Experimental insights into coupled hydraulic, mechanical, and electrical behaviors of granite fractures: Implications for indirect estimation of crustal permeability changes

Abstract

To indirectly ascertain the coupled hydraulic and mechanical behaviors within subsurface rock fracture networks, it is imperative to establish principles linking permeability, geophysical exploration data (such as electrical conductivity and elastic wave velocity), and internal void structure. To enhance our foundational understanding of these aspects, we conducted an experimental investigation into the hydraulic-mechanical-electric coupled behaviors of granite fractures exhibiting various degrees of surface roughness. The study involved two cases: varying the external pressure (i.e., confining pressure) under a constant flow rate, and varying the pore pressure and associated flow rate under a constant external pressure. Laboratory experiments yielded the following key insights: (1) Both the permeability and electrical conductivity of granite fractures exhibited nonlinear reductions with increasing effective stress, followed by increments upon decreasing effective stress. Notably, we observed hysteresis in both parameters during loading and unloading phases. (2) Fractures with rougher surfaces demonstrated increased impedance to fluid and electrical flow. Particularly in instances of highly rough surface fractures, subtle variations in the pore structure resulted in notable discrepancies in the trends of permeability and electrical conductivity alterations. (3) The ratio of hydraulic aperture to electrical aperture was quantified as approximately 0.11 for saw-cut fractures roughened with silicon carbide, while it ranged between 0.18 and 0.37 for tensile mode fractures. Based on these results, we present an indirect estimation method for crustal permeability changes in fractured rocks based on 3-D time-lapse ERT imaging results. According to this method, it is estimated that in the observation period covered by Johnson et al. (2021), crustal permeability at the EGS Collab site may increase by a maximum of 2.1–3.8 times due to the pressure-induced aperture dilation of pre-existing natural fractures, while compressive shadow stress may reduce the crustal permeability by a factor of 0.3–0.5 times the original value.