Rock Physical Modeling of Sheared Fractures: Permeability-Resistivity-Seismic Velocity Relationship Explored via Digital Rock Physics Approach

Abstract

ABSTRACT 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, we could not quantitatively interpret these geophysical monitoring data due to the absence 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 surface roughness and shear displacement. The results show that permeability-resistivity and permeability-velocity relationships are less dependent on any geometric characteristics, but controlled by the percolation based on microscopic flow analysis. Our finding suggests that Overall, resistivity is more sensitive to less connected flow regime, while seismic velocities are more sensitive to connected flow regime. INTRODUCTION The fluid-flow characteristics of fractured geological formations are of critical interest in a number of areas including fluid resources (e.g., geothermal fluids, shale oil, and groundwater), geological storage or disposal, and seismic hazard assessment (fault reactivation, slow slip, and induced seismicity). Numerous studies have revealed that fluid-flow properties are constrained by fracture surface topography, which is also altered by shear displacement and stress (e.g., Chen et al., 2017; Durham and Bonner, 1994; Durham, 1997; Ishibashi et al., 2015; Pyrak-Nolte and Morris, 2000; Watanabe et al., 2008 Witherspoon et al., 1980; Zimmerman et al., 1992). Insitu stress is never constant during geoengineering developments or on the geological time scale, and consequently the aperture distribution and associated hydraulic properties also must change in natural settings. Although direct measurement of permeability changes is challenging, geophysical observations are potential approaches to evaluate the permeability change. Many studies have reported changes in seismic velocities or electrical resistivity that may reflect subsurface stress changes associated with hydraulic stimulation, earthquakes or geothermal fluid production (e.g., Brenguier et al., 2008; Didana et al., 2017; Mazzella and Morrison, 1974; Nimiya et al., 2017; Park, 1991; Sánchez-Pastor et al., 2019Taira et al., 2018; Tsuji et al., 2021; Johnson et al., 2021). It would be advantageous if changes in permeability caused by subsurface stress changes could be linked to these geophysical properties that could be remotely monitored.