A drilling fluid is any fluid which is circulated through a well in order to remove cuttings from a wellbore. This section will discuss fluids which have water or oil as their continuous phase. Air, mist and foam, which can be used as drilling fluids, will not be discussed at this time.
A drilling fluid must fulfill many functions in order for a well to be drilled successfully, safely, and economically. The most important functions are:
1. Remove drilled cuttings from under the bit
2. Carry those cuttings out of the hole
3. Suspend cuttings in the fluid when circulation is stopped
4. Release cuttings when processed by surface equipment
5. Allow cuttings to settle out at the surface
6. Provide enough hydrostatic pressure to balance formation pore pressures
7. Prevent the bore hole from collapsing or caving in
8. Protect producing formations from damage which could impair production
9. Clean, cool, and lubricate the drill bit
Occasionally, these functions require the drilling fluid to act in conflicting ways. It can be seen that items #1-3 are best served if the drilling fluid has a high viscosity, whereas items #4-5 are best accomplished with a low viscosity. Items #6 & 8 are often mutually exclusive because drilled solids will tend to pack into the pore spaces of a producing formation.
