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number of 9 metre lengths of high strength steel pipe that are
screwed together end to end. As the well is deepened, the drill
string is lengthened by screwing additional lengths of pipe to the
top. But now a couple of questions should come to mind. (1) How
do we get these pieces of crushed rock (rock cuttings) up out of
that hole, which may be thousands of metres deep? (2) How do we
keep the high pressured liquids and gases that may be present in
the rock formations, from blowing out? Both of these concerns are
addressed by the drilling mud.
Drilling mud, which is a mixture of clay, high density
solids (weighting material), water and chemical additives is
pumped down through the inside of the hollow drill string (Figure
4.4). Because the drill bit, and hence the borehole, is larger in
diameter than the drill string, a hollow space (the annulus) exists
between the drill string and the walls of the borehole. The mud that
is being pumped down squeezes through holes in the drill bit and
circulates back to the surface through the annulus, carrying the
rock cuttings along with it. Back on the surface, the rock cuttings
are mechanically removed from the drilling mud in the “shale
shaker” and can be examined by the wellsite geologist. The same
mud can then be recirculated through the drill string and the
process continues while drilling is in progress.

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