Geothermal modelling in fault zones with the CIMLIB/EXALIB library

Energy transition requires the use of low-carbon energies such as geothermal energy for the production of electricity or heat. Geothermal exploitation has a number of preferential targets, including fault zones in the context of graben. High temperatures can, indeed, be observed where fluids rise through fault zones, but geothermal processes are complex to understand and to model in such a 3D tectonic context. For instance, seismic observations and then observations at well-scale show structures on different spatial scales that can overlap and interconnect. These structures then present strong heterogeneities in physical properties (e.g. fault core or damage zones). In addition, this knowledge evolves over time, from the exploration to drilling and exploitation phases. One of the challenges of numerical modelling is to represent this complexity while being readily upgradeable in the light of exploration. We are developing an adaptive approach using the CIMLIB/EXALIB library. Geometrical complexity and physical properties are defined by distance functions (level set functions) to geologic objects that are inherited from a geologic modelling software. Coupled fluid flow and heat transport processes are then modelled in 3D with adaptive meshing. The mesh can, indeed, be adapted according to static criteria such as geometry or dynamic criteria such as physical processes. This approach will be illustrated by examples derived from an ongoing GIS-Geodenergies project in the Upper Rhine Graben.