Page 190 - Geothermal Energy Systems Exploration, Development, and Utilization

166 3 Drilling into Geothermal Reservoirs
practice and the prices are coming down to a level where also onshore drilling can
share the positive influence on overall drilling cost. There are numerous examples
for such developments done in the past which are now common practice for the
drilling business on oil and gas as well as on geothermal drilling.
Very often new technology not only improves the drilling progress but also
reduces downhole risks. Some technical trends which are to become ‘‘standard’’
are described below.
3.11.1.1 Topdrive
As described earlier (Section 3.2) the topdrive is the modern type of equipment to
rotate the drillstring; it has become already more or less standard since the last 10
years because of the advantage to be able to rotate the drillstring even while pulling.
This allows back-reaming through ‘‘tight spots’’ and can avoid fishing under some
circumstances.

3.11.1.2 Rotary Steerable Systems (RSS)
These systems are to substitute the conventional directional drilling technique
with DHM and allow to rotate the drillstring all time, while with the conventional
systems the drillstring cannot be rotated during steering phases. Particularly in
deep and deviated hole sections a nonuniform drilling process is caused by the
‘‘stick-slip’’ effect when using conventional directional drilling technique; this can
be avoided by the continuous rotation of the drillstring with RSSs. The reason is
that due to continuous rotation the friction at the contact areas between drillstring
and borehole wall is always in the lower ‘‘sliding’’ mode and avoids the ‘‘sticking’’
mode where the friction is higher. The result is normally a higher ROP and a safer
drilling operation even under adverse conditions.
The first generation of those RSSs were developed for the ultradeep main hole of
the ‘‘Continental Deep Drilling Project of the Federal Republic of Germany’’ (KTB)
in the late 1980s/early 1990s. At this time the focus was to develop self-steering
vertical drilling systems (VDSs) which were able to keep the hole trajectory as close
as possible strictly vertical (Figure 3.30). Several different types of systems had
been investigated, designed and built during this time, with and without integrated
DHM, with and without expandable ribs, ‘‘point the bit’’ or ‘‘push the bit’’, and so
on. One of the early systems is shown in Figure 3.30.
The different types and steering techniques have been improved over the years
for various applications; however, the ‘‘roots’’ of these tools for all tools are leading
back to the efforts which had been made during the KTB project.
Most of the systems use hydraulically actuated expandable ribs in the outer
(stationary) sleeve which are pushed against the borehole wall on the opposite side
to where the trajectory’s direction is to be corrected. Other systems have a fixed
outer diameter of the outer sleeve, and the inner rotating mandrel is pushed to the
right direction inside the outer sleeve.
A recently used system is shown in Figure 3.31 (please note the similarity in
design).