In its most basic form a drilling fluid is composed of a liquid (either water or oil) and some sort of viscosifying agent. If nothing else is added, whenever the hydrostatic pressure is greater than the formation pore pressure (and the formation is porous and permeable) a portion of the fluid will be flushed into the formation. Since excessive filtrate can cause borehole problems, some sort of filtration control additive is generally added. In order to provide enough hydrostatic pressure to balance abnormal pore pressures, the density of the drilling fluid is increased by adding a weight material (generally barite).
In summary, a drilling fluid consists of:
· The Base Liquid
· Water – fresh or saline
· Oil – diesel or crude
· Mineral Oil or other synthetic fluids Dispersed Solids
· Colloidal particles, which are suspended particles of various sizes Dissolved Solids
· Usually salts, and their effects on colloids most is important.
All drilling fluids have essentially the same properties, only the magnitude varies. These properties include density, viscosity, gel strength, filter cake, water loss, and electrical resistance.