Hydraulic and mechanical behaviors of the geothermal reservoirs or the seismic faults are strongly controlled by the characteristics of rock fractures. To monitor and predict the hydraulic-mechanical coupling within the crust, geophysical explorations potentially are the powerful tools. However, there is few established rock physical model to link the hydraulic properties of fracture to the resistivity or elastic wave velocity. For our better interpretation of the exploration data, detailed investigation linking hydraulic properties to the mechanical/electric properties for the fractured rocks is required. Therefore, we explore the link by coupling the laboratory experiments and digital rock modeling on the fractures with different aperture distributions. We conduct the fluid-flow experiments and the numerical modeling on granite fractures. In our modeling, we first digitalized the real granite fractures by 0.1 mm grid system. Then, under the same condition with experiments, we calculate the fluid flow (Lattice Boltzmann Method) and resistivity/elastic wave velocity (finite-element method). Laboratory experiments show that fracture permeability decreases with increasing pressure, and this relationship could be reproduced in our modeling study. We further determine the aperture distributions based on the permeability matching approach. As a result, we successfully constrain the variation of permeability, resistivity and elastic wave velocity as well as fracture stiffness of the rock fracture against the pressure build-up; changes of permeability and resistivity are controlled by connection or disconnection of fluid-flow pathway whereas velocity and fracture stiffness are not. Our results suggest that the evolutions of permeability and flow area associated with aperture closure of fracture can be modeled by the changes of resistivity or fracture stiffness regardless of the roughness of the fracture.

1. Introduction

Mechanical properties of fractured geological formations and fluid-flow in that are of interest in a number of contexts such as 1) developing and monitoring fractured reservoir (e.g., geothermal, shale and groundwater) and 2) elucidating the mechanism of earthquake (e.g., fault-valve model; Sibson, 1992). Although permeability is often discussed for evaluating the potential of reservoir exploration or reoccurrence of the earthquake triggered by pore pressure build-up, local behavior of fluid-flow (e.g., fluid-flow pathway) within fractures is also important because it controls preferential-flow and total thermal response in geothermal area (e.g., Hawkins et al., 2018). To monitor and predict these hydraulic properties and hydraulic-mechanical coupling within the crust, geophysical explorations potentially are the powerful tools. In geothermal fields, the change of resistivity or velocity associated with the hydraulic stimulation, earthquake and geothermal fluid production was detected (e.g., Peacock et al., 2012; Taira et al., 2018). Although these geophysical monitorings could detect the change of reservoir condition, quantitative interpretations about the injected water distribution, permeability enhancement associated with aperture changes of fracture have not been evaluated yet. To monitor these fluid-flow behaviors from the geophysical explorations, we should investigate the basic relationships between the hydraulic (permeability and fluid-flow pathway), electric (resistivity) and mechanical (elastic wave velocity and fracture stiffness) properties of rocks.

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