An oil based mud can be defined as a drilling fluid which has oil as its continuous external phase and the water, if present, is a dispersed or internal phase. The solids in an oil-based fluid are oil-wet, all additives are oil dispersible and the filtrate of the mud is oil. The water, if present, is emulsified in the oil phase. Since the 1930’s it has been recognized that better productivity is achieved from reservoirs when oil based muds rather than Water Based Mud are used to drill through the reservoir. This is largely because the oil does not cause the clays in the reservoir to swell or cause changes in the wettability of the formations. Crude oil was first used to drill through the pay zone, but it suffered from several disadvantages (low gel strength, limited viscosity, safety hazard due to low flash point).
Oil Based Drilling Mud Classifications Types
There are two basic classifications of oil-based fluids; invert emulsions and all-oil muds. The amount of water present will describe the type of oil-based fluid. The oil used in these types of oil-based fluids can range from crude oil, refined oils such as diesel or mineral oils, or the non-petroleum organic fluids that are currently available. The latter type fluids – variously called invert fluids, pseudo-oils, non-aqueous fluids, and synthetic fluids – are now considered to be more environmentally acceptable than diesel or mineral oils.
As a completion fluid (during perforating and workovers), there are three types of oil based muds in common use:
- Full oil (water content < 5%)
- Invert oil emulsions (water content 5 – 50%)
- Synthetic or Pseudo oil based mud
Conventional Oil Based Drilling Fluid
Conventional all-oil muds have oil as the external phase but they are designed to be free of water when formulated or in use. The first, simply referred to as “oil mud,” is defined as having 95-100% of its liquid phase oil, with 0-5% unintentional water. This water is usually unintentionally incorporated into the system through numerous sources, most notably from the drilled formations. Since water is not usually present, asphaltic type materials and clays are required to control the fluid loss and viscosity. Since there is no water added to this system during the formulation and water additions are avoided if possible while drilling, there is only a minimum requirement for emulsifiers. All-oil muds can withstand small quantities of water; however, if the water becomes a contaminant, the mud should be converted to an invert emulsion. If the water is not quickly emulsified, the solids in the mud can become water-wet and will cause stability problems. Water-wet solids will blind the shaker screens and loss of whole mud will occur.
Invert Emulsion Mud
The second type of oil-based drilling fluid is the “invert emulsion” mud. As the name implies, an invert is an emulsion mud utilizing oil as the continuous or external phase. Brine constitutes the discontinuous or internal phase of the mud. Typical oil/brine ratios range from 95/5 to 50/50 in invert emulsion systems. Invert emulsions are oil muds that are formulated to contain moderate to high concentrations of water. Water is an integral part of the invert emulsion and usually contains a salt such as calcium or sodium chloride. Special emulsifiers are added to tightly emulsify the water as the internal phase and prevent the water from breaking out and coalescing into larger water droplets. These water droplets, if not tightly emulsified, can water-wet the already oil-wet solids and seriously affect the emulsion stability. Special lignite derivatives or asphaltites are used as the fluid loss control agents, and bentonite derivatives are used to increase the viscosity and suspension properties of the system. Invert emulsions are usually tightly emulsified, low fluid loss oil muds. An improvement in drilling rates has been seen when the fluid loss control of the system is relaxed, thus the name “relaxed” invert emulsion. Also, the relaxed invert emulsions fluids do not use as much emulsifier as the regular invert emulsion systems.
Oil Mud Applications
Oil muds offer many advantages over water based muds. The high initial cost of the oil-based mud can be a factor in not selecting this type of mud system. However, if the overall drilling costs are considered, the costs accompanying the use of an oil mud are usually less than that for a water mud. Some of the applications of oil-based muds will be described below.
Oil muds are most suited for drilling water sensitive shales. Formulated with the proper salinity, oil muds can prevent water movement from the mud into the shale. In some cases, water can actually be drawn from the shale and could result in strengthening. However, it is also possible, but very uncommon, to draw too much water from the shale (with too high a salinity) and cause a shale to be less stable. It is desirable to have enough salinity to prevent water migration into the shale but not to allow dehydration of the shale. This is the “balanced activity” concept. The required salinity is usually determined through field experience. Shale cores that have not been altered by the oil mud are necessary to accurately determine the salinity requirements.
Oil-mud formulations can usually drill faster than water muds and still provide excellent shale stability. Relaxed filtrate invert emulsions usually have a high oil to water content and some of the additives used to control fluid loss are omitted. These systems do not use the primary emulsifiers, which have been shown to reduce drilling rate, and they do not have the same temperature stability as conventional invert oil muds. The relaxed type oil muds are especially suited to drilling with PDC bits.
Oil muds have the ability to drill formations where bottom hole temperatures exceed water-based mud tolerances, especially in the presence of contaminants. Oil muds have been used at temperatures approaching 550°F. Oil muds can be formulated to withstand high temperatures over long periods of time, unlike water-based muds, which can break down and lead to loss of viscosity and fluid loss control, as well as corrosion.
Invert oil muds will provide gauge hole and do not leach out salt. The addition of salt to the water phase will prevent the salt from dissolving into the emulsified water phase. Water-based mud, even up to saturation and over saturation does not assure that the salts will not be leached out.
Special oil muds provide a good coring fluid for minimum water saturation changes. Oil-based coring fluids freuently give low invasion. These fluids are usually water-free and thus require only a minimal content of emulsifiers. Oil mud emulsifiers are very strong oil-wetting agents and can cause oil-wetting of the formation. Oil-based coring fluids will not introduce any water into the core, so determination of water saturation can be more accurately determined. If low surfactant concentrations are used, then the invading filtrate will not alter residual water saturation.
Oil mud packer fluids are designed to be stable over long periods of time and when exposed to high temperatures. Oil muds provide long term stable packer fluids under condi- tions of high temperature since the additives are extremely temperature stable. Since oil is the continuous phase, corrosion is almost negligible compared to water muds under the same conditions. Properly formulated, oil mud packer fluids can suspend weighting material over long periods of time.
The high lubricity offered by oil muds makes them especially suited for highly deviated and horizontal wells. Along with the higher lubricity, the risk of differential sticking is minimized when using oil muds. An oil mud has a thin filter cake and the friction between the pipe and the wellbore is minimized, thus reducing the risk of differential sticking.
Low Pore Pressure Formations
The ability to drill low pore pressure formations is easily accomplished with oil muds since the mud weight can be maintained at a weight less than that of water. Mud weights as low as 7.5 lb/gal can be achieved. Also, oil-based muds provide better filter cakes.
Corrosion of pipe is controlled since oil is the external phase and coats the pipe. Oil muds offer exceptional corrosion protection due to the non-conductive nature of the oil, and corrosion cells cannot develop since the metal surfaces are oil-wet. The products used in oil mud are very thermally stable and do not degrade to produce corrosive products. Also, bacteria do not thrive in oil muds.
Oil muds are well suited to be used over and over again. The oil mud can be stored for long periods of time, since bacterial growth is suppressed. The oil mud can be conditioned before being used again by reducing the drill solids content with mechanical removal equipment.
Disadvantages of Oil Muds
- The initial cost of oil mud is high, especially formulations based on mineral or synthetic fluids. The high cost can be offset by oil mud buy-back or leasing from the mud service company.
- Kick detection is reduced when using oil muds (compared to that of water based muds) due to high gas solubility in oil muds. Oil muds are costly when lost circulation occurs.
- Greater emphasis is placed on environmental concerns when using oil muds as related to discharge of cuttings, loss of whole mud and disposal of the oil mud. Special precautions should be taken to avoid skin contact because emulsifiers, brine, and lime may promote allergic reactions and irritations. Inhalation of fumes from oil muds can be irritating. Oil muds can be damaging to the rubber parts of the circulating system and preclude the use of special oil resistant rubber.
- Oil muds pose potential fire hazards due to low flash points of vapors coming off the oil mud. Mineral oils and the synthetic fluids have higher flash points than diesel and crude oils. Crude oils should be “weathered” before using in oil muds.
- Additional rig equipment and modifications are necessary to minimize the loss of oil muds.
- Electric logging must be modified for use in oil-based muds. Oil muds are non-conductive there- fore resistively measuring logs will not work in oil muds (SP, resistivity, dipmeters).
- Oil muds require emulsifiers that are very powerful oil-wetting materials, which can also change the wettability of the rock to an oil-wet condition, which provides resistance to oil flow. Oil-based muds are more compressible than water muds, and, therefore, the downhole density may vary considerably from that measured at the surface.
Oil Based Drilling Mud Profucts & Additives:
Oil muds require special products to ensure that the emulsion is extremely stable and can with-
stand conditions of high temperature and contaminants. Oil mud products must be dispersible in
the external oil phase. In our article oil-based mud additives, you will find more infos about the following:
- Primary Emulsifier
- Secondary emulsifiers
- Organophilic lignites
- Organophilic gellants
- Wetting agents
- Polymeric viscosifiers
- Rheological modifiers
- Weighting Agents
Modern oil based muds use low-toxicity base oils and a variety of chemical additives to building good mud properties. The use of oil in the drilling mud does have several disadvantages:
- Higher initial cost
- More stringent pollution controls required
- Reduced effectiveness of some logging tools (resistivity logs)
- Detection of Kicks more difficult due to gas solubility in base oil
However, for some applications oil based muds are very cost-effective. These include:
- To drill and core pay zones
- To drill troublesome formations (e.g. shale, salt)
- To add lubricity in directional drilling (preventing Stuck Pipe)
- To reduce corrosion
In brief, two types of oil based muds are commonly used. An oil mud has less than 5% water. An invert emulsion has a water concentration greater than 5%. The oil based muds are generally used for specific purposes, such as maintaining hole stability in hydratable formations or drilling hydrogen sulfide-bearing zones. While drilling hydratable formations, it is important that the salinity level of an oil based mud be maintained at levels greater than the salinity of the formation being drilled. Mud contamination from hydrogen sulfideor carbon dioxide gas can be controlled with excess lime in an oil based system.
The History Of Oil Based Mud
Historically, diesel has been the primary oil source for the continuous phase of oil based muds. Pollution restrictions, especially in offshore environments, have necessitated the use of a mineral oil phase that is within environmental safety levels. Refineries are now supplying highly processed paraffin based oils that meet these environmental safety standards. Except for a few physical characteristics, these new oils are handled and mixed in a manner similar to diesel oil
The Initiation Of Oil Based Mud
The first oil based mud was crude oil, and was used to complete shallow, low-pressure zones. Although there is no record of its first usage, it probably occurred soon after the advent of rotary drilling. The first patent application for an oil based mud was issued in 1923, but this mud was not a commercial success. Oil Based Muds Company (now Hughes Drilling Muds) was formed by George Miller to manufacture, market, and service the first commercial oil based mud, Black Magic.
Oil Based Mud & Richfield Oil Company
On May 1, 1942, Richfield Oil Company (now ARCO) used Black Magic as a completion mud. Black Magic at that time was composed of air blown asphalt dispersed in a diesel oil that contained naturally occurring naphthenic acid, quick lime, and 5% by volume water. The uses of Black Magic in these early years were as completion fluids for low pressure and/or low permeability sands, coring fluids, and to free Stuck Pipe.
The Black Magic Did Good
This original system performed well when applied properly. However, it had some obvious drawbacks. Asphalt was the primary viscosifier and fluid loss control additive. It did a good job of both but contributed to very high apparent and plastic viscosities and consequently was detrimental to drilling rates when compared to a Water Based Mud of the same density. It was also much more expensive per unit volume than water based mud.
Why They Called It Invert Emulsion Mud
Because it did perform many functions well, the industry then set about to improve on it. From this work came the development of what are called the Inverts or Invert Emulsion Muds. Invert emulsion means that water is emulsified in oil (water-in-oil emulsion). In the earlier years (1940’s), one of the most popular Water Based Mud run was oil in-water. These muds were called oil emulsion systems. Therefore, during the development of invert emulsion systems, the term ”inverts” or invert emulsion was used to differentiate the oil system containing some oil.
The development Of Oil Based Mud Additives
The control of the water base muds is made possible because of the wide variety of additives available for performing specific functions. At this time in history, development of oil mud additives and the technology of oil muds were pointed in the same direction. The first step dealt with the amount of water emulsified. Inverts were developed to contain and tolerate a much greater water volume than true oil muds. Rheology could then be controlled by altering oil/water ratios. This allowed the system to have adequate weight material suspension and filtration control with lower viscosity and gels. Water contamination became a less acute problem with inverts. Oil/water ratios ranged from 55/45 to 70/30
The High Cost Of Oil Mud Preparation
The initial preparation of many oil muds tended to be time consuming and expensive because additives such as asphalt did not blend readily in crude or diesel oils but required heat for adequate dispersion. Muds containing these additives had to be prepared at a mixing plant and hauled to the rig site. Make up costs were also high with true oil mud due to higher volume percentage of oil plus the large additions of asphalt. Water contamination was an acute problem causing excessive viscosity and water wetting of solids, necessitating replacement of the system or at least dilution with new mud.
Adjusting Oil Based Muds Properties
Water contamination of invert emulsions required adjustment of mud properties by the addition of oil and emulsifiers. The principal components in the oil muds could not be added to adjust a single property without affecting most of the other mud properties. Single additives to adjust or control specific mud properties were not available at the time to provide the flexibility and versatility needed for lower cost.
Water in oil emulsions
The original inverts were composed of the same basic ingredients as the true oil muds. The concentrations of materials differed however. Calcium and magnesium soaps were used along with asphalt in small concentrations. Sodium chloride brine was used as the internal phase. The earliest of these systems, No-Blok (Magcobar) and Kenex (Ken Corp., later IMC) did not have any other additives. Although they were more flexible (rheologically) than the true oil mud, they were not as stable. In recent years the base oil in OBMs has been replaced by synthetic muds such as esters and ethers. These muds are generally called synthetic or psuedo oil based muds.
The water in invert emulsion muds is dispersed as small droplets throughout the oil. Emulsifiers coat the droplets, preventing them from coalescing and making the mud unstable (i.e. larger water droplets will settle out and break down the emulsion). A calcium or magnesium fatty acid soap is often used as an emulsifier in an oil based mud. The long hydrocarbon chain of the soap molecule tends to be soluble in oil while the ionic portion tends to be soluble in water. When soap is added to a mixture of oil and water the molecule takes up the position shown in Figure.1 .
This reduces the surface energy of the interface and keeps the water droplets in the emulsion. Other types of emulsifiers can also be used (e.g. naphthenic acid soaps and soaps from tree sap). The effectiveness of an emulsifier depends on the alkalinity and electrolytes present in the water phase and also on the temperature of the mud. To increase the stability the water droplets should be as small and uniform as possible. This is done by shearing the mud by agitators. When oil is added the stability increases, since the distance between droplets becomes greater. This causes a decrease in viscosity. For good mud properties there must be a balance between oil and water. The water droplets help to:
- Support the barite
- Reduce filter loss
- Build viscosity and gel strength
When a drop of liquid is placed on a solid it will either:
- Spread itself over the surface of the solidor
- Remain as a stable drop
The shape that the drop takes up depends on the adhesive forces between the molecules of the solid and liquid phases. The wettability of a given solid surface to a given liquid is defined in terms of the contact angle q (Figure 2).
- For a solid/ liquid interface which exhibits a small contact angle (<90 by=”” degrees=”” figure=”” in=”” is=”” li=”” liquid.=”” nbsp=”” preferentially=”” solid=”” the=”” thus=”” water=”” wet.=”” wetted=””>
- If q = 0 degrees, then the solid is totally water wet.
When two liquids are present and brought into contact with a solid, one of the liquids will preferentially wet the solid. Most natural minerals are water wet. When water wet solids enter an emulsion the solids tend to agglomerate with the water, and settle out. To overcome this problem surfactants are added to the oil phase to change the solids from being water wet to being oil wet. The soaps added as emulsifiers will also act as wettability control agents, but special surfactants are more effective.
The stability of the emulsion can be tested by measuring the conductivity of the mud. The stronger the emulsion the higher the voltage required for an electric current to flow.
A loose emulsion is often due to water wet solids or free water. When water-wet solids are present the surface of the mud becomes less shiny and the cuttings tend to stick to each other and blind the shale shaker. Barite added for density control must also be oil wet otherwise the particles will tend to settle out.
|Fig.2 Contact angles in three phase systems|
The activity of a substance is its affinity or potential for water. All rocks which contain clay will absorb water to some extent. This is because there is a difference between the activity of the shales and the activity of the mud. If the chemical potentials of the shale and the mud were equal the shale would not absorb any water. This would eliminate any swelling of the clays, leading to borehole instability. For balanced activity in an oil-based mud the activity of the mud (Aw) must be adjusted to equal the activity of the formation being drilled. CaCl2 or NaCl may be added to the mud to keep Aw above 0.75. The activity of the shale can be measured by taking samples from the shaker.
Excessive viscosity in an oil based mud may be the result of:
- Too much water content – When water is properly emulsified it behaves like a solid. As the water fraction increases so does the viscosity
- Drilled solids – The solids content affects viscosity in oil based mud in the same way as Water Based Mud. The build up of fine solids(e.g. due to diamond Drill Bit) may produce high PV, YP and gel strengths. Finer shaker screens (120 mesh) should be used to reduce this effect. Water wet solids may also cause problems with high YP
It is recommended that pilot tests should be done to assess the implications of adding drilling chemicals to the mud to control viscosity. Emulsifiers and wetting agents may be added to reduce viscosity.
Water and special viscosifiers (organically treated bentonite) may be added to the mud to increase viscosity.
Only the oil phase in oil based mud is free to form a filtrate, making an oil based mud suitable for formations which must not be damaged. The fluid loss is generally very small with oil based muds (less than 3cc at 500 psi and 300 degrees F). During the test there should never be water present in the filtrate (indicates a poor emulsion). If water is present more emulsifying agent should be added. Excessive filtrate volumes can be cured by adding polymers, lignite etc. (pilot tests are recommended).