Permeability partitioning through the brittle-to-ductile transition and its implications for supercritical geothermal reservoirs

Abstract

Geothermal projects utilizing supercritical water (≥400 °C) could boost power output tenfold compared to conventional plants. However, these reservoirs commonly occur in crustal areas where rocks are semi-ductile or ductile, impeding large-scale fractures and cracking, and where hydraulic properties are largely unknown. Here, we explore the complex permeability of rocks under supercritical conditions using mechanical data from a gas-based triaxial apparatus, high-resolution synchrotron post-mortem 3D imagery, and finite element modeling. We report a first order control of strain partitioning on permeability. In the brittle regime, strain localizes on permeable faults without necessarily increasing sample apparent permeability. In the semi-ductile regime, distributed strain increases permeability both in deformation bands and the bulk, leading to a more than tenfold permeability increase. This study challenges the belief that the brittle-ductile transition (BDT) marks a cutoff for fluid circulation in the crust, demonstrating that permeability can develop in deforming semi-ductile rocks.