Page 77 - Drilling Technology in Nontechnical Language

68 Drilling Technology in Nontechnical Language Second Edition

At the bottom of the casing is the float shoe (a valve made of cement

with some plastic components). When the 17½" (445 mm) diameter drill bit

is run in to continue drilling, the first thing it will encounter is the top of

the float shoe. This and any cement below it is drilled, until the depth that
the previous bit drilled to is reached. After drilling to the original depth
and drilling some virgin formation, the strength of the formation below the
casing shoe has to be tested. Usually the well is displaced to a new mud

system first, and this also ensures that the density of all the mud in the well
is accurately known.

The test is done with some variation of the following procedure (fig.

3–18). The first step is to drill out through the float shoe with the old spud

mud system. Once the float shoe and cement are drilled, pumping of the new
mud commences, while drilling a few feet or meters of new formation. As
the old mud comes out of the annulus, it is normally dumped into the waste
pit. After drilling the new formation, circulation of new mud continues
until all of the old mud is out and all drilled cuttings are circulated out.
Once clean mud returns out of the annulus at the same density as the mud

going in, a uniform fluid of known gradient fills the well.

The drill bit is pulled back until the bit is inside the surface casing. The
blowout preventer is closed so that it forms a seal around the drill pipe.

This leaves the situation as illustrated in figure 3–18.
Fluid is now slowly pumped into the well through the drill pipe.
As the annulus is sealed at the top by the BOP, the well becomes
pressurized because the fluid has no way out. The volume pumped and

the resulting pressure is recorded. At first, the relationship between the
volume pumped and the resulting pressure is proportional. For example,
for every 10 gal (38 l) pumped into the well, the pressure increases by
50 psi (345 kPa). After pumping 40 gal (151 l), the pressure is 200 psi
(1,379 kPa). Eventually, pumping in the same additional volume results
in a lower-than-expected rise in pressure. This point is called the leak off,

because the exposed formation has just started to allow fluid to leak into
it. If any more fluid is pumped into the well, the formation is likely to

completely fracture, which will, of course, reduce its strength.
The actual pressure on the formation when it starts to leak is calculated
by adding the final surface pressure to the hydrostatic pressure of the fluid

in the well. For instance, assume that the fluid in the well has a gradient

of 0.5 psi/ft (11.3 kPa/m) and it is 1,000 ft (305 m) vertically deep, so

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