By 2004/2005 a geothermal pilot plant for electricity production shall be completed at the HDR site at Soultz. This plant will consist of two production wells, one injection well and a reservoir in an ambient temperature of roughly 200?C. As a first step towards this ambitious goal the existing well GPK-2 was deepened from 3.8 km, where a large reservoir was already developed, down to 5 km, where the temperature requirements for an industrial power plant have been met. In a second step, a massive hydraulic stimulation with injection rates of up to 50 l/s was performed in summer 2000.

During this operation a large rock volume was activated for seismic slip. The locations of the microseismic events show that the mechanically affected rock volume is rather heterogeneous in shape having extensions of roughly 2.5 km in length, 0.5 km in width and 1.5 km in height. This is the largest volume ever stimulated in a single operation at Soultz. 3-D analysis and visualization of the seismic data reveals crucial insights into the internal structures of the seismic cloud.

Hydraulic tests before and after the stimulation confirmed that the hydraulic properties of the granite were permanently enhanced. Although the concept of hydraulic stimulation developed in the shallower system of Soultz approved to be also applicable at 5 km, some significant differences came out. The stress conditions as well as the connectivity to large scale fractures/faults seem to differ from what was observed at shallower depth.

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