The Relation Between Fracture Aperture and Hydro-mechanical Properties: An Experimental and Analytical Approach

The focus of this study is to elucidate the relation between elastodynamic and hydraulic properties of fractured media subjected to local stress perturbations in relation to fracture aperture distribution. We describe a multi-scale analytical model that relates permeability to fracture aperture distribution in tensile-fractured Westerly granite samples. This physics-based model is used to scale-up observations of the poromechanical and seismic response of rough fractures in rocks. The model relates fracture aperture to hydraulic aperture in which hydraulic properties such as steady-state flow rate and permeability are calculated using a model derived from the parallel plate approximation. The model accounts for asperity deformation due to fracture closure using quasi-static Hertzian contact theory. Fracture roughness and aperture evolution with stress are imaged using two complementary techniques: (1) high resolution profilometry of fine-scale features and (2) pressure sensitive film at average stresses ranging from 3 to 21 MPa to monitor asperity deformation. The analytical model is constrained by and validated against experiments measuring nonlinear elastodynamic and flow response of this fracture under load. Experiments are conducted on intact then pre-fractured, samples of Westerly granite biaxially-loaded in a pressure vessel with permeability evolution measured from fluid flow-through of deionized water. Oscillations of applied normal stress and pore pressure are applied at amplitudes ranging from 0.2 to 1 MPa and frequencies 0.1 to 10 Hz. The experiments consider the influence of fracture aperture with perturbations at applied normal stresses 5 to 20 MPa. During the dynamic stressing an array of piezoelectric transducers (PZTs) continuously transmit and receive ultrasonic pulses across the fracture to monitor the evolution of elastic response. Concurrent measurements of changes in ultrasonic wave velocity and amplitude due to dynamic stressing are conducted to simultaneously measure the contact acoustic nonlinearity and permeability evolution. Future work includes laboratory experiments with fractures of different roughness and varying amounts of synthetic wear material with investigation of flow and pore-throat clogging/unclogging mechanisms in numerical simulations.