Considering the various tasks of the drilling fluids in oil and gas well drilling, the efficiency of the drilling mud is the result of the accurate and correct selection of the mud formulation. Both water-based and oil-based muds have advantages and drawbacks with specific applications that justify their usage in drilling operations according to them. In general, water-based muds are less toxic and less expensive to produce. Also, water-based muds are more compatible with the environment, specially about releasing drilling wastes into the sea. However, for many drilling operations with critical conditions, such as drilling deep formations with high temperature, drilling in the highly unstable shale layers, horizontal and directional drilling, drilling narrow wells that requires high torque, oil-based drilling muds are recommended to be used. On the other hand, the formulation of oil-based drilling mud is more complicated and their production cost is higher.

The advantages of oil-based drilling mud include high corrosion controlling, increasing therate of penetration (ROP), the possibility of reusing drilling mud in several wells, higher efficiency in high temperature-high pressure conditions and better lubricating the drill bit.

From technical point of view, in some cases, oil-based drilling muds are preferable to water-based drilling muds. For instance, when using water-based muds, more drilling waste is produced than oil-based muds. In addition, oil-based mud has higher resistance against different types of contaminations -such as cement, formation water, anhydrite particles, and etc.- compared to water-based muds.  On the other hand, due to the high cost of preparing oil-based muds compared to water-based muds with similar density, the economic justification of choosing an oil-based muds should be done according to its performance.

Drilling fluids used in Oil-based drilling mud