The current phase of the European Hot Dry Rock Project at Soultz-sous-Forets requires the drilling of two additional deep wells to 5000 m depth into the crystalline basement, to form a module consisting of a central injector and two producers. The first well GPK-2 was drilled to 5000 m in 1999 and stimulated in 2000. The well GPK-3 (the injector) was drilled in 2002 and targeted using microseismic and other data. The bottom hole temperature was 200.6 ℃ and separation between the two wells at the bottom is around 600 m. GPK3 was then stimulated to enhance the permeability between the wells. A number of stimulation techniques were tried including "focused" stimulation, a novel method of injecting simultaneously in two wells. Microseismic monitoring, flow logging and other diagnostic methods were used during these injections. The "sparse" microseismic network at the Soultz site consists of a number of seismic sensors deployed in wells between 1500 m and 3600 m deep with bottom hole temperatures of 130-160 ℃. A 48 channel, 22 bit data digitizing unit was used for data acquisition in conjunction with proprietary software to carry out automatic timing and location in real time. This gave a real time decision-making possibility and control of the reservoir. This was the first time that such an interactive method had been carried out at this site. Around 90 000 micro-earthquakes were triggered during these injections and about 9 000 events were automatically timed and located in real time. These stimulations created a total reservoir volume in excess of 3 km~3. This is the largest stimulated volume in the development of HDR technology to date. The data suggest that "focused" stimulation may have a significant advantage over a single well stimulation technique and may be a way forward for efficient stimulation of larger separations between wells, thus improving the economic viability and acceptance of HDR/HFR/EGS systems. It is recognized that the reservoir creation process generates microseismic events but generation of bigger events (30 events approaching 2ML & one up to 2.9ML during this campaign) may retard the acceptance of this technology in an urban environment. This needs further studies to understand the processes and find a procedure to reduce the incidence of larger events.
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