INTRODUCTION
In-situ stresses are key parameters in the design of a hot-dry-rock (HDR) operation for the extraction and the economic use of geothermal energy. In-situ stresses control the operation pressure to activate pre-existing fracture / joint-systems, determine the underground fluid flow paths, or influence borehole stability in un-cased borehole sections.
The HDR geothermal research site at Soultz-sous-Forets (France) near the western margin of the Upper Rhine Graben is one of the possible candidates for a future European HDR-prototype with a target reservoir temperature of 175°C at only 3.5 km depth. During the first project phase (1986-1990) borehole GPK-1 was drilled to a depth of 2000 m. In the granitic section between 1376 m and 2000 m, eight hydrofrae or injection tests were conducted [1]. These tests were characterized by several technical problems caused by using conventional packer technology in the hostile downhole environment (high temperature of 140 °C, high gas and salt content of the borehole fluid). In spite of the problems, five tests yielded sufficient pressure and fracture orientation data to conduct an inversion type stress analysis [2]. The result of the computations yields the following stress profile for the depth interval between 1458 m and 2000 m:
Sh, MPa = 15.1 + 0.0179 (z, m 1458)
Ss, MPa = 24.8 + 0.0198 (z, m1458)
Sv, MPa = 0.024' z, m
direction of Ss: N 155 ° ± 3°.
Due to the limited data base of only five tests a valid extrapolation of this stress profile to reservoir depth was impossible. The determination of stresses down to reservoir depth, therefore, asked for the conduction of deeper and more reliable hydrofrae stress testing. Such tests also required the development of a new hydrofrae tool based on aluminum packers adequate to the hostile downhole environment.
