| Abstract |
Operation of the enhanced geothermal system (EGS) requires to re-inject cooled fluid, after heat exchange at the surface to the energy production, into the geothermal reservoir. This cold re-injection triggers disequilibrium between fluid and rock and then implies the dissolution/precipitation of minerals, which may potentially cause changes in porosity, permeability, and flow pathways of the geothermal reservoir. As preliminary steps, we first studied the impact of changes in temperature by using a geochemical code (THERMA) which enables us to calculate the changes in equilibrium constants of all primary and secondary minerals and aqueous species as a function of temperature; then we performed numerical simulations in time at different temperatures, but without transport, to study the reaction pathways and to compare the thermodynamic and kinetic approaches in precipitation modelling (KINDIS code). These preliminary results enabled us to identify the key minerals, primary as well as secondary minerals among a large amount of tested minerals, and to get the necessary associated thermodynamic and kinetic data. The goal of what we present lies in the building of a coupled thermo-hydrogeochemical model of the EGS system. The temperature transport was implemented in the one dimensional coupled hydro-geochemical code (KIRMAT) by considering a classical temperature model, i.e. temperatures in fluid and rocks are equal. The simulations we performed with this coupled approach on the Soultz-sous-Forêts reservoir case made appear a great complexity of geochemical reactions, linked to the mineralogical complexity, especially sharp pH/Eh variations which may have a great impact on dissolution/precipitation conditions. |