Influence of Fracture Roughness on the Elastodynamic and Hydraulic Properties of Fractured Rock

Understanding the hydromechanical behavior of fractures in rock has important implications in engineered geothermal systems and unconventional reservoirs which involve fluid injection and hydraulic stimulation for permeability enhancement. We investigate the influence of fracture roughness on the coupling between the elastodynamic fracture properties and permeability evolution of fractures. The aperture size distribution and profile roughness impact both fracture stiffness and permeability. We report on a series of controlled laboratory experiments elucidating the relationship between dynamic stiffness and fracture permeability evolution of intact, pre-fractured and saw-cut Westerly Granite samples with known aperture distribution and roughness. The roughness across two fractured surfaces is captured using optical profilometry. In addition, pressure sensitive films are used to estimate the true contact area and stress distribution at different levels of normal stress. After loading the samples to a certain normal stress, we dynamically perturb the applied stress field to measure fracture dynamic stiffness and permeability enhancement. The effective stress field is perturbed by applying normal stress and pore pressure oscillations with amplitudes varying between ~0.2 and ~1 MPa at frequencies in the range 0.1 up to 250 Hz. The dynamic stiffness of the fracture is estimated based on the velocity and amplitude of ultrasonic waves transmitted across the rough fracture. Concurrently, the evolution of fracture permeability due to dynamic stressing is also measured. The fracture is subsequently sheared with simultaneous ultrasonic and permeability data collected during shearing at various shear-displacement offsets. Finally, laser profilometry is conducted post-mortem on the samples to reconstruct post-shear fracture aperture distribution. This work quantifies the effect of fracture roughness on the seismic, hydraulic and frictional properties of fractured rocks.