Page 29 - Drilling Technology in Nontechnical Language

20 Drilling Technology in Nontechnical Language Second Edition

Normally, both pore pressure and fracture pressure will increase with
depth. However, this is not always the case, and sometimes a change of
lithology can cause sudden changes in fracture pressure. Similarly, pore
pressures can be normal—equivalent to a saltwater gradient of around
0.465 psi/ft—or can change below a rock that is impermeable.
These predictions are very important to the drilling engineer planning
the well. They are so important that a competent engineer will not simply
accept the geologist’s predictions but will also examine data from other
wells in the area to verify that the predictions seem reasonable.
As the well is drilled deeper, at certain depths the drilling stops and
a steel pipe—called casing—is lowered into the well and cemented in
place. Drilling then resumes through the casing, using a smaller drill bit

to fit inside it. The depth at which drilling stops to run casing is called a
casing point, and it is very carefully selected, using the predicted pore and
fracture pressures.
The strength of the rock (ability to hold pressure) at the casing point

must be sufficient to withstand all pore pressures down to the next casing
point, with the addition of a safety margin. This is necessary so that if a
kick occurs and fluids flow into the well under pressure, the well will not

fracture when the BOP is closed to seal the top of the well.

Summary

This chapter examined the basic geological processes that affect
how wells are drilled. It discussed how rocks are formed and showed
the basic rock classifications (igneous, sedimentary, and metamorphic).

That downhole stresses may vary, and how this affects the stability
of the wellbore as well as causing rock faulting, was mentioned. It also
summarized the main sedimentary rocks (shales, sandstones, carbonates,

and evaporites) and briefly mentioned some of the drilling problems that
might arise while drilling through these rocks. The principles of rock
stresses and hydrostatic pressure were explained, along with what happens

when the mud hydrostatic pressure is less than formation fluid pressures.
Finally, the geologists’ contributions to planning a well were examined.

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