Page 73 - Drilling Technology in Nontechnical Language

64 Drilling Technology in Nontechnical Language Second Edition

▪ Cuttings might also settle at some intermediate depth (i.e., not on
the bottom). When the casing reaches this depth, it hits the debris
and starts to push it down ahead of the casing. This will most
likely plug the annulus, and possibly also the casing, causing the
casing to become stuck.
So whenever drilling is stopped to pull out of the hole, the well must be
circulated for long enough to remove all of the cuttings from the annulus
first. This is called circulating clean.

When drilling the surface hole, the formations drilled are generally
weak and unconsolidated. The hole might not be very stable, and the sides
of the hole will tend to crumble slowly (or perhaps quickly). The hole will
enlarge as material falls into the wellbore. This means that the actual
size—or volume—of the hole is not accurately known. Material falling
off the side of the hole is called caving, and the bits of rock are called
cavings. Cavings can be recognized at the surface due to their shape, size,
and appearance. Careful examination of cavings can give indications of
what is causing the wellbore instability.
With the well cleaned up and before pulling out of the hole, it is necessary
to measure whether the bottomhole assembly is vertical. Various tools are
available to do this. The drillers can then tell whether the well is vertical or
if it has started to wander off course while drilling. This process is called
taking a survey.
As pipe is pulled out of the hole, only every third drill pipe connection
is unscrewed to leave three joints of drill pipe screwed together. The rig’s

derrick is high enough to stand 90 ft (27 m) of pipe on the drill floor,
secured in special racks at the top. Three joints of pipe screwed together
is called a stand of pipe. So if there are 45 joints of drill pipe in the hole,
only 15 connections have to be unscrewed and 15 stands of pipe racked in
the derrick. It is much faster that way. The stands are racked in rows, and
when one row is complete, another is started. The drill collars can also be
racked in stands of three joints.
After drilling the pilot hole, it is enlarged to 26" (660 mm) to be able to
run in the 20" (508 mm) casing. In a vertical well in softer formations, this
might be done simply by drilling again with a 26" (660 mm) drill bit. The
potential problem with using a drill bit to enlarge the hole is that if it hits
an obstruction (such as a boulder buried in a softer formation), it might be

deflected and start to drill another hole. This can be avoided by running a

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