Page 13 - Drilling Technology in Nontechnical Language

4 Drilling Technology in Nontechnical Language Second Edition

and cause some real problems not only for drillers but also for seismic
surveying. Biological sediments (such as fossilized coral reefs and coal)

are also significant.
In most areas of the world, sedimentary rocks have been deposited
on top of basement rocks (igneous and metamorphic). The layer of
sedimentary rock above the basement rocks can vary in thickness from
zero (eastern Canada) to 50,000 ft or 15,000 m. Around volcanoes,
igneous and metamorphic rock may be found at or near the surface, with
sedimentary rock underneath.

Plate Tectonics

Below the thin, solid crust at the earth’s surface, the planet core is
molten. This liquid rock may be seen at the surface in active volcanoes. On
top of the liquid rock, the crust consists of plates of solid rock, floating on

the molten rock underneath rather like rafts floating on water.

The earth’s crust is divided into seven major plates (African, Pacific,
Indian-Australian, North and South American, Eurasian, and Antarctic)
and numerous smaller plates (such as the Arabian and Cocos). The plates
are all moving relative to one another. Some plates are moving apart
from each other (e.g., North Atlantic and Southern Indian Ocean), and
new crust is formed as molten rock is exposed and cools down. Some are
converging (e.g., at the western rim of the Pacific plate), where one plate

slides underneath the other, and volcanoes and mountains may be formed.
Others are sliding past one another (e.g., western coastal United States
at the boundary of the North American and Pacific plates). The rate of

movement varies from 1.3 to 17.2 cm/year.
In areas close to the edge of a plate (tectonically active areas), the rocks

are under much greater stresses than normal. This can make it difficult to
drill a hole that remains stable. The sides of an unstable wellbore will tend
to collapse into the hole, enlarging the hole.
Movements of the plates will lead to rocks moving up or down within
the earth’s crust. It will also lead to rock beds becoming folded, broken,
and turned over. Pressures of fluids contained within the rocks can be

drastically changed from the surrounding rock. Stresses within the rock
may become different in different directions. All of these different stresses
can deform and break the rock layers in different ways.

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