| Keywords |
Geothermal reservoir, brine, granite, secondary minerals, thermo-hydraulic-chemical coupled code, fluidrock interactions, mineral reaction rates, porosity, permeability, acidizing treatments, Enhanced Geothermal System (EGS), Soultz-sous-For?ts |
| Abstract |
The development of Enhanced Geothermal Systems (EGS) depends on the creation of permeable fractures. Once the fractures network is created, the success of the long-term exploitation depends on maintaining and enhancing permeability. Sustaining fracture permeability will depend on many variables including rock mineralogy, fluid chemistry, temperature, local stress field, fracture strain rate, and the proximity of natural fractures to the wellbore. Operations exploiting little known, deep heat sources and low permeability reservoirs face new problems involving high temperature and pressure brine-rock interactions. In order to forecast the behaviour of an enhanced geothermal reservoir under exploitation, interaction between flow, heat transfer, transport and chemical reactions must be evaluated. For this purpose, coupled reactive transport modelling can provide useful information, by simulating chemical reactions likely to occur in the system coupled to reactive transport, at large time and space scales. FRACHEM, a thermo-hydraulic-chemical coupled code, was developed especially to forecast the evolution of the EGS project at Soultz-sous-For?ts, Alsace (France). FRACHEM can simulate thermal, hydraulic and fluid-rock interactions within the fractures connecting the injection and the production wells, and determine the dissolution/precipitation reactions of nine minerals in the Soultz granite (carbonates, pyrite, silicated minerals). Reactive transport modelling with FRACHEM code has been used to simulate re-injection of the formation brine after cooling within the 5000-m deep Soultz reservoir. In the first application, the coupled processes of a single fractured zone between two wells were investigated. In the second application, a more complex geometry has been shaped to represent a realistic reservoir model. This model includes two fractured zones of different widths following two different paths with the dimension of the Soultz reservoir. Depending on their distance and the relative exposure, these fractured zones interact on each other. These interactions have been investigated to predict the geochemical evolution and to quantify the impact on the reservoir. Results of numerical simulations for a long-term circulation confirm the role played by carbonates on the evolution of reservoir porosity and permeability. Due to their fast reaction rates, carbonate minerals are responsible for most of the reservoir evolution. Indeed, occurrence of calcite precipitation near the production well tends to decrease the reservoir porosity and permeability, induced by the decrease of the fractures aperture. Silicates and pyrite behaviour is also simulated between two wells, but their influence on the fractures aperture is limited. |