Influence of water–rock interactions on fracture permeability of the deep reservoir at Soultz-sous-Forêts, France

Circulation of geothermal fluids through granitic fractured reservoirs leads to chemical reactions, modifying the porosity and permeability of the rock mass. FRACHEM, a thermo-hydraulic-chemical coupled computer code, was developed specifically to predict changes in the geothermal reservoir of the Soultz-sous-Forêts Enhanced Geothermal System (EGS) located in Alsace, France. This code can simulate fluid–rock interactions and determine the dissolution/precipitation reactions of eight minerals in the Soultz granite (i.e. carbonates, pyrite, silicates and aluminosilicates). Numerical simulation results of long-term fluid circulation through the 5000-m deep Soultz reservoir are comparable to those determined for the shallow reservoir and confirm the role played by carbonates in the evolution of reservoir porosity and permeability. Moreover, experiments with FRACHEM in simulating short-term fluid flow during hydraulic and/or chemical stimulations have demonstrated that the code could prove an efficient tool in reservoir engineering and management.