Page 23 - Drilling Technology in Nontechnical Language

14 Drilling Technology in Nontechnical Language Second Edition

Geologists, being geologists, do not use the engineering convention;
rather, to a geologist, compressive stress is positive and tensile stress
is negative. This is reasonable, since in the earth’s crust (rock), stresses
are normally compressive and only rarely tensile. In a normally stressed
situation, the greatest stress, called sigma 1 (σ ), is the vertical stress.
1
The smallest stress, denoted sigma 3 (σ ), is normally horizontal, and
3
the intermediate stress, σ , is also horizontal, with little or no difference
2
between σ and σ . However, in some cases, σ or σ can be vertical.
2
3
2
3
Understanding the stress state is very important to planning wells, which
is the reason for covering it here.
Wells are sometimes drilled straight down vertically. However, most
wells deviate from vertical to a greater or lesser degree, and wells may even

be planned and drilled so that they finish up horizontal in the reservoir. In
deviated (and especially highly deviated or horizontal) wells, these stresses
can become a significant factor in designing the well, and deciding on the

procedures needed to drill through it successfully can be challenging.
Of particular interest to the drillers is the ability of the rock to
withstand pressure inside a hole drilled in it. This is called the fracture
pressure. Imagine that a hole is drilled into a chunk of rock. Inside the
hole is liquid. If pressure in this liquid is continually increased, at some
point the rock will start to break. Fluid will leak into the rock, creating
growing fractures that extend away from the hole and into the rock. The
fluid pressure creates tensile forces in the rock around the hole, which

eventually causes tensile failure. However, the compressive stresses around
the rock will act to support the rock against the pressure. As most rocks are
weak against tensile forces, the fracture pressure will be very close to the
lowest compressive stress imposed on the rock, σ .
3
As rock compressive stresses generally increase with greater depth,
the fracture pressure also tends to increase with depth.

Hydrostatic Pressure Imposed by a Fluid

Fluid pressures are fundamental to many aspects of oil well drilling.
If downhole pressures are not kept under control, an uncontrolled release
of oil and gas to the surface (called a blowout) can result that might lead
to loss of life, massive environmental damage, damage to underground
reservoirs, and damage to the rig and other surface facilities.

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