151 4. Towards a revisited geothermal conceptual model in the Upper Rhine Graben was drilled at 2560 m and targeted a local normal fault steeply dipping in the fractured granite. This first well, GRT-1, was not permeable enough and was thermally, chemically and hydraulically stimulated. Its hydraulic performance was enhanced by a factor 5 and was considered as a successful geothermal well (Baujard et al., 2017). Then, the second well, GRT-2, was drilled to 3200 m and inclined from the same pad and targeted the same permeable normal fault which was better defined thanks to a new 2D seismic survey (representing 16 km length) done in 2013 calibrated with the geological results of the first well. The highly deviated well, GRT-2, crosscut this local fault and its damage zone and was immediately permeable at the end of the drilling operation without any stimulation operations (Baujard et al., 2017). Thus, the first vertical well could be considered as an EGS-like well whereas the second one was fully hydrothermal because several permeable channels bearing by the normal fault were crosscut and contributed to the flowrate. This Rittershoffen EGS project demonstrated that permeable faults took place at great depth, but the main challenge is to adapt the well trajectories with the complex geometry and internal architecture of those local faults. To fill the gap between the lack of deep knowledge of the geothermal resource (lithology, fault geometry, permeable features) and the design of future geothermal wells, innovative geophysical exploration is one of the main tools. From 2016, GPK-2 is the unique production well at SsF with a mass flowrate of about 30 kg/s. Therefore, both GPK-3 and GPK-4 could be used as reinjection wells. It turns out that it is easier to produce a geothermal fluid with a relevant flowrate, assisted by a production submersible down-hole pump because the wells are artesian, than to reinject in one well only. Thus, it is the reason why GPK-4 became an injection well. Moreover, the fact to use two reinjection wells limits the seismic activity and consequently the occurrence of large felt seismic event. In parallel, from 2016 the geothermal doublet at Rittershoffen is producing a brine at 168 °C in surface with an average flowrate of 80 kg/s. From 2010 to 2019, a new geothermal project was launched close to Strasbourg at Illkirch (Figure 4.1). It targeted a deep normal fault having a vertical off-set of about 800 m located at the interface between the Lower Triassic sandstone and the top crystalline basement. Thus, a new 2D seismic reflection survey (35 km) was acquired in 2015 as well as other geophysical methods (gravity, aeromagnetic). However, this faulted interface was tight during drilling operation. It could be interpreted as the occurrence of secondary argillic clay halo that plugged this fault due to the past activity of the hydrothermal system (Glaas et al., 2021a). Thus, this first highly deviated well, GIL-1, was deepened to 3800 m depth in the crystalline basement which evidenced some permeability indicators and a high fracture density (Baujard et al., 2022, Glaas et al., 2021b). A stimulation program including hydraulic and chemical operations was developed for enhancing the initial productivity conditions. However, due to felt induced seismicity taking place at Vendenheim (Figure 4.1) and generated by another geothermal operator, the local mining authority suspended unilaterally the Illkirch project, and the second well is still pending to the Alsace prefecture decision.
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