149 4. Towards a revisited geothermal conceptual model in the Upper Rhine Graben Firstly, the French-German SsF project started based on the Hot Dry Rock (HDR) concept. It consists in exploiting the vast energy resource that resides as heat in the low-permeability rocks which underlie most continental regions of the Upper crust at practically drillable depths. Thus, it was planned to drill a vertical well in the area of SsF because the geology was very well-known from several thousand of former petroleum wells mainly drilled in the Cenozoic reservoirs where interesting temperatures could be accessible at low drillable depths (Figure 4.2). Therefore, a first vertical well, called GPK-1, was planned to intersect about 1400 m of Cenozoic and Mesozoic sediments before to penetrate into the Carboniferous top crystalline basement. In such theorical HDR concept, at the end of each drilling operation, it was planned to inject fresh water under pressure in order to create artificial fractures that can be used as a heat exchanger. Then, it is needed to drill and then to inject under pressure in a second vertical well for connecting hydraulically the first one by creating newly-formed planar fractures in the deep heat exchanger. Considering the absence of natural permeability, fresh water must be injected from one well (injection well), and by heat transfer taken on the newly created artificial fractures, the cold injected water becomes hotter and could be produced at surface in the second well (production well) for producing steam and thus electricity. The SsF project started with this HDR concept by developing permeability in a crystalline basement lying below the super-hot sedimentary cover. After the drilling of the GPK-1 well in 1987, the main findings were a very high geothermal gradient in the first km with about 110 °C. However, below those depths, the geothermal gradient declined sharply indicating the occurrence of natural fluid circulations within the natural fractures inside the Triassic formations and the crystalline basement, the uppermost sedimentary cover acting as a thermal insulator. It turned out that 140 °C was measured at 2000 m depth in the granite instead of the 200 °C planned initially. The second main finding is the occurrence of a native brine within the crystalline basement proving that the top basement between 1400 and 2000 m depth was not tight as anticipated by many geoscientists (Vuataz et al., 2000). By taking into account those findings, the capacity to develop post-stimulation permeability was investigated considering two cases. First, the top basement was considered as a medium with a residual permeability due to individual natural fractures partly sealed by hydrothermal deposits. Its stimulation could lead to preferential flow paths and thus a rapid cooling. Therefore, a second option was also considered. Indeed, the deepest depths probably correspond to complex fractured rock where closed natural fractures took place within brittle crystalline rocks (Gerard et Kappelmeyer, 1987). In this deeper case, it would be possible to stimulate a volume of fractured rocks and thus to engineer structural linkages between the future doublet for extracting large amounts of heat by circulation through this created down-hole heat exchanger. However, before to drill at great depths, it turned out that for better exploring the Soultz basement an old petroleum well was deepened and fully cored in 1990-1991 from the Middle Triassic limestone to the deep granite from 930 till 2230 m depth.
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