INTRODUCTION TO CEMENT
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TABLE OF CONTENTS
INTRODUCTION – A little history
MANUFACTURING PORTLAND CEMENT
CHEMICAL COMPOUNDS OF CEMENT
COOLING PROCESS
CLASSIFICATION OF CEMENTS
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INTRODUCTION
WHERE CEMENT COMES FROM
A LITTLE HISTORY
Being widely used in the construction of houses and buildings, clay was
also used to consolidate the first wells.
However pozzolana cements were already known back in ancient Roman
times; indeed, the Romans used materials of volcanic origin mixed with lime
which had good compressive strength.
Portland cements were first developed in 1824 by Joseph Aspdin.
This material was obtained by baking a clay and limestone mixture together.
Aspdin called it Portland simply because the quarry where he extracted this
material was on the island of Portland, off the English coast.
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INTRODUCTION
Portland is a base cement mixture; over the years cement used for oil
applications has greatly evolved and can now be used even in the most
severe conditions.
Cements for the oil industry are classified in an API (American Petroleum
Institute) scale and can be adapted to suit needs by adding additives and
inert material.
The first known cementing of wells took place in 1903 in California, when
the Union Oil Co. mixed and pumped some fifty sacks of cement into a well
to isolate a water zone.
The slurry was left to harden for 28 days and then drilling began again.
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INTRODUCTION
A.A. Perkings introduced the plugging casing cementing system in 1910; this
system was very similar to the system still used nowadays.
The cement was mixed on the surface by hand and pumped into the casing,
placing a wooden plug with rubber inserts between the slurry and the
displacement, for which steam was often used.
The first real step forward in cementing was made by E.P Halliburton, in
Oklahoma, when he introduced the funnel mixing system in 1920. At that time
only one type of cement and no additives existed.
In 1949, with the appearance of Dowell, Chemical Process, Halliburton began
to offer two types of cements and three additives. Technologies, types of
cements and additives have gradually developed over the years.
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MANUFACTURING PORTLAND CEMENT
The minerals needed to make cement are extracted from quarries
containing clay and limestone deposits and their by-products.
The percentages of each component depend on where they are extracted
and can easily vary; it is therefore important to remember that the data
indicated in the tables of each cement are indicative only and that the
cement supplier will have to adjust the mixture to guarantee that the
characteristics of the product are within the range envisaged by the relative
standards.
Service companies that use cement in the oil industry must check that the
batches of cement received from suppliers possess the requirements
established in the specifications by means of base laboratory tests and
specific tests when a cementing job is being prepared.
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MANUFACTURING PORTLAND CEMENT
The main minerals in cement are:
CALCAREOUS MATERIALS
LIMESTONE, CEMENT ROCKS, GYPSUM, MARL and ALKALINE WASTE
(waste products of chemical industries containing oxides and calcium
carbonate).
ARGILLACEOUS MATERIALS
CLAY, SHALE, SLATE and ASH.
All these products are finely ground and mixed together in the
required proportions using either a dry or wet process.
This preliminary mixture is passed through a pipe heated to between
1430°C and 1540°C (2600°F – 2800°F) at a set rate and speed. The
temperature and time of exposure result in a chemical reaction and the
material obtained is called CLINKER.
The clinker is ground and gypsum is added; the end product is
Portland Cement.
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MANUFACTURING PORTLAND CEMENT
The following reactions take place during the product baking stage:
100º C
evaporation of free water
600º C
dehydroxylation of clay minerals
900º C
crystallization of dehydroxylated clay minerals
900º - 1200º C
reactions between CaCO3 and CaCO with the
aluminiumsilicates
1250º - 1280º C start of the liquid phase
above 1280º C
the material becomes Clinker
It will then be ground and mixed with gypsum to become CEMENT.
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MANUFACTURING PORTLAND CEMENT
CEMENT MANIFACTURING PROCESS
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MANUFACTURING PORTLAND CEMENT
What is Portland Cement?
CLINKER
+
GYPSUM
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MANUFACTURING PORTLAND CEMENT
GYPSUM CRYSTALS
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MANUFACTURING PORTLAND CEMENT
PORTLAND CEMENT AT 50 µc AND 100 µc.
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MANUFACTURING PORTLAND CEMENT
Crystals contained in a grain of Cement
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CHEMICAL COMPOUNDS OF CEMENT
The main compounds formed during baking are:
•
3CaO SiO2
Silicate
(C3S)
Tricalcium
•
2CaO SiO2
Silicate
(C2S)
Dicalcium
•
3CaO Al2O3
Aluminate
(C3A)
Tricalcium
•
4CaO Al2O3 Fe2O3
Tetracalcium Alumino
(C4AF)
Ferrite
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CHEMICAL COMPOUNDS OF CEMENT
All cements are made in more or less the same way and have the same
ingredients. What distinguishes one from another, besides their final
characteristics, is their grain-size and therefore the amount of wetting water
needed.
Hydration process of Portland Cement
{C3A
{C4AF + CaSO4.2H2O + H2O > CSH + Ca(OH)2
{C2S
{C3S
• maximum gypsum content, 3%
Isothermal reaction of cement:
Average value
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COOLING PROCESS
SLOW COOLING
This allows the components to crystallize, better grinding and better longterm compressive strength.
FAST COOLING
Causes the formation of glass and grinding is therefore more problematic.
The final result has an excellent initial strength but may deteriorate in the
long-term.
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CLASSIFICATION OF CEMENTS
Surface area of 1 gram of particles of some materials.
CLASS H
2600 cm2/g
CLASS G
3200 cm2/g
Sand 100 mesh
>
Micro Cement
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11,000 cm2/g
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CLASSIFICATION OF CEMENTS
In the oil industry, the cement, its constituent materials, the lab equipment
needed to carry out tests and the procedures used to carry out these tests
must all comply with set specifications.
These specifications are listed in detail and described in API
standards and are supplemented with ANSI and ISO specifications.
API stands for America Petroleum Institute; this institute issues the
specifications or standards which cover all the issues mentioned above and
are those referred to for cementing jobs.
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CLASSIFICATION OF CEMENTS
In the API standards, the cements currently used in wells are catalogued
in CLASSES:
-API classes
A, B, C, D, E, F, G, H
Classes A and B are the common Portland cements, which are used only
in cementing jobs where no specific properties are required, normally at
depths of less than 1900 m and temperatures of not more than 80º C.
Class C is also limited to the same depth and temperature constraints as
the previous classes but, having a finer grain-size and higher C3S
content, it offers better compressive strength.
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CLASSIFICATION OF CEMENTS
Classes D, E and F are cements containing an inorganic retarder and are
used only for particular jobs. They are also called Premium cements.
Classes G and H are the most widely used cements in the oil sector,
above all G. They are prepared with stricter criteria than the others which
means that they are more uniform and valid. They are used at any depth
and temperature and with the addition of additives they can cover almost
every need.
While class A, B, C, D, E and F cements are produced by grinding clinker
with calcium sulphate and other additives,
..classes H and G are produced without any additives whatsoever.
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CLASSIFICATION OF CEMENTS
Cements are also classified in the basis of their resistance to the
chemical attack of sulphates which are damaging to cement material in
general.
From this point of view, all 8 classes of API cements can be divided into
three groups:
• Cements with ordinary resistance to sulphates
• Cements with moderate resistance to sulphates
• Cements with high resistance to sulphates
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CLASSIFICATION OF CEMENTS
For example, a class G cement with high resistance to sulphates is different
from a G with ordinary resistance, due to the fact that the maximum content
of Tricalcium Silicate rises from 58% to 65%, the maximum content of
Tricalcium Aluminate decreases from a maximum of 8% to 3% and that
24% of Tetracalcium Aluminoferrite + Tricalcium Aluminate is added in the
G with high resistance to sulphates; this is not required in the other.
Moreover, to comply with the standards, a cement of a certain class must
meet certain physical and resistance requirements specified by API.
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PROPERTIES OF CEMENT SLURRY
The characteristics of a cement slurry can be altered depending on its use.
These characteristics can be summed up as follows:
1. Easy mixing and pumping.
2. Ideal rheological characteristics for the removal of the mud.
3. Rapid development of compressive strength.
4. Capacity to prevent the passage of gas.
5. Maintain a good level of compressive strength over time.
6. Capacity to fix to the surfaces.
7. Elasticity.
8. Capacity to maintain these characteristics at high temperatures.
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WATER / CEMENT RATIO
The water / cement ratio when preparing a slurry depends on the
density required, on the additives, and above all on the API class of
the cement.
Standard properties of cement slurries without additives
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Class API
Density
(kg/l)
Water
(l/100 kg)
Yield
(l/100 kg)
% mixing
water
A and B
1.87
46.2
784
46
C
1.78
56.1
877
56
D
1.97
38.1
698
38
G
1.9
44
764
44
H
1.97
38.1
698
38
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REASONS FOR USING CEMENT
Filling the annular space between the wall of an uncased hole and the
casing is the first operation after the bit has been run out of the hole and the
casing has been run in for the following reasons:
a)
b)
c)
d)
e)
To support the casing and anchor it to the borehole wall.
To prevent the borehole from caving.
To protect the casing from corrosion.
To protect the casing from crushing or bursting.
To isolate adjacent zones of the borehole and prevent the different
formation fluids from mixing.
f) To extend and optimize the life of a well.
g) To increase well safety and control.
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TYPES OF CEMENTING JOBS
Cement slurries can be used in three types of jobs:
a) Cementing of casing or strings (primary)
b) Remedial or complementary cementing
c) Cement plugs.
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CASING CEMENTING
Depending on the order, their position and purpose, casing
can be called:
a)
b)
c)
d)
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Surface casing
Intermediate casing
Production casing
Liner
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CASING CEMENTING
Surface casing
This is the first casing to be positioned and cemented. It is normally run in
to a depth of a few hundred metres and is naturally the one with the
largest diameter.
However, it is not the first casing to be run in hole because the conductor
pipe was run in previously with the aim of supporting the first sand and
gravel layers susceptible to slippage.
Intermediate casing
The intermediate casing is the casing which is run in hole after the
surface casing and before the production casing.
Intermediate casing can be run in hole for several thousand metres and is
the casing whose cementing is most critical for the entire well because it
can cross zones of circulation loss, overpressure, gas presence, cross
flow, swelling shales and unstable brines as well as high temperature
and pressure.
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CASING CEMENTING
Production casing
The casing which reaches the pay from the surface is called production
casing and through which production takes place is called production
casing, even if in practice there will be a production string inside the casing
to bring the fluids to the surface.
Liner
It is not always necessary to position the entire casing from the surface
down; sometimes only a section is needed which covers or lies just above
the pay zone and which ends, anchoring a few hundred metres inside the
upper casing.
At a later stage it will be possible to cement a casing above the liner and
this will be called Liner Tieback.
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MOST COMMON DIAMETERS
The normal dimensions of the casing or liner and in which open hole they are run
in are shown below; the dimensions are given in inches:
casing/ liner dimension
(inches)
open hole dimension
(inches)
20”
18 5/8”
13 3/8”
9 5/8”
7”
5”
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26”
24”
17.5”
12.25”
8.5”
6.5”
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TYPES OF CASINGS
z
z
z
z
z
Conductor pipe
Surfaces
Intermediate
Production
Liner
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MAIN CHARACTERISTICS OF A CEMENT SLURRY
The characteristics required of a cement slurry depend on the job to be
carried out.
Cementing of surface casing generally requires light slurries without any
particular additives except for accelerators.
Instead, cementing of intermediate or production casing sometimes
requires many additives because the casing has to be anchored in
formations which can give rise to a range of problems such as high
temperatures and pressures, instability of the borehole, gas percolation,
invasion of water and others still.
In remedial jobs, to repair cracked casing, or shut-ins of pay and other
levels, in uncased hole or in casing with shots such as intermediate or
production casing, additives may have to be used with care and attention.
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MAIN CHARACTERISTICS OF A CEMENT SLURRY
In any case, the main characteristics of a slurry, are those indicated
in lab test reports; i.e.:
*
*
*
*
*
*
*
Density
Mixing water
Composition of the slurry, with the list and
concentration of the additives
Thickening time
Fluid loss
Free water
Rheology
*
Compressive strength
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MAIN CHARACTERISTICS OF CEMENT SLURRY
The formula of a slurry is directly linked to the lab tests, the results of which are
indicated in the cement lab report and which, as far as possible, should be carried
out on samples of cement, water and additives sent directly from the rig-site.
DENSITY
Density is linked to the cement’s API class and to the additives used in the
formulation of the slurry.
Additives are often in powder form and have a set wetting water requirement.
The density; i.e. the Specific Weight of the slurry is expressed in kg/l, or in lbs/gal
(pounds per gallon).
Its value can vary from around 0.5 kg/l, when mixed with Nitrogen (N2), to around
2.5 kg/l, when mixed with weighted additives.
Increases in the density of a slurry cause it to become thicker and its viscosity
therefore increases; this in turn results in the need for a friction reducer to improve
its rheology.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
Density as a function of the
additives
Cement + weighting additive
16 - 21
Densified cement
16 - 17
15 - 17
Cement + Salt
15 - 16
API Class G or H
API Class C
14 - 15
12 - 15
Cement + Bentonite
Cement + Spherelite
8 - 13
Cement + Nitrogen
4 - 16
6
8
10
12
14
16
18
20
22
Density in lbs/gal, as a function of class and additives
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MAIN CHARACTERISTICS OF CEMENT SLURRY
MIXING WATER
This is the volume of water needed to mix a standard quantity of powdered
cement, which in metric units is expressed in litres of water per 100 kg of
cement while in imperial units it is expressed in gals per sack (1 sack = 1
cubic foot, is generally a quantity of cement of 94 lbs; i.e. 42.6 kg).
For example, for a class “G” cement without additives, the water requirement
is 44 l per 100 kg of cement, or 5.0 gals per sack.
The additives significantly affect this need.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
YIELD
This is an important value on which the calculation of the final volumes, in
bottomhole conditions, depends. There are additives such as Sferelite
whose microsfers tend to break under pressure and so undergo fluid
invasion when they are in the well, increasing the need for mixing water
compared to the 0 pressure conditions on the surface.
It will therefore be the down hole yield, based on the volume of liquid
cement required for the job, which will therefore tell us the quantity of
powdered cement needed.
Yield is expressed in litres of slurry per 100 kg of cement, or in cubic feet
per sack.
For a class “G” cement without additives, these values are around 760 lt /
100 kg, or 1.15 cuft / sk.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
COMPOSITION OF THE SLURRY
A lab report gives the composition or formula of the slurry, indicating in
detail the additives and their concentration as well as the quantity of
mixing water required.
For some, albeit very few, additives, the concentration is expressed as a
percentage of the weight of the volume of mixing water while for most it
is expressed as a percentage of the weight of the cement in powder.
In detail: (% bwc) = by weight of cement
(% bww) = by weight of water
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MAIN CHARACTERISTICS OF CEMENT SLURRY
THICKENING TIME
This is perhaps the most significant and closely examined value in a lab
report.
For a very good reason, because the Thickening Time (T.T.) is the value
which allows us to carry out the job in safe conditions.
Underestimating the time can result in disastrous cementing which can even
lead to the well having to be abandoned in the most serious cases.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
In a lab test the thickening time is found by carrying out the test on a sample of slurry at a
temperature defined as the circulating temperature (BHCT, bottom hole circulating
temperature) at the lowest point of the casing and that is, at the depth of the casing shoe,
which is always lower than the static temperature (BHST, bottomhole static temperature)
at the same point.
In the API standards there are tables which indicate the BHCT on the basis of the
temperature gradient and the type of job to be carried out.
For example, BHST being equal, the BHCT will be lower for a casing job than for a liner
cementing job or a plug or a squeeze.
What affects the thickening time:
a) Temperature, for a casing job this is the circulating temperature at the casing shoe while
for plugs and shoes it is close to static temperature. If the temperature increases the
thickening time shortens and vice versa.
b) Depth; the greater the depth, the higher the quantity of slurry needed and the longer the
pumping times.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
What affects the thickening time: (cont.)
c) Casing and open hole dimension (O.H = open hole). Again, the
larger the dimensions, the higher the volumes and pumping times.
d) Pressure, which is directly proportional to the depth. This value is
applied to the slurry during the lab test. An increase in pressure
shortens the TT value.
e) Presence of open zones, which provoke absorption. These also
affect the final volume of slurry.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
f) Excess. Excess means a factor added, in percentage, to the volume of
slurry positioned in the open hole, which mainly takes into consideration
possible absorption and if there is a Calliper reading available; i.e. a
dimensional mapping of the open hole. The excess can vary from 100%
or more for surface casing to 5% or 10% for deep production casing for
which a calliper is however available.
g) Safety factor. This is a multiplicative coefficient of the final thickening time
based on all the previous factors but which above all takes into account
the possibility that problems may be encountered at the rig site which
could lead to an interruption in pumping. One hour is generally the
minimum assumed for this factor.
The Thickening Time is obtained from a curve...
... an example of a curve is given below.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
Test end: 05:00
hr:mi
Test start
70B
c
30 minutes
THE THICKENING TIME CURVE
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MAIN CHARACTERISTICS OF CEMENT SLURRY
where the red and green curves are the temperature of the oil of the
machine carrying out the test and the temperature of the slurry sample in
the machine respectively and the blue curve is a consistency curve and is
the one which defines the Thickening Time value.
The horizontal numbers (0, 20, 40, 60, 80 and 100), are the slurry’s
consistency reference. Generally, when the slurry consistency curve
crosses the 70 Bc line, that is the moment which defines the slurry’s
pumping limit. However, for reasons of safety, some consider the moment in
which the consistency curve crosses line 50 Bc as the pumping limit.
In the previous chart, the T.T is measured at 05:00; 5 net hours.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
FLUID LOSS
•
Water is an essential component of slurry which gives it the rheological
characteristics which keep it at liquid state for the time needed to pump it to
where it is needed .
If the water content decreases compared to the initial quantity, the slurry’s
viscosity would increase until it can no longer be pumped, giving rise to a
false thickening of the cement.
That is, the cement does not set but it can simply no longer move from
where it is and, containing less water, its final compressive strength will be
less than that needed. However, this would not be the only problem.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
This phenomenon tends to occur while the slurry is flowing up through the
annulus behind the casing because, pressed by the hydrostatic pressure
against the wall of the open hole which acts as a mechanical filter, it tends
to lose its most mobile element; i.e. water. In this way the water separates
from the slurry and enters the rock pores.
The only factors that reduce this loss are the filter cakes which the mud has
left on the borehole wall and above all specific additives in the cement.
To give an idea, a slurry without additives which control fluid loss (Fluid
Loss Additives), easily reach loss values of more than 1000 ml/30 minutes
of water, while with a slurry with specific additives the fluid loss can be
reduced to 20 ml/30 minutes; a big difference.
Experience has taught us that in the case of surface or shallow casing, the
filter cake is generally sufficient to limit water loss of a slurry even if no
specific additives have been added while these additives are indispensable
in the case of bottomhole casing. .
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MAIN CHARACTERISTICS OF CEMENT SLURRY
FREE WATER
•
Free water is the phenomenon whereby a portion of mixing water separates
from the slurry when it is no longer agitated. In vertical wells, free water is
not a particularly serious problem as long as it is limited to just a few
percentage points. However, it can be serious in the case of horizontal
wells in which even just 1% of free water would leave a free channel of
cement in the upper part of the cemented annulus with consequent
communication and therefore the possibility of the exchange of fluids
between adjacent formations.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
COMPRESSIVE STRENGTH
This is the resistance to breakage through compression that a slurry develops
and begins at the moment in which it solidifies.
It is an important value but it is often over-emphasized in the sense that many
operators specifically request a high value, to the detriment of its elasticity.
Compressive strength decrease as the amount of water in the mixture increases
and therefore as the density decrease and is at its maximum in densified
slurries.
Compressive Strength is generally expressed in psi.
For light slurries with a high water content, values of just a few hundred psi of
CS can be expected.
For slurries with a water content in accordance with API standards, C.S. values
will rise to 3000 psi or more’, while in the case of densified slurries; i.e. with low
water content, values of 6000 psi and more are reached.
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MAIN CHARACTERISTICS OF CEMENT SLURRY
RHEOLOGY (Fann Reading)
Cement slurry is classified as a “non Newtonian” fluid; i.e. a fluid whose
viscosity is not constant but varies with the speed at which it flows through a
pipe.
Rheology measurements on slurry allow the characteristics of its flow to be
predicted as well as its capacity to suspend solids.
The PV (Plastic Viscosity) and YP (Yield Point) values can be obtained on
the basis of the apparent viscosity values, determined for example using a
Fann Viscosimeter. These values tell us to what extent a slurry is fluid and if
it is able to transport solids and keep them in suspension.
Moreover, the Flow Index (n’) and Consistency Index (K’) are obtained and
allow us to estimate the pressure loss caused by friction during the motion
of the slurry and its flow type; that is, if it movement is plug, laminar or
turbulent.
Agip KCO
Well Area Operations
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MAIN CHARACTERISTICS OF CEMENT SLURRY
For example, a PV with values of between 15 and 50 is considered
fluid, while above 100 it is very viscous; at the same time, positive YP
values of between 2 and 10 give the slurry transporting properties
without it being too viscous. Negative YP values tell us that the slurry is
not able to transport solids and therefore, as soon as it is left in static
condition, the heavy solids will settle and this should not happen.
Agip KCO
Well Area Operations
Drilling Supervisor Training Course
Drilling Cements
RPW2021A
50