chapter 7 oil base muds

7/27/2019 Chapter 7 Oil Base Muds

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Cod.: RPWA2021A Date : 01/03/2005 Rev: 00 Page: 21

G R O U P

Agip KCO

WELL AREA OPERATIONSDRILLING SUPERVISOR TRAINING COURSE

OIL BASE MUDS


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Agip KCO Code RPWA2021ARev. 00 Date 01/03/2005

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Well Area Operations

INDEX

1.0 INTRODUCTION 3

2.0 USES OF OIL BASE MUDS 4

3.0 DISADVANTAGES OF OIL BASE MUDS 6

4.0 DESCRIPTION OF OIL BASE MUDS 7

5.0 TYPES OF OILS USED 9

6.0 PREPARING OIL BASE MUDS (COMPOSITION) 13

7.0 MIXING PROCEDURES 15

8.0 PROPERTIES OF OIL BASE MUDS 16

9.0 SOLVING OIL BASE MUD PROBLEMS 18

10.0 CALCULATIONS FOR OIL BASE MUDS 20

11.0 GAS SOLUBILITY IN OIL BASE MUDS 21


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1.0 INTRODUCTION

Oil base fluids can be defined as drilling fluids, where oil represents the continuous (or external

or dispersing) phase, and water, if present, represents the discontinuous (or internal or

dispersed) phase. Solids in this type of mud are oil wettable. All additives are dispersible and

emulsifiable and the mud filtrate is oil. Water, if present, is emulsified during the oil phase.

Oil base fluids are classified into two basic groups, invert emulsions and oil base muds, and the

amount of water will determine the type of mud. The oil used varies considerably, from crude to

refined oil (diesel or other mineral oils) and other non-petroleum organic oils which are now

widely available. The latter kind of oils can also be called inert fluids, pseudo oils, non-aqueousfluids or synthetic fluids and are currently considered to have a better environmental rating than

diesel or other mineral oils.

Oil base muds comprise a continuous oil phase and have been designed to be water-free during

make up and use. As water is not present, asphalt materials are needed to control viscosity and

fluid loss; moreover because water is not used in making up these muds, and is not added if

possible, a minimum amount of emulsifier is required. Oil base muds can tolerate small amounts

of water, however beyond a certain threshold the water becomes a contaminant and the mud

should be converted to an invert emulsion. If water is not quickly emulsified, the solidstransported by the mud may become soaked with water and cause stability problems. Solids

soaked with water will obstruct the shale shaker and all the mud will be rejected.

Invert emulsion systems are oil base muds that have been designed to incorporate varying

amounts of water (average to high quantities). Water is an integral part of these muds and may

contain salts such as CaCl2or NaCl. Invert emulsion muds can contain up to 60% of the liquid

phase in water. Special emulsifiers are used to emulsify water as the internal (dispersed) phase

and to prevent coalescence, which would form dispersing water droplets which gradually get

bigger.

These water droplets will wet the solids, previously wetted by oil, if they are not highly

emulsified, and thus compromise emulsion stability. Special lignite or asphalt-based materials

are widely used as fluid loss control agents, while bentonite-based products increase the

viscosity and mud suspension properties. Invert emulsion muds (mud in which the continuous

phase is oil), usually have a strong emulsifying effect, so they are oil base muds with low

filtrates. When filtration control is reduced, drilling rates increase, hence the name relaxed filtrate

oil-mud. Moreover, these relaxed muds need less emulsifier compared to standard invert

emulsion (low filtration) muds.


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2.0 USES OF OIL BASE MUDS

Oil base muds have many advantages over water base muds, but their high initial cost means

other fluids may be chosen. If we consider overall drilling costs though, the associated costs of

using an oil base mud are lower in some cases than using a water base mud. Waste disposal

costs are also an important factor. Some uses of oil base muds are outlined below.

Shale stability - Oil base muds are the most suitable for drilling water-sensitive shales. If these

muds are made up with the right salinity level, they can prevent water from the mud migrating to

the shale. Water migrates through a semi-permeable oily membrane, in a lower salinity to a

higher salinity direction. In some cases, the water may be absorbed by shale, causingconsolidation problems in the shale formation to drill. Water may also migrate in an inverse

direction (when salinity levels are too high), leading to shale formation stability problems.

Naturally, the ideal scenario is a salinity level that prevents water migrating from the mud to the

shale and prevents inverse migration which would lead to dehydration. This concept is defined

as "balanced activity".

The appropriate salinity level is usually determined by field tests. Shale cores which have not

been altered by oil base mud (uncontaminated shale coring) are needed to accurately determine

the optimal salinity level.

Rate of penetration (ROP) Oil base muds usually ensure a better ROP compared to water

base muds, plus they guarantee excellent shale stability. The filtrate from invert emulsion muds

has a high oil content and some additives used for filtration control are omitted. Primary

emulsifiers are not used in these muds, as they decrease the ROP, but thermal stability is not as

efficient as with conventional invert emulsion muds. Relaxed muds are particularly suitable for

drilling with PCD bits.

High temperatures - Oil base muds can be used to drill formations with high bottomhole

temperatures, when water base muds cannot be used, and particularly when contaminants are

present. Oil base muds have been used in temperatures close to 290C. Moreover, the muds

can be made up to guarantee good long-term thermal resistance and this is an advantage over

water base muds which may degrade in extreme conditions, causing viscosity loss, fluid loss

and corrosion and barite decantation.

Drilling salt formations Oil base muds help to form gauge holes (holes with the same

diameter as the drill bit), and they do not wash salt away from the formation. Adding salt to the

aqueous phase of the emulsion will prevent the salt being further dissolved in water. On the


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other hand, water base muds will not prevent the salt being dissolved, even if saturated or

supersaturated.

Coring fluids -Some particular oil base muds are excellent as fluids for native state coring, with

a minimum impact on formation wettability properties. These fluids are usually waterless, and so

only small amounts of emulsifiers are needed. Emulsifiers for oil base muds usually decrease oil

wettability to a very considerable extent and can therefore impact the formation.

Oil base fluids for coring can be used to accurately determine the water saturation level of a

formation, as the fluids do not increase the water content of the core.

Completion fluids (packer fluids to guarantee the hydrostatic head) - Oil base completion fluids

are designed for long-term stability even if subject to high temperatures. This property of oil base

completion fluids is due to the use of additives which are extremely resistant to high

temperatures. Moreover, corrosion is negligible compared to water base muds in equivalent

conditions, as the continuous (or external) phase is oil. If suitably prepared, oil base completion

fluids can suspend weighting materials for long periods of time (years).

Lubrication the high level of lubrication from oil base muds makes them particularly suitable

for highly deviated or horizontal wells. Using oil base muds and increasing lubrication at the

same time decreases the risk of stuck pipe. Oil base muds produce a thin cake and friction

between the drill pipes and walls of the well is minimised, reducing the risk of the bottom hole

assembly (BHA) getting stuck.

Low hydrostatic pressure (low pore pressure) formations - Low hydrostatic pressure

formations can easily be drilled using oil base muds, as the density of these muds can be

maintained at a level below that of water base muds. In fact density can be reduced to 0.88 kg/l.

Corrosion control - Pipe corrosion is negligible, as the emulsion continuous phase comprises

oil, which lines the pipe surface. Oil also provides optimal protection against corrosion as it is anon-conductor and because corrosion cells cannot form as long as the pipe is covered by oil.

Moreover, corrosion products do not form through degradation, as the products used have an

excellent thermal stability, plus bacteria do not proliferate in oil base muds.

Re-use Oil base muds can easily used several times. They can be stored for long periods

without bacteria developing. The muds can also be regenerated before re-use, reducing the

solids content with mechanical removal equipment or by dilution.


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3.0 DISADVANTAGES OF OIL BASE MUDS

The initial cost of an oil base mud is high, particularly for formulations based on mineral oils or

synthetic fluids. However, this cost can be partially offset by using service companies that hire

muds and repurchase muds after use.

Gas kicks (the kick is a negative difference between the casing pressure and formation

pressure) are harder to identify, as gases are very soluble in oil base muds.

Oil base muds are expensive when they absorb during all well operations.

Environmental problems relating to oil base muds and the disposal of drill cuttings, mud loss

and the disposal of oil base muds at the end of operations is now a major concern.

Measures should be taken to avoid the product coming into contact with the skin and inhaling

oil base mud vapours which can cause allergic reactions and/or irritation.

Oil base muds can damage rubber parts of the circulating system, so special oil-resistant

rubbers are needed.

Oil base muds are a potential fire risk because the vapours produced by the mud have a low

flash point. Mineral and synthetic oils have a higher flash point than diesel and crude oil. The

latter should be weathered to eliminate more volatile fractions before use.

Some modifications and additional drilling equipment specifically designed for offshore or

onshore operations and to minimise mud loss are needed.

Electric logging has to be adapted if oil base muds are used. In fact these muds are non-

conductors so the log will not work with these muds (resistivity, SP, dipmeters).

Oil base muds need emulsifiers with a high oil wetting capacity; this can change the wettability

conditions of rocks which may become oil wetted, leading to productivity problems.

Oil base muds have a greater compressibility than water base muds. As a result, major

variations in the surface and bottomhole density may occur (in proportion to the vertical depth

and maximum recorded temperature).


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4.0 DESCRIPTION OF OIL BASE MUDS

Oil base muds need particular products to ensure that the emulsion is stable and can tolerate

contaminants and high temperatures. Products for oil base muds must be dispersible and

emulsifiable in the continuous oil phase.

Primary emulsifiers - Calcium soaps are primary emulsifiers for oil base muds. These are

prepared in the mud, from the reaction between the lime and long-chain fatty acids (C-16 C-

22). Soap emulsions are powerful emulsifiers but their reaction time before emulsion is actually

complete must be considered. Wetting agents will prevent solids from getting water wet, while

the emulsion is forming. Emulsifiers will surround the pulverised water droplets, preventingcoalescence.

Secondary emulsifiers- These are chemical products with a high oil-wetting capacity. Unlike

primary emulsifiers, they do not usually form the emulsion, but wet solids with oil before

emulsification. Secondary emulsifiers prevent solids from getting water-wet and are typically

polyamides or imidazoline.

Organophilic lignites (not hydrophili c) - These lignites are used at high temperatures as fluid

loss additives. They also contribute to water emulsification, particularly at high temperatures.

Organophilic lignites are pre-treated with amines to make them water-repellent. Fluid loss control

is achieved through plugging and this method can be used with high concentrations

(approximately 55 g/l [20 lb/bbl]) without causing an excessive increase in viscosity.

Asphalt addit ives fo r fluid loss control -Gilsonite or asphalt derivatives are normally used.

Gilsonite has a high thermal stability (up to 205C), while the value for asphalt is lower (up to

175C). High concentrations of these additives can cause excessive viscosity and mud gelation,

so the treatment should not usually exceed 44 g/l (15 lb/bbl).

Organophilic gelling agents - Materials such as bentonite, hectorite or attapulgite which havebeen suitable pre-treated with amines to make them compatible with the oil phase (dispersible)

are normally used to determine viscosity. Bentonite is the most common material and is

compatible with mineral oils up to temperatures of approximately 175C. Hectorite should be

used for higher temperatures, and particularly with mineral oil base muds. Organophilic

attapulgite is normally used to improve packer fluid suspension capacities, without considerably

increasing viscosity.

Wetting agents - Wetting agents are used to quickly and effectively oil wet solids that were

previously water-wet. Drill cuttings and weighting material components are usually water-


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wettable and so wetting agents (oil-wetting) have to remove the water from the solids and

replace it with an oil film.

Thickening polymers These additives increase the viscosity of muds in oil when organophilic

bentonite is present. They are used in particular when bentonite is less effective because of high

temperatures; these polymers are effective up to temperatures of around 205C. Polystyrene

sulfonate, which has a high molecular weight, is only effective at temperatures above 120C.

Rheology modifiers These are low molecular weight fatty acids which increase viscosity at

low revs (3 - 6 RPMof a rotational viscometer). Barite can settle or slide downwards, particularly

in deviated wells, and rheology modifiers reduce or eliminate this problem. In any case, theseadditives do not lead to an overall increase in mud viscosity (as they only act at an RPM of 3 and

6).

Weighting agents These agents are used to increase the density of oil base muds. Barite is

the most frequently used weighting material, for densities of up to 2.5 kg/l (21 lb/gal). Hematite

can also be used to increase density, and with a specific weight of 5.0, a density of 2.9 kg/l (24

lb/gal) can be achieved. With equivalent densities, an oil base mud weighted with hematite will

have fewer solids compared to a mud weighted with barite, as barite has a lower specific weight

(4.2 against 5.0).


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5.0 TYPES OF OILS USED

Diesel oil is used in many areas to prepare and maintain oil base muds (it is cheap in Italy,

because of special rates for oil exploration). Crude oil can be used instead when diesel oil is not

readily available in sufficient amounts. In this case, the crude should be analysed before use, as

it could pose a number of safety problems. Mineral and synthetic oils can also be used instead

of diesel, and they have the benefit of being less toxic, yet they cost more than diesel.

A wide range of oils are used to formulate oil base muds and their different physical properties

can have a major impact on the physical properties of the mud. The following oil properties are

determined:

Flash point this indicates an oils volatility. The higher the flash point, the lower the risk that

an oil base mud will catch fire. The flash point of an oil changes as the oil matures, because its

more volatile components evaporate in the atmosphere. Adding water to depleted mud will

usually increase the flash point compared to the base oil. The flash point should be above 65C

(150F).

Aniline point this indicates the relative content of aromatic components in the base oil, which

are particularly harmful for rubber parts in the circulating system. The aniline point should be at

least 60C (140F).

Some oil base mud products, such as organoclays and thickening agents, are affected by the

content of aromatic components in the base oil. As this content decreases, the amount of

thickening agent needs to be topped up or another agent used.

Base oil viscosity this indicates the viscosity of the base oil, which varies considerably

depending on the type of oil. Crude oil will have very high viscosity values because of the

significant asphalt content, while refined oils such as diesel and mineral oils will have far lower

viscosity values. Adding saltwater (brine saturated water) and solids to oil will considerablyincrease the viscosity, however the viscosity of an oil base mud is usually proportional to the

viscosity of the base oil. Lowering the viscosity of mud usually increases the ROP (because

cuttings are removed more effectively).

Aromatics content this indicates the content of aromatic substances or benzene-similar

substances in oil. These components affect oil toxicity, and the higher the content the more toxic

the oil and resulting mud will be. The aromatics content in most mineral oils currently used in

preparing muds is below 1% in weight. Oils for preparing muds are described below:


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Crude oil Crude oil can be used instead of diesel oil to prepare and maintain mud, in areas

where diesel oil is not readily available. Crude oil muds do have some advantages though, which

are outlined below:

Crude oils have lower flash points and combustion temperatures than other refined oils.

Crude oil usually has a higher viscosity than diesel oil and so overall viscosity in the mud will

be greater.

Crude oil must be weathered in the open, before use, to decrease the content of volatile

components which lead to a low flash point.

The aniline point of crude oils is usually low and can cause rubber parts in the circulating

system to deteriorate.

Crude oils may contain impurities and this requires more emulsifier. Pilot tests should be

carried out to determine an appropriate mud formula.

Refined oils Refined oils such as diesel and kerosene oils, are the most frequently used in

preparing and maintaining oil base muds.

Test the aniline point of the diesel oil to check for any potential problems with the deterioration

of rubber parts.

Some diesel oils contain additives which lower the freezing point (they winterize the mud) and

these can affect the emulsifier used in making up the mud.

Diesel oils have a higher aromatics content than mineral oils and these components affect the

toxicity of oil base mud.

Mineral oils mineral oils have a lower aromatics content and so are less toxic than diesel oil.

Mineral oils have a higher flash point than diesel oil and are far safer, particularly in high

temperature conditions.

The viscosity of mineral oils is lower than that of diesel and crude oil, and this also affects the

overall viscosity of oil base mud.

Mineral oils have a low aromatics content (


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as ESCAID110 (EXXON), have an aromatics content below 0.1%. (the oils should always be

analysed for quality).

Unlike diesel oils, mineral oils do not contain surfactants that can modify formation wettability.

Synthetic fluids The base oils in synthetic fluids are non-petroleum based organic

compounds which act in the same way as petroleum-based oils in drilling operations, yet seem

to biodegrade quickly at sea. As in the case of most oil base muds (OBMs), synthetic fluids are

emulsions inverted with the external or continuous phase, represented by synthetic oil, and the

internal phase, represented by brine. A number of synthetic fluids, (mainly C16 C24 long

chains), have come onto the market over the last few years.

PETROFREE (Baroid) An ester produced from the reaction between palm kernel fatty acids

and a patented alcohol. PETROFREE was the first synthetic oil to go on the market and can be

considered a synthetic vegetable oil.

AQUAMUL (Anchor) A diacetal (diester), produced by condensing alcohols (lower molecular

weight diacetals are used as solvents, in cosmetics, perfumes and flavourings).

NOVASOL (M-I) Non-aromatic linear chain hydrocarbons produced from ethylene

polymerisation. Also known as PAO (polyalphaolefin), this product is similar to highly refined, low

toxicity mineral oils such as Amocos SUPERLA White Mineral Oil No. 7. Other hydrocarbons

similar to Novasol, such as ISOTECH (Chevron) and ULTIDRILL (Dowell) have also made a

recent appearance on the market.

BIOMUL (Baker-Hughes Inteq) An alkylate detergent consisting of benzene combined with a

saturated hydrocarbon chain (alkylate detergents, which are used in producing detergents) are

also known as linear alkylbenzenes and can be considered hybrid compounds of low toxicity

diesel and mineral oils.

Properties Synthetic fluids have been designed for use in the field similar to conventional oil

base muds, yet have some differences which are either beneficial (+) or pose problems (-):

a. synthetic fluids are several times more expensive than the oils used in conventional OBMs; (-)

b. synthetic fluids seem to be more biodegradable and they are dispersed more quickly at sea;

(+)

c. synthetic fluids are far more viscous at ambient temperature but thin far more quickly as the

temperature increases; (-)


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d. synthetic fluids do not have a very good thermal stability; (-)

e. synthetic fluids do not hydrate shale as effectively as other OBMs; (+)

Table comparing the functions o f oil base mud products .

FUNCTION M-IBaker-Hughes Inteq

Baroid Dowell

Primaryemulsifier

Versa Mul Carbo Tec L Invermul Emul

Secondaryemulsifier

Versa Coat Carbo Mul Ez mul Fl

Organolignite Versa Lig Carbotrol A9 Duratone Na

AsphaltGilsonite

Versa Trol Carbo Trol Barablok Trudrill S

OrganophilicBentonite

Vg-69 Carbo Vis Geltone II Truvis

OrganophilicHectorite

Carbo Gel Bentone 38 Truvisht

Wetting agent Versa Wet Surf Cote Driltreat O.W.

Rheologymodifier

Versa Mod Six-Up Rm 63 Interdrill

Polymerthickener

Versa HRP Carbo Vsht X-Vis Truplex

Thinner for oilbase muds

Versa Thin Surf Cote OMC Defloc


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6.0 PREPARING OIL BASE MUDS (COMPOSITION)

Product specifications are given for each company and for specific temperature ranges. These

formulations are based on tests in laboratory conditions and formulation values will be lower

when used in the field. Actual requirements are lower, due to the grain size of weighting

materials, the longer shear time while drilling (the mixing energy at well pressure and

temperature) and because the mud in the drill cuttings is incorporated. The listed formulations

can be made up using diesel and mineral oils with a few changes considering the variability of

the formulations in relation to the oil/water ratio and density. Standard service company

specifications (Agip manual 1996) must be followed.

BAKER-HUGHES INTEQ - Product concentration, g/l (lb/bbl)

PRODUCT 95C(200F) 150C(300F) 205C(400F)

CARBO-TEC 20 (7) 28.5 (10) 37 (13)

CARBO-MUL 5.7 (2) 11.4 (4) 17.1 (6)CARBO-TROL 14.3 (5) 20 (7) 42.8 (15)

LIME 14.3 (5) 14.3 (5) 20 (7)

CARBO-VIS, an organophilic thickener, is used in concentrations of 5.7-8.6 g/l (23 lb/bbl) for

O/W muds (O/W emulsions)with 75/25 and 80/20 ratios. When O/W ratios are between 85/15

and 90/10, CARBO-VIS is used in the field in concentrations from 8.6 - 14.3 g/l (3 - 5 lb/bbl).

The concentration of CARBO-VIS will vary as the mud weight changes. More CARBO-VIS will

be needed for lower weight muds.

M-I Product concentration, g/l (lb/bbl)

PRODUCT 95C (200F) 150C (300F) 205C (400F)

VERSA MUL 14.3 (5) 20.0 (7) 20 (7)

VERSA COAT 5.7 (2) 8.6 (3) 11.4 (4)

VERSA TROL 14.3 (5) 14.3 (5) 28.5 (10)LIME 14.3 (5) 14.3 (5) 20.0 (7)

VG-69, an organophilic thickener, is used in concentrations of 5.7-8.6 g/l (2-3 lb/bbl) for O/W

muds with 75/25 and 80/20 ratios. When O/W ratios are between 85/15 and 90/10, VG69 is used

in concentrations above 11.4 and 17.1 g/l (4 - 6 lb/bbl). The concentration of VG-69 will vary as

the mud weight changes. More VG-69 will be needed for lower density muds.


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BAROID Product concentration, g/l (lb/bbl)

PRODUCT 95C (200F) 150C (300F) 205C (400F)

INVERMUL 14.3 (5) 22.8 (8) 28.5 (10)

EZ MUL 5.7 (2) 11.4 (4) 17.1 (6)DURATONE 14.3 (5) 22.8 (8) 28.5 (10)

LIME 8.6 (3) 14.3 (5) 20.0 (7)

GELTONE II, an organophilic thickener, is used in concentrations of 8.6 - 14.3 g/l (3-5 lb/bbl) for

O/W muds with 75/25 and 80/20 ratios. When O/W ratios are between 85/15 and 90/10,

GELTONE II is used in concentrations above 11.4 and 17.1 g/l (4 - 6 lb/bbl). The concentration

of GELTONE will vary as the mud weight changes. More GELTONE will be needed for lower

density muds.

DOWELL Product concentration, g/l (lb/bbl)

PRODUCT 95C (200F) 150C (300F) 205C (400F)

EMUL 8.6 (3) 8.6 (3) 14.3 (5)

FL 8.6 (3) 14.3 (5) 14.3 (5)S - 8.6 (3) 22.8 (8)

LIME 9.6 (3) 25.7 (9) 34.2 (12)

TRUVIS, an organophilic thickener, is used in concentrations of 8.6 - 14.3 g/l (3-5 lb/bbl for O/W

muds with 75/25 and 80/20 ratios. When O/W ratios are between 85/15 and 90/10, TRUVIS, is

used in concentrations above 14.3 - 20.0 g/l (5 - 7 lb/bbl). The concentration of TRUVIS will vary

as the mud weight changes. More TRUVIS will be needed for lower density muds. The

difference in the laboratory and field (well) formulation should not be high, considering that

equipment is now available which can simulate real well conditions or semi-industrial plants.


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7.0 MIXING PROCEDURES

Adding components in the right order, when mixing a mud, optimises product performance. The

order shown below is the most widely used when making up an oil mud, even though each mud

will require some changes in the procedure. The mixing time may vary depending on the amount

of mixing energy from the mud circuit (bit chokes, funnel choke). Organophilic thickeners need

considerable shear to develop their full potential (use the maximum flow rate formula possible).

After a period of downhole circulation, emulsion stability, filtration control properties and rheology

will improve considerably.

1. Add the amount needed for the type of oil.

2. Add the primary and secondary emulsifier, as required.

3. Add fluid loss control additives, as required.

4. Add enough lime Ca(OH)2.

5. Add water; if using brine, always add after lime.

6. Add thickener, as required.

7. If brine is not available, use powder calcium chloride, or if this is not available, add

calcium chloride flakes to the water and add as brine.

8. Mix for several hours to obtain a good emulsion.

9. Add weighting materials to obtain the right density.

Viscosity will be affected by gelling agents to a greater extent, if the gellants are added before

the water and after the calcium chloride. If brine is used, the gellant should be added after and

viscosity will be lower. Electrical stability will be initially lower if salt water is used instead ofcalcium chloride added after the water.

Electrical stability and filtration control will improve after use, because of the mixing energy from

circulation. The mixing procedure above can be used for most muds. The suppliers mixing

procedure should however be analysed, to evaluate if any changes are necessary.


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8.0 PROPERTIES OF OIL BASE MUDS

The weight of an oil base mud ranges from 0.9 Kg/l (7.5 lb/gal) to 2.6 Kg/l (22 lb/gal).

Bottomhole density is affected by temperature and pressure conditions more consistently than

water base muds. The increase in temperature will decrease mud density because of thermal

dilation, while high pressure will tend to increase density, compressing the oil phase.

Viscosity is also affected by temperature and pressure. An increase in temperature will lead to

a decrease in viscosity, while an increase in pressure will increase viscosity. Marsh viscosity is

greatly affected by temperature conditions, so this value is indicative and is not used to select

treatments. The rheological properties of muds are normally evaluated using a rotationalviscometer. Plastic viscosity, the yield point and gel strength are measured (in accordance with

the pseudoplastic rheological model), using a rheometer. The yield-power law model is used for

a more accurate analysis of mud rheology. Drill cuttings and suspended weighting materials are

monitored by analysing the gel strength (for static settling), or recording the value at 3 or 6 RPM

(for dynamic settling). Rheological tests on oil base mud must all be carried out at the expected

bottomhole temperature, as the plastic viscosity of these muds is highly sensitive to temperature

variations. Generally speaking, the higher the temperature, the lower the plastic viscosity.

Plastic viscosity can be reduced if necessary using solids control equipment (decreasing the

solids content) or by diluting the oil base mud

Yield is affected to quite an extent by standard test temperatures, though the effect is more

significant at temperatures above 175C.

The yield point can be increased by adding organophilic clay or water, and decreased by adding

polymer wetting agents for oil (wetting oils), thinners or by diluting with oil. The gel strength

behaves in the same way as the yield point and increases as organophilic clay, water or

rheological modifiers are added. It decreases instead when wetting agents or thinners are used,

or when diluted with base oil.

Electrical stability (ES) is the measurement of the voltage applied to electrodes and put in a

mud sample, when the emulsion breaks. ES depends on the amount of water; so the greater

the volume of water, the greater the ES value. Conductor solids or insoluble salts also generate

a low ES.

New-generation instruments to record ES (sinusoidal wave) guarantee a better reproducibility

and reliability of measurements. Values recorded with the new instruments are about half those


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measured with old instruments. A decrease in ES values shows that the emulsion is breaking up

and emulsifiers and lime need to be added.

HPHT filtration is carried out at bottomhole temperature in static conditions to determine the

emulsion status, the volume of filtrate and quality of the filter cake. Water in the filtrate can

indicate a deteriorating emulsion; thick filter cakes and significant fluid loss are indicative of too

many drilling solids. Fluid loss control additives, emulsifiers and lime should be added, while the

filtrate can be increased by diluting with oil.

Salinity is determined from calcium and sodium chloride for the entire mud. A new analysis

method for this test was recently established by API to determine all types of salt and theirinsolubility in mud. Undissolved calcium chloride can cause water wettability problems and the

content should be reduced by adding water or an oil base mud without salt in the aqueous

phase.

Sodium chloride can be reduced using this method, however this substance does not cause the

same solids wettability problems.

Lime analysis determines the excess lime in mud. Lime is essential for forming the emulsion

when fatty acids are used as emulsifiers. The lime content should be constantly monitored to

prevent adding emulsifiers, when lime is sufficient to activate the emulsifiers already in the mud.

A decrease in the calcium content can indicate the presence of acid gases such as H2S, or CO2

or mud decay caused by high temperatures.

Water activity or the relative humidity of the oil base mud is determined by a hygrometer,

though this does not detect the presence of any insoluble salts.

The relative percentage of oil/water/solids is determined using a distiller, which works up to

temperatures of 345C (650F). Results must be extremely accurate, particularly to correctly

determine salinity. A small mistake in determining the water content can lead to majordifferences in salinity analysis results.

The presence of sulphides in an oil base mud is measured with a Garrett gas train. A whole

rather than filtered mud sample is used. Zinc oxide is usually selected to treat soluble sulphides,

and if H2S is present, the amount of lime should be increased.


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9.0 SOLVING OIL BASE MUD PROBLEMS

PROBLEM INDICATORS AND SOLUTIONS

Low viscosity Add water, emulsifiers and gellants. If temperatures are high, use polymer

thickeners. The listed products affect low-shear viscosity, the yield point and gel

strengths rather than plastic viscosity.

High viscosity Remove low density solids using solids removal equipment and/or by dilution.

Increase the O/W percentage, if the water content is too high. Add oil-wetting

agents to reduce viscosity.

Water-swettable

solids

Remove water wettable solids and add oil-wetting agents (invert the wettability).

Make sure there is no insoluble calcium chloride in the mud. Water-wettable

solids obstruct the shale shakersand lead to low ES values. Water-wettable

solids aresolids that dissolve easily in water.

Low ES Water wettable solids (undissolved salts), concentrations of emulsifiers or lime

which are too low for some weighting agents (e.g. hematite), lead to low ES

values. Chemical treatments are used to solve this problem, apart from when

using hematite. Most muds prepared with mineral oils will have lower ES valuesthan muds made up with diesel oil. In general low density muds have low ES

values.

High

concentration

of low gravity

drilled solids

Mud viscosity will increase while ES will decrease, even if the concentration of

emulsifiers is adequate. Improve the effectiveness of solids removal. Use dual

centrifuges to remove drill cuttings, recovering barite and the oil phase.

High filtrate level Add emulsifiers if the filtrate is in the water. Organolignite will emulsify the water

and decrease the filtrate. Make sure that there is excess lime in the mud. New-formulation oil base muds can have high HPHT values unless they are suitably

sheared and stabilised. Even small amounts of water can sometimes lower the

HPHT in muds with a high O/W ratio. Organolignites are not effective when the

bottomhole temperature is below 65C (150F).(Versa Lig, Duratone, etc).

Ac id gases Acid gases in muds are indicated by a drop in alkalinity and lime values. If

Garret gas train analysis detects H2S, add lime to offset the decrease in

alkalinity. Continue adding lime and add sulphide-specific decontaminants such


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as zinc oxide. Add lime if CO2is present.

Mud losses If the loss is partial, use fibrous oil-wettable materials or solid bridging materials

(which obstruct the pores), such as calcium carbonate. Use the same

technique for fluid and mud loss; minimise cake thickness and avoid stuck pipe

(differential sticking). If all the mud is lost evaluate the feasibility of a squeeze

job using organophilic clays (D.O.B or D.O.C.); cement or convert to a water

base mud until the thief formation has been covered by the casing.

Free surface oil Free surface oil can collect on the surface after periods of inactivity. Agitate the

mud in the pits or add organophilic clay to increase the viscosity.


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10.0 CALCULATIONS FOR OIL BASE MUDS

The following calculations specify how to increase or decrease the oil water (O/W) ratio of an oil

base mud. If water is added to an O/W emulsion, the ratio will decrease; if oil is added it will

increase. The amount of oil needed to increase the O/W ratio can be calculated as follows:

Increase the O/W ratio - add oil (% oil + X) / (% water) = O/W (ratio required)

If you want to decrease the O/W, add water to the emulsion based on the following equation.

Add water to decrease the O/W ratio.

(% water + X) / (% oil) = O/W (ratio required)

Example:

Retort analysis: 52 vol. % oil,

10 vol. % water.

The amount of oil required to obtain an O/W ratio of 88/12 is as follows:

(52 + X)/10 = 88/12

(52 + X)/10 = 7.33

X = 21.3%

= 21.3 l of oil/100 l of mud (0.213 bbl oil/1 bbl mud)

Resulting volume= 100 l of mud + 21.3 l of oil = 121.3 l (1 bbl mud + 0.213 bbl oil = 1.213 bbl)

To convert ratios to a final volume of 100 l, divide the mud and oil volumes by the resulting

volume and multiply by 100.

100 l of mud / 121.3 * 100 l= 82 l of mud (1 bbl mud /1.213 bbl = 0.82 bbl mud)

21.3 l of oil / 121.3 l * 100 = 18 l of oil (0.213 bbl oil / 1.213 bbl = 0.18 bbl oil)


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11.0 GAS SOLUBILITY IN OIL BASE MUDS

Gas is far more soluble in oil base muds than in water base muds.

Gas kicks will not develop rising up the borehole, as in water base muds.

if kick outs occur during circulation, the gas stays in solution and nothing or very little can be

done at the surface level or to the casing pressure.

When gas expands from the mud, the mud level in the mud pit quickly rises, and the casing

pressure increases.

Gas solubility in mud is a function of the amount of oil; once saturated, further gas will lead to

gas kicks similar to those which occur with water base muds.

Because of the solubility of gases in oil base muds, casing pressure (pipe/casing annulus) will

not be much higher than the pressure inside the pipes. This pressure difference is important

when the gas cushion does not dissolve but starts to immediately expand, as in the case of

water base muds. The expansion of the cushion decreases density at the annulus and at the

same time the intake level increases. These two events, in an oil base mud are attenuated and

explode in the final stage of circulation.

A reliable level sensor in the pit which can record even the slightest variation is the best

indicator of a gas kick. All indications should be treated as gas kicks and should not be confused

with possible low casing pressures (which occur when water or oil are produced).

A separator with rotating head designed for large gas volumes (degasser)should be provided

if using an oil base mud.

chapter 7 oil base muds

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