Drilling fluid or drilling mud can be defined as the fluid that circulates in the oil well during the drilling operation.

It comes out and after passing through the drilling string and drill, it returns to the surface through the inner annular oil well space. Drilling fluid is the most important and main part in drilling operations Because without drilling fluid, drilling operations will not be possible.

The most important tasks of drilling fluid are as below. ·Removing drilling cuttings & particles from inside the well (cleaning the bottom of the well) (bring cuttings to the surface) ·        Subsurface pressure control of the well· Suspending drilling chips and releasing them on the surface·        Creating a solid layer on the well wall·   Creating stability in the well·        Reducing the amount of damage to the reservoir formation·Cooling, lubrication and maintenance of part of the weight of the drill and drilling string·  Transmission of hydraulic energy to the drill and drilling string·     Help in gathering information about the formation·  Corrosion and erosion control· Assisting in cementing operations and well completion Most drilling muds have a liquid phase. Drilling mud with liquid phase includes water based mud (WBM) and oil based mud (OBM).1- Water-based mud is a fluid or mud whose main phase is water, and other materials that are added to it are water-soluble materials. Most oil wells use water-based drilling mud because These muds have a lower production cost than oil-based muds and are more affordable & cost benefit.2- Oil-based mud is a fluid that the continuous phase of oil-based mud is made up of liquid hydrocarbon materials, the best of which is diesel(gasoline). Water is also used as an emulsion in this mud, and an emulsifying chemical must be added to the mud to prevent water from leaving the emulsion. Oil-based mud has a high cost and causes environmental pollution.