Page 187 - Drilling Technology in Nontechnical Language

178 Drilling Technology in Nontechnical Language Second Edition

The rate of change of direction is measured in degrees (of directional
change) per 100 ft or 30 m (of hole drilled). This is called the dogleg
severity—a dog’s leg being bent, it is a descriptive expression of a bent
hole. A high dogleg severity should be avoided shallow in the hole because
of the resulting high forces between the pipe and the hole wall.
Vertical wells are never truly vertical. The drill bit will experience side
forces while drilling, for a number of reasons. The well might penetrate
rock bedding planes at an angle other than 90°, and this will tend to make
the bit walk away from vertical. A harder lump of rock within softer rock
can push the bit off its path. Vertical wells often drill in a slight spiral, like
a stretched spring. If a well is planned to be vertical, directional drilling
techniques are still used to keep the bit going as straight and vertical
as possible.

Deviating the Wellbore

In order to make a well deviate from vertical to follow a predesigned
path, it is necessary to develop a side force at the bit. The amount of side
force and its direction are critical to following the designed path. Of course
the hardness of the rock being drilled through and other characteristics
such as bedding plane angles all have an influence.

There are a number of techniques for developing a controlled side force
at the bit. Two techniques from the early days of rotary drilling are jetting
and whipstock. These are normally used to start the well path deviating,
with different techniques used to continue after that initial kickoff away
from vertical.

Jetting

Tricone drill bits have three drilling cones, and in between each cone
is a nozzle (see chapter 6). If a large nozzle was set into one nozzle pocket

and two small nozzles were in the other two, most of the flow of mud
would go through the big nozzle. Drilling fluid comes out of the drill bit

with great force, and if the formation is not too hard, the fluid will erode
the rock away. With one big nozzle directing most of the flow towards one

part of the hole, a pocket will be washed into the rock in that direction. By

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