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Fluid origin and circulation in the heat exchanger of Soultz-sous-Forêts (France) estimated using geochemical and tracer test data.

Abstract : In the framework of the European Hot Dry Rock Energy (HDR) Program, three deep wells (5000 m) have been drilled into a fractured granite basement at Soultz-sous-Forêts, located within the Rhine Graben, in order to develop a deep heat exchanger (GPK-3 as injector and GPK-2/GPK-4 as producers) and produce electricity after the creation of an EGS reservoir. Despite very few representative analyses of deep geothermal fluids, often contaminated by drilling fluids or injected waters, the geochemical data provide new and interesting information on the nature, origin, circulation and deep temperature of these fluids (Sanjuan et al., 2006a and c). They indicate similar chemical and isotopic compositions (NaCl fluids) and high salinity values (TDS about 100 g/l) which suggest a common sedimentary origin and identical water-rock interaction processes. The chemical and gas geothermometers suggest that the native geothermal brine and associated gases are equilibrated with a mineralogical assemblage at temperatures close to 220-240°C (> 200°C measured at the bottom-hole; Sanjuan et al., 2006a and c). According to the Na/Li geothermometer and the 7Li values, these equilibrium reactions would occur in a sedimentary rather than granite reservoir (Fig. 1). Given the location of the Soultz site and these constraints, this reservoir would be situated more eastern, towards the Graben centre where the Triassic sedimentary formations are the deepest and hottest (Fig. 2). From tracer tests carried out after 2000, the natural flux of the native geothermal brine was estimated at 1-1.2 m3/h, which is identical to that calculated for the fluid flux parallel to the Graben strike, based on a convection model and numerical 3D modeling (Bächler, 2003; Sanjuan et al., 2006b and c). During all the production and circulation tests, the tracer tests and geochemical data showed the omnipresence of the native geothermal brine in the discharged fluids even after injection of large amounts of external water into the wells. The existence of at least three fluid flow pathways between the wells GPK-2 and GPK-3 with different effective fluid velocities, which contrasts with a poor hydraulic connection between GPK-3 and GPK-4 (Fig. 3), was highlighted during the fluid circulation loop and the associated tracer test using fluorescein, carried out between July and December 2005 (Sanjuan et al., 2006b and c).
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Bernard Sanjuan, Romain Millot, Chrystel Dezayes, Michel Brach. Fluid origin and circulation in the heat exchanger of Soultz-sous-Forêts (France) estimated using geochemical and tracer test data.. EHDRA Scientific Conference, Sep 2008, Soultz-sous-Forêts, France. 6 p. ⟨hal-00773163⟩

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