Hydraulic Properties of the Rheingraben Basement MaterialUnits

Abstract

The next step in the European HDR-project at Soultz-sous-For&s, France, is the deepening of borehole GPK-2 to app. 5 km, in order to reach rock formation temperatures of app. 200 "C and to prepare a longterm circulation test at higher production temperatures. The success of the longterm circulation test 97JuL12 is based on the permeability increase due to earlier stimulation tests. Hydraulic flow in the granite Rheingraben basement is mainly restricted to faults and fractures, because the rock matrix permeability is low. The economy of a HDR-power-plant is controlled by the flow rate and the production temperature of the circulated water. The knowledge of the permeability of fractures under controlled laboratory conditions is therefore fundamental. A laboratory test program in 1999 will be carried out on cylindrical (6 cm diameter) and /or cubic (max. 10 cm edges) granite samples with a central hole (0.3 cm diameter). In hydraulic injections tests vertical or horizontal fractures will be induced. In consideration of different flow models (radial or straight) the fracture permeability will be determined in dependence on the pressure normal to the fracture plane and on the flow rate. In a next step the dependence of the permeability of fractures on the fracture aperture and on the fracturesurface- roughness will be investigated by shearing and/or thermal treatment of the fracture plane. Additionally in situ hydraulic tests are planned to derive the fracture transmissivity in 5 km depth from pressure oscillation analysis. The results of the planned in situ and laboratory tests will be presented at the conference. In the paper the results of the earlier in situ tests in 1996 and the preliminary permeability laboratory tests mainly on intact rock samples are summarised.