Page 190 - Drilling Technology in Nontechnical Language

Chapter 8 – DIRECTIONAL AND HORIZONTAL DRILLING 181

Steerable motors

A steerable motor is a downhole motor, powered by pumping mud

through it. The lower part of the motor has an adjustable bend (fig. 8–5).
Before running the motor in the hole, several things are done to set it up:
1. The bend is adjusted for the directional performance required of
the motor. The bend will vary from 0° to something less than 2°.
2. The tools above the motor that transmit navigational information
back to the surface are connected to the motor and calibrated, so
that the driller can see which direction the bend points towards
when drilling. These tools, called measurement while drilling or
MWD, are described later.
3. Other components in the system will be adjusted or tailored
for the required directional performance (the required dogleg
severity according to the well design).

Referring to the drawing on the right in figure 8–5, the main components
of the system are shown from the bottom up:
▪ Drill bit. When mud is pumped down the drillstring, the motor
turns. As the bit sits on a bent housing, it does not point straight
ahead. A side force at the bit results from this, which causes the
bit to drill a curved hole.

▪ Undergauge stabilizer. Behind the bit is usually an undergauge
(smaller diameter than the bit) stabilizer. This forms a fulcrum,
with the motor behind acting as a lever, to allow the side force to
be generated at the bit.
▪ Motor. Above the stabilizer is the motor itself, with the bottom
part having the adjustable bend.
▪ Dump valve. At the top of the motor is a dump valve. This allows
mud to be diverted at the top of the motor if need be.
▪ Stabilizer. A stabilizer above the motor acts as the far end of the
lever, exerting an opposing force at the drill bit.

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