160 Geophysics in Geothermal Exploration exploitation (Vidal and Genter, 2018). They are classified as hydrothermal and not strictly EGS wells. Their trajectories were well designed according to the geological and structural context because they crossed out several permeable fractures. The absence of stimulation is a substantial advantage for reducing cost as well as induced seismicity, main nuisance for public acceptance. However, all the geothermal projects in the URG are considered as EGS because they will need to use a reinjection well and to develop induced seismicity related to the geothermal exploitation (Maurer et al., 2020). Conclusion and perspectives In the Upper Rhine Graben, since the earlier development of matrix porosity geothermal projects in sandstones (e.g. Cronenbourg) and HDR projects in deep granitic basement (e.g. SsF) in the 90s, the geothermal concept evolved towards EGS projects by considering the geological properties of the deep geothermal system. The occurrence of fractured reservoirs characterized by natural brine circulations with fractured zones obliged developers to adapt geophysical exploration methods, geophysical well logging strategies as well as technical well design for reaching hydrothermal or EGS geothermal targets. Therefore, by improving the conceptual model of deep geothermal resources in the URG, well productivity has been improved either by stimulation or by optimizing geothermal targets by derisking drilling depths and well design. The depths of the wells have been divided by factor 2 between SsF and Rittershoffen, the first km of the top basement being highly fractured, hydrothermally altered and permeable. Consequently, the flowrate is higher than 70 kg/s at Rittershoffen compared to the 30 kg/s at SsF only. In terms of surface geophysical methods, only 2D seismic reflection was used in the past for shallower petroleum targets. By considering the importance of the fractured reservoirs, 2D or even better 3D seismic is now routinely used for deep geothermal resource exploration in the URG for imaging the top basement as well as the fault system at seismic scale. In terms of depth penetration, some progresses were also done from surface to great depths. For instance, some years ago, at Rittershoffen the second well, GRT-2, was targeted based on a specific 2D seismic line. For new geothermal projects based on a doublet, well design and their geothermal targets are defined before any drilling operation based on surface geophysical methods and specific treatments (inversion, machine learning, 3D modelling, …). Permeable faults or fractures lying into a deep basement hidden by a thick sedimentary cover are still challenging to image based on surface geophysical methods. Thus, there is a real need for combining various geophysical methods and treatments to propose a multi-physics image of deep fractured geothermal reservoirs in the URG in order to explore with low risks and thus exploit more sustainably the deep geothermal resource.
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