Electrical resistivity, permeability, and normal stiffness of fractured crystalline rocks: Simultaneous laboratory measurements subjected to hydromechanical loading

Abstract

This study presents the first simultaneous measurements of the hydraulic, electrical, and elastic properties of fractured crystalline rocks under uniaxial loading. To elucidate the fluid-related transport in hydrothermal systems based on geophysical measurements, the correlation between permeability and measurable geophysical properties should be compre-hensively understood. However, no study has investigated detailed simultaneous changes in the permeability and electrical resistivity of rough-walled fractures under hydromechanical deformation. Herein, this study simultaneously measured these properties in fractured granite and gabbro samples through laboratory experiments. During hydromechanical loading, normal stress was increased up to 50 MPa while a KCl solution was injected into void space. Resistivity was estimated based on electrical impedance and compared with permeability and fracture-specific stiffness changes under elevated stress. Consequently, increasing stress increased resistivity and stiffness for ∼ 1 decade while decreased permeability for ∼ 3 decades. The resistivity increase was lower in granite samples than that in the gabbro sample owing to lower matrix resistivity. Based on the measured matrix properties, this study estimated the electric aperture and hydraulic aperture. The ratio of electric aperture to hydraulic aperture decreased with increasing stress, reaching 0.3-0.4 at stress of > 15 MPa. These results suggest that the permeability-resistivity relationship is nonlinear and matrix effect can be insufficient at higher stress such as geothermal reservoirs. Meanwhile, the permeability-stiffness relationship might be controlled primarily by the rock type, while the resistivity-stiffness curves were regulated by the fracture surface state. These different sensitivities suggest that simultaneous measurements of the electrical and elastic properties aid in interpreting permeability changes.