Crustal Fault Zones (CFZ) as Geothermal Power Systems: 3D Variation of Permeability and Related Processes

Abstract

The Pontgibaud Crustal Fault Zone (CFZ) in the French Massif Central provides an opportunity to evaluate the high-temperature geothermal potential of these naturally permeable zones. Previous 2D modeling of heat and mass transfer in a fault zone highlighted that a subvertical CFZ concentrates the highest temperature anomalies at shallow depths. By comparing the results of these large-scale 2D numerical models with field data, the depth of the 150°C isotherm was estimated to be at a depth of 2.5 km. However, these results did not consider 3D effects and interactions between fluids, deformation and temperature. A combined multidisciplinary approach makes it possible to better constrain the variation of permeability in 3D. This variation is then integrated into 3D numerical modeling. Based on a thermo-poroelastic assumption, a preliminary 3D THM numerical model is presented. A first parametric study highlights the role of permeability, stress direction and stress intensity on fluid flow. In particular, three different convective patterns have been identified: finger-like, blob-like and double-like convective patterns. Finally, a large-scale 3D numerical model of the Pontgibaud CFZ, based on THM coupling and the comparison with field data (temperature, heat flux, electrical resistivity), allows us to observe the convective pattern at the origin of the rise of the 150°C isotherm to economically exploitable depths. This result is obtained for values of permeability, intensity and direction of stresses comparable to the parametric study. Although simplified hypotheses have been used, 3D field data have been reproduced. Finally, we tried to understand the observed effects of poroelasticity on fluid flow.