Imaging Elastodynamic and Hydraulic Properties of In Situ Fractured Rock: An Experimental Investigation Exploring Effects of Dynamic Stressing and Shearing

We describe laboratory experiments to elucidate the relationship between nonlinear elasticity and permeability evolution in fractured media subjected to local stress perturbations. This study is part of an effort to measure fluid pathways and fracture properties using active-source acoustic monitoring during fluid injection and shear of rough fractures. Experiments were conducted with L-shaped samples of Westerly granite fractured in situ under triaxial conditions with deionized water subsequently circulated through the resulting fractures. After in situ fracturing, we separately imposed oscillations of the applied normal stress and pore pressure with amplitudes ranging from 0.2 to 1 MPa and frequencies from 0.1 to 40 Hz. In response to normal stress and pore pressure oscillations, fractured Westerly granite samples exhibit characteristic transient softening, acoustic velocity fluctuations, and slow recovery, together with permeability enhancement or decay, informing us about the coupled nonlinear elastodynamic and poromechanical rock properties. Fracture interface properties (contact asperity stiffness, aperture) are then altered in situ by shearing, which generally decreases the measured elastic nonlinearity and permeability change for both normal stress and pore pressure oscillations. Plain Language Summary We conducted laboratory experiments to better understand the properties of fractures subjected to dynamic stresses. This is part of a larger effort to image how fluids flow through fractures and other fracture properties using continuous ultrasound monitoring during fluid injection and shearing rough fractures. During experiments samples of granite were fractured inside our apparatus capable of applying stresses found in the shallow subsurface. Water was circulated through the resulting fractures allowing us to measure the ability for fluid to flow through. After fracturing, we separately imposed oscillations of horizontal stress and fluid pressure (inside the fracture) with different amplitudes and frequencies. During these oscillations an array of ultrasound sensors continuously monitored the fracture, which we use to analyze the resulting evolution of elastic properties of the fractures. The fracture roughness (among other properties) is then altered during the experiment by shearing, which generates wear products at the interface of the fracture. We document the evolution of fluid flow and changes in sound wave speed (from ultrasonic monitoring) as a function of dynamic stressing and shear offset.