Dynamic stressing of naturally fractured rocks. On the relation between transient changes in permeability and elastic wave velocity

We report on laboratory experiments designed to explore the fundamental processes that impact fluid permeability during drilling and fluid injection. Coupled ultrasonic and permeability measurements are presented for Westerly granite samples fractured in situ and used to illuminate the relationship between nonlinear dynamic stiffness and fracture permeability indexed via fracture aperture. We perturb the effective stress field using normal stress and pore pressure oscillations. The velocity of ultrasonic waves transmitted across the fracture is used to infer the evolution of elastic properties and the fracture nonlinearity. Changes in permeability are measured concurrently. We observe that relative changes in wave velocity and permeability, due to both normal stress and pore pressure oscillations, are correlated, such that larger drops in wave velocity (higher nonlinearity) correspond to larger increases in permeability. Our observations suggest that dynamic stressing is more likely to enhance the permeability of fractures that exhibit greater nonlinearity.