146 Geophysics in Geothermal Exploration water already present in the underground. Based on an extensive deep geoscientific characterization including geophysical exploration and geophysical logging, this chapter explains how a better knowledge of the geothermal resource allows improving the conceptual model of this deep resource and thus how to optimize geothermal targets by derisking drilling depths and well design. 4.1 Geothermal development in the Upper Rhine Graben In Western Europe, deep geothermal energy started in the 90s by a French-German scientific cooperation in the Upper Rhine Graben, at Soultz-sous-Forêts (SsF), France. This research site was the location of many deep geothermal wells reaching 5 km in the Carboniferous crystalline basement, various geophysical logging including borehole imagery logs, well testing and hydraulic circulation. After an extensive phase of purely scientific research on the underground, the site slowly evolved to a pre-industrial site by building a first binary plant producing electricity in 2008. However, due to severe corrosion induced by the high salinity of natural geothermal fluids, after the dismantling of the first plant, a second but more robust binary plant was constructed and operated in 2016 by Electricité de Strasbourg. Since, it has been producing electricity on the French grid with an installed capacity of 1.7 MWe. From the SsF experiment, many spin-offs have been created or new competitors tried to duplicate the Soultz concept in the Upper Rhine Graben, mainly in France, Germany and Switzerland. Thus, Landau and Insheim geothermal plants were developed in the Rhine-Palatinate in Germany whereas Rittershoffen plant and Illkirch sites were developed in Alsace (Figure 4.1). Rittershoffen, which is a real SsF cousin, is producing 24 MWth of energy since 2016 to a biorefinery with geothermal wells having a reservoir depth divided by two in comparison with SsF reservoir depth. Therefore, after several decades of geothermal research, exploration and development in Northern Alsace, two geothermal plants are commercially operating in France (SsF and Rittershoffen). In the German part of the URG, three plants are also operating (Landau, Insheim, and Bruchsal) (Figure 4.1). The Bruchsal geothermal doublet was drilled in the 80s in a fractured reservoir on the Eastern side of the URG and penetrated Permo-Triassic sandstones considered as a hydrothermal fractured/faulted reservoir (Kolbel et al., 2020). Those sites have penetrated the sedimentary formations of the graben and, for some of them, the deeper crystalline basement (Soultz, Rittershoffen, Insheim, Landau). In Switzerland, at Riehen, a heat plant has been operating from several decades but is located on the Eastern shoulder of the URG (Figure 4.1). This site deliver heat from geothermal hot water pumped in the Middle Triassic reservoir composed of fractured carbonates. In the past, for various reasons, several geothermal projects were stopped in the URG (Trebur, Brühl, Cronenbourg, Basel). For instance, at Cronenbourg in the suburb of Strasbourg, the unique deep geothermal well at 3220 m, drilled in 1980, was not
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