Mud has many important functions and corresponding necessary properties.
The main functions of drilling fluids include providing hydrostatic pressure to prevent formation fluids from entering into the well bore, keeping the drill bit cool and clean during drilling, carrying out drill cuttings, and suspending the drill cuttings while drilling is paused and when the drilling assembly is brought in and out of the hole. The drilling fluid used for a particular job is selected to avoid formation damage and to limit corrosion. –wiki
Task 1 and 2: The single most important function is to 1) remove the cuttings away from under the bit and 2) transport them from the bottom to the surface. Flushing the cuttings from the bottom require a high flushing effect. This is achieved by inserting small jet nozzles in the rock bit and thereby creating a large pressure drop. Typically 50% of the pump pressure is placed here for this very purpose. The other 50% is pure friction loss through the thin, long drill string, and the concentric hole between the wellbore and the drill pipe. The wellbore is made up by the drilling bit, and the concentric hole is referred to as the annulus.
Friction loss is an important part of Task 2, and is studied separately. Here it will be revealed that the viscosity of drilling fluids is not a constant parameter for a given drilling fluid; it varies with shear rate (which is created by the pumping action of the mud pump). Viscosity of the mud is in fact surprisingly high at low pump rates, but decreases with increasing pump flow rates.
Task 3: Pump pressure loss in the annulus has an additional importance compared to other friction drops. Annular friction pressure adds itself onto the hydrostatic wellbore pressure, and may create difficult conditions from time to time.
Task 4: Maintaining a stable wellbore. This task includes many sub tasks, like chemical stability, mechanical stability and filtration control.
The drilling fluid must be designed to take care of other tasks also, such as;
- Cool and lubricate the bit and the drill string.
- Avoid losing mud into natural or induced cracks.
- Bring information back to the surface.