Page 146 - Geothermal Energy Systems Exploration, Development, and Utilization

122 3 Drilling into Geothermal Reservoirs
3.2.3
Directional Drilling

Directional drilling is done if a borehole has to be steered into a specific direction
and/or into a target. It also allows drilling several wells from one single site into
different targets. To implement directional drilling, special equipment such as
those listed and described below is needed.

3.2.3.1 Downhole Motor (DHM)
It consists of a rotor which is connected via a flexible joint to the drill bit, and a stator
connected to the drillstring. The stator has a ‘‘bent housing’’ with an adjustable
bent (Figure 3.8). The rotation of the rotor is done by pumping drill mud through
the motor. The downhole motor (DHM) will deviate the borehole direction into
the direction of the bent as long as the drillstring is not rotated; if the drillpipe is
rotated slowly no preferred direction is given anymore and the trajectory will be
(more or less) straight.
DHMs are also used as an alternative to rotary/top drive for straight hole drilling
if it is more economical due to the higher ROP, so that the higher daily costs are
overcompensated by the ROP increase achieved with it.

3.2.3.2 Rotary Steerable Systems (RSS)
Rotary steerable systems (RSSs) are a modern alternative to DHMs; they allow
steering while rotary drilling, that is, with continuous rotation of the whole
drillstring (Figure 3.9). They have some advantages, for example, a more constant
WOB because the drillstring can slide better in a highly deviated borehole due to

Coupling
Dump valve

Stabilizer

Adjustable
bent housing
Bearing pack
Driveshaft

Power
section
Figure 3.8 Downhole motor. (From Economides, Watters, and Dunn-Norman, 1998.)