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.

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

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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.

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 G = density of mud in ppgH = depth of the hole in feet.

This hydrostatic head will counter the formation

pressure in order to avoid a blowout while drilling.

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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 = 0.052GH4,650 = 0.052 x G x 10,000G = 8.94 ppg

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.

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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.

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)

Types of drilling fluids

1. Water-base mud

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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.

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.

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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 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

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Yb = Bingham yield point, lb/100 ft2

= Torque readings from instrument dial at 600 and 300 rpm.

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.

Finer particles in the mud stream are removed with desanders,

desilters and centrifuges. Each of these items of solids-control

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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.

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

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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.

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.

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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.

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.

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.

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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 solidVm1 = 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

= m2Vm2 m1(Vm2 Vs)

sVs m1Vs = m2Vm2 m1Vm2

Vs(s m1) = Vm2(m2 m1)

As to units, the densities may be in any consistent set.

The corresponding weight to add is

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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

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