DRILLING FLUIDS
The key to making the rotary drilling system work is
the ability to circulate a fluid continuously down
through the drill pipe, out through the bit nozzles and
back to the surface.
The drilling fluid can be air, foam (a combination of air
and liquid or a liquid.
Liquid drilling fluids are commonly called drilling mud.
All drilling fluids, especially drilling mud, can have a
wide range of chemical and physical properties. These
properties
are
specifically
designed
for
drilling
conditions and the special problems that must be
handled in drilling a well.
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Purpose of Drilling Fluids
1.
Cooling and lubrication.
As the bit drills into
the rock formation, the friction caused by the
rotating bit against the rock generate heat.
The heat is dissipated by the circulating drilling
fluid. The fluid also lubricates the bit.
2.
Cuttings removal. An important function of the
drilling fluid is to carry rock cuttings removed
by the bit to the surface.
The drilling flows
through treating equipment where the cuttings
are removed and the clean fluid is again pumped
down through the drill pipe string.
3.
Suspend cuttings.
There are times when
circulation has to be stopped. The drilling fluid
must have that gelling characteristics that will
prevent drill cuttings from settling down at the
bit. This may caused the drill pipe to be stuck.
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4.
Pressure control. The drilling mud can be the
first line of defense against a blowout or loss of
well control caused by formation pressures.
The hydrostatic head produced by the mud in
psi is = 0.052 x G x H
where
This
G = density of mud in ppg
H = depth of the hole in feet.
hydrostatic
head
will
counter
the
formation pressure in order to avoid a blowout
while drilling.
For example, Lets say a well is being drilled in a
salt-water basin (pressure gradient of 0.465
psi/ft), the pressure in the formation at 10,000
feet would be expected to be:
10,000 x 0.465 = 4,650 psi
The weight of mud required to counter this
pressure is calculated as follows.
P = 0.052GH
4,650 = 0.052 x G x 10,000
G = 8.94 ppg
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5.
Data source. The cuttings that the drilling mud
brings to the surface can tell the geologist the
type of formation being drilled.
6.
To wall the hole with impermeable filter cake.
This will give a temporary support to the wall of
the borehole from collapsing during drilling.
Drilling fluid can solve problems
Many drilling problems are due to conditions or
situations that occur after drilling begins and for
which the drilling fluid was not designed.
Some of these problems can be solved by adding
materials
to
the
drilling
fluid
to
adjust
its
properties.
Other cases, it may be necessary to replace the
drilling fluid being used with another fluid system.
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The most common changes is the mud weight or
density.
Weighting material is added when high-
pressure formations are expected.
Some of the problems are:
1. Lost circulation
Lost circulation can occur in several types of
formations, including high permeable formations,
fractured formations and cavernous zones.
Lost circulation materials can be added to the mud
to bridge or deposit a mat where the drilling fluid
being lost to the formation. These materials include
cane and wood fibres, cellophane flakes and even
padi husks were used in oil drilling in Sumatra.
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2. Stuck pipe
Stuck pipe can occur after drilling has been halted
for a rig breakdown, while running a directional
survey
or
when
conducting
other
nondrilling
operation.
The drill pipe may stick to the wall of the hole due to
the formation of filter cake or a layer of wet mud
solids on the wall of the hole in the formation.
3. Heaving or sloughing hole
This occurs when shales enter the well bore after
the section has been penetrated by the bit. To solve
this problem, drilling is suspended the hole is
conditioned (by letting the mud in circulation for a
period of time)
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Types of drilling fluids
1. Water-base mud
This fluid is the mud in which water is the continuous
phase. This is the most common drilling mud used in
oil drilling.
2. Oil-based mud
This drilling mud is made up of oil as the continuous
phase. Diesel oil is widely used to provide the oil
phase. This type of mud is commonly used in swelling
shale formation.
With water-based mud the shale will absorb the
water and it swells that may cause stuck pipe.
3. Air and foam
There are drilling conditions under which a liquid
drilling fluid is not eh most desirable circulating
medium. Air or foam is used in drilling some wells
when these special conditions exist.
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Mud Properties
1. Mud density or mud weight
Mud weight is measured by means of a mud balance.
The weight of water is 8.33 ppg. The mud weight can
be increased by adding barite (barium sulphate).
Barite has a specific gravity of between 4.2 – 4.3.
Other materials can be used to increase mud weight
such as ilmenite (S.G of 4.58)
2. Mud viscosity
Mud viscosity is difficult to measure but in the field
the Marsh funnel and the Fann V-G meter is
commonly used.
The Marsh Funnel is filled with mud, the operator
then notes the time, removes his finger from the
discharge and measures the time for one quart (946
8
cm3) to flow out. Marsh funnels are manufactured to
precise dimensional standards and may be calibrated
with water which has a funnel viscosity of 26  0.5
sec.
In using Fann V-G (Viscosity-gel) meter, readings are
taken at 600 rpm and 300 rpm.
The viscosities are defined as follows:
p = 600 - 300
aF = ½ 600
Yb = 300 - p
Where
p = plastic viscosity, cp
aF = apparent viscosity, cp
Yb = Bingham yield point, lb/100 ft2
 = Torque readings from
instrument dial at 600 and 300
rpm.
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From these relationships:
Yb = 2(aF - p)
aF = p + ½ Yb
True yield point:
Yt = ¾ Yb
Yield point is influenced by the concentration of solids,
their electrical charge, and other factors. If not at
the proper value, it can also reduce drilling efficiency
by cutting penetration rate, increasing circulating
pressure, and posing the danger of lost circulation.
3. Gel strength
The gel strength of a mud is a measure of the shearing
stress necessary to initiate a finite rate of shear.
With proper gel strength can help suspend solids in the
hole and allow them to settle out on the surface,
excessive gel strength can cause a number drilling
problems.
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4. Filtration
The filtration, water loss or wall building test is
conducted with a filter press.
The rate at which filtrate will invade permeable zone
and the thickness of the filter cake that will be
deposited on the wall of the hole as filtration takes
place are important keys to trouble-free drilling
Drilling Fluid treating and monitoring equipment
In addition to the main mud pumps, several items of
mud treating equipment are found on most rigs. Much
of this equipment is aimed at solids removal, including
shale shakers, desanders, desilters and centrifuges.
Shale shakers remove larger particles from the mud
stream as it returns from the bottom of the hole.
Shakers are equipped with screens of various sizes,
depending on the type of solids to be removed.
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Finer particles in the mud stream are removed with
desanders, desilters and centrifuges.
Each of these
items of solids-control equipment is applicable only
over a certain range of particle sizes.
In addition to removing solids, mud handling equipment
may also include a mud degasser to remove entrained
gas from the mud stream. Degassing the drilling fluid
is sometimes necessary when small volumes of gas flow
into the well bore during drilling.
Additional equipment include mixers to agitate mud in
the tanks, smaller pumps to various duties and
equipment for adding chemicals and solid materials to
the mud system.
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Drilling hazards
The following are some of the most common hazards in
drilling and can be overcome by proper control of the
mud properties.
1.
Salt section hole enlargement
Salt section can be eroded by the drilling fluid and
causes hole enlargement. These enlargement will
require larger mud volume to fill the system and in
case of casing the hole, larger cement volume is
required.
To avoid these problems a salt saturated mud
system is prepared prior to drilling the salt bed.
2.
Heaving shale problems
Areas with shale sections containing bentonite or
other hydratable clays will continually absorb
water, swell and slough into the hole.
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Such beds are referred to as heaving shales and
constitute
a
severe
drilling
hazard
when
encountered.
Pipe sticking, excessive solid buildup in the mud
and hole bridging are typical problems.
Various treatments of the mud are sometimes
successful, such as
 Changing mud system to high calcium
content by adding lime, gypsum etc which
reduces the tendency of the mud to
hydrate water sensitive clays.
 Increasing circulation rate for more
rapid removal of particles.
 Increasing mud density for greater wall
support
 Decreasing water loss mud
 Changing to oil emulsion mud
 Changing to oil-based mud.
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3.
Blowouts
Blowout is the most spectacular, expensive and
highly feared hazard of drilling.
This occurs when encountered formation pressure
exceed the mud column pressure which allows the
formation fluids to blow out of the hole.
Mud density or the mud weight is the principal
factor in controlling this hazard.
In drilling a blow out preventer (BOP) stack is
always attached at the top of the conductor pipe.
In case of a gas kick (a sign that may lead to a
blow out) the BOP stack can close the annular
space between the drilling pipe and the conductor
pipe or casing or shut the whole hole (with a blind
ram of the BOP).
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4.
Lost Circulation
Lost circulation means the loss of substantial
amount
of
drilling
mud
to
an
encountered
formation.
Lost circulation materials are commonly circulated
in the mud system both as a cure and a continuous
preventive.
These materials are the fibrous materials such as
the hay, sawdust or padi husk and lamellated (flat
and platy) materials such as mica, cellophane.
Drilling Mud Calculations
The most common mud engineering calculations are
those concerned with the changes of mud volume and
density caused by the addition of various solids or
liquids to the system.
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The first step is to compute the system volume, which
is the sum of the mud in the hole and surface pits.
Consider then the volume and density change of a mud
(or water) resulting from the addition of solids.
Two basic assumptions must be made:
1. The volumes of each material are additive.
2. The weights of each material are additive.
Expressions for these assumptions:
Vs + Vm1 = Vm2
sVs + m1Vm1 = m2Vm2
where
Vs = volume of solid
Vm1 = volume of initial mud
Vm2 = final volume of mixture
s = density of solid
m1 = density of initial mud
m2 = density of final mud
Solving for Vs :
sVs + m1Vm1 = m2Vm2
sVs = m2Vm2  m1Vm1
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= m2Vm2  m1(Vm2  Vs)
sVs  m1Vs = m2Vm2  m1Vm2
Vs(s  m1) = Vm2(m2  m1)
Vs 
Vm2 (m2  m1 )
s  m1
As to units, the densities may be in any consistent set.
The corresponding weight to add is
sVs 
sVm2 (m2  m1 )
s  m1
Example:
A 9.5 lb/gal mud contains clay (S.G.=2.5) and fresh
water. Compute (a) the volume % and (b) the weight %
clay in this.
Solution:
(a) From the equation
Vs 
Vm2 (m2  m1 )
s  m1
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volume of solid 

Vs
 100
Vm2
(m2  m1 )
 100
s  m1
(b) Weight % solids 
 (  m1 )
s Vs
 100  s m2
 100
m2 Vm2
m2 (s  m1 )

9.5  8.33
 100  9.4%
(2.5)(8.33)  8.33

20.8(9.5  8.33)
 100  20.6%
9.5(20.8  8.33)
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