3D basin-scale groundwater flow modeling as a tool for geothermal prospection of the Geneva Basin, Switzerland-France

Switzerland supports the energetic transition by promoting the development of geothermal energy among other renewable energies. In particular, the Canton of Geneva is actively prospecting the Geneva Basin, generating a large dataset of geophysical and geological information. This large dataset of the Geneva Basin is used here to constrain geologically complex numerical models of fluid flow. Previous and ongoing projects demonstrated the geothermal potential of the Geneva Basin but a consistent basin-scale fluid flow model of the area has yet to be defined.

We use MRST (Matlab Reservoir Simulation Toolbox) for which we recently developed a geothermal module. The module is available with the last MRST release (2019b) and it is used to build up a 3D basin-scale dynamic model of the Geneva Basin. The goal of our numerical study is to investigate the large-scale control of tectonic structures and lithological hetherogeneities on fluid flow in the basin.

The static model is derived from active seismic and gravity inversion data. Petrophysical data and geo-location of faults are obtained from the existing literature. The resulting heterogeneous model takes into account the main geological facies, observed in the basin. We define a reference simulation with standard initial conditions (geothermal gradient and hydrostatic pressure topographically corrected) and a basal incoming heat flux. We consider a single-phase pure water compressible laminar flow in porous media. The geothermal module solves the mass and energy conservation equations using a fully implicit finite-volume discretisation with two-point flux approximation and single-point upstream mobility weighting.

We design a parametric study along three main axis: tectonic structures (i.e. faults), petrophysical and thermal properties and perform twenty three simulations running for 500 000 years to reach an equilibrium flow (steady-state). Our results show that fluid flow is driven by the hydraulic head of the topographic highs bounding the basin. Hotter fluids are found in the centre of the basin where we propose to focus geothermal exploitation in the future. Our results represent, to our knowledge, the first example of 3D basin-scale fluid flow modelling used as a preliminary prospection method for the assessment of geothermal resources.