159 4. Towards a revisited geothermal conceptual model in the Upper Rhine Graben A methodology was established by using the thermal logs of the deep geothermal wells of Rittershoffen and SsF and applied to the temperature profiles measured in the gradient wells as an exploration tool. The basic idea is to try to estimate the temperature at the top of the geothermal reservoir. Thus, a series of seven shallow wells (<200 m) were drilled in Northern Alsace and temperatures were measured at equilibrium. Then, by using detailed lithostratigraphy and other relevant geological information, temperatures were extrapolated linearly till the top of the Middle Triassic limestone (Maurer et al., 2018). Above this geological interface, the geothermal gradient is conductive and shows a very high slope. This method is effective in volcanic environments for locating the heat source. It has been adapted to the URG reservoirs for delineating hottest zones related to local faults at the depth corresponding to Middle Triassic layers. 4.4 Optimizing borehole design according to the geological knowledge of the reservoir Geophysical exploration was not really used for HDR because there was no need of reconnaissance of a deep heat exchanger nor to identity geothermal permeable reservoirs. However, based on extensive structural characterization by drilling in the URG, faults and fractures are highly dipping (>70°) and drilling vertical wells present a high probability to not cross these nearly vertical structures. Some recent wells in the URG (GRT-2 in Rittershoffen, GIL-1 in Illkirch, two wells in Vendenheim) are deviated in order to intersect a maximum of nearly vertical faults and fractures. However, a deviated well is more complicated to drill, to log and to exploit, and consequently more expensive. Insofar the cost of the drilling follows an exponential law corelated to the drilled length. It is the reason why the most recent wells are not drilled into the deep granitic basement only like in SsF but target the fracture network in the overlying Lower Triassic sandstones as well as the first kilometer of crystalline basement just below the interface with the sedimentary cover. It has been demonstrated from extensive structural analysis of the SsF wells that the first km of the top crystalline basement is much more naturally fractured than the overlying Triassic sandstones as well as the deeper basement. By discovering that the fractures are highly dipping and locally permeable, well design evolved from vertical to deviated or even inclined wells trajectories. Therefore, the first wells were vertical at Soultz (GPK-1, GPK-2) but slightly evolved to a more complex design like GRT-2 in Rittershoffen that was deviated and drilled parallel to the main local faults identified by 2D seismic. The hydraulic performance of this second well was so good, that stimulation operations were not needed (Baujard et al., 2017). The open-hole sections of the geothermal wells are generally aligned with the orientation of the principal horizontal stress (SsF, Rittershoffen). Some wells in Brühl, Insheim or Rittershoffen were not stimulated and presented a sufficient natural permeability for industrial
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