Page 44 - 4822

geologist as oil returning with the mud is a good indic ation that
there are hydrocarbons down below [3].
Drilling mud also helps to cool and lubricate the drill bit
and provides a protective and stabilizing coating on the walls of
the borehole, which helps to seal off porous formations and keep
the wellbore from caving in.
Although the drilling mud does help prevent the wellbore
from caving in and does help to seal off porous formations that
have been drilled, a more permanent and reliable method to
accomplish this is to case the well. Casing the well involves lining
the borehole with steel pipe (Figure 4.4) and cementing it in place.
To case a well, the drill string and bit are removed from the hole.
The drill string, of course, cannot be removed in one piece. Every
third joint in the drill string will be unscrewed and the stands of
pipe will be stacked against the derrick. When all of the drill string
and the bit have been removed, the casing is lowered into the mud-
filled hole by screwing together the 12 metre lengths of casing
pipe end to end. The casing is slightly smaller in diameter than the
borehole and is secured in place by pumping cement into the
annular space between the outside of the casing and the borehole.
When the cement sets, drilling will continue using a slightly
smaller bit than before. As the well is deepened, additional casings
of concentrically smaller diameters are added as needed to seal off
the deeper formations. Although the wellsite geologist can get
many hints of the types of rock formations in the well from the
rock cuttings, and sometimes gets hints of the presence of oil or
gas by the small amounts that come up with the drilling mud, more
information is needed to properly analyse the rock formations that
have been drilled. To help obtain this information, the geologist
has some associates who go by the name of the logging crew.
This logging crew does not come equipped with chainsaws
or axes, but with geophysical logging devices. These logging tools
are lowered into the mud- filled hole (usually prior to casing being
set in that part of the hole) and measure the electrical, acoustic,
and radioactive properties of the different layers of rock. The
results of these logging measurements are analyzed to determine

44