The aim of this study is to assess the geothermal potential of a reservoir in the Limagne area (Massif Central, France). Located in the Clermont-Ferrand basin, the Limagne area is a Tertiary graben belonging to West European rift system characterized by a geothermal anomaly with Tertiary clastic reservoirs. Measurements from old exploration geothermal wells show about 90°C at 1.5km depth.
The first step is dedicated to better understand the geometry of geology of the target zone. Various data, such as field measurements, drill holes and seismic profiles were collected and independently interpreted over the last 25 years. These data are reinterpreted through the construction of a 3D geological model. Inconsistencies are checked and turned into a coherent 3D interpretation by using the 3DGeoModeller software developed by BRGM. The new interpretation consists in a 35km x 30km x 3km 3D model extending from the basement to the surface of the basin.
The result shows the basement geometry and the sedimentary deposition. The later is made of four sedimentary sequences; each one is formed of detrital formations at the bottom, followed by alternating layers of detrital and carbonated sediments, and formations of marls and carbonates at the top of the sequence. The whole domain is crossed by a network of normal faults affecting both the basement and the sedimentary formations.
The second step consists in investigating the geothermal significance of the area. The geological model is meshed into a 3D unstructured Finite Element mesh using meshing capabilities from GEOWATT AG. A diffusive temperature model is applied and temperature field is computed using the Finite Element code FRACTure. Thermal conductivity of the various geological units were measured in the lab and corrected for porosity effects. Thermal boundary conditions such as surface temperature and surface heat flow are derived from available data. Results of the model show good agreement with thermal data in deepest part of reservoir. Above that level, temperature profiles recorded in wells are strongly deformed. This deformation may be due to local water circulations or because the thermal equilibrium was not released at the time of measurements.
As a final result, the total amount of geothermal energy available for each aquifer is computed, and the geothermal potential (recoverable energy) is mapped over the entire model. The most interesting geothermal target is identified (S1-DET, sandstone); the total amount of recoverable energy in this aquifer is estimated to more than 500 PJ in the modelled area.
This work is supported by ADEME (French Agency for Energy and Environment) and benefits from the ENGINE European coordination action (Enhanced Geothermal Innovative Network for Europe).
