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with a powerful built-in electric motor mat turns the pipe and bit. And, in special cases, a slim
downhole motor, usually powered by drilling fluid but in some cases by electricity, rotates the bit.
A long metal housing with a diameter a little less than the hole's holds the motor. The bit screws
onto the end of it.
Generally, the latest rotary rigs use a top drive to rotate the pipe and bit. However, rigs using
rotary tables have been around a long time and many drilling companies still own and use them.
Moreover, rotary tables are simple, rugged, and easy to maintain. Rotary rig owners often use
downhole motors where they have to rotate the bit without rotating the entire string of pipe. Such
situations occur when the rig is drilling a slant, or directional hole, a hole that is intentionally
diverted from vertical to better exploit a reservoir. (A later chapter in this book covers directional
drilling in more detail.)
Regardless of the system used to rotate the bit, the driller, the person operating the rig,
allows some of the weight of the pipe to press down on the bit. The weight causes the bit's cutters to
bite into the formation rock. Then, as the bit rotates, the cutters roll over the rock and scrape or
gouge it out.
Fluid Circulation
By itself, rotating a bit on pipe does not get the job done. The cuttings the bit makes must be
moved out of the way. Otherwise, they collect under the bit cutters and impede drilling. Recall that
the crew on a cable-tool rig had to stop drilling and bail the cuttings. A rotary rig crew does not
have to bail cuttings, because the rig circulates fluid 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.

4 Find synonyms in the text for the following words.

techniques to drill
steamboat fluid
rig bailer
clay rotate
bit motor
mud seesaw
caving

5