Page 204 - Drilling Technology in Nontechnical Language

Chapter 8 – DIRECTIONAL AND HORIZONTAL DRILLING 195

Wellbore stability

A stable wellbore occurs when the hole stays the same size as the bit
that drilled it. An unstable wellbore may get larger all the way round, or
it may become larger in one direction than perpendicular to it, or it can
become smaller in some formations, such as thick salt. A stable wellbore
is desirable because this makes it easier to control direction, run into and
pull out of the hole, and cement casing.
Rock strengths and stresses were mentioned in chapter 1. When
a hole is drilled away from vertical, the stresses acting on the wellbore
wall change in such a way that the difference between the maximum and
minimum stress acting on the well becomes greater. The maximum stress
increases as the well gets more horizontal. This can make it difficult to

keep the wellbore nice and round; parts of the wall will tend to fall off
due to the stresses in the wellbore wall. Drilling directional wells then
requires special considerations to prevent problems arising from an
unstable wellbore.

Logging

Running tools into the hole on wireline starts to get difficult when hole

angles get over about 50°. It is worse if the wellbore wall itself is jagged
rather than smooth.

Summary

This chapter examined some of the tools and techniques used to drill a
well along a defined path. Some of the reasons for drilling directional wells

were mentioned, such as to drill many wells from a single surface location
or to exploit a reservoir as effectively as possible. Next, the process of
kicking off a well was described, and then how to continue to guide the
well path after the kickoff, including rotary and motor assemblies.
Horizontal and multilateral wells are more recent developments
in directional drilling. These wells show the state that the art has now
reached.
Finally, this chapter examined the techniques of surveying the wellbore
and the main tools in current use.

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