A three-year research program (2010-2012) associated with the geothermal exploitation of the Soultz-sous-Foręts power plant is on-going with a scientific and technical monitoring. Several hydraulic circulation tests have been performed that involve one production well, GPK-2 and two reinjection wells, GPK-1 and GPK-3 (Figure 1): a long term circulation for about 11 months in 2010, and two short term circulation tests in 2011. During the 2010 exploitation, geothermal fluid discharge from GPK-2 reached a volume of about 500 000 m3 by producing at 18L/s for a temperature of 164°C. A tracer test was conducted and showed the good connection between GPK-3 and GPK-2. In 2010, more than 400 induced micro-seismic events of low magnitude occurred. Geochemical monitoring of the fluid discharged from GPK-2 indicates that the chemical composition of this fluid becomes closer to that of the native geothermal brine (NGB) because only 4-8% of injected freshwater between May and October 2010 and 1-4% in February 2011 remain in the production fluid. In 2011, geothermal fluid discharge from GPK-2 reached a volume of about 300 000 m3 by producing at 24L/s for a temperature of 159°C. The strategy was to increase the reinjection flow rate in GPK-1 and simultaneously minimize it in GPK-3 in order to decrease reinjection pressure. Induced seismic activity was very low with only 5 micro-earthquakes in 2011. The observed improvement of well productivity is interpreted as a self-cleaning of the fracture network during geothermal production. In parallel, many research works have been carried out for characterizing scaling (sulfate, sulfide) and the natural radioactivity derived from natural brines circulating within a deep fractured granite reservoir. Such scaling is preferentially located in the cold part of the geothermal installations (reinjection side). On-site corrosion study on several kinds of materials indicates a corrosion rate of about 0.2mm/year at re-injection conditions. Down-hole pump technology was also tested in various geothermal conditions during exploitation. In April 2011, occurrences of cuttings (granite particles) at high flow rate, generated abrasion of the production pump reinforcing its damaging. In October 2011, significant vibrations during high rate pumping were observed and interpreted as the main cause of pump destruction. Gross thermal power was relatively stable during production whereas the gross electrical power was quite variable. Environmental nuisances such as noise, vibration, seismic activity, and natural radioactivity have to be carefully investigated in order to evaluate their impact on the local population and then on public acceptance.

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