The composition of the oil-based muds
Oil phase
Oil-based muds were originally composed of an oily phase, which often consisted of diesel, crude oil, and crude oil derivatives. Despite the fact that initially oil-based muds were produced based on the use of diesel, to reduce the environmental impact, mineral oils were replaced.
One of the important points in using oil-based muds is the possibility of ignition of the drilling fluid or its vapors. This issue can pose a risk to the active personnel in the project as well as the stability of the well. Therefore, the use of an oily phase with low flammability can highly reduce the existing risks. Moreover, oils are categorized into the harmful substances, and long- term contact with them are not recommended.
Water phase
To improve the performance of oil-based muds, the water phase is added as a suspension in the oil phase. The water in the oil-based mud is in the form of fine droplets dispersed in the oil phase. Emulsifiers coat the droplets to prevent droplet coalescence and mud instability. Usually, Calcium and Magnesium soapy fatty acids are used as emulsifiers in oil-based muds.
Oil-based mud Additives
To ensure that the prepared drilling fluid has the necessary properties to drilling different types of formation, specific materials should be added to the mud, which are called mud additives. Each of these materials are used to improve a feature of the mud which are explained below.
Emulsifiers
Calcium or Magnesium fatty acids are often used as an emulsifier for oil-based muds. Fatty acids are organic acids found in natural fats and oils.
Soap substances dissolve in oil using their long hydrocarbon chain and also dissolve in water using their ionic part. When the soap agents enter the mixture of oil and water, the soap molecule accumulates at the contact surface of the oil and water, therefore one end dissolves in the water and one end in the oil. This greatly reduces the contact surface energy and enables the formation of a stable emulsion.
OBM Primary Emulsifier and OBM Secondary Emulsifier are the most common emulsifiers used in drilling mud.
OBM Primary Emulsifier increase emulsification between water and oil phases, increase temperature stability and viscosity, minimize filtration and help to form thin filter-cake in wellbore surface.
OBM Secondary Emulsifier to increase thermal stability, improve resistance to contaminants and wetting solids in OBM systems.
To increase the emulsifying power, Lime is added to the combination of oil, water and emulsifying agents.
The efficiency of an oil-based mud emulsifier depends on the alkalinity and electrolytes in the water phase. Also, some emulsifiers may decompose at high temperatures.
Viscosity
Emulsified water can increase oil viscosity and reduce cost of mud. If a small amount of extra emulsifiers is added to the oil, it will increase the viscosity of the oil. Viscosity increase is also achieved by adding solids to the mud.
OBM-Viscosifier and asphalts are important additives for viscosity control. Some of the heavy hydrocarbons in the asphalt enter the mud, while the less soluble components are dispersed as colloidal solids. The high molecular weight polar molecules present in asphalts are likely to form solids that are preferentially oil-compatible. OBM-Viscosifier easily spreads in oily base muds and forms colloids.
Weighting Agent
Barite is the main weighting agent in oil and water-based muds. The weakness of Barite in oil-based muds are:
- In oil-based muds, due to low gel resistance, Barite deposition may be severe.
- Barite can cause accumulated on drill string or formation, in case of not having proper rheological properties in OBM.
- It is not reservoir friendly and can damage productive zones permanently.
Calcium Chloride 96%, Calcium Chloride 78%, Hematite, Sodium Chloride are other materials that can increase weight in drilling mud. Calcium carbonate can also be used to produce muds with a low density in reservoir sections. Calcium carbonate will not damage reservoirs as it is highly acid-soluble and can be removed in downhole.
Loss Circulation Materials (LCMs)
While drilling with OBMs, curing loss circulation is critical due to OBM’s high cost compared to water based muds. Usually, loss circulation happens in fractured or vuggy formations. To cure downhole losses, LCMs are used to alleviate the problem in minimum time. Some of the common LCMs are named below:
Sometimes in severe or complete lossesliquis compounds containing Bentonite and cement are used to cover cracks in the formation and control loss circluation.
Filtration
Oil-based muds are used in drilling through formations that might be damaged by water. In addition, oil-based muds must have good filtration properties and high-quality require filtration control additives. The amount of filtration in oil-based mud is usually low. Because when this type of drilling mud comes into contact with the wall of the well or the formation, first a small amount of oil penetrates into the formation. Then, the water phase in the form of impermeable membranes on the wall of the formation prevents oil from penetrating into the formation. In addition, due to the high surface tension between oil and formation water, the oil phase in oil-based muds cannot easily enter the hydrophilic formations.
OBM Filtration Control can be used to adjust filtration properties. HPHT filtration of oil based muds are usually controlled to be less than 3 cc/30min.
Alkalinity Control and Lime Content
Lime is used to an acceptable extent to maintain the alkalinity of oil-based muds. Sufficient Lime content is required for high emulsion stability and good performance of emulsifiers. Lime content of oil based muds are usually controlled to be more than 2 Lb/Bbls.
lubrication
Graphite Powder can be used for lubrication purposes in drilling operations with oil-based mud.