The Soultz-sous-ForÍts Enhanced Geothermal System (EGS), established in the Rhine Graben, North of Strasbourg (France), has been investigated since the mid 1980ís. The final goal of this project is to extract energy from a forced fluid circulation between injection and production boreholes within a granitic basement rock. This accelerated circulation of hydrothermal brine takes place within the fractured reservoir, at a depth of 5000 m, but different thermodynamic and chemical processes can modify the properties of this reservoir. The aim of this study is to predict the long-term behaviour of this system under circulation. To reach this objective, many aspects as heat, hydraulic, fluid transport and geochemical processes have to be taken into account, and their impact on porosity and permeability evolution has to be evaluated. In this study, we have examined in details the water-rock interactions on fluid flow, as well as the variations of reservoir properties with production time.

The fluid used in the Soultz system is the formation brine characterised by a total dissolved solid of 100 g.kg-1, a high temperature (200?C) and a pressure of 500 bars. Several Thermo-Hydraulic and Chemical (THC) coupled codes are available to model the behaviour of hot diluted fluids or cold brines, but not for hot hypersaline brines. Consequently, a new THC code had to be built for the Soultz reservoir conditions. Instead of creating a totally new modelling programme, two existing codes, FRACTure and CHEMTOUGH 2, have been combined in a code called ěFRACHEMî. FRACTure determines the thermal and hydraulic processes whereas CHEMTOUGH simulates the reactive transport. This last code has been modified because of the high salinity of fluid: the Pitzer formalism was incorporated into the code to determine activity coefficients of selected chemical species. Indeed FRACHEM code takes into account precipitation/dissolution reactions of some minerals, i.e carbonates, quartz, amorphous silica, pyrite and some aluminosilicates.

Presently, the evolution of reservoir properties is simulated in a 2-D simplified model. A fractured zone of 650 m long, surrounded by an impermeable matrix, links the injection and production wells. Injected fluid is kept at 65?C at a rate of 25 L.s-1 and flows through the fractured zone, which has an initial temperature of 200 ?C. The fluid circulation is carried out in a closed loop for a period of 5 years. The results given by the simulation code revealed that geochemical processes have some influence on reservoir properties.

Simulations indicate that carbonates reactions are predominant because of their fast reaction rates, compared to those of other minerals. Carbonates tend to rapidly dissolve near the injection point, whereas precipitation processes occur near the production point, due to re-heating of the fluid. Other minerals dissolve and precipitate all along the fractured zone but in negligible amounts.

As a consequence, porosity of the fractured zone increases near the injection point but decreases near production point. We can estimate that porosity doubles near injection point, is maintained constant between 200 and 400 m and then decreases of about 40 % between 400 m and the production well, at a distance of 650 m. Some simulations have been carried out in order to minimize adverse effects of porosity and permeability variations. For example, temporarily reverse fluid circulation, injection of acid solutions or circulation of brines with various compositions have been tested. The results seem to exhibit an improvement of reservoir properties caused by re-mobilization of some minerals.

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