In the drilling process, mud performs a number of functions requiring different physical and chemical properties and it is useful to consider how these are affected by the use of an oil based fluid.
The first function of the mud is to transport cuttings to the surface, which in general requires a velocity of around 100-200 ft/min and a viscosity of 30-50 secs/qt. The mud should have sufficient viscosity and gel strength to ensure that the cuttings are transported from the cutting surface and that they do not sink back to the bottom should circulation be interrupted. Secondly, the mud also works as a cutting fluid to lubricate and cool the bit and to ensure that it is working on a clean formation, and to assist cutting via the energy of the fluid jets from the bit nozzles.
The third essential function of the mud is as a primary means of pressure control. The hydrostatic head of the column of mud serves to counter-balance the formation pressure at any depth and must be amenable to adjustment to compensate for variations in pressure. Also, by measuring flow rates in and out of the hole, the volume of mud in the tanks and by monitoring whether the mud contains gas, fluids or hydrocarbons, a large amount of information about what is happening at the bit can be obtained.