| Abstract |
Several geothermal plants are in operations or in development in the French part of the Rhine Graben. Due to the French mining authority, an environmental survey is routinely ongoing on those sites in terms of seismological monitoring, GPS measurements and Peak Ground Velocity (PGV) monitoring. From 2008 to 2013, a first binary power plant at Soultz-sous-Forêts, established in a deep fractured granitic massif, has resulted in the development of the EGS (Enhanced Geothermal System) concept. After the dismantling of the previous facilities on 2014-2015 due to corrosion issues related to the geothermal brine, a complete renovation of the plant, with a new geothermal loop and a new ORC (Organic Rankine Cycle) unit, has been built. The commissioning of the renovated plant started in 2016 to exploit a geothermal brine at 150-160°C with a flowrate of about 30 l/s to produce a net power of about 1.4 MWe since the beginning of July 2016. Currently the brine is pumped out of GPK-2 well and reinjected into two wells, GPK-3 and GPK-4. The geothermal project located at Rittershoffen, 6 km east of Soultz-sous-Forêts, in Northern Alsace, exploits a geothermal brine trapped in the fractured hard rocks. This geothermal project is designed to produce 24 MWth (170°C, 70 l/s) which is delivered to a bio-refinery located 15 km away. Two deep wells have been drilled between 2012 and 2014 to 2500 m TVD (True Vertical Depth) for targeting local normal-faults located close to the interface between the clastic Triassic sediments and the top crystalline basement. The second well was good enough hydraulically after drilling operation and thus, it was not necessary to enhance its natural permeability. The geothermal plant commissioning started in May 2016. The Illkirch-Graffenstaden deep geothermal project plans the construction of a power plant located 10 km south from the city of Strasbourg to produce electricity and heat for a district heating with a maximum of expected thermal power of 20 MWth. The project involves the drilling of two wells (doublet) at a depth of 2700 m TVD, to produce a geothermal fluid at a flow rate of 70 l/s and a temperature around 150 °C. The targeted reservoir is a fractured zone in the Buntsandstein sandstones and the upper granitic basement. The drilling of the first well is currently ongoing. In order to detect any rise of micro-seismicity induced by the geothermal operation, the mining authorities required the deployment of a permanent seismic network. At least, five surface stations around each geothermal plant is required as well as a geodetic station. Several thresholds have been established in PGV (peak ground velocity) to legislate the exploitation of the geothermal plants. Since 2012 for both Soultz-sous-Forêts and Rittershoffen geothermal plants, and since 2015 for Illkirch-Graffenstaden, the micro-seismicity activity has been monitored by permanent seismic networks and by temporary surface networks installed during strategic operations (drilling, development of wells, commissioning of a plant). A geodetic monitoring is also ensured for each site. Since 2016 an almost continuous production and re-injection loop was performed in both geothermal fields of Rittershoffen and Soultz-sous-Forêts. As low magnitude seismic events are associated with deep circulations of such projects, the micro-seismic activity has been carefully monitored in real–time since the beginning of the production. Since then, a couple of thousands of induced low magnitude earthquakes were detected (Mlmax = 1.7), all located in the vicinity of the injection wells, GRT-1 for Rittershoffen and GPK-4 for Soultz-sous-Forêts. So far, no induced seismicity was detected during the drilling of the first well GIL-1 in Illkirch-Graffenstaden. Non felt induced seismicity observed in Rittershoffen and in Soultz-sous-Forêts highlights the need to perform a continuous seismic monitoring, even during the exploitation phase of power plants, which is now explicitly required by the French mining authorities for getting the exploitation license. The Soultz and Rittershoffen geothermal plants have a availibity higher than 90% and produces energy from a fractured granite rocks with no felt induced seismicity |