7 Oil Based Muds

OIL BASED MUDS

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OIL BASE MUDS Nowadays, the common name of these systems is nonaqueous drilling fluids N.A.D.F. or simply nonaqueous fluids NAF . They generally include: OBM (Oil Base Mud) • Conventional Inverted Emulsion • Relaxed Filtrate Inverted Emulsion • All-Oil

Base Oil: Diesel Mineral Oil

Low Toxic EMO (Enhanced Mineral Oil)

SYNTHETIC BASED MUD Ester PAO (Poly Alpha Olefine) from Ethylene ETHER ACETAL LAO (Linear Alpha Olefine) IO (Internal Olefine) LP (Linear Parafine)

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OIL BASE MUDS The advantages of OBM for drilling and completion fluids can be resumed as follows: • • • • • • •

Clays don’t hydrate and swell Wellbore stability is improved Reduced problems in salts and anhydrite formations Reduced wellbore enlargement Stabilized mud properties Increased resistance to contaminants Reduced problems when productive sandstone contains clays

OBM gives also some undesirable characteristics: • • • •

Flammability Compounds that cause failure in rubber goods (o-ring, pumps and BOP elements) Not great carrying capacity Gas solubility

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OIL BASE MUDS

Inverted Emulsion Fundamentals

Inverted Emulsion Drilling Fluids are a mixture of two immiscible liquids: Oil (or synthetic) and water. Water is the dispersed phase and the oil is the continuous phase. Water is broken up into small droplets and uniformly dispersed in the oil (or synthetic) and prevented from coalescing by surfactants. To adequately emulsify the water in oil, there must be sufficient chemical emulsifier to form a film around each water droplet. The smaller the water droplets, the stronger in stability the emulsion. In addition, uniform droplet size make the emulsion more stable

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OIL BASE MUDS (cont’d) To obtain droplets which are small and uniformed in size, energy in form of shear is required. For this reason, the best way to prepare a stable OBM is to use high pressure pump (Triplex) and special high shear device in mud pits. The stability is improved when the mud is circulating through the bit jets. If water content is increased in the system: • the size of water droplets increases • the P.V. increases • the stability decreases

If oil content is increased in the system: • the stability increases • the density decreases

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OIL BASE MUDS SYNTHETIC FLUIDS

The synthetic fluids have the same behaviour of conventional oil-based mud •

Properties – There are some differences that may be desirable peculiarities (+) or may cause usage problems (-) between the synthetic fluids and conventional oil-based muds.

a)

Synthetic fluids are more expensive than oils used in the conventional OBM; (-)

b)

Synthetic muds are more biodegradable and disperse more rapidly into the sea water (+)

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OIL BASE MUDS

c)

The synthetic muds are more viscous at ambient temperature but they loose viscosity more rapidly as the temperature increases; (-)

d)

Synthetic muds do not have great thermal stability; (-)

e)

Synthetic muds do not hydrate like the other OBM; (+)

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OBM - INTRODUCTION

An oil-based fluid is a drilling fluid in which the oil represents the continuous phase while water the discontinuous phase. Solids in this kind of mud are oil wet, all additives are dispersible and emulsifiable and the mud filtrate is oil. Water is emulsified in the oily phase.

Oil-base muds have an oil continuous phase and they have been formulated to be without water in the preparation and use. The inverted emulsion systems are oil-based muds that must incorporate variable quantities of water from average to high, usually, oil/water ratio is 60/40 – 70/30 large annular section, 80/20 – 90/10 in 81/2 – 6’’ phases.

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OIL-BASE MUDS APPLICATIONS

Clay stability Oil-base muds are the most suitable to drill water soluble clays. If prepared with the correct salinity, these muds can prevent water migration from mud to clay (Osmosys). The water migration through the semi permeable oily membrane, happens from the lower salinity to the higher salinity. Sometimes, water can be absorbed by clay and problems could arise regarding the consolidation of the clay formation to be drilled. Water can also follow the inverse migration (with too high salinity) with consequent troubles for the stability of the clay formation. The ideal would be a salinity which would prevent water migration from the mud to the clay and the inverse migration that would cause the dehydration. This is called “balanced activity”. The correct salinity is generally determined by experience in the field. Clay cores not alterated by oil-based mud (non contaminated clay corings) are necessary for a correct determination of the optimal salinity.

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OIL-BASE MUDS APPLICATIONS

•

Rate of Penetration – Oil-base muds usually offer better rate of penetration performance compared to water base muds. Moreover, they guarantee an excellent clay stability. The filtrate of the inverted emulsion muds, contains a high oil value and reducing consumption of the additives to control the filtration. These fluids do not use the primary emulsifiers as they can decrease the ROP but they are not as stable as conventional inverted-emulsion muds. The relaxed muds are particularly fit for drilling with PCD bits.

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OIL-BASE MUDS APPLICATIONS

•

High Temperature – The advantage of Oil-base is to allow the drilling of formations where the bottom hole temperature exceeds the maximum tolerance for water-base muds, especially in presence of contaminants. The oil-based muds have been used, up to a temperature of approximately 290°C. Moreover, oil-based muds can be prepared to guarantee a good thermal stability for long periods of time. This is an advantage compared to water-based muds that in extreme conditions can deteriorate and cause loss of viscosity, filtrate, corrosion phenomena and barite settling.

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OIL-BASE MUDS APPLICATIONS

Drilling into a salt formation –Oil-base muds contribute to the formation of “gauged holes”, (holes with the same diameter as the drilling bit) because they do not wash the salt formation away. Salt addition to the emulsion water phase, will prevent a further dissolution of salt in water. On the other hand, water-base muds even if saturated or oversaturated do not ensure dissolution of salt from the formation. Coring fluids – Particular oil-based muds are excellent fluids for native state coring, with low changes to the formation wettability properties. These muds are generally without water and as a consequence, they need only small quantities of emulsifiers. The emulsifiers for oil-based muds usually increase the oil wettability dramatically therefore, they can affect the formation. The oily fluids for coring, do not increase the water content of the core, allow the correct determination of the water saturation level of the formation

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OIL-BASE MUDS APPLICATIONS

Completion fluids (packer fluid – to ensure the hydrostatic charge) – Completion fluids, are planned to be stable for long periods of time even if exposed to high temperature. The extended thermal stability of the oil-based muds is due to the choice of additives which are extremely resistant to high temperatures. Moreover, as the continuous phase is oil, the corrosive phenomena are minimal compared to the water-based muds at the same conditions. If well prepared, the completion oil-based muds, can keep the weighting material in suspension for long period of time.

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OIL-BASE MUDS APPLICATIONS

Lubrication- Oil-base muds are particularly suitable for highly deviated or horizontal wells because of their high lubrication properties. The oil-based muds, increase the lubrication and decrease the possibility of stuck pipes. Oil-base muds create a thin filter cake and the friction between the drill string and the well wall is minimized reducing the stuck pipe risk

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OIL-BASE MUDS APPLICATIONS

Low pressure pores – The capacity of drilling formations with low hydrostatic pressures is easily obtained through the oil-based muds because the density of these muds can be kept at a lower level than water-muds. The density can be lowered till 0,88Kg/L.

Corrosion control –The drill string corrosion is minor because the continuous phase of the emulsion is made of oil and this creates a film on the pipes surfaces. The oil offers an exceptional protection against corrosion because of its non-conducting nature. As long as the pipe is coated with oil, the formation of corrosion cells is impossible. Moreover, because of the excellent thermal stability of the products used, there’s no formation of degradable corrosion products.

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OIL-BASED MUDS APPLICATIONS

Reuse of the oil-based muds – The oil-based muds can be reutilized many times. They can be stored for long periods without bacterial activity development. The oil-based muds can be regenerated before their usage, reducing the percentage of solids using the solid removal equipment instead of dilution.

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OIL-BASED MUDS DISADVANTAGES

•

The initial oil-based mud cost, especially for some formulations based on mineral or synthetic oils is high. However, it is possible to counterbalance the cost through Service Companies specialized in mud hiring or firms that sell and repurchase the mud after the use.

•

A gas kick is more difficult to detect because of the high solubility of gases in oil based mud.

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OIL-BASED MUDS DISADVANTAGES

•

Oil-base muds are expensive when we have partial or total loss of circulation during all well operations

•

Great attention is given to the environmental problems linked to the use of oil-based muds. The problem of drilling cuttings removal, the loss of mud and the oil-based muds removal at the end of the works.

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OIL-BASED MUDS DISADVANTAGES

•

It is necessary to take precautions to avoid contact with the skin and to avoid the inhalation of oilbase mud vapours which can cause allergic reactions a/o irritations.

•

Oil-base muds, can damage the rubber parts of the circulation system and require the use of special oil-resistant rubbers.

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OIL-BASED MUDS DISADVANTAGES

•

Oil-base muds have a high fire risk because of the low flammability point of the vapours developed by the mud. Mineral and synthetic oils have a higher flammability point than diesel and crude oil.

•

Modifications and extra drilling equipment to minimize mud losses on the surface are required, in particular for Offshore and Onshore operations.

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OIL-BASED MUDS DISADVANTAGES

•

The "electric logs" operations must be modified using oil-based muds. The latter, are nonconducting and as a consequence, some of the logs do not work properly in these muds.

•

Oil-base muds have a higher compressibility than water-based muds. As a consequence, there can be strong variations between the density measured on the surface and at the bottom. (Proportional at the vertical depth and at the higher temperature)

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Primary Emulsifiers The calcium soaps are primary emulsifiers for oil-based muds. These are prepared in the mud because of reaction between lime and long chain fatty acids (from C-16 to C22). The soaps emulsions are strong emulsifiers agents but their time of reaction must be considered before the emulsion completion. Oil wetting agents will prevent solids from being wetted by water while the emulsion is forming. The emulsifiers will surround the water nebulized drops preventing their assembling.

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Secondary emulsifiers – They are high oil-wetting power chemical products. Usually, these products are not directly involved in the formation of the emulsion like the primary emulsifiers but they wet with oil the solids before the emulsion. Their task is to prevent solids from wetting with water. Such additives are polyammides or imidazoline.

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Organophilic lignites – These are used at high temperatures like additives for the filtration losses. Moreover, they help the emulsion process of water especially at high temperatures. Such lignites are pre-treated with amines to make them water-repellent. The loss of filtration control, is obtained through plugging (plugging of the formation pores) and can be used up to high concentrations without causing an excessive increment of viscosity till about 55 g/L (20 lb/bbl).

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Asphaltic additives to control filtration loss – Usually, they are represented by gilsonite or asphalt derivates (up to 205°C). The asphalt is not so stable (up to 175°C). High concentrations of these additives can cause an excessive viscosity and mud gelation. Therefore, the treatment usually do not exceeds the 44 g/L (15lb/bbl).

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Organophilic clays or gellings – Viscosity is due to materials such as, bentonite, hectorite or attapulgite which are pre-treated with amines to be compatible with the oily phase of mud. Bentonite is the most used and it is compatible with mineral oils up to a temperature of approximately 175°C. For higher temperatures, in particular with mineral oil-based muds, hectorite should be used. The organophilic attapulgite, is usually used to improve the suspension capacity of the packer fluids without increasing viscosity.

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Wetting agents – Supplementary agents to rapidly wet the solids, that were water wetted, with oil. The drilling cuttings and weighing components are usually water wettable and the wetting agents (oil-wetting) must remove water from solids and replace it with an oil film.

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Polymers Viscosifiers – Additives which increase oil-based mud viscosity in presence of organophilic bentonite. They are used especially when the bentonite performance is reduced by high temperatures. These viscosifiers, ensure good performance up to approximately 205°C. The polystyrene sulphonate at high molecular weight becomes effective only when temperatures are higher than 120°C.

•

Rheolgical modifiers – Low molecular weight fatty acids. They cause the viscosity increasing at low speed from 3 to 6 rpm of the fann viscosimeter. Barite can sag or skid downwards, in particular this problem can occur in deviated wells. These additives reduce or remove this problem. These additives does not create problem of viscosity increasing (since they affect only the reading at 3 and 6 RPM).

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OIL-BASED MUDS PRODUCTS DESCRIPTION

•

Weighting materials – They are added to weight up oil muds. Barite is the weighting agent most frequently used. It allows a density of 2.5 kg/L (21 lb/gal). Using hematite that can increase density as well, a 2.9 kg/L (24 lb/gal) density can be reached. For equal densities, an oil-base mud weighted with hematite will have a solids content lower than one weighted with barite because of its minor specific weight (4,2 vs 5,0).

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CHARACTERISTICS OF BASE OILS

Flash Point of Flammability Point– It is a measure of the oil volatility. The higher the flammability point, the lower the risk for the oil to flare up. The flammability point changes according to the age of the same oil. The aging is due to the progressive evaporation in the atmosphere, of the most volatiles oil components. Water addition to mud, will cause an increase in the mud flammability point compared to the base oil. The flammability point should be higher than 65°C.

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CHARACTERISTICS OF BASE OILS

•

Aniline point – It is an indicator of the relative content of the oil aromatic components. These are particularly harmful for the rubber parts of the circulation system. The aniline point should be about 60°C (140F).

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CHARACTERISTICS OF BASE OILS

•

Base-oil viscosity– It is a measure of the base-oil viscosity. Crude oil will have high viscosity values because of the high content of asphalts, while the refined oils such as diesel and mineral oils will have very low viscosities. Salt water and solids addition will increase the oil viscosity. However, the viscosity of an oil-based mud is proportional to the oil-base viscosity. Usually, lowering mud viscosity causes an increasing in the penetration speed.

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CHARACTERISTICS OF BASE OILS

•

Aromatic content – A measure of the aromatic substances content or similar to benzene, in oil. These components affect the oil toxicity. The higher the aromatic content, the more toxic the oil becomes and consequently the more toxic the mud produced from it is. The majority of mineral oils used in the muds preparation, have an aromatic content lower than 1% in weight.

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TYPES OF BASE OILS USED The types of base oils used for the preparation of muds are described as follows: 1)

Diesel – It is the most common base oil used in the last 50 years. In the last 10 years, it has been changed from low toxic and extra low toxic refined oil with a very small amount of aromatics components. - Density: 0-83 – 0.86 SG - Viscosity: 3-4 centistokes at 106°F - Flash Point: 150°F (65-5 °C) - Pour Point: 14°F (-4°C) - Aniline Point: 149°F (65°C) - Aromatics Content: 18-30%

2)

Crude Oil – Crude oil can be used instead of diesel in those areas where diesel is not sufficiently available for the preparation and maintenance of the mud. Crude oils have a flammability point and combustion temperature lower than refined oils. Crude oil has usually a higher viscosity than diesel. As a consequence, the mud viscosity will be greater.

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TYPES OF BASE OILS USED

•

The aniline point of crude oils, is generally low and this can cause the deterioration of the rubber parts of the circulation system.

•

Crude oils

can have impurities and as a consequence, an increasing of the emulsionant

concentration is necessary. Therefore, perform pilot test is important in order to determine the most correct mud formulation. •

In the last years, the use of crude oil is very limited because of environmental and health motivations

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TYPES OF BASE OILS USED

Refined Oils –Refined oils such as diesel and kerosene are the most frequently used for the preparation and maintenance of oil-base muds. •

Test the diesel aniline point to determine if deterioration problems of the rubber parts could arise.

•

Some diesel can have additives which can lower the freezing point and can affect the emulsionant used for mud preparation.

•

The aromatic content of Diesel is higher than in mineral oils. These components affect the toxicity of the oil-based mud.

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TYPES OF BASE OILS USED Mineral oils – Mineral oils have a lower aromatic content. Therefore, they are less toxic than diesel. •

The flammability point of mineral oils is higher than diesel. They are safer especially for high temperature applications.

•

The viscosity of mineral oils is lower than diesel and crude oil. This will affect the viscosity of the oil-base mud.

•

Density:

0.80 – 0.86 S G

•

Viscosity:

2-3 centistokes at 106°F

•

Flash Point:

210 °F (99°C)

•

Pour Point:

- 0.4 °F (-18°C)

•

Aniline Point: 169 °F (76°C)

•

Aromatics:

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TYPES OF OILS USED •

Mineral oils have a low aromatics content (239°F (115°C)

•

Pour Point: -74°F (-59°C)

•

Aniline Point: 194°F (90°C)

•

Aromatics: NIL

•

Aromatics PAH: 0.001%

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TYPES OF BASE OILS USED

•

Synthetic fluids – Base oils in the synthetic fluids, are organic compounds of non-petroleum origin which behave, in drilling operations, in the same way as the oils derived from petroleum but they are rapidly biodegradable in the ocean. Synthetic muds are inverted emulsions with the external or continuous phase represented by the synthetic oil and the internal phase by salt water (brine). Different synthetic fluids mainly with chain lengths between C16 and C24 have been introduced on the market in recent years

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PRODUCT FOR OBM AND FUNCTION FUNCTION

M-I

Baker-Hughes Inteq

Baroid

Dowell

Primary emulsionant

Versa Mul

Carbo Tec L

Invermul

Emul

Secondary emulsionant

Versa Coat

Carbo Mul

Ez mul

Fl

Organolignite

Versa Lig

Carbotrol A9

Duratone

Na

Asphaltic Flagilsonite

Versa Trol

Carbo Trol

Barablok

Trudrill S

Organophilic Bentonite

Vg-69

Carbo Vis

Geltone II

Truvis

Organophilic Hectorite

————

Carbo Gel

Bentone 38

Truvisht

Wetting agent

Versa Wet

Surf Cote

Driltreat

O.W.

Rheology modifier

Versa Mod

Six-Up

Rm 63

Interdrill

Polymer viscosifier

Versa HRP

Carbo Vsht

X-Vis

Truplex

Oil-base muds thinner

Versa Thin

Surf Cote

OMC

Defloc

Comparative table of the products for oil-based muds

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OIL-BASED MUD PREPARATION (COMPOSITION) EXAMPLE OF CONVENTIONAL I.E. OBM IN THIS CASE VERSA SYSTEMS M-I

FUNCTION

PRODUCT

D=8.5 (1.02) – 10 (1.2)

O/W RATIO 80/20 150°C (300°F) D=14 (1.44) – 14 (1.68)

O/W RATIO 90/10 205°C (400°F) D=17 (2.04) – 18 (2.16)

O/W RATIO 70/30 TEMP. 95°C (200°F)

CONTINUOUS PHASE

BASE OIL

0.625 – 0.588

0.609 – 0.552

0.584 – 0.558

DISPERSED PHASE

WATER

0.265 – 0.249

0.150 – 0.136

0.064 – 0.061

SALT (FOR BRINE)

CaCl 2

33 (94) – 31 (88)

18.5 (53) 16.8 (48)

8 (23) – 7.5 (21)

PRIMARY EMULSIONANT

VERSA MUL

7 (20) 7 (20)

8-9 (23-26)

8-9 (23-26)

SECONDARY EMULSIONANT

VERSA COAT*

1-2 (3-5.5)

1-2 (3-5.5)

2-3 (5.5-8.5)

FLUID LOSS AGENT

VERSA TROL

5 (14)

7 (20)

8-9 (23-26)

ALKALINITY PROVIDER

LIME

7 (20)

8 (23)

9 (26)

ORGANOPHILIC CLAY

VG-69

2-4 (5.5 -11)

2-3 (5.5-8.5)

1-1.5 (2.8-4.3)

WEIGHTING AGENT

BARITE

23 (65) – 104 (306)

226 (644) – 333 (950)

504 (1440) – 557 (1590)

M-I PRODUCT CONCENTRATION lb/bbl (Kg/m3) Mud Density: ppg (kg/l) *in this system the secondary emulsionant is also the wetting agent.

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MIXING PROCEDURES 1.

Add the necessary quantity of the type of oil chosen.

2.

Add the primary and secondary emulsifier as requested.

3.

If requested, add the additives for the filtrate control.

4.

Add the opportune quantity of lime Ca(OH)2.

5.

Add the quantity of water needed, if salt water must be used (brine), add it always after the lime.

6.

Add the quantity of viscosifier needed.

7.

If salt brine is not available, use calcium chloride in powder form or, if not available, add calcium chloride in scales to water and add it as salt water (brine) .

8.

Mix for several hours to ensure a good emulsion.

9.

Add the weighting materials to reach the required density.

•

Electrical stability and filtration control will improve after use, thanks to the mixing energy generated by the circulation.

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OIL-BASED MUDS PROPERTIES

•

The weight of an oil-based mud, oscillates between 0.9 Kg/L (7.5 lb/gal) till 2.6 Kg/L ( 22 lb/gal). The bottomhole density is more consistently affected by temperature and pressure conditions than water-based muds. The temperature increasing will decrease the mud density because of thermal expansion phenomena, while the high pressure will increase the density compressing the oily phase.

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OIL-BASED MUDS PROPERTIES

•

Viscosity is influenced either by temperature and pressure. As the temperature increases, the viscosity decreases. On the other hand, the pressure increasing causes a viscosity increasing. Marsh viscosity, is strongly influenced by temperature conditions. This kind of measurement, however, has an indicative function and it is not used for the determination of possible treatments. Usually, the rheological properties of mud are controlled through a rotary viscosimeter. The plastic viscosity, yield point and gel strenght are measured (according to the pseudoplasitc rheological model) with a rheometer. A most accurate analysis of the mud rheology, is done according to the “Power Law” model. Drilling cuttings and weighting materials in suspension are monitored through the analysis of the gel strength for static settling or through reading at 3 or 6 rpm for dinamic settling.

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OIL-BASED MUDS PROPERTIES

•

Rheological tests on oil-based mud must be done at the bottom hole temperature as the plastic viscosity of these muds is very sensible to temperature variations. In general, the higher the temperature, the lower the plastic viscosity. If necessary, plastic viscosity can be decreased through the solids control equipment (decreasing the solids content) or diluting the mud with the oil.

•

The yield is quite influenced by the temperatures in which tests are made. However, the dependence of the yield point on temperature is even more stressed above 175°C. The yield point can be increased with organoliphic clay, or water while it may be lowered through wetting agents polymers for oil, fluidizing or diluting with oil. The gel strength behaves like the yield point. It increases adding organoliphic clays water or rheological modifiers and on the other hand, it will decrease with the use of wetting agents, fluidizers and diluting with the base oil

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OIL-BASED MUDS PROPERTIES

•

Electric stability it is the measure of the tension applied to the electrodes inserted in the mud sample at the emulsion breaking. The electrical stability depends on the quantity of water present. The higher the water quantity, the lower the value of the electrical stability. The presence of solids conductor or insoluble salts, brings a low E.S. (electrical stability) value. The new E.S. gaugers (sinusoidal wave) ensure higher reproducibility and reliability. The values checked by the new instruments are approximately the half of the value of that obtained by the old ones.

•

A decay of the E.S. values indicate that the emulsion stability is failing and usually, emulsionants and lime addition is necessary.

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OIL-BASED MUDS PROPERTIES

•

The HPHT filtration is done at bottom hole temperature conditions to determine: the state of the emulsion, the filtrate volume and the quality of the filter cake. Water in the filtrate can indicates the emulsion decay. Thick filter cakes and high loss of filtration are index of an excessive drilling solids content. Operate lowering the filtration through agents for the filtration control, emulsifiers and lime.

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OIL-BASED MUDS PROPERTIES

•

The determination of the calcium and sodium chlorides contents, is carried out on mud. A new method of analysis , for this test, has been recently established by API to determine each type of salt and its insolubility in the mud. The undissoluted calcium chloride may cause water wettability problems and should be reduced adding water or oil-based mud without salt in their water phase. The presence of sodium chlorides can be removed in the same way but this is not the cause of the same solids wettability problems.

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OIL-BASED MUDS PROPERTIES

•

The lime analysis determines the excess of lime in the mud. Lime is essential for the formation of the emulsion when fatty acids, such as emulsifiers are used.

•

The calcium content decreasing can be the signal of acid gas presence such as H2S or CO2 or mud degradation because of high temperatures.

•

Water activity or the relative oil-based mud humidity, is determined with an hygrometer. However, it does not detect insoluble salts presence.

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OIL-BASED MUDS PROPERTIES

•

The percentage of oil/water/solids is determined with a distiller, which operates till a temperature of 345°C (650°F). The results must be precise especially in order to determine the salinity correctly. Even a small mistake in the determination of the water content can cause great differences in the analysis of salinity.

•

Sulphures presence in an oil-based mud is measured with the “Garrett gas train”. A sample of the whole mud (not filtrated) is used. The zinc oxide is the preferred compound in the treatment of soluble sulphures. Moreover, with H2S presence it is necessary to increase the lime quantity.

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TROUBLESHOOTING OIL MUDS PROBLEM

Low viscosity

SIGNALS AND SOLUTIONS

Add water, emulsifiers and gellants. If the temperatures are high use polymeric viscosifiers. The products listed affect low-shear viscosity: the yield point and the gel strength instead of plastic viscosity.

High viscosity

Remove low density solids through the solid removal equipment a/o through dilution. Increase the O/A (O/W if the water content should be too high. Add oil-wetting agents to reduce viscosity.

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TROUBLESHOOTING OIL MUDS

PROBLEM

SIGNALS AND SOLUTIONS

Remove the water wet solids and add oil-wetting agents Water wettability solids

(invert the wettability). Be sure that insoluble calcium chloride is not in the mud. Water wettability solids, obstruct the screens and bring low E.S. values (electrical stability). Water wettability solids easily dissolve in water.

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TROUBLESHOOTING OIL MUDS

PROBLEM

Low E.S.

SIGNALS AND SOLUTIONS

Water wettability solids, inadequate concentrations of emulsionants or lime, in relationship with the emulsionant and some weighting agents (such as the hematite) cause low E.S. values. In all these situations, apart from hematite, we operate through chemical treatments. The majority of muds prepared with mineral oils will have lower E.S. values than the values in which diesel was used. In general, low density muds, have a low E.S.

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TROUBLESHOOTING OIL MUDS

PROBLEM

SIGNALS AND SOLUTIONS

High concentration of

Mud viscosity will become high while the electrical stability

drilled solids (Low gravity)

will decrease even if the emulsionant concentration will be adequate. Improve the treatment for solids removal. Use double centrifuges to remove drilling cuttings, recovering barite and the oily phase.

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TROUBLESHOOTING OIL MUDS

PROBLEM

High Filtrate

SIGNALS AND SOLUTIONS

Add emulsifiers in case of presence of water on the filtrate. The organolignite will emulsify the water and will decrease the filtrate. Ensure excess of lime in the mud. New oil-base muds formulations can have high HPHT until they are properly sheared and stabilized. Sometimes, even small quantities of water can lower the HPHT in the high percentage muds O/A(O/W). The organolignite are not effective when the bottom hole temperature is lower than 65°C (150°F).

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55

TROUBLESHOOTING OIL MUDS

PROBLEM

Acids Gas

SIGNALS AND SOLUTIONS

They are detected in the mud because of alkalinity and lime values drop. If the Garret gas train test detects the H2S presence, to compensate the alkalinity decreasing, add lime. Continue the lime addition and add specific decontaminants for sulphures such as zinc oxide. If CO2 is present, add lime.

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56

TROUBLESHOOTING OIL MUDS

PROBLEM

Mud losses

SIGNALS AND SOLUTIONS

If the loss is partial, use oil-wettable fibrous material or obstructing solids materials like calcium carbonate. Use the same technique for the losses of filtration and mud and minimize the thickness of filter cake to avoid the differential sticking. If the loss is total, you could squeeze (cement under pressure) with organoliphic clays (D.O.B or D.O.C.); cement or convert to a water-base mud untill the leak area will be covered by the casing

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57

TROUBLESHOOTING OIL MUDS

PROBLEM

Free oil on the surface

SIGNALS AND SOLUTIONS

After a period of inactivity, free oil can gather on the surface. Shake the mud in the tank or add organophilic clay to increase viscosity.

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OIL-BASED MUDS CALCULATIONS

The following series of calculations show how to increase or decrease the oil/water (O/A) percentage of an oil-base mud. If you add water to an emulsion O/A, the percentage will decrease. If it is necessary to increase it , add oil. The quantity of oil needed to increase the O/A percentage can be calculated as follows:

•

Increase the O/A percentage – add oil (% oil + X) / (% water) = O/W (desired)

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OIL-BASED MUDS CALCULATIONS

•

On the other hand, if you want to decrease the O/A percentage, then you have to add water to the emulsion according to the following equation:

•

Decrease the O/A percentage- add water (% water + X) / (% oil) = O/W (desired)

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OIL-BASED MUDS CALCULATIONS

Example: Mini Still analysis (retort):

52% of oil in volume

10% of water in volume How much oil is necessary to have a percentage O/A of 88/12 Hence: ( 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)

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OIL-BASED MUDS CALCULATIONS

Example: (cont’d) Volume = 100 L of mud + 21.3 L of oil = 121.3 L (1 bbl mud + 0.213 bbl oil

= 1.213 bbl)

Convert the percentages to the final volume of 100 L divide respectively the volumes of mud and oil for the resultant volume and multiply for 100 L mud / 121.3 * 100 L= 82 L of mud (1 bbl mud /1.213 bbl = 0.82 bbl mud) 21.3 L oil / 121.3 L * 100 = 18 L oil (0.213 bbl oil / 1.213 bbl = 0.18 bbl oil)

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GAS SOLUBILITY IN OBM

•

Gas solubility in oil-base muds is many times greater than water-based muds

•

Gas kick phenomena won’t have an expansion, raising through the hole as in water-based muds.

•

In the kick out phenomena, during the circulation, the gas is in solution and will have little effect on the circulating pit level or on the casing pressure.

•

When the gas expands from the mud, a rapid increase in the level in the mud tank and of the casing pressure occur.

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GAS SOLUBILITY IN OBM

•

Gas solubility in the mud, is function of the oil quantity in the mud. After having reached the saturation level, further gas influx, will generate gas kick phenomena similar to the ones in the water-base muds.

•

Because of gas solubility in the oil-based muds, the pressure at the casing (annulus drillstring/casing) won’t be much higher than the pressure in the drillstrings. This difference of internal pressure drillstrings/annulus, appear immediately when the gas cushion does not dissolve but expands at once like water muds. The expansion of the cushion reduces the density at the annulus and at the same time the pit level increases. These two phenomena in the oil-based mud are attenuated and they explode in the final phase of the circulation

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64

GAS SOLUBILITY IN OBM

•

A reliable and sensible level sensor in the tank detects the smallest variations in the mud volume is the best gas kick indicator. All signals must be treated like gas kick. Do not mixed up by possible low annulus pressures (like in case of oil or water flowing).

•

If you want to use an oil-based mud you should have a special degasser able to separate big volumes of gas from mud.

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Well Area Operations Drilling Supervisors Training Course

Drilling Muds

RPW2021A

65