153 4. Towards a revisited geothermal conceptual model in the Upper Rhine Graben Therefore, EGS sites in the URG could be considered as convective-dominated systems characterized by the occurrences of some open natural fractures. The natural fluids circulating within the fracture/fault system could be pumped via suitable borehole trajectories. The shallower Soultz wells were drilled till 2000 m vertically intersecting a steeply dipping fracture system. Therefore, the probability that vertical boreholes intersect subvertical natural fractures was quite low and complex stimulation strategies were needed to connect the open-hole section of the geothermal wells with the partly permeable natural fracture system. For more recent and future projects in the URG, inclined or deviated wells could be drilled into the nearly vertical fracture system that allowed easier connections to the most convective and permeable fractures (Vidal and Genter, 2018). Then, from the purely HDR concept developed at SsF for creating from scratch a down-hole heat exchanger, the occurrence of natural brines trapped with complex steeply dipping fractures, the initial geothermal concept evolved to Enhanced or Engineered Geothermal Systems (EGS) in order to enhance the hydraulic flowrate. Generally, there is no need to restimulate the geothermal wells during long-term exploitation. There is no decline of the production neither increases of the reinjection pressure, which is below 20 bars both at SsF and Rittershoffen. Then, the stimulated fractured reservoir acts as a hydrothermal reservoir partly reconnected to the far-field due to the impact of the stimulation techniques. Therefore, we can state that there is a kind of physical continuum between EGS, with no or low initial permeability and hydrothermal system, which are prone to be hydraulically improved by post-drilling stimulations. 4.3 Pre-exploration phase The structural and geological context of the SsF area in the URG can be outlined by a thick sedimentary cover of 1400 m made of Mesozoic (Permo-Triassic to Middle Jurassic) and Cenozoic formations lying on the top of a Carboniferous granite horst, limited by local faults striking NNE-SSW to NE-SW dipping 60 to 70° West. There is a huge unconformity between the Mesozoic and the Cenozoic sedimentary successions due to an emersion or an uplift before the Cenozoic. By comparison with the Paris Basin geology, that means that many Mesozoic sedimentary units are lacking like the top Jurassic and all the Cretaceous. As this area was characterized by several tectonic phases during the emplacement of the Rhine Graben, the geophysical methods deployed on surface and sometimes in the wells must consider the specific geological background of this area. During purely HDR development at SsF, geophysical exploration was rather limited because the basic idea was not to find an aquifer or a specific geological unit but deep-seated brittle rocks showing very low matrix porosity and high temperature conditions. Therefore, there was no real geophysical exploration phase at SsF even if research based on surface electrical methods were done between 1977 and 1979 in order to target deep geothermal resources in the basin (Baudu et al., 1980).
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