Drilling mud is viscous fluid injected into boreholes. It is used for lubricating, cooling, and protecting the drill, and providing support for the borehole. It also serves to remove debris from the borehole. It usually consists of water and clay with additives to reach the desired level of viscosity. These fluids help prevent other fluids from entering the borehole. Some other functions include keeping the drill bit clean and cool, and carrying out the drill cuttings. The type of drilling fluid selected is to reduce the amount of damage and corrosion.

There are 3 main types of drilling muds:

1. Water-based – It is water containing clays and other chemicals. Used to cool and lubricate equipment, and enhance rate of penetration.
2. Non-aqueous – Mud with a base fluid of a petroleum product. Need environmental consideration before use. Used to increase lubricity and cleaning abilities.
3. Gaseous – Compressed air or other gas is pumped down the bore hole. A wide range of gases are used.

Figure: Drilling mud surrounds the drill bit in the borehole while removing the cutting to the surface.