150 Geophysics in Geothermal Exploration This newly exploration well with a low borehole diameter brought new geological characterization about the natural fracture system. The main finding was the confirmation of highly fractured zones with associated hydrothermal alterations in this shallow basement reservoir. Natural fractures are organized in clusters and intense argillic hydrothermal alteration took place in the granite evidencing the impact of paleo or recent fluid circulations (Genter et al., 2010). It was also shown that the natural fracture density in the granite was more than three times higher than in the Permo-Triassic sandstone (Genter et al., 1997). Then, in order to achieve hotter temperature conditions, it was planned to deepen the GPK-1 geothermal well from 2000 to 3600 m in 1992 and to drill a second geothermal well, GPK-2, from surface to 3890 m in 1995 which is located about 500 m apart from GPK-1. The temperatures measured at total depth of those two wells GPK-1 and GPK-2, were around 160 °C and thus still far from the 200 °C expected. Many tests were conducted in those wells including hydraulic testing, tracer tests, hydraulic stimulations and circulation tests in this intermediate reservoir (Schill et al., 2017). Induced seismicity was monitored both in surface but also in some former oil wells deepened till the top basement (Cuenot et al., 2008; Dorbath et al., 2009). Finally, GPK-2 was deepened to 5058 m measured depth for reaching 200 °C at total depth and two additional deep geothermal wells, GPK-3 and GPK-4 were drilled to about 5000 m and reached 200 °C (Genter et al., 2010). As the HDR concept was still in mind, it was planned that GPK-2 and GPK-4 would be two production wells and GPK-3 an injection well. All these new drill pads were drilled from the same platform for optimizing the geothermal operations (stimulation, circulation test and future exploitation). Natural permeability was observed in the granitic sections of all wells mainly related to hydrothermally and fractured sections (Evans et al., 2005). Even if permeability indicators were observed in each well, natural artesian flowrate was too low for a viable economic production. Therefore, each well, which has an open-hole section between 4500 and 5000 m, was hydraulically and chemically stimulated. These stimulations enhanced significantly the hydraulic yield of the three reservoir sections (2000 m, 3500 m, 5000 m), in some instances by about two orders of magnitude (Schill et al. 2017). Kohl et al. (1997) shown that complex hydraulic flow regimes are not restricted to near-well vicinities but rather extend large distances until reaching high capacity far-field faults. The most effective method for enhancing the flow, was the hydraulic stimulation rather than the chemical ones. In the follow-up geothermal projects such as at Landau and Insheim, the concept of enhancing the naturally most productive reservoir level at the top of the granitic basement was applied, as well as specific hydraulic stimulation techniques (Schindler et al., 2010). Several long-term circulation tests including tracer tests were carried out at SsF for demonstrating that the deep wells are connected after stimulation on a large open geothermal reservoir producing a very saline brine (Sanjuan et al., 2006). From 2011, the new Rittershoffen geothermal project, located less than 10 km from SsF, was launched for producing heat at high temperatures (170 °C) and high flowrate (>70 L/s) for providing geothermal heat to a biorefinery. The first vertical well
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