| Keywords |
Geothermal reservoir, FRACHEM, SHEMAT, brine-rock interactions, geochemical modelling, Pitzer activity coefficients model, thermodynamic, kinetic reaction rate, porosity-permeability relationship, Enhanced Geothermal System (EGS), Soultz-sous-For?ts |
| Abstract |
Coupled numerical simulations of heat transfer, fluid flow and chemical reactions in geothermal systems are complex because of the highly heterogeneous geology, high temperatures, elevated pressures and often high salinity of the formation fluids. Codes such as FRACHEM and SHEMAT have been developed to forecast the long-term evolution of exploited geothermal reservoirs in order to determine how fluid circulation within geothermal reservoirs can modify the rock properties. FRACHEM is a thermo-hydraulic-chemical coupled program developed from the combination of two existing codes: FRACTure, a 3-D finite element code for modelling hydraulic, fluid and heat transport and elastic processes, that was developed originally for the study of flow-driven interactions in fractured rock; and CHEMTOUGH2, a modified version of the TOUGH2 simulator able to simulate the coupled transport of water, vapour, noncondensable gas and heat in porous and fractured media. FRACHEM has been developed especially to simulate the behaviour of the fractured granitic EGS reservoir at Soultzsous- For?ts (Alsace). Consequently, it contains specific implementations, such as the Pitzer formalism used to determine activity coefficients. The precipitation/ dissolution reaction rates of carbonates (calcite, dolomite), quartz, amorphous silica, pyrite and some alumosilicates (K-feldspar, albite, illite) are computed according to kinetic expressions deduced from published experimental data for high-temperature NaCl brines. FRACHEM also allows the coupling of porosity and permeability to chemical processes occurring within the reservoir. SHEMAT, the "Simulator for HEat and MAss Transport", has been developed in several phases and is today a general purpose reactive transport code for a wide variety of thermal and hydrogeological problems in two and three dimensions. SHEMAT solves coupled problems involving fluid flow, heat transfer, species transport and chemical water-rock interaction. It is a finite difference code that solves the flow and transport equations on a Cartesian grid. The chemical modules for calculating the activityconcentration relationships underlying the mineral reaction processes are based on the one hand on Pitzer?s equations and on the other hand on Debye-H?ckel?s theory. SHEMAT is particularly well suited to quantify the effect on flow and transport of chemically induced changes in the pore space of deep sandstone aquifers and was successfully employed for the prediction of the 30-year behaviour of geothermal fluid production systems. The Soultz-sous-For?ts Enhanced Geothermal System (EGS), established in the Rhine Graben, North of Strasbourg (France), has been investigated since the mid 1980?s. 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. The two codes have been applied to simulate fluid circulation in the enhanced geothermal system of Soultz-sous-For?ts. The same geometrical model and identical thermodynamic and kinetic input data have been used. The specific features of each code concerning calculation schemes and coupling mechanisms are presented. Such differences could typically lead to differences in numerical simulation results. Focus has been on the evolution of reservoir temperature, calcite and quartz reaction rates and porosity evolution. |