Environmental Engineering Reference
In-Depth Information
440
drill bit and for reducing the friction between borehole wall and drill pipe. Most of
the drilling fluids are water based and contain bentonite or other tixotropic mate-
rials. Adjustment of fluid density is achieved by adding salts or barite. The stabil-
ity of bentonite drilling fluids becomes a major problem at temperatures above
150 °C since they start to degenerate significantly /10-6/ and 190 °C seems to be
the limit for water based drilling fluids. This does not mean that they cannot be
used at these rock temperatures since the rock is cooled as long as the circulation
of the fluid is maintained, but it can become a problem during longer breaks of
circulation. In hard crystalline rock formations brine with a friction reducing agent
proved to be very efficient at temperatures up to 200 °C.
For many of the exploitation schemes mentioned above directional drilling is
essential. This technique was pushed forward for offshore drilling in oil and gas
reservoirs where multiple wells are drilled from the bottom of a single well. To-
day, even the drilling of a several kilometre long horizontal well section from the
bottom of a vertical well is possible. In most cases down-hole motors are used for
rotating the drilling bit under these circumstances. These are either turbines or
Moineau-motors driven by the drilling fluid pumped through them by strong in-
jection pumps at the surface. Today down-hole motors with a driving power of
more than 1,000 kW are available. Directional drilling is also possible with the
conventional rotary technique.
Drilling direction is continuously monitored by using the so-called "Measuring
While Drilling (MWD)" technique. A pressure pulse generator transmits the sig-
nals of directional sensors installed in the bottom part of the drill pipe via the
drilling fluid to the surface. Reverse signal transmission and a hydraulically
driven actuator allow adjustment of the drilling direction at any time and depth.
The technique is successfully applied in soft rock formations at temperatures up to
150 °C.
There is only little experience with this technique in hard crystalline rock at
high temperature and depth. Its application in the Hot-Dry-Rock (HDR) project
Soultz (see Chapter 10.3) showed that the more intense vibrations of the drill
string in this type of rock requires some improvement of this technique.
Well completion.
To prevent a collapse of the borehole and protect freshwater
aquifers at more shallow depths the geothermal well like any other deep well is
cased down to the reservoir by inserting and cementing in steel pipes (Fig. 10.3).
This is done in several stages. After the drilling of the first 15 m or so the large
diameter conductor pipe is set and cemented. This pipe has a diameter of 24 to
30 in and lends structural support to the well and the well head. After drilling
another 30 to 100 m the surface casing (13 3/8 in outer diameter) is set and ce-
mented in. This casing provides a more sound foundation and protects the fresh-
water aquifers from contamination /10-6/. After reaching the top of the reservoir
the technical casing is set and cemented in. This casing needs the most complex
design considerations. The design has to take into account the stresses expected
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