Circulating System

A fluid called mud circulates through the drilling bit as it cuts through rock. The fluid lubricates the bit, removes rock cuttings, stabilizes the wall around the hole, and controls the pressure in the wellbore. The mud is a suspension of chemicals and minerals, such as bentonite clay in water or sometimes oil.

Workers blend the mixture in the mud-mixing shack. The mud pumps push the fluid up the standpipe and into the drill pipe through the kelly, or through fittings in a top drive mechanism. After passing through the drill bit, the mud and cuttings circulate back to the surface through the space outside the pipe, known as the annulus, and into the return line.

The shale shaker, a vibrating screen, then separates the cuttings from the mud. The cuttings flow into lined pits or tanks, while the drilling mud flows through the de-silter and is recirculated. More fluid is added through the mixer as the drilling progresses to greater depths. When drilling is completed, the remaining fluid is trucked away for disposal or reuse at another site.

Petroleum Communication Foundation. Our Petroleum Challenge: Exploring Canada's Oil and Gas Industry, Sixth Edition. Calgary: Petroleum Communication Foundation, 1999. With permission from the Centre for Energy.