The Upper Rhine Graben (URG), in the border area of southwestern Germany and eastern France, has suitable conditions for deep geothermal applications using enhanced geothermal systems (EGSs). Sandstone formations are used as geothermal reservoirs, where in situ brines are in contact with the rock at the surfaces of fracture networks. Alteration reactions influence the fluid flow in the geothermal reservoir, and therefore, play a major role in the sustainable operation of the geothermal installation. For a better understanding of brine–rock interaction in EGSs, batch experiments at reservoir conditions have been performed with typical temperatures of 200 °C. Sandstone samples with a natural rock fabric were used. Sample surfaces represent rock fractures. Three different fluids with a composition close to natural URG brines have been used for the experimental series lasting up to 128 days: Na–Cl, Na–Ca–Cl, and Ca–Cl solutions all with an ionic strength of 2m. Quartz dissolution and the removal of illite cement are the main reactions of the primary mineral assemblage, independently of the experimental solution composition. K-feldspar shows a high degree of dissolution only in the Ca–Cl solution. In general muscovite shows a high resistance to solution. The fluid compositions reflect the processes observed in the solid samples over time. On the sample surface massive amounts of zeolites precipitated during the hydrothermal experiments. In the Na–Cl series analcime is the sole zeolite phase. Analcime and wairakite occur on samples from experiments with Na–Ca–Cl solution. In the Ca–Cl series wairakite is the dominant zeolite phase, accompanied sparsely by epistilbite crystals. The zeolite phases have been identified and characterized by electron microprobe and synchrotron X-ray diffraction analyses. Reaction mechanisms have been derived from the observations of the solid and fluid samples. The main processes involve dissolution of quartz, illite, and partly K-feldspar, together with consumption of dissolved (earth-)alkali elements from solution. This results in the precipitation of zeolites, depending on the experimental solution composition. Based on the overall reactions, the volume of the solid phases increases, depending on the parent mineral. The observations in the experiments have been transferred to natural systems and are discussed in the context of URG geothermal systems. The experimental results show a large influence of brine–sandstone interaction on surfaces of fracture networks in EGSs. The observed processes have the potential to hamper the fluid flow and decrease the efficiency of geothermal installations.

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