PET 315: DRILLING FLUID

TECHNOLOGY
BY
ENGR. DR. E.E. OKORO
&
ENGR. (MRS) K.B. ORODU 1
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 INTRODUCTION

• Drilling Fluid Technology involves:

• The sciences of geology,

• Chemistry and physics, and

• The skills and applications of engineering.

• The goal is to utilize the available equipment and materials to attain at

lowest cost the desired objective of Drilling Operations.

DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 2

 DRILLING FLUID/ MUD
• The term DRILLING FLUID encompasses all of the compositions used to aid the production and
removal of cuttings from a borehole.

• This broad definition purposely places no restriction on the type of tools employed or on the
objective.

• Some specific examples of the application of drilling fluids are:

• Water poured into the hole while boring a foundation footing with an auger;

• Air introduced to blow cuttings from a blast hole;

• Mud made twice as heavy as water to control tectonic forces in mineral exploration;

• Foam as a conveyor of cuttings from a hole being drilled for water in glacial drift;

• Bentonite slurry employed to maintain a stable wall while excavating a cutoff trench;

• An Oil base mud used to drill for corrosive gases and reactive shales
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 3
 DRILLING FLUID/ MUD
• The simplest drilling fluid is a dirty mixture of water and clay, often called Mud

• Drilling fluids are any fluids which are circulated through a well in order to remove
cuttings from a wellbore

• The drilling fluid in drilling process can be seen as the equivalent to the blood in
human body, the mud pump is the heart and drilled out cuttings as the slag
product

• The mud cleaning system at the surface corresponds to the kidney and the lungs

• The fluid is pumped down the drill string, through the nozzles of the bit, and
returns back up the annulus between the drill string and the wellbore walls,
carrying the cuttings produced by the bit action to the surface
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
THE WELL CIRCULATION SYSTEM

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 DRILLING FLUID/ MUD
• The drilling fluid is related either directly or indirectly to almost every drilling
problem

• This is not to say that the drilling fluid is the cause or solution of all drilling
problems, but it is a tool that can often be used to alleviate a problem situation

• The successful completion of an oil well and its cost depend to a considerable
extent on the properties of the drilling fluid

• The cost of the mud itself is relatively small, but the choice of the right fluid and
maintenance of the right properties while drilling profoundly influence total well
cost

• For example, the number of rig days required to drill to total depth depends on the
rate of penetration of the bit and avoidance of delays cased by caving/ swelling
shales, stuck pipe, loss circulation, etc
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 6
 FUNCTIONS OF A DRILLING FLUID
• Suspend cuttings (drilled solids), remove them from the bottom of the hole
and the wellbore, and release them at the surface
• Control formation pressure and maintain wellbore stability
• Seal permeable formations
• Cool, lubricate and support the drilling assembly
• Transmit hydraulic energy to tools and bit
• Minimize reservoir damage
• Permit adequate formation evaluation
• Control corrosion
• Facilitate cementing and completion
• Minimize impact on the environment
• Inhibit gas hydrate formation
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 7
FUNCTIONS OF A DRILLING FLUID

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BASIC WELLBORE PRESSURES

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OVERPRESSURE AT CLAY/ SAND BOUNDARIES

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 DRILLING FLUIDS/ MUDS COMPOSITION
• In its most basic form, a drilling fluid is composed of:

• A liquid (either water or oil) and

• Some sort of viscosifying agent

• Whenever the hydrostatic pressure is greater than the formation pore pressure
(and the formation is porous and permeable) a portion of the fluid will be flushed
into the formation.

• Since excessive filtrate can cause borehole problems and to prevent such; some
sort of filtration control additive is generally added.

• In order to provide enough hydrostatic pressure to balance abnormal pore

pressures, the density of the drilling fluid is increased by adding a weight material
(generally barite).
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
HYDROSTATIC PRESSURE

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 DRILLING FLUIDS/ MUDS COMPOSITION
In summary, a drilling fluid consists of:
• The Base Liquid
• Water - fresh or saline
• Oil - diesel or crude
• Mineral Oil or other synthetic fluids

• Dispersed Solids
• Colloidal particles, which are suspended particles of various sizes

• Dissolved Solids
• Usually salts, and their effects on colloids most is important

• All drilling fluids have essentially the same properties, only the magnitude varies.
These properties include density, viscosity, gel strength, filter cake, water loss, and
electrical resistance.
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 PRINCIPAL COMPONENTS OF DRILLING FLUIDS

• Drilling fluids can be classified on the basis of a principal component

• These components are

(1) water
(2) oil, and
(3) Gas

• Frequently two-and sometimes all three-of these fluids are present at the
same time, and each contributes to the properties of the drilling fluid

• When the principal constituent is a liquid (water or oil), the term mud is
applied to a suspension of solids in the liquid
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CLASSIFICATION OF DRILLING FLUIDS ACCORDING TO
PRINCIPAL CONSTITUENT

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 CLASSIFICATION OF DRILLING FLUIDS/ MUDS
• Non Dispersed: They consist of Spud muds, Natural muds and other lightly treated
systems generally used for Shallow wells or Top hole drilling

• Dispersed: At greater depths or where hole conditions may be problematic. These

products are also effective filtrate reducers

• Calcium Treated: They inhibit the swelling of shale and clay formations; thus, are
used to control hole enlargement and prevent formation damage

• Polymer: Muds incorporating long chain, high-molecular weight chemicals are

effective in increasing viscosity, reducing filtrate loss and stabilizing the formation.
Example, Bentonite extenders

• Low solids: This includes systems in which the amount and type of solids are
controlled. Total solids should not range higher than about 6% to 19% by volume
(and clay <3% by vol.). They improve penetration rate
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 CLASSIFICATION OF DRILLING FLUIDS/ MUDS
• Saturated salt: Saturated salt systems have a chloride ion concentration of
189000ppm. Saltwater systems have a chloride content from 6000 to 189000ppm
and at its lower level is called Seawater systems

• Workover/ Completion: These fluids are specialized systems designed to minimize

formation damage, must be compatible with acidizing and fracturing operations,
and capable of reducing formation impairments

• Oil/ Synthetic: Oil-based fluids are used for high temperature wells, deviated holes
and wells where pipe sticking and hole stabilization is a problem. They consist of
two types of system:
• Invert emulsion mud – water-in-oil fluids and have water (up to 50%) as the dispersed phase
while oil as the continuous phase. Emulsifier and water concentrations are varied to control
rheological and electrical stability

• Synthetic fluids – designed to duplicate the functions of oil-based muds, without the
environmental hazards. Types are esters, poly alpha olefins and food grade paraffin
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 CLASSIFICATION OF DRILLING FLUIDS/ MUDS
• Air, Mist, Foam and Gas: four basic operations are included in this specialized category according
to the IADC. These include:

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CLASSIFICATION OF DRILLING FLUID SYSTEMS

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CLASSIFICATION OF PARTICLES IN DRILLING FLUIDS

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 NORMAL DRILLING FLUIDS
• In the field, a normal fluid generally means there is little effort expended to control the range of
properties.
1. It is used where no unexpected conditions occur
2. The mud will stabilize, so its properties are in the range required to control hole conditions
3. The chief problem is viscosity control
Formations usually drilled with this type of mud are shales and sands.

• Since viscosity is the major problem, two general types of treatment are used:

1. Water soluble polyphosphates

(a) they reduce viscosity
(b) can be used alone or with tannins
(c) if filter cake and filtration control is required - add colloidal clay to system

2. Caustic Soda
(a) they also reduce viscosity
(b) used under more severe conditions than phosphate treatment
The upper portions of most wells can use “normal” muds DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
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 SPECIAL DRILLING FLUIDS
• These drilling fluids are made to combat particular abnormal hole
conditions or to accomplish specific objectives. These are:

1. Special Objectives
(a) Faster penetration rates
(b) Greater protection to producing zones

2. Abnormal Hole Conditions

(a) Long salt sections
(b) High formation pressures 22
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 LIME BASE MUDS
• Water base mud

• Treated with large amounts of caustic soda, quebracho (thinner & dispersant),
and lime. Added in that order

• Ratio of 2 lb caustic soda, 1.5 lb lignosulfonate and 5 lb lime per 1 barrel of mud

• Will go through a highly viscous stage, but will become stable at a low viscosity

Good points
(a) can tolerate large amounts of contaminating salts
(b) remains fluid when solids content gets high

Weakness
• It has a tendency to solidify when subjected to high bottom-hole temperatures23
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 EMULSION MUDS - OIL IN WATER
• Oil can be added to any of the normal or special muds with good results

• No special properties necessary

• Natural or special emulsifying agents hold oil in tight suspension after mixing

• Oils used are:

(a) Crude oils
(b) Diesel
(c) Any oil with an API gravity between 25 and 50

• Oil content in mud may be 1% to 40%

• Advantages are:
(a) very stable properties
(b) easily maintained
(c) low filtration and thin filter cake
(d) faster penetration rates
(e) reduces down-hole friction

• Major objection is that the oil in the mud may mask any oil from the formations 24
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 INHIBITED MUDS
• Muds with inhibited filtrates

• Large amounts of dissolved salts added to the mud

• High pH usually necessary for best results

• Designed to reduce the amount of formation swelling caused by filtrate - inhibit

clay hydration

Disadvantages
(a) Need specialized electric logs
(b) Requires much special attention
(c) Low mud weights cannot be maintained without oil
(d) Hard to increase viscosity
(e) Salt destroys natural filter cake building properties of clays 25
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 GYPSUM BASE MUDS
• A specialized inhibited mud
(a) contained large amounts of calcium sulfate
(b) add 2 lb/bbl gypsum to mud system
(c) filtration controlled by organic colloids

Advantages
(a) mud is stable
(b) economical to maintain
(c) filtrate does not hydrate clays
(d) high gel strength

Disadvantages
(a) fine abrasives remain in mud
(b) retains gas in mud
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 OIL BASED MUDS
• Oil instead of water used as the dispersant

• Additives must be oil soluble

• Generally pre-mixed and taken to the wellsite

Advantages
(a) will not hydrate clays
(b) good lubricating properties
(c) normally higher drill rates

Disadvantages
(a) expensive
(b) dirty to work with
(c) requires special electric logs
(d) viscosity varies with temperature 27
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 INVERTED EMULSIONS
• Water in oil emulsion. Oil largest component, then water added. Order of
addition is important

• Have some of the advantages of oil muds, but cheaper. Somewhat less
stable

SALT WATER MUDS

• Can be used either completely or partly saturated

• Weight can vary up to 10 lb/gal when saturated

• No filter cake building properties, easily lost to porous formations

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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 LOW SOLIDS MUDS
• Keeps amounts of clays in the mud at a minimum, which promotes faster and safer drilling

• Three ways to remove solids from mud

(a) water dilution
(b) centrifuging
(c) circulate through large surface area pits

• When clays are removed, a minimum of viscosity control chemicals are needed

• When viscosity and gel strength become too low, clay solids are replaced by organic or
suspended material – polymers

OTHER ADVANTAGES
(a) good for drilling with large pumps and high mud volumes
(b) always give faster drilling

PROBLEMS
(a) excessive dilution a problem
(b) can become expensive
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 SILICATE MUDS
• Composed of sodium silicate and saturated salt water

• Has a pickling effect on shales which prevents heaving or sloughing

• Corrosive, expensive and gives poor electric log results

LIME-TREATED MUDS
• Similar to lime based mud - differ only in degree

• A compromise attempt at overcoming the high temperature gelation problem

(a) use less lime than lime-base mud
(b) not nearly so resistant to salt contamination
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID ADDITIVES
Alkalinity and pH Control
• Designed to control the degree of acidity or alkalinity of the drilling fluid.
• Most common are lime, caustic soda and bicarbonate of soda.

Bactericides
• Used to reduce the bacteria count. Paraformaldehyde, caustic soda, lime
and starch preservatives are the most common.

Calcium Reducers
• These are used to prevent, reduce and overcome the contamination effects
of calcium sulfates (anhydrite and gypsum).
• The most common are caustic soda, soda ash, bicarbonate of soda and
certain polyphosphates. 31
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID ADDITIVES
Corrosion Inhibitors
• Used to control the effects of oxygen and hydrogen sulfide corrosion.
• Hydrated lime and amine salts are often added to check this type of corrosion.
Oil-based muds have excellent corrosion inhibition properties.

Defoamers
• These are used to reduce the foaming action in salt and saturated saltwater mud
systems, by reducing the surface tension.

Emulsifiers
• Added to a mud system to create a homogeneous mixture of two liquids (oil and
water).
• The most common are modified lignosulfonates, fatty acids and amine
derivatives. 32
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID ADDITIVES
Filtrate Reducers
• These are used to reduce the amount of water lost to the formations.
• The most common are bentonite clays, CMC (sodium carboxymethylcellulose)
and pre-gelatinized starch.

Flocculants
• These are used to cause the colloidal particles in suspension to form into
bunches, causing solids to settle out.
• The most common are salt, hydrated lime, gypsum and sodium tetraphosphates.

Foaming Agents
• Most commonly used in air drilling operations.
• They act as surfactants, to foam in the presence of water.
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID ADDITIVES
Lost Circulation Materials
• These inert solids are used to plug large openings in the formations, to prevent
the loss of whole drilling fluid.
• Nut plug (nut shells), and mica flakes are commonly used.

Lubricants
• These are used to reduce torque at the bit by reducing the coefficient of friction.
• Certain oils and soaps are commonly used.

Pipe-Freeing Agents
• Used as spotting fluids in areas of stuck pipe to reduce friction, increase lubricity
and inhibit formation hydration.
• Commonly used are oils, detergents, surfactants and soaps.
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 DRILLING FLUID ADDITIVES
Shale-Control Inhibitors
• These are used to control the hydration, caving and disintegration of clay/
shale formations.
• Commonly used are gypsum, sodium silicate and calcium lignosulfonates.

Surfactants
• These are used to reduce the interfacial tension between contacting
surfaces (oil/water, water/solids, water/air, etc.).

Weighting Agents
• Used to provide a weighted fluid higher than the fluids specific gravity.
• Materials are barite, hematite, calcium carbonate and galena.
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ADDITIVES: THE BLOOD IN DRILLING AND COMPLETION FLUIDS FORMULATION
WEIGHTING MATERIALS

VISCOSIFIER

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ADDITIVES: THE BLOOD IN DRILLING AND COMPLETION FLUIDS FORMULATION
DISPERSANTS/DEFLOCCULANTS

FLUID-LOSS CONTROL AGENTS

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ADDITIVES: THE BLOOD IN DRILLING AND COMPLETION FLUIDS FORMULATION
SHALE STABILIZERS

LUBRICANTS, EMULSIFIERS AND SURFACTANTS

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ADDITIVES: THE BLOOD IN DRILLING AND COMPLETION FLUIDS FORMULATION
CORROSION INHIBITORS

LOST-CIRCULATION MATERIALS

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ADDITIVES: THE BLOOD IN DRILLING AND COMPLETION FLUIDS FORMULATION
COMMON CHEMICALS

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ADDITIVES: THE BLOOD IN DRILLING AND COMPLETION FLUIDS FORMULATION
CEMENT ACCELERATORS

STRENGTH RETROGRESSION

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ADDITIVES: THE BLOOD IN DRILLING AND COMPLETION FLUIDS FORMULATION
WORK-OVER AND COMPLETION FLUID PRODUCTS

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 DRILLING FLUID PROPERTIES
• The large number of functions that have to be performed has inevitably led to the
formulation of complex systems

• In order to effectively control the properties of a mud, we need to know how specific
changes in mud composition will affect the properties

• The liquid and solids content of a mud form the foundation which ultimately determines
the performance of the mud

• Chemical treatment should be thought of as a means of making small or selective

adjustments in mud properties and not as the primary controlling component of a mud

• The primary source of control for rheological and filtration properties is the control of the
amount and type of solids in the fluid
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 DRILLING FLUID PROPERTIES
DENSITY
• It is the weight per unit volume, expressed either in pounds per gallon or pounds
per cubic foot or compared to the weight of an equal volume of water as Specific
gravity (SG)
• The pressure exerted by a static mud column depends on both the density and the
depth
• In order to prevent the inflow of formation fluids and to lay a thin, low
permeability filter cake on the hole walls; the pressure of mud column must
exceed the pore pressure
• The pore pressure depends on the depth of the porous formation, density of the
formation fluids and the geological conditions
• These geological conditions are Normal pressured formations (which have a self-
supporting structure) and Abnormally pressures or geopressured formations 44
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
DENSITY

• The starting point of pressure control is the control of mud density

• The weight of a column of mud in the hole necessary to balance formation

pressure is the reference point from which all pressure control calculations are
based

• The required weight of the mud column establishes the density of the mud for any
specific case

• The correct drilling fluid density is dependent on the down hole formation
pressures

• The density is adjusted by soluble salts or by the addition of solids, termed

weighting agents. 45
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 DRILLING FLUID PROPERTIES
VISCOUS OR FLOW PROPERTIES
• The flow properties of the drilling fluid play a vital role in the success of the drilling
operation

• These properties are primarily responsible for removal of the drill cuttings, but influence
drilling progress in many other ways

• In the upper hole, water alone may be sufficient, but at greater depths more viscous
fluids are required

• The behaviour of fluids is governed by flow regimes, the relationships between pressure
and velocity

• There are two such flow regimes:

• Laminar flow which prevails at low flow velocities and is a function the viscous properties of the fluid
• Turbulent flow which is governed by the inertial properties of the fluid and is only indirectly influenced
by the viscosity
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SCHEMATIC DIAGRAM OF LAMINAR AND TURBULENT FLOW REGIMES

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 DRILLING FLUID PROPERTIES
CONTROL OF FLOW PROPERTIES AT THE WELL
• It is comparatively easy to formulate a mud with suitable properties but more
difficult to maintain those properties while drilling

• Because of dispersion of drilling solids into the mud, adsorption of treating agents
by drilling solids and contamination by formation fluids

• The influence of drilling fluids on well performance is most critical in pipe/hole

annulus

• Thus, samples are taken directly from the flow line and tested immediately before
any thixotropic change takes place

• The tests include: PV, YP, AV, Gel strength and the power law constants (n and k)
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
CONTROL OF FLOW PROPERTIES AT THE WELL
• Knowledge of these parameters provides the information necessary for day to day
control of the mud rheology

• The PV & K depend largely on the bulk volume of solids present and on the
viscosity of the suspending liquid

• YP and the gel strengths depend more on the presence of colloidal clays and on
contamination by inorganic salts

• Either the YP/PV ratio, or flow index n, may be used to characterize the shear
thinning properties of the mud

• The decrease in effective viscosity with increase in shear rate is known as shear
thinning; the higher the YP/PV ratio the > the shear thinning 50
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
FILTRATION PROPERTIES
• The ability of the mud to seal permeable formations exposed by the bit with a
thin, low permeability filter cake is necessary for successful hole completion

• For a filter cake to form, it is essential that the mud contain some particles of a
size slightly smaller than that of the pore openings of the formation

• These particles known as bridging particles are trapped in the surface while the
finer particles are carried deeper into the formation

• The rate of filtration and increase in cake thickness depends on if the surface of
the cake is being subjected to fluid or mechanical erosion during the process

• When the mud is static, the filtrate volume and the cake thickness increase in
proportion to the square root of time 51
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
FILTRATION PROPERTIES
• For Dynamic condition, the surface of the cake is subjected to erosion at a constant rate;
thus, when rate of growth of filter cake equals the rate of erosion, the thickness of the
cake and rate of filtration remain constant

• The permeability of the filter cake depends on the particle size distribution in the mud
and on the electrochemical conditions

• The filtration properties required for the successful completion of a well depend largely
on the nature of the formation to be drilled

• Stable formations with low permeabilities (carbonate & shales) can usually be drilled with
little or no control of filtration properties

• In permeable formation, filtration properties must be controlled in order to prevent thick

filter cakes from reducing the gauge of the borehole
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 DRILLING FLUID PROPERTIES
FILTRATION PROPERTIES
Excessive filtration and thick filter cake build up are likely to cause the
following problems:

1. Tight hole, causing excessive drag.

2. Increased pressure surges, due to reduced hole diameter.
3. Differential sticking, due to an increased pipe contact in filter cake.
4. Excessive formation damage and evaluation problems with wireline logs.

• Most of these problems are caused by the filter cake and not the amount of
filtration because the aim is to deposit a thin, impermeable filter cake.
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
FILTRATION PROPERTIES
• The standard fluid loss test is conducted over 30 minutes

• The amount of filtrate increases with direct proportion to the square root of the time.
This can be expressed by the following;

Q2 = (Q1 x T2)/T1

Where: Q2 is the unknown filtrate volume at time T2

Q1 is the known filtrate volume at time T1

• Pressure also affects filtration by compressing the filter cake, reducing its permeability
and therefore reducing the filtrate

• Small plate-like particles act as the best filter cake builders and bentonite meets these
requirements
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
pH
• Drilling muds are always treated to be alkaline (i.e., a pH > 7)
• The pH will affect viscosity, bentonite is least affected if the pH is in the range of 7
to 9.5
• Above this, the viscosity will increase and may give viscosities that are out of
proportion for good drilling properties
• For minimizing shale problems, a pH of 8.5 to 9.5 appears to give the best hole
stability and control over mud properties
• The corrosion of metal is increased if it comes into contact with an acidic fluid
• From this point of view, the higher pH would be desirable to protect pipe and
casing
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
pH
• The optimum control of some mud systems is based on pH, as is the detection and
treatment of certain contaminants

• A mud made with bentonite and fresh water, for example, will have a pH of 8 to 9

• Contamination by cement will raise the pH to 10 to 11, and treatment with an

acidic polyphosphate will bring the pH back to 8 or 9

• Other reasons for pH control include maintenance of lime-treated muds,

mitigation of corrosion, and effective use of thinners

• Measurement of pH is made by comparing the colour developed on immersing a

paper strip impregnated with certain dyes with the colour of reference standards
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 56
 DRILLING FLUID PROPERTIES
FLUID LOSS CONTROL
• This is a fundamental property of the drilling fluid and becomes important when
porous formations are being drilled, particularly when those formations may
contain gas or oil

• Several types of materials are used to control fluid loss in water base muds

• Basically, these materials reduce the fluid loss either indirectly by acting on the
clay solids to improve their filtration control characteristics, or directly by acting as
a colloid or water thickener

• In most cases they affect the flow properties as well as the fluid loss

• Basically, lignosulfonate and lignite are used to improve the filtration control
performance of bentonite
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
FLUID LOSS CONTROL
• Lignite is different from lignosulfonate in that it also acts as a colloidal material

• In high weight muds where additional bentonite cannot be used because of viscosity
considerations, addition of lignite will reduce fluid loss without causing excessive
increases in viscosity

• Contaminating ions, such as calcium and magnesium, not only cause flocculation but also
dehydration and aggregation of bentonite

• Lignosulfonate appears to protect the bentonite from these harmful effects and lignite
precipitates the ions, thereby removing the source of the problem

• Since their ability to control fluid loss is tied directly to the colloidal solids content of a
mud, care must be taken to provide sufficient bentonite that is well hydrated and
dispersed
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 DRILLING FLUID PROPERTIES
ALKALINITY
• Its measurements are made to determine the amount of lime in lime treated muds

• The mud is titrated to determine the total amount of lime, soluble and insoluble

CATION EXCHANGE CAPACITY (CEC): METHYLENE BLUE TEST

• It serves to indicate the amount of active clay in a mud system or a sample of shale

• The test measures the total CEC of the clays present and is useful in conjunction with the
determination of solids content as an indication of the colloidal characteristics of the clay
minerals

• Shale cuttings can be characterized and some estimations can be made regarding mud-making
properties and possible effects on hole stability
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
ELECTRICAL CONDUCTIVITY
• The electrical stability test is used as an indication of the stability of emulsions of
water in oil

• The resistivity of water muds is measured and controlled to permit better

evaluation of formation characteristics from Electrical logs

• Salt is used to lower the resistivity and fresh water is the only means of raising
resistivity

LUBRICITY

• The requirement for lubrication is especially critical in directional and crooked

holes, and in avoidance of wall-sticking 60
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID PROPERTIES
OTHER RELATED PROPERTIES
• Liquids and solids content

• Sand content

• Corrosivity

• Formation protection

• Temperature tolerance

• Chemical Properties
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 61
 DRILLING FLUID SELECTION
• Over the years a considerable number of drilling mud formulations have been
developed to suit various sub-surface conditions

• Selection of the best fluid to meet anticipated conditions will minimize well cost
and reduce the risk of stuck pipe, loss of circulation and kicks

• Consideration must be given to obtaining adequate formation evaluation and

maximum productivity

• The task of selecting the proper fluids for each particular situation is the job of the
mud engineer, who is educated in the functions & properties of muds, and has
expertise to choose the right fluids for the right applications, keeping in mind the
restrictions of expense, time, and performance

• Considerations that affect the choice of muds to meet specific conditions are given
in the next slide
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID SELECTION
• Well’s design • Hole Instability
• Anticipated formation pressures & rock • Hole Contraction
mechanics • Hole Enlargement
• Formation chemistry • Mud-Making Shale
• Montmorillonite cause rapid rises in
• Permeability Impairment viscosity as drilled solids become
• Temperature incorporated in the mud
• Environmental regulations • Geopressured Formations
• Shallow formation can be drilled with
• Logistics, and economics unweighted muds
• Location • Geopressured formations must be drilled
with weighted mud so as to exceed the
• High Temperature formation pressure by a safe margin
• Fast Drilling Fluid
• Characteristics: low density, low viscosity
and low solids content
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 64
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS

• One of the contributing factors to unstable mud properties is contamination

• Mud contaminants are materials that enter the drilling fluid and adversely
alter its physical and or chemical properties and can also be detrimental to
the environment

• If not minimized or treated out they could result in hole problems

• What constitutes a contaminant in one mud system could not necessarily be

a contaminant to another

DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 65

 DRILLING FLUID CONTAMINANTS
• Fluid contaminations constitute the major problem in drilling fluids during
drilling operations, these contamination challenges has resulted into:

• Wellbore instability
• Loss of time during drilling operations amidst others

• Aside from the well known salt water and rock salt contaminations, other
major source of contamination are:

• The sea water (containing magnesium and calcium ion) and

• Clay particles
• Anhydrite and gypsum, acid gases, hydrocarbons and heavy metals 66
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS
THE SEA WATER (CONTAINING MAGNESIUM AND CALCIUM ION)

• Magnesium ions and calcium ions both present in seawater are detrimental to
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

• Caustic soda will 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 will remove magnesium, but the resulting calcium
levels will be very high and are undesirable
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS
CLAY PARTICLES

• It occurs during drilling and depend on formation composition

• Calcium montmorillonite clay particles due to lower degree of ionization and

hydration will be more unstable as colloidal solution

• This may lead to a greater instability in the clay suspension and cause
flocculation

• To curtail this flocculation dispersants are employed

DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU 68
 DRILLING FLUID CONTAMINANTS
ANHYDRITE AND GYPSUM

• Anhydrite (CaSO4) and Gypsum (CaSO4.2H2O) have nearly the same chemical
composition; but gypsum has an attached water of crystallization which made it
more soluble than anhydrite

• The Calcium which is gotten from gypsum or anhydrite is a contaminant in some

water base mud

• Calcium contamination from the anhydrite formation severally limits the hydration
of bentonite and will flocculate hydrated bentonite, causing sharp increases in the
fluid loss and the viscous properties of the drilling fluid

• Their initial effect on a polymer mud system is high viscosity, high gel strength,
increase fluid loss and decrease pH 69
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS
SALT CONTAMINATION
• In the course of drilling, some rock types can be encountered which can produce a
severe effect on the bentonite type of mud system

• The effects are increase in the chloride content of the mud, increase in mud pit
level due to formation water influx, high water loss with thick spongy cake and
reduction in bentonite concentration

• The effect is not pronounced in some polymer most especially the KCl polymer

• Three naturally occurring types of rock salts are: NaCl (common salt), potassium
chloride and carllites (KMgCl3.6H2O)

• The mechanism of contamination in the case of salt is based on action by the

predominant cation and sometimes pH
70
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS
HYDROGEN SULPHIDE
• Hydrogen Sulphide (H2S) can result in severe pitting of the various elements in the
drill string, bit bearings and mud pump expendables

• Large influxes from the formation can result in sudden drill string failure as well as
present a hazard to rig personnel

• It can be identified by the reduction of mud pH, rotten egg odor, and viscosity and
fluid loss increase due to pH reduction

• It is an acid gas and reduce the pH of the mud very fast by neutralizing the OH
radical

• In order to offset the harmful aspect of the H2S, the pH must be increased to at
least 11 or a safer level of 12 by adding caustic soda or lime
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS
HYDROCARBON AND TEMPERATURE
• Oil and condensate can cause the viscosity to rise undesirably in water base muds

• Insufficient quantities of oil and condensate can thin an oil base mud and cause
the weighting materials to fall out of the mud

• Many water base muds suffer from thermal degradation

• In general, the operating temperature range of specific muds should not be

exceeded unless temperature extending chemicals are added to these systems

• Effects of high temperature will be to reduce the effectiveness of chemical

additives, increase the fluid loss, and to create gelatin of muds laden with drilled
solids

• In general, the lower the drilled solids content, the easier it is to maintain the
desired properties at higher temperatures 72
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS
HEAVY METALS CONTAMINATION AND POSSIBLE SOURCES
• Metals can be incorporated into the mud as a result of sever pitting of various
elements in the drill string, bit bearings & other metallic components of the rig

• The site of the petroleum deposit can contribute to heavy metals contamination of
the mud

• If the deposits occur in a mineralized area possibly while drilling, those mineral
ores can be incorporated into the drilling mud

• They are present in all crudes and Some quantities of crude are drilled out with the
mud and in the course of performing this operation; heavy metals can be
introduced into the mud

• Drilling mud metals could cause environmental harm if they are present in water
column or sediments in soluble forms
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 DRILLING FLUID CONTAMINANTS
• The rheology of drilling fluid is typically characterized with the parameters of yield
point, plastic viscosity, and low shear rate yield point

• The yield point, which is the second component of resistance to flow in a drilling
fluid, is a measure of the electrochemical forces in the fluid which determines the
hole cleaning ability of the drilling fluid

• The PV is the resistance to flow of the drilling fluid caused by mechanical friction, it
is related to the type and concentration of the solids in the drilling fluid

• The LSYP is related to the sag character of the fluid

• A desirable drilling fluid has a low PV but good low shear viscosity (LSYP 7-15
lb/100ft2) and a good yield point within API recommended range

• The ability of the mud to suspend barite is more dependent on the gel strength
and low shear viscosity
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DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU
 CHARACTERISTICS OF COLLOIDAL SYSTEM
• Colloids are not a specific kind of matter but particles whose size falls between the smallest
particles that can be seen with an optical microscope and that of true molecules, but they may be
of any substance

• It is more correct to speak of colloidal systems, since the interactions between two phases of
matter is an essential part of colloidal behavior

• Colloidal systems may consist of solids dispersed in liquids (e.g., clay suspensions), liquid droplets
dispersed in liquids (e.g., emulsions), or solids dispersed in gases (e.g., smoke)

• One characteristic of aqueous colloidal systems is that the particles are so small that they are kept
in suspension indefinitely by bombardment of water molecules, a phenomenon known as the
Brownian movement

• Another characteristic of colloidal systems is that the particles are so small that properties like
viscosity and sedimentation velocity are controlled by surface phenomena

• Surface phenomena occur because molecules in the surface layer are not in electrostatic balance;
i.e., they have similar molecules on one side and dissimilar molecules on the other 75
DR. OKORO E. E. SCHOOL OF PETROLEUM ENGR. CU