Bentonite, a type of clay primarily composed of montmorillonite, is a crucial component in drilling mud due to its exceptional swelling and viscosity properties. When mixed with water, bentonite forms a gel-like substance that is vital for various functions in the drilling process. It acts as a lubricant, cools the drill bit, removes drill cuttings, and stabilises the borehole walls. 

Bentonite-based drilling mud reduces friction between the drill string and the borehole walls, extending the lifespan of the drill bit and other drilling equipment. It also helps dissipate heat generated during drilling, preventing overheating.