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while the bit drills and the fluid carries the cuttings up to the surface.
As mentioned earlier, crew members attach a rotary bit to hollow pipe,
instead of to braided cable. The pipe is thus a conduit: a powerful
pump on the surface moves fluid down the pipe to the bit and back to
the surface. This fluid picks up the cuttings as the bit makes them and
carries them to the surface where they are disposed of. The pump then
moves the clean mud back down the hole.
The fluid is usually a special liquid called "drilling mud".
Don't be misled by the name, however. Although the earliest drilling
muds were not much more than a plain, watery mud (recall that the
Hamil brothers supposedly filled a pit with water and ran cattle
through it to make it muddy), drilling mud can be a complex blend of
materials. What's more, sometimes it isn't a liquid, which is why a
better name for drilling mud is "drilling fluid." A fluid can be a liquid,
a gas, or a combination of the two.
As you now know, one advantage of a rotary rig is that
workers do not have to worry about soft formations caving in on the
bit and sticking it. Just as the Hamils prepared the mud to stabilize the
hole at Spindletop, today's drillers also prepare, or condition, the
drilling mud to control formations. Besides keeping boreholes from
caving in, circulating mud performs several other important functions.
For example, it moves the cuttings away from the bit and cools and
lubricates it. It also keeps formation fluids from entering the hole and
blowing out to the surface. Indeed, circulating drilling fluid has so
many advantages that cable-tool drilling is virtually obsolete.
Although companies may use a cable-tool rig in a few special cases,
more often they use rotary rigs. Several kinds of rotary rig are
available for drilling on land and offshore. Let's look at the major
types.

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