Using the mud rotary method, the bore hole is advanced in rock and overburden by rapid rotation of a drill bit mounted at the end of drill rods.
The bit breaks material at the bottom of the hole into small pieces. These cuttings are removed by pumping drilling fluid (water, or water mixed with bentonite or fluid enhancers) down through the drill rods and bit and up the annulus between the bore hole and the drill rods. The fluid cools the bit, stabilizes the bore-hole walls and prevents fluids flowing between the hole and surrounding earth materials, reducing cross-contamination between aquifers.
Drilling fluid circulates in the bore-hole as it pumps down through the rod string, where it picks up drill cuttings and carries them to the surface.