Page 21 - Drilling Technology in Nontechnical Language

12 Drilling Technology in Nontechnical Language Second Edition

Historically, thick salt beds were hard to see through seismically. More
modern techniques allow good images to be created below thick salts,
and this opens up exploration opportunities. The Gulf of Mexico has a

lot of thick salt deposits, and now significant reservoirs are being found
below them.

Rock Strengths and Stresses

The strength of a rock will vary depending on the type of stress applied
to the rock (compressive, tensile, or shear) and may also vary depending
on the direction that the stress is applied (fig. 1–8). Most rock has little

tensile strength; it will pull apart relatively easily. However, rock can have
great compressive strength, especially if the compressive force is applied
at right angles to the bedding plane of a sedimentary rock. Compressive
strength is important; a rock with high compressive strength is harder to
drill through, but it also tends to be more stable (less likely to fall to pieces)
once a hole is drilled through it.

Fig. 1–8. Comparison of rock stresses

When a rock is buried in the earth, it is subjected to stresses from the
rock around it. The weight of the overlying rock (the overburden) will
apply a vertical stress to the rock. In a normal area, where there is little
or no tectonic activity, the horizontal compressive stresses will tend to

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