| Abstract |
The first phase of the Hot Dry Rock (HDR) Project in Urach Spa begann in 1977/78 with the Urach 3 drilling hole. A depth of 3334 m within a metamorphic gneiss rock was achieved. An extension of the drill hole to 3488 m depth followed in a second phase in the year 1982/83, where a temperature of 147? C was attained. A single hole circulation system was tested. Basic results concerning the temperature field, joint system, stress field and hydraulic behaviour of the rock were achieved. Due to high flow impedance the concept was turned back to a doublette system. To reach higher temperatures for realizing a HDR pilot plant, futher investigations had to be obtained. The Urach 3 drillhole had to be extended from 3488 m to higher depths, where the required rock temperature of > 170?C was expected. The final depth was reached in a third phase, at 4444 m. The bottom hole temperature at true vertical depth of 4394,72 m was determined with 170?C. It can be proved that the temperature gradient is constant with 2.9 ?K/100 m depth. Temperatures expected at 4500 m depth are in the range of 175?C. As main lithological units, metamorphic rocks such as biotite-gneiss, anatexite and diatexite were determined in the extended drill hole. The different crystalline units are effected by brittle deformation. The resulting fracture system is sealed by hydrothermal products (clays, carbonates, sulphates). The aperture of fractures is in the range of some tenth to tenn millimeters. Subvertical sinistral strike-slip shears and faults which correspond to the most intense cataclastic structures, strike N 170? E. Televiewer and FMI logs show a general North-South orientated joint system and borehole breakouts around N 80-120? E. The orientation of maximum horizontal stress direction was determined with N 170? E. Wireline Hydrofrac stress measurements at 3352 m depth yields values of Shmin between 41-50 MPa. Estimated stress magnitudes of Anelastic Strain Recovery (ASR) measurements on cores from around 4425 m depth yields values of Sh = 63 MPa. Within a carried out study (1990-1996), local boundary conditions, infrastructure, user potential and a preliminary utilisation concept (3 MWel, 17 MWth) were evaluated. Due to the results of the investigations, it is proposed that the Urach site located in a widespread tectonic horizontal strike-slip system is suitable for a HDR demonstration project. The results can be applied in Southern German and Northern Swiss regions and in other areas of Europe. Many potential consumers of geothermal energy produced by the HDR concept are situated close around the Urach 3 drill site. |