Page 22 - Drilling Technology in Nontechnical Language

Chapter 1 – DRILLING GEOLOGY 13

be less than the vertical compressive stress, and the horizontal stresses
will be a similar intensity from all directions. However, where tectonic or
other forces act to distort the rock, stresses may differ depending on the
direction in which the stress is measured. These unequal stresses may lead
to failure of the rock perpendicular to the direction of the greatest stress,
so that the hole is less stable in one direction—the hole may become oval
or eye shaped. It is possible to measure the hole size in different directions.
If the hole is bigger in one direction relative to its size in the perpendicular
direction, the highest and lowest horizontal stresses are different. The
greater the size difference, the greater the stress difference.

Principal stresses

All of the stresses present in a particular piece of rock can be resolved
into three mutually perpendicular planes of stress, which are all compressive
or tensile (not shear). These three stress planes are called the principal

stresses (fig. 1–9).
The Greek lowercase sigma (σ) is used to represent stresses within
materials. In engineering, the convention is that tensile stresses are positive
and compressive stresses are negative.

Fig. 1–9. The principal stresses in rock

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