Permeability partitioning through the brittle-to-ductile transition and its implications for supercritical geothermal reservoirs
Gabriel Meyer, Ghassan Shahin, B. Cordonnier, Marie Violay
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. In this study, we reveal that permeability in experimentally deformed ductile granite increases by more than an order of magnitude. We demonstrate that permeability varies spatially and that the distribution of strain across the brittle-ductile transition leads to a corresponding partitioning of permeability.