Chapter 7 Oil Base Muds

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WELL AREA OPERATIONS DRILLING SUPERVISOR TRAINING COURSE

OIL BASE MUDS

Cod.: RPWA2021A

Date : 01/03/2005

Rev: 00

Page: 21

Code RPWA2021A Rev. 00 – Date 01/03/2005 Page 2 of 21

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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

Well Area Operations


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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-aqueous fluids 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 solids transported 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 CaCl2 or 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, causing consolidation 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 290°C. 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 a non-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 nonconductors 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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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 – C22). 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, preventing coalescence. 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 hydrophilic) - 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 additives for fluid loss control - Gilsonite or asphalt derivatives are normally used. Gilsonite has a high thermal stability (up to 205°C), while the value for asphalt is lower (up to 175°C). 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 have been 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 175°C. 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 205°C. Polystyrene sulfonate, which has a high molecular weight, is only effective at temperatures above 120°C. Rheology modifiers – These are low molecular weight fatty acids which increase viscosity at low revs (3 - 6 RPM of a rotational viscometer). Barite can settle or slide downwards, particularly in deviated wells, and rheology modifiers reduce or eliminate this problem. In any case, these additives 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 oil’s 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 65°C (150°F). • 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 60°C (140°F). 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 considerably increase 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 (<1.0%) compared to diesel oil, and this makes their use more acceptable in environmental terms. Moreover, this is an advantage for the safety of personnel, as the oils are less hazardous in the event of contact. Low toxicity mineral oils, such



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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 Amoco’s 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 of oil base mud products. FUNCTION Primary emulsifier Secondary emulsifier Organolignite Asphalt Gilsonite Organophilic Bentonite Organophilic Hectorite Wetting agent Rheology modifier Polymer thickener Thinner for oil base muds

M-I

BakerHughes Inteq

Baroid

Dowell

Versa Mul

Carbo Tec L

Invermul

Emul

Versa Coat

Carbo Mul

Ez mul

Fl

Versa Lig

Carbotrol A9

Duratone

Na

Versa Trol

Carbo Trol

Barablok

Trudrill S

Vg-69

Carbo Vis

Geltone II

Truvis

————

Carbo Gel

Bentone 38

Truvisht

Versa Wet

Surf Cote

Driltreat

O.W.

Versa Mod

Six-Up

Rm 63

Interdrill

Versa HRP

Carbo Vsht

X-Vis

Truplex

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 95°C(200°F) 150°C(300°F) 205°C(400°F) 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 VERSA MUL VERSA COAT VERSA TROL LIME

95°C (200°F) 14.3 (5) 5.7 (2) 14.3 (5) 14.3 (5)

150°C (300°F) 20.0 (7) 8.6 (3) 14.3 (5) 14.3 (5)

205°C (400°F) 20 (7) 11.4 (4) 28.5 (10) 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 INVERMUL EZ MUL DURATONE LIME

95°C (200°F) 14.3 (5) 5.7 (2) 14.3 (5) 8.6 (3)

150°C (300°F) 22.8 (8) 11.4 (4) 22.8 (8) 14.3 (5)

205°C (400°F) 28.5 (10) 17.1 (6) 28.5 (10) 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

95°C (200°F)

150°C (300°F)

205°C (400°F)

EMUL FL S LIME

8.6 (3) 8.6 (3) 9.6 (3)

8.6 (3) 14.3 (5) 8.6 (3) 25.7 (9)

14.3 (5) 14.3 (5) 22.8 (8) 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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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 of calcium 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 supplier’s mixing procedure should however be analysed, to evaluate if any changes are necessary.



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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 rotational

viscometer. 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 175°C. 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 their insolubility 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 345°C (650°F). Results must be extremely accurate, particularly to correctly determine salinity. A small mistake in determining the water content can lead to major differences 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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SOLVING OIL BASE MUD PROBLEMS PROBLEM

Low viscosity

INDICATORS AND SOLUTIONS 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

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

solids

Make sure there is no insoluble calcium chloride in the mud. Water-wettable solids obstruct the shale shakers and lead to low ES values. Water-wettable solids are solids 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 values than muds made up with diesel oil. In general low density muds have low ES values.

High

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

concentration

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

of low gravity

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

drilled solids 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. Newformulation 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 65°C (150°F). (Versa Lig, Duratone, etc). Acid 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 CO2 is 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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