Poromechanics of Particle Remobilization and Interface Stiffness of Dynamically Stressed Fractured Rock

Abstract

A range of natural (teleseismic waves) and industrial sources (hydraulic stimulation) of stress perturbations in the subsurface can alter the poromechanical response of fractured rock which may reactivate faults or otherwise breach reservoir seals. We describe laboratory experiments that seek to illuminate the physical processes that dictate these effects. Our experiments focus on the roles of fracture interface stiffness and fracture infill (sediment, proppant or fault gouge) and how they modulate the hydraulic and elastodyamic properties of fractured rock. Experiments were conducted on Westerly granite using flat surfaces with uniform roughness subject to triaxial stresses while monitoring fluid permeability evolution using deionized water. Thin layers of quartz powder within fractures were used to study the role of infill and the poromechanical response of clogging and unclogging of pore throats. Pore pressure oscillations of amplitude 0.2–1 MPa and frequency of 1 Hz were used to simulate subsurface dynamic stresses. Before, during, and after the oscillations we monitored fracture elastic properties using piezoelectric transducers (PZTs). We measured stress‐induced changes in transmitted ultrasonic wave velocity and amplitude to estimate the nonlinear elasticity of the fracture concurrent with permeability evolution. We find that increasing gouge layer thickness up to fracture roughness results in enhanced permeability and a greater nonlinearity in the elastodynamic response. Thicker layers, comparable to or larger than the fracture roughness, result in reduced permeability enhancement and elastodynamic response. Our results offer insights for techniques to monitor the evolution of subsurface fractures during energy production and dynamic stressing via distant earthquakes.