148 Geophysics in Geothermal Exploration More recently, in the Strasbourg area (Vendenheim), a new competitor came for developing geothermal projects for producing electricity. However, after the drilling of two deep wells at about 6 km measured depth, a series of man-made seismic events were felt on surface during well testing or various operations between 2019 and 2021 with a local maximal magnitude of 3.9 (Schmittbuhl et al., 2021). It turned out that social acceptability became a real issue in this urban area because some structural damages were observed on houses around Strasbourg as well as on the German side around the city of Kehl. About 4000 requests from inhabitants have been made due to the occurrence of cracks observed on the walls of their houses. In parallel, by using the precautionary principle, the Illkirch geothermal project which was under development South of Strasbourg at the same time, was suspended by the French mining authorities in order to anticipate any further issue related to manmade seismicity. Now, the Vendenheim project has a new geothermal owner, but the site is still stopped by the mining authorities. In parallel, the Illkirch project is still suspended even if it did not cause any nuisances. The operator is waiting for the possibility of drilling at least a second geothermal well if the social acceptability is acceptable. Moreover, new geothermal projects are under development in Northern Alsace in the area of SsF with a geothermal well that could be planned for 2025 for exploiting the reservoirs at the fractured interface between the deep sediments and top basement. In parallel to the geothermal development of the URG, a new era is rising with the booming of geothermal lithium. Several French and German companies aim to extract the geothermal lithium which is dissolved into the geothermal brine with a concentration ranging between 160 and 210 mg/L. After a phase of research for extracting lithium from the brine and then producing lithium carbonate of battery grade quality, several companies are currently obtaining geothermal and lithium leases in order to conduct pre-feasibility and feasibility studies before investment decisions for conducting industrial projects for producing lithium. 4.2 Evolution of the geothermal concept during the SsF adventure The basic concept started on the Hot Dry Rock (HDR) concept considering that many geothermal wells reached interesting temperatures, but their flowrates were too low, even though fully dry, for reaching viable economic balance. Therefore, based on previous geothermal research dealing with the creation of artificial heat exchanger by hydraulic fracturing in US, UK, Japan or Germany (Figure 4.2), a jointed European research site was selected in Alsace, Eastern part of France, based on well-known temperature field measured in the former oil mines and oil wells close to Pechelbronn where the Schlumberger brothers did their first geophysical well logging in 1927 for detecting hydrocarbon reservoirs based on electrical measurements.
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