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Geophysical and mineralogical impacts of fluid injection in a geothermal system: the Hot Fractured Rock site at Soultz-sous-Forêts, France

Abstract This paper deals with the geothermal system at Soultz-sous-Forêts, an important research site in France of the European Hot Fractured Rock project. A pilot plant is ready for electric energy production of up to 25–30 MW. Several research projects were developed in France, Switzerland, and Germany in order to study various scientific aspects of the geothermal system in parallel with the industrial development. Of primary scientific interest are the geophysical, geological, and geochemical conditions in the geothermal reservoir as well as the predictable consequences of future exploitation on the quality of the reservoir and on induced environmental problems, including: induced seismicity; rock alteration and possible formation of secondary minerals along the water pathways; and evolution of porosity and permeability in the deep aquifers. This paper focuses on two aspects: the geochemical evolution of minerals in the reservoir due to thermal gradients and fluid flow; and the evolution of physical properties of rocks during fluid injection in the wells. The petrophysical study shows that the fluid-rock interactions can be extensive in the reservoir matrix. The experimental study suggests that the fluid-mineral interactions are dynamic and that clay minerals can precipitate.