In geotechnical engineering , drilling fluid is used to aid the drilling of boreholes into the earth , o ften used while drilling oil and natural gas wells and on exploration drilling rigs , and also for much simpler boreholes like water wells . Liquid drilling fluid is often called drilling mud. The three main categories of drilling fluids are water-based mud (which can be dispersed and non-dispersed), non-aqueous mud, usually called oil-based mud, and gaseous drilling fluid, in which a wide range of gases can be used. The main functions of drilling fluids include : providing hydrostatic pressure to prevent formation fluids from entering into the well bore, keeping the drill bit cool and clean during drilling, carrying out drill cuttings, and suspending the drill cuttings while drilling is paused and when the drilling assembly is brought in and out...