CHAPTER 3:

DRILLING FLUIDS

DRILLING ENGINEERING I(CGE577)

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CONTENTS

Definition of drilling fluid

Mud Circulation system

Functions of drilling fluid

Types of drilling mud

Drilling mud additives

Drilling Mud Testings

Solid Control

Drilling Mud Calculations

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Definition of Drilling Fluid

A drilling fluid, or drilling mud, is any fluid that is used in a drilling operation in which that fluid is circulated or pumped from the surface, down the drill string, through the bit, and back to the surface via the annulus.

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Mud Circulation System

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1. Mud is sucked and pumped from the

mud pits to the drilling apparatus.

2. Pipes and hoses, connects pump to

drilling apparatus.

3. Mud Return line, returns mud from

hole.

4. Shale shaker, shaker/sieve that

separates rock cuttings from mud.

5. Reserve pit, collects rock cutting

separated from the mud.

6. Mud pits, where drilling mud is mixed

and recycled.

7. Mud-mixing hopper, where new mud

is mixed and then sent to the mud

pits.

FUNCTIONS OF DRILLING MUD

The main function of drilling mud is to bring the drill cuttings from the wellbore to the surface

The drilling mud must have suitable density and viscosity to transport the cuttings to the surface during its circulation process

Enough gel strength is needed to suspend the cuttings when it is in static condition (not circulating)

Mud must be designed such that it can :

Carry cuttings to surface while circulating

Suspend the cuttings while not circulating

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FUNCTIONS OF DRILLING MUD (CONT)

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The drilling mud also prevent formation fluid from entering the wellbore thus preventing the influx Enough density has to be determined to

prevent influx but not too high to break the formation.

Formation breakdown will cause mud flow into the formation and subsequent loss circulation

The drilling mud tends to seep into permeable sands and form filter cake on the borehole wall

The mud filtrate will reduce productivity by mean of clay hydration leads to permeability reduction.

Filter cake which is too thick can cause stuck pipe due to high differential pressures

FUNCTIONS OF DRILLING MUD (CONT)

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Protect the wall of the wellbore By creating pressure inside the hole that is

greater than the reservoir pressure, drilling mud prevents the flow of reservoir fluids into the borehole. Thus, prevent cave-in and collapse.

This is done by adjusting the density (mud weight) of the liquid. The weight of the mud can be increased by adding heavy minerals such as barite.

The mud has to seal off the permeable formations to avoid damages

It will form a thin impermeable mud cake (or filter cake) at the borehole wall

The cake should not be too thick, otherwise, it may cause stuck pipe

The mud cake also protects the borehole from caving-in and collapse.

FUNCTIONS OF DRILLING MUD (CONT)

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The drilling mud helps to maintain wellbore stability with correct density

Borehole stability problems can occur in troublesome formation such as instable shales, highly permeable zones, lost circulation and overpressure zones

The most common problem is shale instability, due to: The shale instability can be caused by the pressure differential in the

borehole and formation, and clay hydration

The interaction between clay and mud filtrate containing water during hydration process will cause hydration stress and lead to borehole instability.

The hydration of the clays can be overcome by using non-water based mud or inhibited muds

OTHER FUNCTIONS OF DRILLING MUD (CONT)

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The rotating process of drillbit will generate a lot of heat, which will expedite the wear out, unless the bit is cooled. The drilling mud helps to cool the bit down and lubricate the cutting process

Obtaining downhole information

Transmit hydraulic horsepower to the bit and allow maximum penetration rate

Minimize torque and drag of the drillstring to decrease wear and possible failure as well as stuck pipe

Drilling Mud Design Exercise

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Fresh water has a density of 62.3 lbm/ft3 which gives a pressure gradient of 0.433 psi/ft.

For 10 000 ft well, the bottom hole pressure due to a full Column of water is .. psi.

At this depth normally pressured formation have a pressure gradient of 0.465 psi/ft. Giving a formation pressure at 10 000ft of psi.

If we used water as a drilling fluid at this depth, formation fluid will the wall, causing a .

Drilling fluid can be classified into liquid, gas-liquid mixture (foam) or gas (air)

Liquid mud is further classified into water based mud (WBM) or oil based mud (OBM).

Both types of mud contains a few percentages of solid composition (clay, sand, barite)

Fresh water or salt water (sea water) is used as continuous phase for WBM

TYPES OF DRILLING FLUID

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Common Mud Additives

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The typical composition of water based mud is water (the largest %), active solids, low density inactive solids and high density inactive solids.

The active solids are solids, which will react with water (e.g clays). The inactive solids are solids, which do not react with water (e.g sands, limestone and Barite. Sand and limestones is the low density solids whereas barite is the high density solid)

Clay in the water based mud helps to increase viscosity to carry drill cuttings and suspend Barite.

The other function of clay is to create impermeable mud cake.

Illite is the type of clay that is non-expandable while montmorillonite is the type that is expandable or absorb water .

Water Based Mud (WBM)

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Additives are used in the WBM to control the viscosity, density, fluid loss, pH and contaminants removal.

Viscosity

Example of viscosity control additive is Wyoming Bentonite clay.

The Wyoming bentonite has 100bbl/ton yield.

In order to reduce the viscosity of the mud, solid content, number of particles per unit volume and attractive forces between the particles should be reduced.

The additives used to reduce viscosity include phosphates, lignites, lignosulphate and Tannins, those are called the thinners or dispersants.

WBM

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Density

Weighting material such as Barite that has higher denstity is normally used to increase the mud density in high pressure zones.

The other weighting materials are calcium carbonate and lead sulphide.

WBM

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Fluid loss control

The ideal filter cake is thin and impermeable.

Thick filter cake will lead to pipe sticking due to differential pressures. Impermeable filter cake will avoid filtrate from losing into the formation. The filtrate invasion will lead to formation damage and tight spot in the hole due to permeability reduction.

Materials those are used to control the fluid loss are clays (Bentonite), starch, CMC (sodium caboxyl-methyl cellulose), polyacrylates, lignosulphate and polyanoinic cellulose).

pH

Low pH will cause hydrogen embrittlement and corrosion to the drillstring and casing.

Caustic soda (NaOH) and caustic potash (KOH) can be used to increase the pH.

Contamination in drilling mud

The contaminants from cement could enter drilling mud and change its properties.

Common contaminants are like calcium (treated with soda ash NaCO3), carbon dioxide (treated with CaOH), hydrogen sulphide (treated with NaOH), Oxygen (treated with sodium sulphite).

WBM

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WBM

WBM is less expensive compared to OBM due to ready supply of water

The major disadvantage of WBM is it may lead to shale instability from clay hydration

Several WBM added with salt and polymer was introduced as inhibited mud to reduce the clay hydration process.

Type of inhibited mud

Calcium treated mud

Saltwater mud

Lignosulphanate treated mud

Potassium Chloride (KCl) combined with PHPA (partially-hydrolyzed polyacrylamide)

Mixed-metal hydroxide (MMH) mud

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Oil based mud consists of oil (50-80%), water (up to 30%), barite, salt, clays, sand, etc.

The oil based mud could either be

Full oil (low water content of about 5%)

Invert oil emulsion (with water content up to 50%)

Or Synthetic based mud (pseudo oil)

Water Phase in the Oil-Based Mud

Water in an oil mud will increase the viscosity of the mud.

Since mud density is related to viscosity, as mud density is increased, water content must be decreased to prevent excessive mud viscosity.

Water content will gradually increase during drilling as water from formation will be absorbed by the mud and vice versa.

OBM

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Viscosity Control in Oil-Based Muds

The viscosity of OBM is controlled by the amount of water and solid content

Mud viscosity can be increased with emulsified water and to a lesser extent with soaps.

Mud viscosity can be further increased with the addition of solids to the mud such as adding asphalts and amine-treated bentonite.

Filtration Control

Oil based muds have excellent filtration properties and rarely need filtration control additives. Thus, oil muds are especially suitable for drilling in formations easily damaged by water.

If necessary, fluid loss can be additionally controlled with asphalt, polymers, manganese dioxide (MnO2) or amine-treated lignite.

OBM

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Density Control

Like water based mud, barite is the main density control additive for oil-based muds.

Alkalinity Control

Lime is used to control the pH of oil muds between 8.5 and 10, or even higher values if CO2 and/or H2S are present.

Control of Solids and Water Content in Oil-Based Muds

There are a few methods that can be used to control the solids and water content in oil based mud.

Screening is often the only economical method of solids control of oil muds.

Hydrocyclones and centrifuges cannot normally be used on oil muds due to the loss of oil.

OBM

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If screening cannot maintain the desired solids level, dilution is used where part of the mud is discarded and water or oil are added as make-up.

Use of Oil Muds for Freeing Stuck Pipe

The differential pressure sticking problems happened when the drillstring is held against the mud cake by wellbore pressure.

The oil based mud can be used to release the stuck pipe.

A volume of oil mud is displaced sufficient to fill the annular region where the pipe is stuck and then compression, tension and torque are applied until the pipe is free. Oil mud of equal density with the wells water-based mud will prevent flow of the oil mud up the annulus.

OBM

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Lost Circulation Problem

Lost circulation zone could occur when there is fracture or very high permeability zone.

A mixture of diesel oil and bentonite or diesel oil-bentonite-cement is sometimes used to seal off a fractured formation in which drilling fluid is being lost.

The diesel oil slurry is pumped down the drillstring and water-based mud is pumped down the annulus. When they meet in the formation they set to a stiff paste and stop the loss of drilling fluid. About 2 parts of slurry to 1 part mud are required

OBM

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Composition of Muds

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OBM

Advantages Disadvantages

Not reactive with shale Expensive

More lubricating effect May create pollution

Reduce risk of differential sticking (thin mud cake)

Required special logging tool

Higher temperature stability Complex formulation

Low formation damage Difficult to control mud rheology

Higher penetration rate

Good hole for better cementing

Reduces corrosion

Advantages and disadvantages of OBM

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Synthetic based mud (Ester Based drilling fluid)

Advantages:

Environmental Friendly

Biodegradable

Local Product Derived from Palm Oil

Disadvantages:

Suitability/Properties

High Cost

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DRILLING MUD TESTS

Mud has to be tested to determine the mud quality and other rheological properties

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Mud density is measured directly using mud balance

The density is reported as ppg (lbs per gallon), lbs/ft3, psi/ft, psi/1000 ft, kg/l, and specific gravity (SG)

1. Density Test

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Viscosity in simple word is the resistance to flow

Marsh funnel only gives comparative viscosity but not other rheological properties.

The standard dimension for marsh funnel is 12 long, 6 diameter at the top and 2 long, 3/16 diameter tube at the bottom.

The viscosity is measured as time to flow one quart (946 ml) of fluid into the beaker.

The standard time for fresh water at 75 degree F is 26 sec/quart

2. Viscosity Test (Marsch Funnel)

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Rheological properties are used to design and evaluate the hydraulics and to assess the functionality of the mud system.

The rheological properties of mud such as Plastic Viscosity, Yield Point and Gel Strengths are measured using rotational viscometer.

The shear stress at different rotational rates are measured using viscometer

The Plastic Viscosity (PV) is measured as the dial reading at 600 rpm minus the dial reading at 300 rpm. The unit is centipoise

The Yield Point is given by the dial reading at 300 rpm minus the Plastic Viscosity and the unit is lb/100 ft2.

3. Rheology Test (Viscometer)

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3. Rheology Test (PV & YP)

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Plastic Viscosity (cP) Yield Point (lbs/100 ft2)

Depends on the friction between solids and liquid

A measure of the attractive forces between active clay particles in the mud

under flowing conditions

Represent the shear rate viscosities encountered at the drill bit

Used to evaluate the ability of a mud to lift cuttings out of annulus

Low PV indicates that the mud is capable to drill rapidly because of the low viscosity of mud exiting at the bit

Higher YP indicates that drilling fluid has ability to carry cuttings better than

a fluid similar density but lower YP

High PV is caused by a viscous base fluid and by excess colloidal solids.

PV can be lowered by dilution to reduce the solids content in the mud

YP can be lowered by adding deflocculant and increased by adding

flocculant

3. Rheology Test (PV & YP)

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Muds behave with non Newtonian fluid flow properties as their viscosity is not only influenced by temperature and pressure but is also strongly related to the velocity at which the mud flows through the hydraulic system.

The drilling fluid velocity and the resulting rate of shear at the walls of the conduits play an important role on the viscosity of the fluid pumped.

For this reason, it is important to know the viscosity in the full range of shear rate usually considered for hydraulic calculations.

Traditionally, oil industry uses the Bingham Plastic model to represent drilling fluid behaviour.

Drilling fluid in static condition has its internal structure strength that required pressure to initiate its flow called as gel strength.

This internal structure strength indicates the ability of the drilling fluid to suspend cuttings when not circulating

The gel strength can also be determined using viscometer

The rotor speed is set low speed of 3 rpm after leaving at static condition for specific time

If the mud is left in static condition for 10 secs then it is reported as the initial or 10 second gel

The mud can be reported also as 10 minutes and 30 minutes gel

The unit for gel strength is the same as Yield Point, which is lbs/100ft2

3. Rheology Test (Gel Strength)

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3. Rheology Test (Gel Strength)

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The filtrate analysis to determined filter cake building properties of drilling mud is measured using filter press apparatus

In the experiment, the rate of fluid flow through filter under specific pressure and temperature are measured

Measured also the thickness of the solid residue on the filter paper/ ceramic disc

4. Filtrate AnalysisWELL CONSTRUCTION

PETROLEUM ENGINEERING SKG-10

5 Drilling Fluid

DRILLING FLUIDS PROPERTIES

Filtrate and Filter Cake

Mud press

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Liquid and solid content of drilling mud is determined using the water retort kit

The kit contains heater to heat up the measured mud until it vaporized

The condensed fluid (water/oil) are collected in a graduated tube and recorded as percentage from total volume.

The volume of solid is found by subtracting liquid content from 100%

5. Liquid and solid content

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pH is the term used to express the concentration of H+ ion in an aqueous solution

Mud must always be treated to be alkaline ( pH 7 9.5)

If mud pH is below 7 (acidic)

Corrosion problem occurs (which can be caused by CO2 and H2S)

If mud pH is above 9.5 (too alkaline)

Mud viscosity increases

Shale instability occurs

pH is measured by pH paper or electronic pH meter

pH can be increased by adding lime or caustic soda

6. pH Test

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Alkalinity of filtrate or mud is determined by the amount of acid needed to titrate the solution containing phenolphthalein indicator.

The filtrate or mud is added with phenolphthalein. Acid is titrated to the solution until the colour changes. The number of ml of acid required is reported as the filtrate or mud alkalinity

7. Alkalinity Test

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The chloride content test is used to determine the salt contamination from the formation

The filtrate or mud is added with phenolphthalein. Acid is titrated to the solution until the color changes

25 to 50 ml of distilled water and a small amount of potassium chromate solution is added.

The solution is stirred continuously while adding silver nitrate until the color changes.

The chloride content is given by (ml of silver nitrate x 1000/ml of filtrate sample)

8. Chloride Contest Test

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SOLID CONTROL

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Solid control may be defined as the control of the quantity and quality of suspended solids in the drilling fluid so as to reduce the total well cost.

The equation to estimate the volume of solids entering the mud system whilst drilling is:

SOLID CONTROL (EXERCISE)

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Calculate the amount of cuttings to be removed by the solids control equipment if a well in East sea field consists of the following conditions: Average formation porosity, = 0.22

Hole diameter (in) = 24

Rate of penetration (ft/hr) = 58

SOLID CONTROL (Cont)

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Solid control is aimed at inert solids in the mud

These may include sand, silt, limestone and barite.

All of these solid (except barite) are considered to be undesirable since:

1. They increase frictional resistance without improving lifting capacity.

2. They cause damage to the mud pumps, leading to higher

maintenance costs.

3. The filter cake formed by these solid tends to be thick and

permeable. This leads to drilling problems (stuck pipe, increase drag) and possible formation damage.

Therefore it is very important to remove all of these solids

However, solids like barite and bentonite should be retained

SOLID CONTROL (Cont)

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There are 3 basic methods used to control solids content:

Screening

Settling

Dilution

1. Screening of Solids

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A shale shaker uses a vibrating screen to separate solids according to size.

Solids too large to pass through a given mesh size will be discarded while the finer material undergo further treatment.

2. Settlings

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The solids can be separated from the mud by natural settling mechanism.

Solids will settle out more readily if: The solid particle are large and heavy The mud is light and has a low viscosity The gravitational force can be increased

by mechanical means

Natural settling is far too slow, so mechanical devices are introduced (forced settling)

These devices include hydrocyclones & centrifuges

3. Dilution of Solids

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After passing through all screening and settling stages there will be a very fine solids content which remains in the mud. These solids can either be discarded or diluted.

Due to limited capacity of the active system some mud usually discarded (together with desirable solids and other chemicals) before remainder can be diluted and conditioned for re-circulating)

DRILLING FLUID DESIGN

Drilling fluids must be carefully designed while drilling a well in order to satisfy various regulatory and environmental standards, as well as achieve the highest performance

CONSIDERATION MUST BE GIVEN TO:

1. Impact of fluid on the formation or pay zone

2. Economics or cost of fluid/ additives

3. Logistics and availability of components/ additives

4. Environmental impact

5. Well design

6. Temperature

Refer: Article on Designing and Managing Drilling Fluid (April 1994) for further details

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Material Balance Equation

Used to calculate volume and densities of drilling fluid when two or more insoluble materials are mixed together.

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Density of Mud Additives

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Exercise 1

When 150 lbm of API barite, 40 lbm of bentonite and 1 bbl of water are mixed, what is the resulting density?

Ans: 11.2 ppg

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Exercise 2

When 70 lbm of calcium chloride is mixed with 1 bbl of water, what is the resulting density?

Ans: 9 ppg

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Exercise 3

Calculate how many sacks of barite are required to increase the density of an 800 bbl mud system from 12.7 ppg to 14.5 ppg. Assume one sack contains of 100 lb of barite.

Given: 1 bbl of barite = 15 sacks

S.G. of barite = 4.25

Density of water = 1.0 g/cc = 8.33 ppg = 62.4lb/cuft

Ans: 1033 sacks

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