK
Mg(OH)2
H2O
H2O
1 crystal
H2O
Kaolinite
H2O
Chlorite
(Mg(OH)2 = brucite sheet)
KK
H2O
Illite
(K = potassium)
Plate-like, non-swelling clays
1 crystal
1 crystal
H2O
b
H2O
1 crystal
H2O
H2O
1 crystal
a
Mg(OH)2
KK
c
1 crystal
H2O
H2O
H2O
Chlorite type
Illite type
H2O
“a” Has the properties of chlorite
“b” Has the properties of montmorillonite
“c” Has the properties of illite
H2O
Montmorillonite
Plate-like, swelling clays
Mixed-layer clays
Figure 6: Clay structure comparison.
Cation Exchange Capacity (CEC)
The quantity
of cations
per unit
weight of
clay is…
the CEC.
The compensating cations that are
adsorbed on the unit-layer surface may
be exchanged for other cations and are
called the exchangeable cations of the
clay. The quantity of cations per unit
weight of clay is measured and reported
as the CEC. The CEC is expressed in
millequivalents per 100 g of dry clay
(meq/100 g). The CEC of montmorillonites is within the range of 80 to
150 meq/100 g of dry clay. The CEC
of illites and chlorites is about 10 to
40 meq/100 g, and for kaolinites it is
about 3 to 10 meq/100 g of clay.
The Methylene Blue Test (MBT) is an
indicator of the apparent CEC of a clay.
Clay Chemistry
4B.7
When this test is run on a mud, the
total methylene blue exchange capacity of all the clay minerals present in
the mud is measured. It is normal
procedure to report the Methylene
Blue Capacity (MBC) as the equivalent amount of Wyoming bentonite
required to obtain this same capacity.
It is important to note that the test
does not directly indicate the amount
of bentonite present. However, an estimate of the amount of bentonite and
solids in the mud can be calculated if
one considers that the average drill
solids have about 1/9 the CEC of bentonite, and if the amount of drill solids
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
…any cation
to the left
will replace
any cation
to its right.
Clay Chemistry
present in the mud is calculated from
a retort analysis. This estimation of
the quantity of added bentonite and
drill solids can be made more exact
by measuring the MBC of the drill cuttings. This procedure can be helpful
in estimating both the amount and
quality of the clays in the mud.
In order to have some idea of which
cations will replace other cations in
the exchange positions, the following
is generally accepted and is arranged
in decreasing preference:
H+ > Al3+ > Ca2+ > Mg2+ > K+ > NH4+ >
Na+ > Li+
In other words, any cation to the
left will replace any cation to its right.
The relative concentration of each
cation also affects this cation-exchange
preference. Even though calcium is
more difficult to replace than sodium,
if the ionic concentration of Na+ is
significantly higher than Ca2+, then
sodium will displace calcium. Cation
exchange may result from a change in
temperature since many compounds
have different solubility-to-temperature
relationships. Some of the common
calcium salts, such as CaSO4, decrease
in solubility at high temperatures while
most sodium compounds increase in
solubility. As the Na+/Ca2+ concentration increases, there is a tendency for
the Ca2+ on the clay to be replaced by
Na+ from solution.
Composition of Clay-Water Muds
In most
areas,
commercial
clays…are
added to
water when
preparing a
water-base
mud.
In most areas, commercial clays, such
as M-I GEL and GEL SUPREME, are added
to water when preparing a water-base
mud. The clays serve a dual purpose:
(1) to give viscosity to the drilling fluid,
and (2) to deposit a filter cake that will
seal permeable formations in order to
limit filtration losses and prevent stuck
pipe. In some areas, drilling can be performed starting with water and allowing the drill solids to be incorporated,
resulting in sufficient properties to allow
the well to be drilled. In other situations,
polymer-base systems are used where no
clays are added to the formulation.
Clay-water muds have water as the liquid continuous phase in which certain
materials are held in suspension and
other materials dissolved. Numerous
mud additives are used to obtain special
properties but, basically, all components
can be divided into three categories.
1. The water phase is the continuous
phase of the mud. Depending on
location and/or available water,
this may be freshwater, seawater,
Clay Chemistry
4B.8
hard water, soft water, etc. It is
not uncommon to use a variety
of brine solutions from salty up
to saturation as the base liquid
to build a water-base system.
2. The reactive-solids phase is composed of commercial clays, incorporated hydratable clays and shales
from drilled formations that are held
in suspension in the fluid phase.
These solids are treated chemically
to control the properties of the drilling fluid. Various additives will be
used to obtain desirable properties.
3. Inert solids refer to those solids in
suspension that are chemically inactive. These may be inert drill solids
such as limestone, dolomite or sand.
Barite is added to the drilling fluid
to increase the fluid density and is
also an inert solid.
The remainder of this chapter will
discuss the behavior of the reactive
solids in the water phase and how
this affects mud properties.
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CHAPTER
4B
Clay Chemistry
HYDRATION
The thickness
of the
adsorbedwater film
is controlled
by the type
and amount
of cations…
OF CLAYS
The bentonite crystal consists of
three layers: an alumina layer with a
silica layer above and below it. The clay
platelet is negatively charged and has a
cloud of cations associated with it. If a
significant amount of these cations are
sodium, the clay is often called sodium
montmorillonite. If they are primarily
calcium, then the clay is called calcium
montmorillonite.
Depending on the cations present,
the interlayer spacing of dry montmorillonite will be between 9.8 (sodium)
and 12.1 Å (calcium) and filled with
tightly bound water. When dry clay
contacts freshwater, the interlayer
spacing expands, and the clay adsorbs
a large “envelope” of water. These two
phenomena allow clays to generate
viscosity. As shown in Figure 7, calcium-base bentonites only expand to
17 Å, while sodium bentonite expands
to 40 Å.
The thickness of the adsorbed-water
film is controlled by the type and
amount of cations associated with the
clay. Water adsorbed to the large, flat,
planar surfaces comprises the major
part of the total water retained by
hydratable clays. Divalent cations
such as Ca2+ and Mg2+ increase the
attractive force between platelets, thus
decreasing the amount of water that
can be adsorbed. Monovalent cations
such as Na+ give rise to a lesser attractive force and allow more water to
penetrate between the platelets.
Calcium montmorillonite
Ca2+
17 Å
Ca2+
Silica
Alumina
Silica
Ca2+
10 - 12 Å
Ca2+
Hydration
water
1-2µ
Na+
Na+
+ water
Na+
Sodium or calcium
montmorillonite
Na+
Na+
Na+
40 Å
Na+
Na+
Sodium montmorillonite
Figure 7: Comparison of swelling for calcium and sodium montmorillonite.
Clay Chemistry
4B.9
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CHAPTER
4B
A cation
may serve
as a bond
to hold the
clay mineral
particles
together…
…colligative
properties
are basically
measurements
of the
reactivity
of the clay.
Clay Chemistry
Because sodium bentonite swells four
times as much as calcium bentonite,
sodium bentonite will generate four
times the viscosity. A more comprehensive discussion of the role which
calcium-base exchange plays in calciumtreated systems is covered in the WaterBase Systems chapter.
Smectite, in addition to adsorbing
water and cations on external surfaces,
absorbs water and cations to surfaces
between layers in its crystalline structure. The ability of smectite to adsorb
water is much greater than other clay
minerals. The ability to adsorb water,
the quantity of exchangeable cations
(CEC) and the surface area are closely
related phenomena that are sometimes
termed colligative properties of clay.
These colligative properties are basically measurements of the reactivity
of the clay. Because CEC is easy to
measure, it is a practical method to
assess clay or shale reactivity. The CEC
of clay can be measured with a methylene blue titration. When measuring
the CEC, 0.01 N methylene blue solution is used so the number of milliliters of methylene blue solution
needed to reach the end point is
equal to meq/100 g. The range of
CEC for pure clay mineral materials
is shown in the following table:
Clay
Smectite
Illite
Chlorite
Kaolinite
CEC (meq/100 g)
80 - 150
10 - 40
10 - 40
3 - 10
Table 2: CEC range for pure clay mineral materials.
Smectite is clearly much more reactive
than other clay mineral materials. Shales
containing smectite are the most watersensitive and hydrate the most. Shales
containing other clay minerals have
less ability to hydrate but still may be
Clay Chemistry
4B.10
water-sensitive. Most shales contain several types of clay in varying amounts.
The reactivity of a shale depends on the
types and amounts of clay minerals present in the shale. Often the CEC is a
better measure of clay reactivity than
the mineralogical analysis inferred from
X-ray diffraction analysis.
CATIONIC
INFLUENCE ON HYDRATION
As pointed out previously, the relative
replacing power of one cation by
another is shown in the following series:
H+ > Al3+ > Ca2+ > Mg2+ > K+ > NH4+ >
Na+ > Li+
A cation may serve as a bond
to hold the clay mineral particles
together, thereby decreasing hydration. Multivalent cations tie layers
together more firmly than monovalent cations, usually resulting in aggregation of the clay particles. Potassium,
a monovalent cation, is the exception
to the rule. The adsorbed cations may
become hydrated and attract a water
envelope with a definite shape. The
size and shape of the hydrated cation
affects its ability to fit between interlayer clay surfaces and influences both
clay swelling and clay hydration. Spaces
within the crystalline montmorillonite
layers is 2.8 Å. Small ions, like potassium, that can fit between clay layers
are more easily and permanently
exchanged. In addition, cations that
become large when hydrated expand
the interlayer distances to promote clay
hydration. Calcium is a good example,
having a hydrated diameter of 19.2 Å.
Lithium is another example, having
three water molecules and a hydrated
diameter of 14.6 Å. Monovalent cations
with large hydrated diameters cause the
most swelling and dispersion. Multivalent
cations with small hydrated diameters
are the most inhibitive.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
Table 3 lists the ionic diameter
(crystalline) and hydrated diameter
of cations common to drilling fluids.
Once hydrated cations are adsorbed
in the interlayer region, they can be
dehydrated with time and exposure to
high temperatures so that the interlayer distances actually shrink and
become less reactive (see ion fixation
discussed in the following section).
Cation
Chemical
reactions
between
clay and
potassium
ions are
unique…
…smectite
clays in the
Gulf of
Mexico have
undergone at
least some
degree of
alteration…
Li+
Na+
K+
NH4+
Mg2+
Ca2+
Al3+
Ionic Diameter Hydrated Diameter
(Å)
(Å)
1.56
14.6
1.90
11.2
2.66
7.6
2.86
5.0
1.30
21.6
1.98
19.2
1.00
18.0
Table 3: Ionic radii and hydration radii
of common cations.
CLAY
REACTIONS WITH POTASSIUM IONS
Chemical reactions between clay and
potassium ions are unique when compared to other ions. The ion exchange
model does not fully explain the interaction of potassium with clay. Special
attention will be paid to this process
because of the widespread use of potassium in drilling and completion fluids
to stabilize reactive shales. Even in U.S.
offshore applications, where the potassium level must be maintained below
5% for environmental reasons, this
small concentration of ions can help
stabilize active shale formations because
ion fixation can occur in some smectite clays when they are exposed
to potassium.
According to Eberl (1980), there are
two ways that potassium can become
associated with clay minerals:
1. Ion exchange (discussed earlier).
2. Ion fixation.
Clay Chemistry
4B.11
The ion exchange reaction is governed by the law of mass action; that
is the rate of exchange depends on
the concentration of the ions (i.e. the
higher the ratio of K ion to Na ion,
the faster the rate of exchange
of K+ for Na+).
In addition to ion exchange, ion fixation will occur in clays with a high
layer charge. This increases the selectivity of the clay for potassium by an
order of magnitude. Montmorillonite
clays, such as Wyoming bentonite and
some gumbo-type shales which were
deposited in potassium-depleted environments, are selective to potassium.
Based on theoretical calculations, Eberl
finds that potassium fixation in smectite clays will occur if the layer charge
is high and will shift the equilibrium
toward preferential cation exchange
with potassium.
In the Gulf Coast, the smectite content of the shales and gumbos is derived
from the weathering of igneous and
metamorphic rock or recycled sedimentary smectite ultimately derived
from igneous and metamorphic rock.
Furthermore, the smectite clays in the
Gulf of Mexico have undergone at least
some degree of alteration by the process
known as burial diagenesis.
This diagenetic alteration can be subdivided into a two-step reaction. The
first step is the creation of high-layercharged smectite by the substitution of
aluminum for silicon in the tetrahedral
layer of the smectite. The high-layercharged smectite is then converted to
illite (actually mixed-layer illite/smectite)
by fixation of potassium. This fixation
of potassium occurs in nature even with
a high sodium-to-potassium ratio in the
pore solution.
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CHAPTER
4B
…linking
processes
must be
understood
in order to
understand
and control
rheological
changes in
drilling
fluids.
Clay Chemistry
There is enough potassium available
to allow smectite layers to transform
to illite layers in many geological settings. In other geological settings, the
complete transformation cannot occur
because potassium is in short supply. In
geological settings where transformation
of smectite layers to illite layers has been
limited by the unavailability of potassium, high-layer-charged smectite can
diagenetically develop. This is the case
with Gulf Coast smectite clays. They
will generally have a high-layer charge,
and a higher portion of the charge will
arise in the tetrahedral layer which
should be more selective toward potassium at lower temperatures. Therefore,
when potassium becomes available from
drilling mud, even seawater muds with
a high sodium-to-potassium ratio,
the conversion of high-layer-charged
smectite layers to illite layers will
occur. The effect of this reaction is
to stabilize the shale.
In some gumbo shales, the highlayer-charged smectite layers coexist
with lower-layer-charged smectite layers. The low-layer-charged smectite
layers will not fix potassium and, in
cases where the potassium concentration is greatly exceeded by sodium,
will behave according to classical ion
exchange theory. Thus, increasing the
potassium-to-sodium ratio in the mud
will help saturate the low-layer-charged
smectite layers with potassium and
provide additional shale stabilization.
CLAY-PARTICLE-LINKING
PROCESSES
important to the rheology of clay suspensions. These linking processes must
be understood in order to understand
and control rheological changes in
drilling fluids.
The thin flat, plate-like particles of
clay have two different surfaces. The
large face or planar surface is negatively
charged and the thin, edge surface is
positively charged where the lattice
is disrupted and a broken bond surface exposed. These electrical charges
and exchangeable cations make up an
electrical force field around the clay particles that determines how these particles interact with one another. If the
exchangeable ions are dissociated from
the clay surface, the repelling force
between the flat negatively-charged
plates is large, and the plates will be
dispersed from one another. Complete
dispersion is rare and probably can only
occur in dilute suspensions of purified
sodium montmorillonite. Usually, some
degree of linking between particles occurs.
Clay particles associate in one of
the following states: aggregation,
dispersion, flocculation or deflocculation (see Figure 8). They can be
in one or more states of association
at the same time with one state of
association predominating.
Aggregation
(face to face)
Dispersion
Flocculation
(edge to face)
(edge to edge)
Deflocculation
In addition to knowing the amount
and quality of the clays in a mud, it is
necessary to know the state of association of the clay particles. The various
linking processes of clay particles are
Figure 8: Association of clays.
Clay Chemistry
4B.12
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CHAPTER
4B
…deflocculating chemicals are often
referred to
as mud
thinners.
…yield of
clays is
defined as
the number
of barrels of
15-cP mud
that can be
obtained from
one ton of
dry material.
Clay Chemistry
Aggregation (face-to-face linking)
leads to the formation of thicker plates
or packets. This decreases the number
of particles and causes a decrease in the
plastic viscosity. Aggregation can be
caused by the introduction of divalent
cations to the drilling fluid, such as
Ca2+. This could occur from additions
of lime or gypsum or by the drilling of
anhydrite or cement. After an initial
increase, the viscosity will decrease
with time and temperature to some
value lower than it was originally.
Dispersion, the reverse of aggregation, leads to a greater number of particles and to higher plastic viscosities.
Clay platelets are normally aggregated
before they are hydrated and some dispersion takes place as they hydrate. The
degree of dispersion depends on the
electrolyte content of the water, time,
temperature, the exchangeable cations
on the clay and the clay concentration.
Lower salinity, longer times, higher
temperatures and lower hardness lead
to more dispersion. Even Wyoming
bentonite will not completely disperse
in water at room temperature.
Flocculation refers to edge-to-edge
and/or edge-to-face association of particles, leading to the formation of a
“house of cards” structure. This causes
an increase in viscosity, gelation and
fluid loss. The severity of this increase
is a function of the forces acting on
the linked particles and the number
of particles available to be linked.
Anything that increases the repelling
forces between particles or shrinks
the adsorbed water film, such as the
addition of divalent cations or high
temperature, promotes flocculation.
Deflocculation is the dissociation
of flocculated particles. The addition of
Clay Chemistry
4B.13
certain chemicals to the mud neutralizes
the electrochemical charges on the
clays. This removes the attraction that
results in edge-to-edge and/or edge-toface bonding between clay particles.
Since deflocculation results in a reduction in viscosity, deflocculating chemicals are often referred to as mud thinners.
Deflocculation also aids in allowing the
clay particles to lay flat in the filter cake
to reduce fluid loss.
YIELD
OF CLAYS
The yield of clays is defined as the
number of barrels of 15-cP (centipoise)
mud that can be obtained from one
ton of dry material. Figure 9 illustrates
why 15 cP was chosen as the defining
value for yield. The critical part of the
curve for all types of clay appears at
15 cP. Large additions of clay up to
15 cP promotes little viscosity increase,
whereas, small amounts of clay have a
pronounced effect on viscosity above
15 cP. This is not only true with commercial clays but for hydratable drill
solids as well. It is also relevant that
a 15-cP clay suspension will support
barite in weighted-mud systems.
This graph can be very useful to the
mud engineer. For a given viscosity of
the various clays, data relative to slurry
density, percent solids by weight, yield
in barrels-per-ton, percent solids by
volume and pounds solids per barrel
of mud may be obtained.
For example, about 20 lb/bbl of bentonite (M-I GEL) is required to produce
a 15-cP viscosity mud. From the graph,
then, it would contain 51⁄2% solids by
weight, yield 100 bbl/ton, have 21⁄2%
solids by volume and weigh about
8.6 lb/gal.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
Clay Chemistry
8.5
9.0
Slurry density (lb/gal)
10.0
10.5
9.5
11.0
11.5
12.0
60
Sub-be
ntonit
e
M-I GEL
50
SALT GEL
Specific gravity of solids = 2.4
40
30
Na
tiv
ec
lay
Viscosity (cP)
4B
20
10
0
0
5
10
200 100 75
2
10
15
25
30
Solids (% wt)
35
50 40 30 25
20 18 16
14
Yield (bbl/ton of 15-cP mud)
4
20
20
6
30 40
50
8
10
12
14
Solids (% vol)
16
100
Solids (lb/bbl)
150
75
40
12
18
45
10
20
200
50
9
25
8
30
250
Figure 9: Viscosity curves resulting from different clay solids.
The yield would be less if a clay takes
up less water. By comparison, if subbentonite was used to produce a 15-cP
viscosity mud, it would contain 18%
solids by weight, yield only 28 bbl/ton,
have 81⁄2% solids by volume and would
weigh almost 9.4 lb/gal.
Clays have many applications in
drilling muds. Increasing the viscosity
of a drilling mud may best be accomplished with the least amount of solids
by adding a clay which has the highest yield (M-I GEL). Lower fluid-loss
values can be obtained with bentonite
since coarse and medium-sized particles are normally produced from the
formation. The quality of the mud will
Clay Chemistry
4B.14
be improved by utilizing high-quality
Wyoming bentonite.
M-I GEL and GEL SUPREME are both
Wyoming bentonites. They differ in
that M-I GEL is treated with very small
amounts of polymer (peptized) to
increase its yield, while GEL SUPREME
is a non-treated bentonite. M-I GEL
meets API Specification 13A, Section 4
“Bentonite” specifications. GEL SUPREME
meets API Specification 13A, Section 5
“Non-Treated Bentonite” specifications.
M-I also sells OCMA bentonite which
meets API Specification 13A, Section 6
“OCMA Bentonite” specifications.
NOTE: OCMA is the acronym for Oil
Companies Materials Association.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
FACTORS AFFECTING THE
YIELD OF CLAYS
Hydration and dispersion of dry clay
are greatly affected if the makeup
water contains salt or various metallic
ions. For example, many drilling
muds are prepared with seawater for
economy and convenience. A typical
analysis of seawater might contain the
following components:
Components
Sodium
Chloride
Sulfate
Magnesium
Calcium
Potassium
Bromine
Other components
Parts per Million
(mg/l)
10,550
18,970
2,650
1,270
400
380
65
80
Viscosity (cP)
Hydration and
dispersion of
dry clay are
greatly
affected if the
makeup water
contains salt
or various
metallic ions.
30
25
20
Salt solution
15
Calcium solution
10
5
0
0
50
0
1.5
100
150
200
250
Salt (mg/l x 1,000)
3.0
4.5
6.0
7.5
Calcium (mg/l x 1,000)
300
9.0
Figure 10: Viscosity effect when adding
bentonite to water containing various
concentrations of salt or calcium.
NOTE: Brackish water could contain the same components,
but at different concentrations.
…hydration
of freshwater
clays decreases
rapidly with
increasing
concentrations
of these ions.
Water containing any salt concentration can be saturated with an additional salt. Saturated saltwater contains
about 315,000 mg/l sodium chloride.
Approximately 120 lb/bbl of salt is
required to saturate freshwater.
Figure 10 shows the effect of various
concentrations of these ions upon the
hydration of bentonite. In general, it
can be stated that the hydration of
freshwater clays decreases rapidly with
increasing concentrations of these ions.
This phenomenon is more apparent
in Figures 11 and 12. Demonstrated in
these examples is the hydration of two
identical cubes of bentonite, the first
in freshwater and the second in salty
water. Figure 11 shows the bentonite
cube initially in a beaker of freshwater
and then again 72 hr later. Hydration
and consequent swelling is readily
apparent. Figure 12 shows the bentonite cube initially in the salty water
and again 72 hr later. It is obvious that
little or no hydration has occurred.
Water containing calcium or magnesium is referred to as “hard” water. To
obtain more viscosity from the clay, one
Clay Chemistry
4B.15
Initial
72 hr later
Figure 11: Hydration of bentonite in freshwater.
Initial
72 hr later
Figure 12: Hydration of bentonite in salty water.
practice is to “soften” the water with
soda ash and caustic soda to precipitate calcium and magnesium. When
high chloride concentrations exist, the
only method of reducing the concentration is by dilution with freshwater.
When the makeup water is salty,
SALT GEL (attapulgite) may be used
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
Figure 13: Photomicrograph of attapulgite particles.
…attapulgite
clay will
build similar
viscosity in
any type of
makeup
water…
for achieving viscosity. Attapulgite is
a unique mineral. Its crystalline structure is needle-like as shown in Figure
12. Its ability to build viscosity is independent of the makeup water. At the
same concentration, SALT GEL in any
type water would give about the same
viscosity as M-I GEL in freshwater. The
ability to build viscosity does not depend
on hydration, but rather upon the
extent to which the bundles of needles
are sheared. The resultant viscosity is
created by two elements:
1. The formation of brush-heap structures by the shearing forces. A simple analogy would be similar to
straw stirred into water.
2. Attractive forces between particles
created by broken-bond charges on
the edges of needles broken by the
shearing force.
Since the attapulgite clay will build
similar viscosity in any type of makeup
1
⁄4 bbl of 30-lb/bbl
prehydrated bentonite slurry
+
3
water, a question might be “Why not
always use attapulgite?” The answer
would be (1) greater cost, (2) lack of
filtration control due to particle shape
and (3) rheology characteristics are
more difficult to control.
Bentonite can be used as an effective
viscosifier in saltwater if it is first prehydrated in freshwater then added to the
salty water. It is beneficial to maintain
a 9 to 10 pH and treat the prehydrated
bentonite slurry with a deflocculant
before adding it to the salty water.
In this way, the initial flocculation
followed by a loss of viscosity from
dehydration in the saltwater environment is reduced.
This is shown in Figure 14. A slurry
consisting of 30 lb/bbl bentonite was
prepared and allowed to hydrate. It
was then added to a barrel equivalent
of water having a concentration of
100,000 mg/l sodium chloride.
From this figure, it is obvious that
the clay is dispersed in the saltwater
and the rheological properties show
that the clay is performing its function. Much of this viscosity will be
eventually lost through dehydration
over time, but a portion will always
remain. The resultant viscosity will
always be substantially higher than
making an addition of dry clay
directly to the saltwater.
⁄4 bbl of 100,000-mg/l
NaCl water
=
1 bbl slurry with properties
AV = 47, PV = 15 and YP = 63
Figure 14: Addition of prehydrated bentonite to saltwater.
Clay Chemistry
4B.16
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CHAPTER
Clay Chemistry
70
60
60
50
50
40
Viscosity (cP)
Viscosity (cP)
4B
40
30
30
20
20
10
10
0
0
1
2
3
4
Salt (mg/l x 1,000)
5
6
0
1
2
3
4
5
Calcium x 100 (mg/l)
6
Figure 15: Effect of calcium on prehydrated bentonite.
…all drilling
muds…
should
have a pH
above 7.
An entirely different reaction occurs
when salt or calcium is added directly
to a bentonite slurry prepared and
hydrated in freshwater. Figures 15
and 16 demonstrate this reaction.
To be noted is the initial increase
and subsequent decrease in viscosity
previously discussed under clay
particle associations.
Figure 15 represents initial viscosity
increase due to flocculation caused by
the addition of the divalent cation Ca2+.
This in turn causes aggregation of the
particles and a viscosity decrease due
to dehydration and decreased number
of particles.
Figure 16 shows essentially the same
thing except that flocculation and the
aggregation are caused by mass action of
the Na+ due to its high concentration.
EFFECT
OF PH
Figure 16: Effect of salt on prehydrated bentonite.
considerations, since pH does affect viscosity, is selection of the most desirable
pH range for optimizing the rheological properties of the drilling fluid. From
the graph it can be observed that the
viscosity of a bentonite suspension is
lowest in the pH range of 7 to 9.5. This
is one reason why most water-base
drilling fluids are run in this range.
Increased dispersion of clay results
when pH is above 9.5, increasing the
viscosity of the drilling fluid.
70
60
50
Viscosity (cP)
0
40
30
20
It is relevant at this time to also consider the effect of pH on the yield of
bentonite. Figure 17 illustrates the viscosity of a bentonite slurry as the pH
is varied. Most all drilling muds are
treated to be alkaline, i.e., they should
have a pH above 7. One of the primary
Figure 17: Effect of pH on Wyoming bentonite.
Clay Chemistry
Revision No: A-0 / Revision Date: 03·31·98
4B.17
10
0
0
2
4
6
pH
8
10
12
CHAPTER
4B
Clay Chemistry
In previous discussions, the emphasis
has been toward getting the most viscosity from the smallest addition of
material. The significance of pH is that
the viscosity created by values above
10 is sometimes out of proportion to
what is considered to be desirable mud
properties. For obvious reasons, such
as safety and corrosion, drilling muds
are rarely operated in the acidic range
with a pH below 7.
Principles of Chemical Treatment
Viscosity is
the result of
frictional as
well as electrical forces
existing in a
mud system.
Viscosity is the result of frictional as
well as electrical forces existing in a
mud system. As drilling progresses,
solids are incorporated into the drilling mud. They will be ground and
broken into very fine particles, causing an increase in the viscosity of the
mud, unless these solids are removed
from the system. Drilling various contaminants will also cause flocculation
and an increase in viscosity.
Evaluating the rheological properties
of the mud will enable the mud engineer to quickly determine the cause
of trouble and the proper treatment
to reduce viscosity. Water is effective
for reducing viscosity if solids are
high, but it is not the most economical treatment if abnormal viscosity is
caused by chemical flocculation (as
indicated by a high yield point and
gels). There are organic and inorganic
anionic additives that can be used to
effectively reduce flocculation.
The primary effect of anionic viscosityreducing chemicals is believed to be a
neutralization of residual broken-bond
cationic charges. The mechanism of
this action in water-clay suspensions
is to reduce that portion of viscosity
due to attractive forces between the
particles without substantially affecting
Clay Chemistry
4B.18
that portion of viscosity due to hydration of the clay minerals. Anionic materials are adsorbed on the edges of the
clay particles to satisfy the residual
broken-bond cationic charges. Anioniccharged chemicals commonly used for
the treatment of drilling mud include
phosphates, tannins, humic-acid lignins
(lignite), lignosulfonates and lowmolecular-weight synthetic polymers.
This adsorption changes the balance of
forces acting on the clay particle from
an attractive force (flocculation) to a
repulsive force (deflocculation). Instead
of being drawn together, the particles
repel or tend to avoid contact with
one another.
Chemical treating agents reduce
flocculation in clay-water drilling fluids by one or more of the following
mechanisms.
1. Removing the contaminant
by precipitation.
2. Reducing the effects of the contaminant by complexing the
contaminate (sequestering).
3. Neutralizing flocculation by
satisfying cationic charges on
the clay particles.
4. Encapsulating or forming a protective film around the clay particle.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
PHOSPHATES
The two principal phosphates used in
drilling mud are:
1. Sodium Acid Pyrophosphate (SAPP)
pH of 4.8.
2. Sodium Tetraphosphate (STP or
PHOS) pH of 8.0.
…phosphates
are powerful
anionic
dispersants…
Phosphates
are used
principally
in low-pH
muds and
spud muds.
These phosphates are powerful
anionic dispersants and only a small
treatment will produce maximum viscosity reduction. The amount of treatment for simple dispersion rarely
exceeds 0.2 lb/bbl. This means that for
a 1,000-bbl system, only 200 lb would
be required to thin the fluid. The phosphates can be added directly through
the hopper or from the chemical barrel.
If added from the chemical barrel,
approximately 50 lb of phosphate is
mixed with a barrel of water. The solution is then added directly to the mud
uniformly over one circulation.
Phosphates are used principally in
low-pH muds and spud muds. They
lower viscosity in two ways: (1) they
neutralize attractive forces by being
adsorbed on the surface of solids, and
(2) they remove calcium and magnesium. The low pH of SAPP and its
ability to remove calcium makes it an
excellent treating agent for cement
contamination. The phosphates are
seldom used by themselves in mud
treatment; rather, they are used to
supplement control along with caustic soda and an organic thinner. If
SAPP (pH of 4.8) were used continuously by itself, the mud would eventually become acidic. This could be
detrimental and lead to severe corrosion and excessive viscosity. PHOS
Clay Chemistry
4B.19
has a more neutral pH (8.0) that
makes it more applicable for routine
mud-thinning treatments.
The application of phosphates for
treatment is limited. The materials are
not effective mud thinners at moderate
temperatures. If the mud temperature
is much in excess of 175°F (79.4°C), the
phosphates revert to orthophosphates.
As orthophosphates, they may become
flocculants rather than deflocculants.
This does not rule out the application
of phosphates for sequestering calcium
at higher temperatures. As orthophosphates, they still have the ability to
decrease calcium, although their thinning power is decreased. The phosphates also do not perform effectively
at higher salt concentrations.
LIGNITE
The basic lignite used for viscosity control is TANNATHINT (pH 3.2). Lignite is less
soluble at low pH, so to be effective the
pH of the mud must be in the alkaline
range or the lignite must be presolubilized in a high-pH slurry before being
added to the mud system. Caustic soda
is usually added with low-pH lignite
additives. In field use, the ratio of caustic soda to TANNATHIN will range from
1:6 to 1:2. The lignins are best added
through the mud hopper. TANNATHIN
performs best in mud systems with
pH values that range from 9 to 10.5.
CAUSTILIGT is a causticized lignite
that has a pH of about 9.5. K-17T is
a potassium hydroxide-neutralized
lignite with a pH of about 9.5.
XP-20T (pH 10) is a prereacted chrome
lignite used primarily in conjunction
with SPERSENEE (chrome lignosulfonate).
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Lignite
additives
help form
oil-in-water
emulsions…
Clay Chemistry
It complements the performance of
SPERSENE in M-I’s Chrome Lignosulfonate
System (CLS or SPERSENE/XP-20 system).
As an integral part of the SPERSENE/XP-20
mud system, XP-20 is a drilling-fluid
stabilizer and emulsifier. It decreases
fluid loss and contributes to the inhibitive properties of the mud. It is the
primary thermal stabilizer in the hightemperature DURATHERME system. The
application of XP-20 is not limited
to SPERSENE/XP-20 and DURATHERM
systems, and can be used in a wide
variety of deflocculated water-base systems for fluid-loss control, thinning
and increased thermal stability.
Lignite additives help form oil-inwater emulsions and are generally not
effective at high calcium concentration.
They are only moderately effective at
higher salt concentrations.
LIGNINS
Lignins are a group of products similar
to lignite and lignosulfonate that come
from chemically-treated tree bark.
QUEBRACHOE is a lignin/lignite blend
designed to provide thinning and fluidloss control. In general, tannin products
are more soluble than alternative chemicals in lower-pH muds. They are more
effective at lower temperatures and
high-salinity environments as compared
Clay Chemistry
4B.20
to lignite-base additives. Tannins are
usually more expensive and provide a
shorter-term effect as compared to
both lignite and lignosulfonate. Desco,
from Drilling Specialties Co., a chrome
lignin, and Desco CF, a chrome-free
lignin, are widely used as thinners.
LIGNOSULFONATES
The lignosulfonates include SPERSENE,
a chrome lignosulfonate; SPERSENE CF,T
a chrome-free lignosulfonate; and
SPERSENE I, a ferro-chrome lignosulfonate. These additives are versatile
materials that have wide applications
in many deflocculated water-base systems. They work well at all levels of
alkaline pH, can be used at elevated
salt levels and are effective in the
presence of higher calcium levels.
Lignosulfonate additives have a low
pH (about 3.0). For this reason, caustic
soda should be added along with all
SPERSENE treatments. The amount of
caustic will vary according to the type
mud being run, but usually at one
part caustic for four parts SPERSENE. It
not only reduces viscosity and gel
strength, but when used in sufficient
quantities, it reduces water loss and
provides an inhibitive environment.
Additions of SPERSENE are generally
made through the mixing hopper.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4B
Clay Chemistry
APPLICATION
Plastic
viscosity,
yield point
and gel
strengths
are the
important
factors.
Figure 18 demonstrates the changes of
viscosity that occur when plastic viscosity and yield-point values are altered by
chemical contamination and treatment.
The data can be analyzed to determine
the effect on both the funnel viscosity
and apparent viscosity by varying material additions to promote changes, then
interpreting the rheological values.
Whether measured in seconds/quart
with the funnel or in cP with the viscometer, the apparent viscosity is composed of two components: (1) solids
content and nature of these solids, and
(2) the electro-chemical attraction
between the solids.
As contaminants are introduced and/
or the solids content is increased, the
viscosity increases. If the Marsh-funnel
viscosity increases, then the apparent
viscosity will usually increase. It is also
true that if one decreases, the other
will usually decrease. However, if only
the apparent viscosity were measured,
this value is of little use for mud control. Plastic viscosity, yield point and gel
strengths are the important factors. Plastic
viscosity is more of a measure of the
structural viscosity that is determined
by the solids concentration. Yield point
and gels are more a measure of clay
hydration and flocculation. Yield points
and gels in clay free xanthan polymer
muds are less affected by normal contamination, flocculation/deflocculation
and anionic chemical thinning.
Clay Chemistry
4B.21
The principles of chemical treatment
in a clay-water fluid are shown in
Figure 18:
1. Introducing 1⁄2 lb/bbl of cement
caused flocculation to occur due
primarily to calcium contamination. Both the funnel viscosity
and the apparent viscosity increase.
Examination of the graph reveals
that this viscosity change was
brought about by increasing the
yield point (increased attractive
forces or flocculation). Little or no
change was experienced in plastic
viscosity because plastic viscosity is
due primarily to solids.
2. Ten % water was added to demonstrate that water has little effect on
reducing yield point (flocculation).
Water does not remove calcium,
which is the cause of flocculation
or high attractive forces. Water can
only increase the separation of the
solids, but does not change the
association of the clays or alter
yield point.
3. The addition of 1 lb/bbl of PHOS
(for removing calcium) produces a
tremendous decrease in both the
funnel and apparent viscosity. This
was brought about by lowering the
yield point. The yield point was
reduced because PHOS reduces calcium and deflocculates the clay particles. It is also shown that this
addition of chemical had little or
no effect upon plastic viscosity.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
Clay Chemistry
100
80
Funnel vi
scosity
Funnel viscosity (sec/qt)
120
60
80
Apparent viscosity
Plastic viscosity
AV, PV and YP (cP or lb/100 ft2)
4B
60
40
Yield point
20
A
B
C
D
E
F
G
H
I
0
Base
mud
10%
water
1/2
lb/bbl
cement
200
lb/bbl
M-I BAR
1
lb/bbl
PHOS
10%
water
1/4
lb/bbl
PHOS
1/4
lb/bbl
PHOS
1/4
lb/bbl
cement +
10 lb/bbl
clay
10%
water
Figure 18: Principles of chemical treatment.
Clay Chemistry
4B.22
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CHAPTER
4B
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Clay Chemistry
4. The second viscosity increase (Part
D) was caused by the addition of
200 lb/bbl of barite (inert solids) to
increase mud weight. The apparent
viscosity change is almost the same as
before, but for an entirely different
reason. This viscosity change resulted
from raising plastic viscosity. The
addition of more solids increased
the friction between solids because
the total surface area of the solids
increased. Yield point increased only
slightly because the solids are closer
together. Any attractive force will
be more effective because the distance between particles is reduced.
However, the funnel and apparent
viscosity increased primarily because
of increased plastic viscosity. The correct mud treatment here would be to
add water.
5. 1⁄4 lb/bbl of PHOS was added to
demonstrate that a slight reduction
in viscosity can be obtained by lowering the yield point, and to also show
that chemical treatment alone will
not reduce high viscosity from solids.
Viscosity remained high even after
the treatment.
6. Adding water is the correct treatment
to reduce viscosity. Ten % by volume
water was added and the plastic viscosity was reduced. Both the funnel
and apparent viscosities decreased
significantly because they are a function of plastic viscosity. The yield
point decreased only slightly.
7. Adding both a chemical contaminant
and reactive solids causes the third
viscosity increase, increasing both
yield point and plastic viscosity. The
1
⁄4 lb/bbl of cement increased the yield
Clay Chemistry
4B.23
point as in Part A. Plastic viscosity
was increased by adding 10 lb/bbl of
clay for the same reason as the viscosity increased in Part D by the introduction of solids. There is, however,
one great difference. The clay solids
hydrate and take up water. With less
free water available, the friction is
increased considerably with only a
small amount of solids. For a unit volume of solids, hydratable drill solids
will always increase viscosity more
than inert solids. The correct treatment here is the addition of both
chemical thinners and water for dilution to lower both plastic viscosity
and yield point.
8. Addition of chemicals lowered
viscosity for the same reason as
in Part C.
9. Addition of water lowered viscosity
for the same reason as in Part F.
The following generalization can be
made for the most economic control
of flow properties to obtain optimum
conditions:
1. An increasing yield point, accompanied by little or no changes in the
plastic viscosity, may be reduced or
controlled by the addition of chemical thinners in a clay-water system.
2. An increasing plastic viscosity,
accompanied by little or no changes
in yield point, may be reduced or
controlled by water or the use of
mechanical solids-control equipment
to discard undesirable solids.
3. Simultaneous large increases in
both yield point and plastic viscosity can be reduced or controlled by
both of the above.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
Introduction
A contaminant is
any type
material…
that has a
detrimental
effect on the
physical or
chemical
characteristics
of a drilling
fluid.
A contaminant is any type material
(solid, liquid or gas) that has a detrimental effect on the physical or chemical
characteristics of a drilling fluid. What
constitutes a contaminant in one type
of drilling fluid may not necessarily be
a contaminant in another.
Low-gravity, reactive solids are contaminants all drilling fluids have in common. These solids consist of drilled
solids incorporated into the system or
through over-treatment with commercial clays. Economically, drilled solids
and the problems associated with their
control have greater impact on mud
costs than other type of contamination. However, the primary focus here
is on the following common chemical
contaminants of water-base muds:
1. Anhydrite (CaSO4) or gypsum
(CaSO4•2H2O).
2. Cement (complex silicate of
Ca(OH)2).
3. Salt (rock salt, makeup water, seawater, magnesium, calcium and sodium
chloride, and connate water).
4. Acid gases, including carbon dioxide
(CO2) and hydrogen sulfide (H2S).
With the exception of the acid
gases, these chemical contaminants
are directly related to ion exchange
reactions with clays. Therefore, the
concentration of the clay-type solids
in a water-base mud has a direct relationship on how severely the chemical
contaminant affects the mud properties.
The Methylene Blue Capacity (MBC) is a
good indication of the concentration of
clay-type solids. Muds with MBC levels
below 15 lb/bbl are less affected by
chemical contamination.
An ion exchange reaction can occur
when sodium bentonite is exposed to
chemical environments containing
high concentrations of other metallic
Contamination and Treatment
4C.1
ions, initially flocculating, then possibly
chemically converting the bentonite
to a lower-yielding clay. This affects
the amount of adsorbed water and the
size, shape and association of particles,
resulting in unstable rheology and
fluid-loss control.
The severity of these contaminants
made it necessary to develop mud systems that could tolerate them. These
systems include lignosulfonate muds,
low-colloid polymer muds, lime muds,
gyp muds and salt muds. Many of these
systems are deliberately pretreated with
lignosulfonate, salt (sodium chloride)
and calcium-containing materials such
as lime or gypsum. Therefore, when
additional concentrations of these contaminants are encountered, they have
minimal effect on the systems.
The primary purposes of this
chapter are:
• To reveal the source(s) of each
chemical contaminant.
• To describe how each affects mud
properties.
• To describe how to use mud property
changes to identify the contaminant.
• To describe how to treat the mud to
restore the original properties.
Since changes in physical mud properties such as increased rheology and
fluid loss due to flocculation are similar
regardless of which chemical contaminant is present, the changes in physical
properties indicate only that a contaminant exists. An analysis of the changes
in chemical properties is necessary to
identify the contaminant. Therefore, the
sources, effects and treatment options
of each chemical contaminant are discussed in detail. A quick-reference guide
and tables, in metric and English units,
are included at the end of the chapter
(see Tables 2, 3 and 4).
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
Anhydrite or Gypsum Contamination
Anhydrite
and gypsum
are both
calcium
sulfate
and nearly
identical in
chemical
composition.
The solubility
of CaSO4 is
controlled
by pH,
salinity and
temperature.
There are few areas in the world where
anhydrite or gypsum is not drilled.
Anhydrite and gypsum are both calcium
sulfate and nearly identical in chemical
composition. Gypsum (CaSO4•2H2O),
with its attached water, is more soluble
than anhydrite (CaSO4). The severity of
this contaminant depends primarily on
the amount drilled. If only a small
amount of a contaminant is encountered, it can be tolerated by precipitating the calcium ion. If large amounts
are encountered, the mud system
should be converted to a calcium-base
system. Both lime and gyp-base calcium
systems can tolerate anhydrite or gypsum contamination without adversely
affecting the mud properties.
The initial effect of calcium contamination on a bentonite-base mud
system is high viscosity, high gel
strengths and increased fluid loss.
The extent to which these properties
are affected is a function of concentration of the contaminant, the concentration of reactive solids and the
concentration of chemical deflocculants in the drilling fluid.
As shown below, when calcium sulfate solubilizes in water, it ionizes into
calcium and sulfate ions.
Ca2+ + SO42–
CaSO4
The solubility of CaSO4 is controlled by pH, salinity and temperature. Increased pH and temperature
decreases the solubility of gyp while
increased mud chlorides increases
the solubility. The solubility of calcium sulfate is reversible and will
reach some level of equilibrium
with the chemical environment.
Contamination and Treatment
4C.2
DETECTION FACTORS
The first indication of anhydrite or
gypsum contamination is an increase
in physical properties, including funnel
viscosity, yield point and gel strengths.
Chemical tests must be performed to
identify which chemical contaminant is
present since the increase in these physical properties is also the first indication
of other types of chemical contamination. The main indications of gyp or
anhydrite contamination include:
1. An increase in filtrate calcium. This
may not be apparent initially if there
is an excess of carbonate, bicarbonate or phosphate ions present in the
mud, or if the pH of the mud system
is being increased. But when the solubilized gyp depletes these chemicals,
a reduction in pH occurs because the
pH of gyp (6 to 6.5) is very low. This
reduction in pH will result in a large
increase in filtrate calcium, since the
solubility of calcium is inversely
proportional to pH.
2. Reduction of the pH and alkalinity,
and an increase in filtrate calcium,
are the most reliable indicators.
3. Due to the relatively limited solubility of anhydrite and gypsum,
cuttings may contain traces of the
mineral. Many times, this is evidenced by the presence of small,
white, mushy balls of acid-soluble
material on the cuttings.
4. The qualitative test for the sulfate ion
should indicate an increase. However,
this test also detects the sulfonate ion.
The test is meaningless if lignosulfonate is used as the primary deflocculant unless a comparison is made
with uncontaminated mud.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
TOLERATING
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THE CONTAMINANT
Treating the mud for
gyp/anhydrite contamination:
1. Increase the concentration of deflocculant in the system. Lignosulfonate
and lignite are both effective deflocculants in the presence of calcium.
This treatment may be sufficient,
depending on the amount of anhydrite or gypsum drilled. Lignite
chelates the calcium ion, thereby
removing it. If there is too much
calcium, soda ash (Na2CO3) may be
required to precipitate it out.
2. The pH must be maintained in the
range of 9.5 to 10.5 with caustic soda
(NaOH) or caustic potash (KOH). This
pH range limits the solubility of gyp
and enhances the performance of
the lignosulfonate.
3. Any one of the following chemicals
may precipitate an increase in filtrate calcium. Precipitating the calcium with a source of carbonate
ions is extremely effective. Due to
the low pH of anhydrite/gyp (6 to
6.5), soda ash is the preferred carbonate because it has a higher pH
(11 to 11.4) than bicarbonate of
soda (8 to 8.5). When soda ash is
mixed in water, the pH increases
due to the formation of a hydroxyl
ion, as shown:
2Na2CO3 + H2O
HCO3 + CO3 + 4Na+ + OH– (pH <11.3)
If calcium ions are present, they are
precipitated as insoluble CaCO3 (limestone). This is the reaction of soda ash
and gypsum:
Na2CO3 + CaSO4
Na2SO4 + CaCO3 (pH >11.3)
Contamination and Treatment
4C.3
A similar reaction occurs when
sodium bicarbonate is used as the precipitant. By-products of the reaction
are chemical compounds such as calcium bicarbonate (Ca(HCO3)2), a highly
soluble material (depending on the pH).
With additional caustic soda to maintain the pH above 9.7, the bicarbonate
ion converts to carbonate. It then reacts
with the filtrate calcium to precipitate
CaCO3. However, the interim period
during which the bicarbonate ion is
present can create problems almost
as serious as the contamination itself.
Therefore, soda ash is preferred over
sodium bicarbonate. Do not over-treat
with soda ash or bicarbonate. Use
Table 2 to calculate the amount of
additive needed.
Phosphates also have the ability to
complex filtrate calcium. This reaction
produces an insoluble calcium phosphate. Common available materials
of this type are:
Sodium Acid Pyrophosphate (SAPP)
– Na2H2P2O7 (pH 4.8)
Sodium Tetraphosphate (STP or
PHOS) – Na6P4O13 (pH 8.0)
Phosphates are limited by their
relatively low temperature stability
(approximately 200°F). They convert
to orthophosphates above this temperature. As such, they are not effective as
deflocculants but are still capable of
removing calcium. However, soda ash
is the preferred product for treating
out calcium from anhydrite or gyp at
temperatures above 200°F.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
CONVERTING
THE SYSTEM
TO A CALCIUM-BASE SYSTEM
…add calcium sulfate
to convert
the system
to a calciumbase mud
system.
When massive sections of anhydrite
or gypsum are drilled, the amount
of contamination makes it virtually
impossible to maintain desirable
flow properties and fluid-loss control.
Therefore, it is necessary to add calcium sulfate to convert the system
to a calcium-base mud system.
The mud may be converted to a gyp
mud by treating with caustic soda, lignosulfonate and additional gypsum. A
gyp mud is a low-pH system, but large
amounts of caustic soda are required to
maintain the pH in the desired range of
9.5 to 10.5. A viscosity hump (increase)
will occur as the additional gyp is
added, but with proper water, caustic
and lignosulfonate additions, the mud
will break over after one circulation,
and the viscosity will decrease. Gypsum
is added until it has no detrimental
effect on the mud properties and then
maintained in excess (5 to 8 lb/bbl) to
feed the chemical reactions occurring.
Typical calcium levels range from 600
to 1,200 mg/l in a gyp mud, depending
on the pH.
The mud may also be converted to
a lime mud by applying the chemical
treatment just outlined. To convert to
a lime mud, additional lime is added
instead of gypsum and is maintained
in excess. To maintain lime in excess,
most of the lime must remain insoluble. Therefore, the pH of the lime mud
must be controlled in excess of 11.5 by
additions of caustic soda and lime. The
caustic soda reacts with the calcium
sulfate to produce additional lime as
shown by the following equation:
2NaOH + CaSO4
Ca(OH)2 + Na2SO4
The resulting lime-treated mud
requires an abnormal amount of caustic soda to maintain excess lime if large
amounts of anhydrite or gypsum are
drilled. Therefore, a gyp mud is usually
preferred. Both muds require the addition of a fluid-loss control agent that
is not too calcium sensitive such as
POLYPAC,T POLY-SALE, RESINEX,T etc.
Cement Contamination
…cement can
have very
detrimental
effects on
the mud
properties.
The probability of drilling cement
exists on every well drilled. The only
circumstances under which cement is
not a contaminant is when clear water,
brines, calcium-base muds and oil-base
muds are used, or when the cement is
well cured. The most widely used mud
system is the low-pH bentonite system.
In this case, cement can have very
detrimental effects on the mud properties. The severity of the contaminant
depends on factors such as previous
chemical treatment, solids type and
concentration, the amount of cement
Contamination and Treatment
4C.4
drilled, and the extent to which it has
cured in the hole. Keep in mind that
bulk barite is occasionally contaminated with cement during transportation or at the rig and can cause severe
cement contamination, even when it’s
not expected.
The initial effect of cement contamination is high viscosity, high gel
strengths and loss of fluid-loss control. This is the result of an increase
in the pH and the adsorption of the
calcium ion onto the clay particles,
causing flocculation.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
Cement is a complex silicate of lime,
Ca(OH)2. When solubilized in water
or the water phase of a drilling fluid,
an abundance of hydroxyl ions (OH–)
is produced.
Ca2+ + 20H– (pH <11.7)
Ca(OH)2
Cement contamination
at high…
(BHT) must
be treated
quickly and
totally…
The above reaction is reversible
and represents an equilibrium
between the concentration of cement
and the pH of the mud. The solubility
of lime decreases as the pH of the
mud increases. When the pH exceeds
11.7, lime is precipitated from solution. Therefore, the lime becomes
practically insoluble at a pH greater
than 11.7 and provides an excess or
reserve of unreacted lime due to the
undissolved cement.
The primary indication of cement
contamination is a substantial increase
in pH, Pm and in the calculated excess
lime content as measured by the Pm
and Pf.
If the amount of cement drilled is relatively small, there is little problem. The
contaminated mud can be disposed of
at the shaker or it can be treated with
deflocculants and precipitants. The stage
of the drilling operation is significant
when treating cement contamination.
Minor contamination can be effectively
treated to leave a satisfactory fluid in the
hole as a packer fluid, but in many cases
insufficient time is taken to properly
condition the mud. Cement contamination at high Bottom-Hole Temperatures
(BHT) must be treated quickly and
totally to avoid high-temperature
gelation or solidification.
The following options should be
considered when a large amount of
cement is drilled:
1. If the drilling operation is at an intermediate or final stage, using water
rather than the drilling mud to drill
Contamination and Treatment
4C.5
the cement should be considered.
This can only be done when the
cement is in a hole that is fully cased
and there is no communication of
pressure through the cement.
2. If the drilling mud must be used, the
resulting contamination problems
can usually be resolved if the drilling operation is at an intermediate
stage. There is usually adequate time
remaining at this stage of the operation for treatment and gradual dilution to properly condition the mud.
3. If the well is in the completion stage,
sufficient time should be allotted to
properly treat the cement contamination, or a gelation problem could
develop. If a lime mud is used as the
packer fluid, the mud could simply
be treated with deflocculants and
dilution.
The effect of pH on the solubility of
the cement makes treatment with precipitants difficult unless there is time
for dilution and reduction of the pH.
The hydroxyl ions produced by the
cement increases the pH, rendering the
calcium (cement) insoluble. Therefore,
a severely contaminated mud may typically have low flow properties due to
the calcium ion exchange reaction,
high pH, high alkalinity, high Pm, low
filtrate calcium and generally high fluid
loss, depending on the chemical concentration of the mud. Caution should
always be exercised when treating
cement-contaminated mud. Low flow
properties derived from tests made at
low temperatures may not reflect the
condition of the mud at the bottom of
the hole, especially at bottom-hole temperatures above 275°F. High-temperature
gelation can be a serious problem with
cement-contaminated muds.
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CHAPTER
4C
Contamination and Treatment
TOLERATING
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THE CONTAMINANT
Treating the drilling fluid
for cement contamination:
1. Increase the concentration of deflocculants in the system. Lignosulfonate
and lignite perform well in the presence of calcium over a broad pH
range. If additional fluid-loss control
is desired, TANNATHIN,T RESINEX and
XP-20 T are very effective in a highcalcium environment. Most cement
contamination problems can be adequately tolerated in this manner.
However, if an excessive amount of
cement is drilled, the mud can be
converted to a low-lime system if
temperatures allow.
2. Cement increases alkalinity when it
becomes soluble. Therefore, it is not
necessary to add caustic soda with
the deflocculants. The low pH of
deflocculants such as lignite and
SAPP offset some of the hydroxyl
ions generated by the cement. This
aids in reducing the pH and Pm,
which increases the solubility of
the cement (and calcium), allowing
precipitation.
CH2
CH2
CH
3. Available filtrate calcium may be precipitated with bicarbonate of soda or
SAPP. There are different opinions as
to which of these is the most effective, but sodium bicarbonate reduces
pH and Pm just as SAPP does.
4. If cement is drilled with a polymer system, the polymers will be
hydrolyzed by the high pH and
precipitated by the calcium (Ca)
(see Figures 1 and 2). Therefore, it
is necessary to reduce the pH and
precipitate the calcium (Ca2+) out
as soon as possible.
5. In this case citric acid (H3C6H5O7)
is the additive to use. It precipitates
cement as calcium citrate and reduces
the pH. Treating with citric acid:
2(H3C6H5O7 •H2O) + 3Ca(OH)2 →
Ca3(C6H5O7)2 ↓ + 8H2O
6. Using solids-removal equipment to
discard the fine particles of cement
is another method to reduce the
contamination. This removes the
cement before it can be dissolved
at a lower pH.
CH
CH
CH2
CH2
CH2
CH
CH2
CH
CH
+2NH3
+2OH–
C
O
C
NH2
O
C
NH2
O
C
O–
O
C
O–
O
C
O–
O
O–
Figure 1: Hydrolysis of polyacrylamide into polyacrylate at high pH, liberating ammonia gas.
CH2
CH2
CH
CH
CH
C
C
C
CH2
CH2
CH2
CH2
CH
CH
CH
C
C
C
+Ca2+
O
O–
O
O–
O
O–
O
O–
O
O
Ca
OO
Figure 2: Precipitation of polyacrylate by calcium.
Contamination and Treatment
4C.6
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Phosphates
effectively
remove
calcium
and aid in
deflocculating
the contaminated fluid.
If a massive
amount of
cement must
be drilled,
then some
form of
acid will be
required…
Contamination and Treatment
Sodium bicarbonate is an excellent
treating agent for cement contamination as it precipitates calcium and
reduces pH. Depending on the pH of
the fluid, sodium bicarbonate forms
carbonate (CO32–) and bicarbonate
(HCO3–) ions which will precipitate
calcium to form calcium carbonate
(limestone) as shown below:
NaHCO3 + Ca(OH)2 →
NaOH + H2O + CaCO3 ↓
The use of bicarbonate reduces the
pH. This is the result of the reaction
of HCO3 with the NaOH required to
convert the HCO3 to CO3.
When using bicarbonate of soda,
the objective is to provide a source
of carbonate and bicarbonate ions to
combine with the excess lime and calcium ions. A frequent problem associated with this method of treatment is
over-treatment with carbonate material. If carbonates are present in excess
of that required to precipitate the calcium, mud problems associated with
the carbonates is possible. It is often
better to undertreat cement contamination initially, then observe the
results before additional treatments.
Another approach to treating
cement is the use of SAPP (phosphate). Phosphates effectively remove
calcium and aid in deflocculating the
contaminated fluid. The commonly
used phosphates convert to orthophosphates above temperatures of
±200°F. They are no longer effective
as deflocculants in this form, but still
remove calcium effectively.
An additional benefit in using phosphates is pH, Pf and Pm reduction. SAPP
has a pH of 4.8 and PHOS has a pH of
8.0. Another benefit is that there are no
harmful by-products resulting from this
reaction that can create other mud
problems. SAPP is preferred if the Pm
needs considerable reduction.
If a massive amount of cement must
be drilled, then some form of acid will
be required. Even if bicarbonate is used
to remove all calcium and excess lime,
a undesirably high pH often results.
In these cases, citric acid, SAPP, acetic
acid or a low-pH lignite or lignosulfonate must be used in combination
with bicarb.
NOTE: Soda ash should not be used
to treat cement contamination due to
its high pH.
Carbonate Contamination
Chemical contamination from soluble
carbonates is one of the most misunderstood and complicated concepts in
drilling-fluid chemistry. Carbonate/
bicarbonate contamination usually
results in high flow line viscosity,
Contamination and Treatment
4C.7
high yield point and progressive gel
strengths, and could result in solidification of the mud. These increases
in viscosity occur as the carbonates
and/or bicarbonates flocculate the
clay-type solids in the mud.
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CHAPTER
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Contamination and Treatment
The source of carbonates and
bicarbonates are:
1. Carbon dioxide (CO2) from the air
is incorporated into the mud through
mud mixers in the pits and through
mud mixing and solids-removal
equipment discharges. As the CO2
dissolves, it becomes carbonic acid
(H2CO3) and is converted to bicarbonates (HCO3) and/or carbonates (CO3)
depending on the pH of the mud.
2. Over-treatment with soda ash or
bicarbonate of soda when treating
cement or gyp contamination.
3. CO2 gas intruding from the
formation and formation water.
4. Bicarbonates and/or carbonates
from by-products of the thermal
degradation of lignosulfonate and
lignite at temperatures above 325°F.
5. Some impure barite contains
carbonate/bicarbonate ions.
The following chemical equations
illustrate how CO2 dissolves to form
carbonic acid (H2CO3) and is converted to bicarbonates (HCO3) and/or
carbonates (CO3) depending on the
pH of the mud. These equations
show that the chemical reactions
are reversible as a function of pH.
Therefore, CO3 can revert back to
HCO3 or even CO2 if the pH is
allowed to decrease.
H2CO3
CO2 + H2O
–
H2CO3 + OH
HCO3– + H2O and
OH–
CO32– + H2O
HCO3–
This is also illustrated graphically in
Figure 3 which shows the distribution
of carbonic acid (H2CO3), bicarbonate
(HCO3) and carbonates (CO3) vs. pH.
Contamination and Treatment
4C.8
100
CO32 -
H2CO3
80
Percent
4C
HCO3-
60 Increasing salinity
40 shifts curves to
the left.
20
HCO3-
0
0
2
4
HCO36
8
pH
10
12
14
Figure 3: Carbonate-bicarbonate equilibrium.
USING CHEMICAL ANALYSIS TO IDENTIFY
CARBONATE/BICARBONATE CONTAMINANT
The pH/Pf method of carbonate/bicarbonate analysis is based on the amount
(ml) of 0.02 N sulfuric acid (H2SO4)
required to reduce the pH of a mud filtrate sample from an existing pH to an
8.3 pH. This covers the pH range in
which hydroxyls and carbonates exist.
Table 1 shows that if no carbonates
exist, very little caustic soda is required
to achieve typical pH ranges of drilling fluids, and that the corresponding
Pf is also low. As examples, 10 pH,
0.0014 lb/bbl caustic soda and 0.005 Pf
or 11 pH, 0.014 lb/bbl caustic soda and
0.05 Pf. The Pf is low because the concentration of OH ions (caustic soda)
is low. A very small concentration of
hydrogen (H) ions from the sulfuric
acid (H2SO4) is required to convert the
OH ions to water (HOH) and reduce the
pH to 8.3, (the Pf end point). However,
if carbonate ions exist, not only must
the OH– ions be neutralized as above,
but each carbonate ion must be converted to a bicarbonate ion by the addition of 0.02 N sulfuric acid to reach the
Pf end point of 8.3 pH. This makes the
Pf higher (for an equivalent pH) in a
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Once the
concentration
of carbonates
is known, the
concentration
of calcium
required to
precipitate it
out can be
calculated.
Contamination and Treatment
filtrate that contains carbonates compared to a filtrate that does not. This
difference in the Pf where carbonates
exist compared to the Pf where no carbonates exist, and a pH measurement
of the concentration of hydroxyls
makes it possible to calculate the concentration of carbonates and bicarbonates. Once the concentration of
carbonates is known, the concentration of calcium required to precipitate
it out can be calculated.
pH
7
8
9
10
11
12
13
14
NaOH
(lb/bbl)
0.0000014
0.000014
0.00014
0.0014
0.014
0.14
1.4
14
Pf
OH–
(cc 0.02N H2SO4) (ppm)
0.000005
0.0017
0.00005
0.017
0.0005
0.17
0.005
1.7
0.05
17
0.5
170
5
1,700
50
17,000
Table 1: Relationship of pH and alkalinity for pure water.
The Pf/Mf method of carbonate/bicarbonate analysis is based on the amount
(ml) of 0.02 N sulfuric acid required to
reduce the pH of a mud filtrate sample
from an existing pH to an 8.3 pH and
a 4.3 pH, respectively. This range also
covers the pH range in which carbonates, bicarbonates and carbonic acid,
exist. As discussed above, if no carbonates exist, very little caustic soda is
required to achieve typical pH ranges
of drilling fluids and that the corresponding Pf and Mf values are low.
However, if carbonate/bicarbonate ions
exist, not only must the hydroxyl ions
be neutralized, but each carbonate ion
must be converted to a bicarbonate
ion by the sulfuric acid to reach the Pf
end point of 8.3 pH. This makes the Pf
higher for an equivalent pH where carbonate/bicarbonate ions exist compared
to a mud with no carbonate/bicarbonate ions. When all the carbonates have
Contamination and Treatment
4C.9
been converted to bicarbonates and the
8.3 Pf end point has been reached, more
0.02 N sulfuric acid must be added to
convert all the bicarbonates to carbonic acid before the 4.3 Mf end
point is reached.
If no carbonate/bicarbonate ions
exist, no reaction with bicarbonates
occur in the pH range of 8.3 to 4.3.
Therefore, the Mf will be only slightly
higher than the Pf. But if the carbonate/bicarbonate concentration is high,
the Mf will also be much higher than
the Pf. Since this is not a qualitative
analysis like the pH/Pf method, some
guidelines must be established.
1. If the Mf is less than 5 ml of 0.02 N
sulfuric acid, there is usually no
carbonate problem.
2. If the Mf is greater than 5 ml of
0.02 N sulfuric acid, and the Mf/Pf
ratio is increasing, carbonate contamination is a strong possibility
and a more quantitative method of
determination (such as the pH/Pf
or Garrett Gas Train (GGT)) should
be used.
NOTES:
1. High concentrations of lignite (or
organic salts such as acetates or formates) in muds can cause a high
Mf. These organic chemicals buffer
the pH between 4.3 and 8.3.
2. API 13B-1 describes a Pf/Mf calculation for hydroxyl (OH), carbonate
(CO3) and bicarbonate (HCO3) concentrations, as well as an alternative
P1/P2 method.
The GGT may also be used to determine the quantity of carbonates. This
device estimates the total carbonates in
mg/l as CO3. Total carbonates include
HCO3 and CO3. To convert mg/l to
millimoles/liter, divide mg/l by 60.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
TREATING A CARBONATE/BICARBONATE
CONTAMINANT
It may
take several
applications
of lime or
gyp over
several
circulations
to completely
treat out the
carbonates.
Treating this contaminant is complicated since HCO3 and CO3 ions can
exist together at various pH levels.
Only the CO3 ion can be treated with
free calcium to form the precipitate
CaCO3. The coexistence of CO3 and
HCO3 form a buffer compound that
remains at the same pH level but at
increasing Pf or Mf levels. As the carbonate/bicarbonate buffer zone is
formed, the Pf increases while the pH
remains relatively constant. It is not
until a pH of 11.7 is attained that all
of the bicarbonate ions are changed to
carbonate ions. Therefore, bicarbonate/carbonate ions coexist in the pH
range of 8.3 to 11.7 (see Figure 3). The
concentration of HCO3 is insignificant
at a pH greater than 11.7.
Since calcium bicarbonate [Ca(HCO3)2]
is too soluble to form a precipitate,
HCO3 ions should be converted to
CO3 ions with hydroxyl (ions). To
convert HCO3 to CO3, the pH should
be increased to at least 10.3 but not
above 11.3. When free calcium is added
to CO3, the two react to form CaCO3.
Calcium carbonate (CaCO3) is a relatively insoluble precipitate. Therefore,
150 to 200 mg/l free calcium should
be maintained in the system. If the pH
is less than 10.3, lime (Ca(OH)2) should
be used to increase the pH because it
is both a source of hydroxyl ions and
a source of calcium to precipitate the
carbonates. If the pH is between 10.3
and 11.3, both lime and gyp should be
used together to provide a source of
Contamination and Treatment
4C.10
calcium without changing the pH. If
the pH of the mud is above 11.3 where
calcium is not very soluble, gyp should
be used as both a source of calcium
and to reduce pH. The concentration
of lime and gyp required is provided
in Graphs 1 and 2 and in M-I computer programs such as PCMODE 3 and
M-I QUICK CALC-II.E These graphs are
based on the Pf/pH method of treatment. Therefore, the pH and Pf values
must be measured accurately. An accurate pH meter is absolutely necessary
to use these graphs.
The reaction of lime and gyp to form
calcium carbonate is illustrated below:
Treating with lime:
(CO32–) + Ca(OH)2
CaCO3 ↓ + 2(OH–)
Treating with gyp:
(CO32–) + CaSO4
CaCO3 ↓ + (SO42–)
It may take several applications of lime
or gyp over several circulations to completely treat out the carbonates (CO3).
Serious mud problems can occur if
carbonate/bicarbonate contamination
is not properly identified and treated.
If the pH of the mud is allowed to drop
to 10 or less, carbonates, which are
beneficial in small concentrations, are
converted to bicarbonates. Very high
viscosities and gel strengths can develop
if this occurs. Adding a large concentration of deflocculants and caustic appears
to deflocculate the mud but what really
occurs is that the addition of caustic
soda converts the bicarbonates to carbonates. This results in a large reduction
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CHAPTER
4C
Contamination and Treatment
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Yield point (lb/100ft2)
_______________________
60
CO3
50
HCO3
40
30
fraction (FW) of the fluid must be
known to use the graphs. Graph I has
been modified to allow 20 millimoles/
liter of carbonates to remain in the
mud after treatment.
20
METHOD A (USING
10
On Graph I, enter the vertical axis at
the Pf value and proceed horizontally
to the intersection of the pH line. From
this point, proceed downward to the
intersection of the horizontal axis and
read the carbonate concentration in
millimoles per liter. Then go upward
to the top of the graph, and read the
pounds per barrel of lime required to
precipitate the carbonates. Graph II
indicates the concentration and treatment of bicarbonates and is used in the
identical manner. Enter the vertical
axis at the Pf value, go across to the
pH line and upward to the top of the
graph to determine the pounds per barrel of lime required to precipitate the
bicarbonates. Add the amount of lime
from both graphs, and then multiply
by the water fraction (FW) to determine
the treatment necessary.
0
0
20 40 60 80 100 120 140 160 180 200
Millimoles/liter
Figure 4: Effect of bicarbonate and
carbonate concentration on yield point.
in viscosity (see Figure 4). This phenomenon will recur until the problem is
properly identified and treated.
The addition of deflocculants may
be required to improve flow properties once the carbonate ions have
been neutralized.
Treatments determined by the following methods are designed to remove all
bicarbonates and all but 20 millimoles/
liter carbonates. In Graphs I and II,
a quantitative analysis of carbonate
and bicarbonate ions is shown with
corresponding additives necessary for
precipitation. The pH, Pf and the water
Contamination and Treatment
4C.11
LIME ONLY)
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CHAPTER
4C
Contamination and Treatment
_______________________
_______________________
.1
_______________________
100.0
80.0
60.0
_______________________
40.0
_______________________
14.0
13.9
13.7
_______________________
20.0
_______________________
Pf (cm3)
_______________________
_______________________
13.6
10.0
8.0
6.0
13.3
4.0
12.9
13.1
_______________________
13.4
13.2
13.0
12.7
2.0
12.5
_______________________
_______________________
13.8
13.5
_______________________
_______________________
Lime required (lb/bbl)
.5 1
2 3 4 6 8 10
12.3
1.0
0.8
0.6
12.1
12.8
12.6
12.4
12.2
0.4
12.0
11.9 11.8
11.7 11.6
0.2
11.5
_______________________
11.0
0.0
Example:
pH = 10.7; Pf = 1.7;
Wf = 0.80
<10
1
3
60
ppm
6
10
30
60 100
Millimoles/liter
600
6,000
CO3 (millimoles/liter)
For constant pH
200 300
600 1,000
60,000
.1 .5 1 2 3 4 6 8 10
20 30 50
Gyp required (lb/bbl)
Graph I: Concentration and treatments for carbonates.
Example:
pH = 10.7; Pf = 1.7; FW = 0.80
From Graph I:
CO3 = 33.5 millimoles/liter
Lime required = 0.34 lb/bbl
From Graph II:
HCO3 = 3.3 millimoles/liter
Lime required = 0 lb/bbl
Total treatment =
(0.34 lb/bbl + 0 lb/bbl)(.80)
= 0.27 lb/bbl lime
METHOD B (USING
CONSTANT pH)
LIME AND GYP FOR
On Graph I, enter the vertical axis at
the Pf value and proceed horizontally
to the intersection of the pH line. From
Contamination and Treatment
4C.12
this point, proceed downward to the
intersection of the horizontal axis and
read carbonate concentration in millimoles per liter. Continue downward
and read pounds per barrel of gyp
required for precipitation of the carbonates. Graph II indicates the concentration and treatments of bicarbonates
and is used in the identical manner.
The bottom scale reflects the quantity
of lime and gyp necessary for precipitation at a constant pH. Requirements of
additives necessary for treatment of
carbonates and bicarbonates are then
added together and adjusted for the
water fraction of the mud.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
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100.0
80.0
60.0
_______________________
40.0
_______________________
_______________________
20.0
.5
11
_______________________
10.0
_______________________
8.0
6.0
_______________________
4.0
_______________________
_______________________
_______________________
_______________________
Pf (cm3)
_______________________
_______________________
Lime using lime only (lb/bbl)
.2 .3 .4 .5 .6 .8 1.0
2.0 3.0 4.0 6.0 8.0 10.0
.1
.0
11
.5
10
.0
10
2.0
5
9.
1.0
0.8
0.6
Example:
pH = 10.7
Pf = 1.7
Wf = 0.80
0
9.
0.4
0.2
0.0
1
3
6
10
30
60 100
HCO3 (millimoles/liter)
200 300
600
1,000
Gyp required (lb/bbl)
For constant pH
.1
.2
.3 .4 .5 .6 .8 1.0
2.0 3.0 4.0 5.0 6.0 8.0 10.0
and lime required (lb/bbl)
.1
.2
.3 .4 .5 .6 .8 1.0
2.0 3.0 4.0 5.0 6.0 8.0 10.0
Graph II: Concentration and treatments for bicarbonates.
Example:
pH = 10.7; Pf = 1.7; FW = 0.80
From Graph I:
CO3 = 33.5 millimoles/liter
Gyp required = 0.8 lb/bbl
From Graph II:
HCO3 = 3.3 millimoles/liter
Gyp required = 0.1 lb/bbl
Lime required = 0 lb/bbl
Total treatment = (0 lb/bbl)(0.80) =
0 lb/bbl lime and
(0.8 lb/bbl + 0.1 lb/bbl)(0.80) =
0.72 lb/bbl gyp
Quantities of lime or lime and gyp
determined from the graphs must be
Contamination and Treatment
4C.13
multiplied by the water fraction (FW)
to determine the required treatment.
When using the old API P1/P2
method of determining carbonates,
Equivalent Parts per Million (EPM) is
used. To convert EPM to millimoles/liter,
EPM is divided by the valence. For
example: 80 EPM of carbonate is the
same as 40 millimoles/liter carbonate
because CO3 has a valence of 2, and
20 EPM of bicarbonate is the same as
20 millimoles/liter bicarbonate because
HCO3 has a valence of one. The current API 13B-1 P1/P2 method reports
mg/l. Divide mg/l carbonate by 60
to obtain millimoles per liter (61
for bicarbonates).
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
Salt Contamination
Halite…is
the most
frequently
drilled salt
and is the
major
constituent
of most
saltwater
flows.
The presence
of halite
can be
confirmed by
an increase
in chlorides.
The three naturally occurring types of
rock salt encountered in drilling operations are halite (NaCl), sylvite (KCl)
and carnallite (KMgCl3•6H2O); also,
see the chapter, “Drilling Salt.” These
salts are listed in the order of increasing solubility. Two other common salts
are magnesium chloride (MgCl2) and
calcium chloride (CaCl2). These two
salts do not occur naturally in the crystalline form due to their extreme solubility. However, both can occur singly,
together or with other dissolved salts
in connate water.
A saltwater flow can be far more
detrimental to flow properties than
drilling into rock salt since the salts
are already solubilized and react with
the clays more rapidly. When a saltwater flow occurs, the mud density
must be increased to control the flow
before time can be taken to condition
mud properties.
The mechanism of contamination
in the case of salts is based on cation
exchange reactions with the clays, mass
action by the predominant cation and
sometimes pH. The only systems on
which dissolved salts have little or no
effect are clear water, brines, oil-base
muds and some low-colloid polymer
systems. Whether the source of salt is
from makeup water, seawater, rock salt
or from saltwater flows makes little difference on bentonite-base mud systems.
The initial effect is high viscosity, high
gel strengths, high fluid loss and a large
increase in the chloride content with
smaller increases of hardness in the mud
filtrate. Salt base-exchanges with the
clays to flush the calcium ion from the
clay particles, resulting in an increase in
hardness. Detecting a chloride increase
does not define the problem well
enough to know how best to treat
the mud since the chloride test does
Contamination and Treatment
4C.14
not indicate what metallic ion(s) are
associated with the chlorides.
For a better understanding of
these salts and how to treat the contamination in each case, the salts
and/or their associated cations are
described separately.
HALITE (NaCl)
Halite (common table salt) is the most
frequently drilled salt and is the major
constituent of most saltwater flows. The
initial effect on drilling mud is flocculation of the clays caused by mass action
of the sodium ion. The funnel viscosity,
yield point, gel strengths and fluid loss
will all increase when halite is encountered. The presence of halite can be confirmed by an increase in chlorides. The
clays dehydrate with sufficient sodium
and time. In doing so, the particle size
is decreased due to the reduction in
adsorbed water. The released water
rejoins the continuous phase of the
mud, which may result in a slight
reduction in plastic viscosity. But the
dehydrated clay particles flocculate,
causing a high yield point, high gel
strengths and a high fluid loss. The fluid
loss will increase in direct proportion
to the amount of salt incorporated
into the mud.
Treating the mud involves adding
enough deflocculant to maintain desirable flow properties and dilution with
freshwater to obtain suitable rheology.
Chemical treatment must be continued
until the clays have been deflocculated.
Additional caustic is required to raise
the pH. This depends on how much
salt is drilled and whether there is a sufficient amount to dehydrate all the
clays in the system. If the pH is reduced
to less than 9.5, the pH may need to be
increased with caustic soda for the acidbase deflocculants to become soluble in
order to be effective.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
The
complex salt
“carnallite”
is relatively
rare.
Contamination and Treatment
Pure halite has a pH of 7. Therefore,
the more halite drilled, the more caustic
is required to maintain the pH above
9.5. Halite also has an effect on the
instruments used to measure pH. If pH
paper is used, the accuracy of this paper
is affected by the chloride concentration
and will indicate a lower pH as the chlorides increase. If the halite is pure, a pH
reduction greater than one unit is not
expected up to complete saturation of
the mud. However, pure halite is rarely
encountered. Associated minerals such
as anhydrite are usually present to some
extent, which will increase the filtrate
calcium. Therefore, some caustic soda
will usually have to be added along with
the deflocculants to maintain the pH in
the proper range.
A greater degree of accuracy can be
attained if a pH meter is available. The
normal probe used in this instrument
contains a potassium chloride (KCl)
solution and is inaccurate when measuring pH in high-sodium-content
solutions. Therefore, a special saltcompensated probe should be used
to measure pH in high-salt muds.
The chloride titration is used to indicate the degree of saturation of NaCl
solutions, since a quantitative measurement for sodium does not exist for field
use. Since neither sodium nor chlorine
can be economically precipitated from
the mud, there is no alternative but to
tolerate the concentration of halite that
enters the mud system. Dilution with
freshwater is the only economical way
to reduce the chloride concentration.
If massive salt or frequent salt
stringers must be drilled, the mud
should be saturated with salt to avoid
washout and hole collapse. Whether
saturated or not, a fluid-loss-control
agent is usually necessary at concentrations greater than 10,000 mg/l. This
could be one of several materials such
,
as RESINEX, POLY-SAL, SP-101,T MY-LO-JELE
Contamination and Treatment
4C.15
POLYPAC and prehydrated bentonite.
The addition of bentonite is usually
recommended regardless of the specific
type of salt contamination. Under these
circumstances, dry bentonite should
not be added directly to the system.
The bentonite should be prehydrated
and protected with additional chemical
before adding it to the active system.
SYLVITE (KCl)
The response of the mud properties to
sylvite contamination and the treatment of the mud for sylvite are identical
to that for halite. If the mud contains
no chlorides other than those obtained
from drilling the sylvite salt, the chloride titration value would be an accurate
measurement of the potassium ion concentration. However, this is rarely the
case. It is not uncommon to find these
salts interbedded. The quantitative titration for the potassium ion can be used
to identify the salt as either pure sylvite
or partially sylvite for geologic purposes.
It is important to know the type of
salt to be drilled. Since the solubility
of sylvite is slightly greater than halite,
a massive sylvite salt section drilled
with a saturated halite fluid would still
wash out to some extent, although not
as severely as if freshwater is used. It is
difficult to prepare a saturated KCl
fluid with desirable flow properties,
fluid-loss control and good suspension
characteristics. However, if required
(and assuming that hole conditions
would permit), a clear, saturated KCl
fluid could be used.
CARNALLITE (KMgCl3•6H2O)
The complex salt “carnallite” is relatively rare. However, it does occur to
some extent in parts of the United
States, South America, Europe and the
Middle East. The most notable occurrence is in Northern Europe underlying the North Sea drilling area. This
is the Zechstein salt that consists of
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Mud
problems
associated
with
carnallite
are severe…
Contamination and Treatment
interbedded halite, sylvite and carnallite. Mud problems associated with
carnallite are severe and two-fold:
1. When solubilized, there are two
strong cations (calcium and magnesium) acting on the clays to cause
flocculation and dehydration. Mud
treatment would not be too difficult
if this were the only problem.
2. In the presence of hydroxyl ions
(OH–), the magnesium from the dissolved carnallite precipitates as magnesium hydroxide (Mg(OH)2). This
precipitate (Mg(OH)2) is a thick jellylike substance that acts as a viscosifier.
At the relatively low pH of 9.9, there
are sufficient hydroxyl ions present
for the precipitate to have a profound
effect on mud viscosity. Magnesium
can only be precipitated by caustic.
This reaction starts occurring with as
little as 0.03 lb/bbl of caustic soda.
Therefore, caustic soda should not be
used if it can be avoided. A viscosity
increase will occur if magnesium is
precipitated by caustic soda.
Most drilling muds are run in an alkaline state to maximize the performance
of the clays and other chemicals used to
treat drilling fluids. They are also run in
the alkaline state to minimize corrosion. Calcium should be removed by
raising the pH and treating with soda
ash to neutralize the calcium as CaCO3.
Sodium sulfate (Na2SO4) is one chemical available to control filtrate calcium
in high magnesium content fluids. The
chemical reaction is as follows:
Na2SO4 + Ca2+ → 2 Na+ + CaSO4↓
This reaction does not affect the
potassium or magnesium content.
However, it will control filtrate calcium
to a maximum of 400 mg/l. This is the
equilibrium solubility of CaSO4, or gypsum, and is indicated by the reversible
segment of the above equation.
Saltwater Flows
The solubility
of most common salts is
directly proportional to
temperature.
Connate waters can contain a broad
spectrum of salts. The origin of these
salts is directly related to the origin of
the sediments themselves. Since marine
sediments are deposited in seawater,
they usually contain salts similar to
those found in seawater. However, with
most of the water driven off in the compaction process, the salt concentration
can be considerably higher.
The solubility of most common salts
is directly proportional to temperature.
As the temperature of a salt solution
increases, the solubility of the salt in
that solution increases. A solution saturated with a certain salt at surface temperature is capable of holding more salt
in solution at elevated temperatures. In
addition, other chemical reactions such
as leaching of minerals from sediments
by groundwater can enrich connate
Contamination and Treatment
4C.16
waters with additional anions and
cations. Many of these can be detrimental to drilling fluids. Those waters highly
enriched in calcium and magnesium are
the most detrimental.
In the case of high-magnesiumcontent water, the relationships
previously discussed under carnallite
apply. The indicators for magnesium
are as follows:
1. A rapid pH reduction occurs.
2. Mud thickens with additions of
caustic soda or soda ash.
3. Titrate for magnesium ion
concentration.
The total hardness titration is
reported as soluble calcium in mg/l.
However, this titration also detects magnesium. To verify the presence and concentration of magnesium, calcium and
magnesium must be titrated separately.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
Indicators for a high-calcium-content
water flow, assuming little or no
magnesium present would be:
1. A lesser effect on pH.
2. A positive mud response to caustic
soda or soda ash additions.
3. Titrate for the true calcium ion
concentration.
The terms high-calcium- or highmagnesium-content connate water is
used because sodium salt is nearly
always present. This illustrates the
point that the chloride determination
can be misleading. For instance,
observe the variance in the ratio of
chloride to the associated metallic
ions in the salts previously discussed.
NaCl
KCl
MgCl2
CaCl2
KMgCl3
-
Halite
Sylvite
Magnesium chloride
Calcium chloride
Carnallite
In a pure solution of one salt, that salt
can be identified by the chloride content and titrating for the cations (except
sodium). However, in mixed-salt solutions the problem becomes complex to
the point of becoming an academic
exercise. This is especially true since
only calcium, potassium and magnesium can be easily identified in the
field. The chlorides not associated with
the identifiable cations are assumed to
be associated with the sodium ions.
Hydrogen Sulfide (H2S) Contamination
The most serious and corrosive contaminant discussed in this chapter is
hydrogen sulfide (H2S) gas. This gas is
destructive to tubular goods and toxic
to human life. The appropriate personal
protective equipment and worker
safety measures should be taken immediately if H2S is identified. Hydrogen
sulfide gas originates from:
1. Thermal deposits.
2. As a formation gas.
3. Biologic degradation.
4. Breakdown of sulfur containing
materials.
Hydrogen sulfide gas can be identified
by the:
1. Reduction of pH of the mud.
2. Discoloration of mud (to dark
color) due to the formation of FeS
from barite.
3. Rotten egg odor.
4. Viscosity and fluid loss increase due
to pH reduction.
5. Formation of a black (FeS) scale on
steel drill pipe.
Since H2S is an acid gas, the pH
of the mud is quickly reduced by
Contamination and Treatment
4C.17
neutralization of OH–. In order to offset the harmful aspects of the H2S gas,
the pH must be increased to at least
11, or a safer level of 12, by adding
caustic soda or lime. The following
chemical reaction describes the alkaline application to H2S. This can also
be seen in Figure 5.
HS + H2O
H2S + OH–
–
HS– + H2O
H2S + OH
S2– + H2O
HS– + OH–
100
HS-
H2S
10
% of total sulfides
The most
serious and
corrosive
contaminant
discussed in
this chapter
is…(H2 S).
1
0.1
S20.01
0
2
4
6
8
pH
10
12
14
Figure 5: Distribution of sulfides with pH.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
The bisulfide (S2–) ion may be removed
by a reaction with SULF-XT (zinc oxide)
to form zinc sulfide, which is insoluble.
S2– + Zn2+ → Zn S–
…oil-base
mud…acts
as a filming
agent in the
presence
of H2S.
A 1-lb/bbl treatment of SULF-X
removes approximately 1,100 mg/l
sulfides. NOTE: Check local environmental
regulations and aquatic toxicity requirements prior to using any zinc compound.
No more than 2 lb/bbl of SULF-X should
be used to pretreat for H2S. The addition
of 1 to 2 lb/bbl of SPERSENE is recommended when pretreating.
To protect tubular goods from the
corrosiveness of H2S, an oil-base mud
may be recommended. The oil acts
as a filming agent in the presence of
H2S. Hydrogen embrittlement is the
cause of tubular destruction due to
the H0 (atomic hydrogen) entering the
small, highly stressed metal pores and
re-forming as H2 (molecular hydrogen)
gas, causing an expansion in the volume of the hydrogen molecule, which
ruptures the metal.
H2S is no less toxic in oil-base muds
than in water-base base muds. Actually,
more safeguards should be used with
oil-base muds than with water muds
due to the solubility of H2S in oil. The
detection of hydrogen sulfide in mud
is tested by two methods:
1. Garrett Gas Train (GGT).
2. Hach test.
Both tests are fast, simple and have
easy-to-define results, but the Garrett
gas train is more accurate and gives a
quantitative result. The procedure is
described in RP 13 B.
If H2S is detected in mud when
using the soluble filtrate test on the
garrett gas train, action should be
taken to:
1. Immediately raise the pH to at least
11.5 to 12 with caustic soda.
2. Buffer the pH with lime.
3. Begin treatments with SULF-X
to remove soluble sulfides from
the system.
If it appears that H2S is bleeding in
(flowing) from the formation, the mud
density should be increased to shut off
the flow of gas into the wellbore.
The chemistry of hydrogen sulfide
gas is quite complex. The actions
described above are recommended
to minimize the toxic aspects of this
corrosive contaminant.
Quick Reference for Recognizing
and Treating Contaminants
CEMENT
CONTAMINATION
Symptoms
1. Increased viscosity and gel strengths.
2. Increased pH, Pm and Pf
(particularly Pm).
3. Increased fluid loss.
4. Increased excess lime and soluble
calcium (later).
Treatment
1. Depending on the type of system in
use, SAPP or TANNATHIN and sodium
Contamination and Treatment
4C.18
bicarbonate may be used to lower
the pH and precipitate the soluble
calcium (see Tables 2 and 3). The
clay particles are then free to react
with the thinner or deflocculant
in use.
2. Large treatments of water and
SPERSENE to control flow properties.
Additions of bentonite are made to
obtain desired fluid loss after flow
properties are under control.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
_______________________
MAGNESIUM
_______________________
Symptoms
1. Unstable yield point and fluid loss.
2. High levels of hardness after treating
calcium with soda ash.
Treatment
NOTE: The following treatment is for minor
levels of contamination, such as from seawater. DO NOT use caustic when treating
massive magnesium contamination (such
as from carnalite).
1. Raise pH of mud to 11 with caustic
soda or caustic potash (KOH) to
remove magnesium (see Tables 2
and 3).
2. Maintain pH at this level to prevent
magnesium from resolubilizing
from Mg(OH)2.
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
CONTAMINATION
GYPSUM
OR
ANHYDRITE CONTAMINATION
Symptoms
1. Increased viscosity and gel strengths.
2. Increased fluid loss.
3. Increased soluble calcium.
4. Possible decrease in Pf and pH.
Treatment
1. Precipitate or sequester soluble calcium with phosphates or soda ash
(see Tables 2 and 3). Reduce viscosity
with treatments of lignosulfonates
and caustic soda. Lower fluid loss
with treatments of bentonite,
POLYPAC or RESINEX.
2. Allow gypsum or anhydrite to
remain in the system to give a soluble calcium level above 600 mg/l.
Control viscosity with lignosulfonate
treatment, pH with caustic soda, and
fluid loss with bentonite and POLYPAC.
SALT
CONTAMINATION
Symptoms
1. Increased viscosity.
2. Increased fluid loss.
3. Increased soluble chloride
and calcium.
4. Decrease in pH and Pf.
Contamination and Treatment
4C.19
Treatment
1. Dilute NaCl concentration with
water if the salt formation is to be
cased off shortly after drilling. Treat
the fluid with lignosulfonates for
viscosity control; caustic soda and
lime on a 1:2 ratio for pH and Pf
control; POLYPAC UL and bentonite
for fluid-loss control.
2. If the salt is not to be cased off, and
the formation is to be left exposed
for a long period of time, saturate the
system with sodium chloride (salt)
to prevent further hole enlargement.
Control the viscosity with treatments
of lignosulfonates plus caustic soda
and lime. Small treatments of POLYPAC
are effective for viscosity control if
solids are controlled in the proper
range. Control fluid loss by additions of starch and/or POLYPAC and
prehydrated bentonite additions. If
starch is used for fluid-loss control,
maintain NaCl at 190,000 mg/l to
prevent fermentation of the starch,
or use a biocide.
SALTWATER-FLOW AND
GAS-KICK CONTAMINATION
Symptoms
1. Increase in mud pit level.
2. Increase in rate of returns
from wellbore.
Treatment
1. Shut pump down.
2. Pick up off bottom to clear
the kelly bushing.
3. Close in the well with BOP.
4. Measure the drill pipe pressure and
calculate the additional mud density
required to balance the kick.
5. Increase the mud weight to the
required density while circulating
the kick out at a reduced pump rate.
6. If it is a gas kick, remove the
gas from the system by the use
of surface circulating equipment
and de-gassers.
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
_______________________
Contamination and Treatment
7. If it is a saltwater flow, dump saltwater at the surface (if possible),
then condition the fluid with additional deflocculants and caustic
soda. Dilution of the NaCl ion concentration with freshwater may be
required. Small treatments of lime
and caustic soda may also be
required for pH and Pf control.
_______________________
_______________________
CARBONATE
_______________________
Symptoms
1. High gel strengths.
2. Increasing Pf with constant pH.
3. Increased difference between Pf
and Mf.
4. Increasing carbonate or
bicarbonate levels.
_______________________
_______________________
_______________________
_______________________
_______________________
CONTAMINATION
Contaminant
Carbon dioxide
Contaminating Ion
Carbonate
Gypsum and anhydrite
Bicarbonate
Calcium
Lime or cement
Calcium and hydroxyl
Hard or seawater
Hydrogen sulfide
Calcium and magnesium
Sulfide (H2S, HS–, S2–)
Treatment
1. Raise the pH to 10.3 to 11.3.
2. Add lime and/or gyp, two soluble
sources of Ca to remove carbonates
as CaCO3 (see Tables 2 and 3).
HYDROGEN
SULFIDE CONTAMINATION
Symptoms
1. Decreasing alkalinities.
2. Slight foul odor (rotten egg)
at the flow line.
3. Mud or pipe turns black.
Treatment
1. Increase pH to 11 to 11.5 with
caustic soda.
2. Buffer with lime.
3. Add SULF-X (see Tables 2 and 3).
Treatment
Gyp to reduce pH
Lime to raise pH
Lime to raise pH
Soda ash
SAPP
Sodium bicarbonate
Sodium bicarbonate
SAPP
Citric acid
Caustic soda
SULF-X (zinc oxide*)
plus sufficient caustic
soda to maintain the
pH above 10.5
Treating Concentration
(lb/bbl)
mg/l x Fw x 0.00100
mg/l x Fw x 0.000432
mg/l x Fw x 0.00424
mg/l x Fw x 0.000928
mg/l x Fw x 0.000971
mg/l x Fw x 0.00735
lb/bbl excess lime x 1.135
lb/bbl excess lime x 1.150
lb/bbl excess lime x 1.893
mg/l x Fw x 0.00116
mg/l x Fw x 0.00091
*Other zinc compounds such as chelated zinc or zinc carbonate may also be used. An excess should always be maintained in the system.
NOTES:
1. Fw is the fractional % of water from retort.
2. Excess lime = 0.26 (PM – (Pf x Fw)).
Table 2: Chemical treatment in U.S. units.
Contamination and Treatment
4C.20
Revision No: A-0 / Revision Date: 03·31·98
CHAPTER
4C
Contamination and Treatment
Contaminant
Carbon dioxide
Gypsum and anhydrite
Contaminating Ion
Carbonate
Bicarbonate
Calcium
Lime or cement
Calcium and hydroxyl
Hard or seawater
Hydrogen sulfide
Calcium and magnesium
Sulfide (H2S, HS–, S2–)
Treating Concentration
(kg/m3)
mg/l x Fw x 0.00285
mg/l x Fw x 0.00121
mg/l x Fw x 0.00265
mg/l x Fw x 0.00277
mg/l x Fw x 0.002097
kg/m3 excess lime x 3.23
kg/m3 excess lime x 3.281
kg/m3 excess lime x 5.4
mg/l x Fw x 0.00285
mg/l x Fw x 0.002596
Treatment
Gyp to maintain pH
Lime to raise pH
Soda ash
SAPP
Sodium bicarbonate
Sodium bicarbonate
SAPP
Citric acid
Caustic soda
SULF-X (zinc oxide*)
plus sufficient caustic
soda to maintain the
pH above 10.5
*Other zinc compounds such as chelated zinc or zinc carbonate may also be used. An excess should always be maintained in the system.
NOTES:
1. Fw is the fractional % of water from retort.
2. Excess lime (kg/m3) = 0.074178 (PM – (Pf x Fw)).
Table 3: Chemical treatment in metric units.
pH
Pm
Pf
Mf
↑
↑
↑
↑
↑
↑
— ↑pH↓
11.5
↑
↑
↑
↓
↓
↓
↓
—
↑
—
—
↑
↑
↑
↓
↓
↓
↓
↑
↑
↑
—
↑
—
↑
↑
↑
↓
↓
→
↑
↑
—
↓
—
↑
—
↑
↑
↑
↓
↓
↓
↓
—
↑
—
—
↑
—
—
—
—
—
—
—
↑
↑
FV
PV
↑
—
↑
—
↑
—
—
↑
—
—
Gypsum or
anhydrite
FL
↑
Contaminant WT
Cement
—
YP Gels
Cl–
Ca2+ Solids
↑
—
Salt
Carbonate or
bicarbonate
H2S
Old
↑
↑
Solids
New
↑
↑
↑
↑
↑
↑
↑
↑
↑
↑
↑
Treatment
Bicarb, or SAPP,
or thinner, bicarb
and citric acid
Caustic, dilution
water and
thinner, or soda
ash (plus fluidloss polymer)
Caustic, dilution
water, thinner
and fluid-loss
polymer
pH <10.3: lime
pH 10.3 to 11.3:
lime and gyp
pH >11.3: gyp
Caustic, lime and
zinc source
(SULF-X)
Dilution
water and
solids-removal
equipment
Dilution water,
solids-removal
equipment and
thinner
Contamination and Treatment
4C.21
↑
↑
↑ Increase ↓ Decrease — No change
Slight increase
Slight decrease
Table 4: Quick reference for recognizing and treating contaminants.
Revision No: A-0 / Revision Date: 03·31·98