An experimental investigation of the coupling between elastodynamic and hydraulic properties of naturally fractured rock at the laboratory scale

We report on a series of laboratory experiments designed to simulate local effective stress field fluctuation and its influence on the evolution of permeability and dynamic stiffness in fractured samples of Westerly Granite. L-shaped samples are loaded with tri-axial stresses and fractured in situ. The fracture is subsequently sheared in two 4-mm steps. Oscillatory changes in the local effective stress field are imposed through application of normal stress or pore water pressure oscillations with varying amplitudes and frequencies. Active ultrasonic data (ultrasonic waves transmitted across the fracture) is used to monitor the evolution of wave velocity and attenuation before, during and after dynamic stressing. Throughout the experiment, the evolution of permeability is concurrently measured to determine the relationship between fracture permeability and nonlinear elastodynamic properties (stress-dependency of wave velocity and attenuation). Our results to date indicate 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 correspond to larger increases in permeability. Shearing of the fracture reduces the nonlinearity measured during normal stress oscillations for both rock samples. After shearing, the oscillations become generally less effective in enhancing the fracture permeability.