Decoupling the poromechanics of particle remobilization and interface stiffness of dynamically stressed tensile fractured rock

Understanding the poromechanical response of fractured rock is necessary for applications ranging from hydraulic stimulation of the subsurface to teleseismic impulses from earthquakes that may reactivate faults or otherwise breach reservoir seals. We describe laboratory experiments that seek to decouple the role of fracture interface stiffness and fracture infill (sediment transport) in the hydraulic and elastodyamic properties of fractured rock. Experiments are conducted on multiple samples of Westerly granite with different uniform roughness (silicon carbide grit-roughened or milled) that were loaded under triaxial stresses in a pressure vessel while permeability evolution is measured from the flow-through of deionized water. In some experiments, thin layers of synthetic wear (gouge) material are added to the fracture interface to simulate mature, sheared, fractures whose poromechanical response is dominated by clogging and unclogging of pore throats. Oscillations of pore pressure and normal stress are applied at amplitudes ranging from 0.2 to 1 MPa at 1Hz. The experiments also consider the influence of fracture aperture with effective stress perturbations applied at normal stresses ranging from 5 to 20 MPa (reducing aperture with increasing effective normal stress). Before, during, and after the dynamic stressing, an array of piezoelectric transducers (PZTs) continuously transmits and receives ultrasonic pulses across the fracture to monitor the evolution of fracture stiffness and fluid transport due to the dynamic stressing. These allow evaluation of stress-induced changes in transmitted ultrasonic wave velocity and amplitude to estimate the contact acoustic nonlinearity of the fracture interface concurrent with permeability evolution. We compare the results for samples with and without synthetic wear (gouge) material to understand the role and evolution of fracture stiffness and clogging-unclogging mechanisms in pore throats of porous and fractured media.