9:00 AM - 9:15 AM
[SEM12-01] Universality and diversity of resistivity-permeability relationship of faulted rocks
★Invited Papers
Keywords:Permeability, Resistivity, Fault, Fracture density, Digital Rock Physics, Anisotropy
The subsurface resistivity structure is a good indicator to evaluate the crustal fluid and its transport behaviors. Recent advances in electromagnetic observations have detected changes in electrical resistivity due to crustal stress changes, and possibly predicted the changes in subsurface fluid flow. The heterogeneity in subsurface permeability considerably impacts geothermal and seismic activities; however, in-situ fluid flow is difficult to estimate. It would be beneficial if we could estimate subsurface permeability using resistivity that can be remotely monitored. There have been proposed permeability models estimated based on resistivity for porous media; however, such permeability estimation models have rarely been studied for fractured rocks. Faults are ubiquitous in the shallow subsurface and dominate subsurface heat and mass transport due to their high permeability. The paucity of a rock physics model for fractured rocks has prevented quantitative interpretation of the observed data, despite technological advances in electromagnetic exploration. Therefore, this study performed numerical simulations of various fracture models with varying fracture surface roughness, shear displacement, degree of chemical erosion, and matrix porosity to investigate universality and diversity in the changes in permeability and resistivity as elevated stress.
Digital rock fractures with controlled roughness and shear displacement were synthesized based on the natural rough surfaces of granitic fractures (Aji granite and Inada granite). For each fracture model, normal loading was applied up to 100 MPa using a half-space-based dry contact model. Based on deformed rough surfacers, fracture flow, and electrical resistivity were simulated by using the lattice Boltzmann method and finite element method, respectively. Each simulation input was determined based on laboratory experiments.
The numerical results show that changes in fracture permeability and electrical resistivity at elevated normal stress were affected by both roughness and shear displacement. The permeability anisotropy in the direction parallel/perpendicular to the shear displacement is also significant at higher stress conditions. On the other hand, the relationships between permeability-resistivity show less dependence on roughness, shear displacement, and size of fractures. The log-log plot of the permeability-resistivity shows inflection, where the percolation of the flow path occurred. The trend of the relationship could be correlated with Archie’s cementation factor based on the equivalent channel model. These results suggest that the permeability evolution of faults can be formulated with changes in resistivity regardless of fracture seize and its microstructures. However, both matrix porosity and fracture spacing alter this relationship, changing the sensitivity of electrical observation to permeability changes.
Digital rock fractures with controlled roughness and shear displacement were synthesized based on the natural rough surfaces of granitic fractures (Aji granite and Inada granite). For each fracture model, normal loading was applied up to 100 MPa using a half-space-based dry contact model. Based on deformed rough surfacers, fracture flow, and electrical resistivity were simulated by using the lattice Boltzmann method and finite element method, respectively. Each simulation input was determined based on laboratory experiments.
The numerical results show that changes in fracture permeability and electrical resistivity at elevated normal stress were affected by both roughness and shear displacement. The permeability anisotropy in the direction parallel/perpendicular to the shear displacement is also significant at higher stress conditions. On the other hand, the relationships between permeability-resistivity show less dependence on roughness, shear displacement, and size of fractures. The log-log plot of the permeability-resistivity shows inflection, where the percolation of the flow path occurred. The trend of the relationship could be correlated with Archie’s cementation factor based on the equivalent channel model. These results suggest that the permeability evolution of faults can be formulated with changes in resistivity regardless of fracture seize and its microstructures. However, both matrix porosity and fracture spacing alter this relationship, changing the sensitivity of electrical observation to permeability changes.