Geothermal energy has been a subject of great interest since the 1990’s in the Upper Rhine graben, where the first European Enhanced Geothermal System (EGS) pilot plant has been developed, in Soultz-sous-Forêts (SsF), France. The geothermal potential of this Franco-German region is the result of a thermal gradient anomaly coupled with a dense naturally fractured network, allowing the exploitation of hydrothermal brine circulating in the granitic reservoir. Thanks to the development of the EGS techniques, two industrial geothermal plants in SsF and Rittershoffen (France) were commissioned in 2016 to produce electricity (1.7MWe) and superheated water for industrial needs (24 MWth), respectively. Other additional geothermal sites are presently under development in the Strasbourg area (Vendenheim and Illkirch-Graffenstaden, France) and other plants are operating on the German side of this Tertiary graben. The SsF and Rittershoffen sites have experienced a successful continuous exploitation since their launch with plant availability higher than 90%, allowing to characterize the chemical evolution of the brines during their operational phase. For this purpose, the geothermal fluids were regularly analyzed for physicochemical parameters, major and trace elements, radionuclide concentration and gas content and composition in order to monitor any potential impact of the exploitation phase on the fluid properties. The heat capacity, density and viscosity were also measured in-situ once on the Rittershoffen brine to obtain the actual physical parameters of this energy vector in the heat process. Data collected from 2016 to 2020 indicates only minor variations of the brine chemical features. During this period, more than 3 and 10 Mm3 of geothermal brine circulated in the SsF and Rittershoffen sites, respectively. Such observations mean that long-term geothermal activity does not have a major impact on the reservoir composition. Considering the proximity of the wells of both plants ( less than 7 km), the chemical stability observed in the brines over time also suggests the sustainability of a geothermal co-exploitation activity in a fractured reservoir for energy needs. Moreover, the geographical homogeneity of the brine composition could be interpreted as a proxy of the significant size of the geothermal reservoir. The preliminary data on Illkirch geothermal fluid appears consistent with the chemical features expected for an Upper Rhine graben brine produced from the granitic basement. However, additional analysis should be performed in the future to better characterize the brine composition of this site.

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