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Most of the mud used in a drilling operation is recirculated in a continuous cycle:
1. Mud is mixed and kept in the mud pit.
2. A pump draws it out of the mud pit and sends it, through the hollow center of the drill pipe, down into the borehole.
3. Mud emerges from the drill pipe at the bottom of the borehole where the drill bit is grinding away at the rock formation.
4. Now the mud begins the return trip to the surface carrying the pieces of rock, called cuttings, that have been scraped off the formation by the bit.
5. The mud rises in the annulus, the space between the drill pipe and the walls of the borehole. The typical diameter of a drill pipe is about 4 inches (10 centimeters). At the bottom of a deep well, the borehole might be 8 inches (20 centimeters) in diameter.
6. At the surface the mud travels through the mud return line, a pipe that leads to the shale shaker.
7. The shale shakers consist of a series of vibrating metal screens which are used to separate the mud from the cuttings. The mud drips through the screens and is returned to the mud pit.
8. The rock cuttings slip down the shale slide to be disposed of. Depending upon environmental and other considerations, they may be washed before disposal. Some of the cuttings are taken to be examined by geologists for clues about what is going on deep down inside the well.
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