Coupled thermal-hydraulic-chemical modelling of enhanced geothermal systems

The study investigates thermal-, hydraulic- and chemically coupled processes of enhanced geothermal systems (EGS). On the basis of the two existing numerical codes, the finite element program FRACTURE and the geochemical module of CHEMTOUGH, FRACHEM was developed, to simulate coupled thermal - hydraulic - chemical (THC) processes, accounting for the Soultz specific conditions such as the high salinity of the reservoir fluid and the high temperatures. The finite element part calculates the thermal and hydraulic field and the geochemical module the chemical processes. According to the characteristics of the Soultz EGS reservoir, the geochemical module was modified. (i) The Debye-Huckel approach was replaced by the Pitzer formalism. (ii) New kinetic laws for calcite, dolomite, quartz and pyrite were implemented. (iii) The porosity - permeability relation was replaced by a new relation for fractured rock. (iv) The possibility of re-injecting the produced fluid was implemented. The sequential non-iterative approach (SNIA) was used to couple transport and reactions. Sensitivity analyses proved the proper functionality of FRACHEM, but highlighted the sensitivity of the SNIA approach to time steps. To quantify the FRACHEM results, a comparative simulation with the code SHEMAT was conducted, which validated FRACHEM. Coupled THC processes in a fractured zone in the Soultz reservoir at 3 500 m (T-0 = 165 degrees C), which occur as a result of the injection of fluid ( T (inj) = 65 degrees C) at one end of the zone and the production at the other end, were modelled for 2 yr. Calcite is the most reactive mineral and therefore the porosity and permeability evolution results from the calcite reactions: near the injection point, porosity and permeability increase and near the production well they decrease. After 2 yr, the system seems to be very close to steady-state. Therefore, mineral dissolution and precipitation during the circulation of the fluid in the reservoir do not represent a limiting factor on the EGS reservoir ( at 3 500 m depth) performance at the Soultz site. Finally, the numerical transmissivity was compared to the transmissivity of the 1997 circulation test. The fact that the transmissivity decreases during the circulation test, when thermomechanical effects are factored out, points to geochemical processes in the reservoir such as the precipitation of calcite. These findings highlight the importance of THC coupled EGS reservoir models. The integration of geochemical considerations is therefore indispensable for integrated simulations of EGS systems and predictions of its performance.