Geochemical Modelling of Fluid-Rock Interactions in the Soultz-sous-Forêts Enhanced Geothermal System

Abstract

The geochemical modeling of fluid-rock interactions has been intensively applied to the Enhanced Geothermal System (EGS) of Soultz-sous-Forets (France). Modeling was combined with experimental approaches in order to predict possible mass transfer associated to the circulated hot fluids from the granitic reservoir at a 5 km depth and 200°C, the surface power system, and in the re-injection wells at 60°C. The formation of carbonates and dissolution / precipitation of silicates were simulated as possible risks or advantages for future exploitation of the geothermal site. In the short term, the precipitation of carbonates appears to be the more important process of the two because of their kinetic properties. But in the long term, and particularly after tens of years of industrial exploitation of the geothermal system (which started in May 2008), the possible dissolution of rock-forming silicates in some areas of the reservoir and the precipitation of other minerals like clay minerals (illites and / or chlorites) in others may be significant enough to change the porosity and the permeability of the exploited reservoir. The dynamics of silicate minerals have clearly been detected with a complementary experimental approach of water-rock interaction under strong thermal gradient.