Simultaneous changes in electrical resistivity and elastic wave velocity with fracture permeability evolution of synthetic faults

Fluid flow through faults controls subsurface transport processes, considerably impacting seismic hazards and geoengineering such as enhanced geothermal systems (EGS). Recent advances in geophysical techniques have detected changes in electrical resistivity and seismic velocity due to crustal stress changes, and possibly predicted the changes in subsurface fluid flow. However, these geophysical monitoring data could not be quantitatively interpreted due to the paucity of rock physical models for fractured rocks. This study investigated changes in permeability, resistivity, elastic wave velocity, and their respective relationships at elevated normal stress by performing numerical simulations of various fracture models with changing fracture's surface roughness and shear displacement.

We synthesized digital rock fractures with controlled roughness and shear displacement based on the natural rough surfaces of granitic fractures. For each fracture model, we numerically applied normal loading up to 100 MPa by a half-space-based dry contact model. Based on deformed rough surfacers, we simulated fracture flow by the lattice Boltzmann method and calculated geophysical properties by the finite element method. Each simulation input was determined based on laboratory experiments.

As a result, changes in fracture permeability, electrical resistivity, and elastic wave velocity at elevated normal stress were affected by both roughness and shear displacement. On the other hand, the relationships between permeability-resistivity and permeability-velocity show less dependence on roughness and shear displacement. The log-log plot of the permeability-resistivity shows inflection at the fracture permeability of ~10-11 m2, where the permeability-velocity trend is almost saturated. The microscopic flow analysis also supports this concordance. Although the threshold stress of these inflection points (i.e., percolation threshold) differs with fractal characteristics, both percolations occur at a similar fraction of the contact area (~20%). These results suggest that the permeability evolution of faults can be correlated with changes in resistivity and velocity, of which the fracture contact state possibly controls trends.