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440 10 Utilisation of Geothermal Energy 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 materials. Adjustment of fluid density is achieved by adding salts or barite. The stability of bentonite drilling fluids becomes a major problem at temperatures above 150 째C since they start to degenerate significantly /10-6/ and 190 째C 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 째C. 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. Today, 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 injection 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 signals 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 째C. 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 cemented in. This casing provides a more sound foundation and protects the freshwater 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