Alteration of ultrasonic signatures by stress-induced changes in hydro-mechanical properties of fractured rocks

In this study, we evaluate the changes in ultrasonic signatures (i.e., frequency content, velocity, amplitude, and attenuation) due to stress-induced alteration of fracture aperture/permeability. Flow-through experiments were performed on artificially-fractured phyllite specimens along with the concurrent measurements of ultrasonic signatures under different stress conditions. Increasing pore pressure led to fracture opening, as indicated by increases in both mechanical and hydraulic apertures. In addition, we observed that increase in confining pressure (and decrease in pore pressure) led to increases in ultrasonic velocities, ultrasonic amplitudes, and fracture specific stiffness, and decrease in ultrasonic attenuations. It was found that time-frequency partitioning depends on hydraulic aperture. The higher frequency band, for both P- and S-waves, was insensitive to the changes in stress conditions; the lower band was sensitive to the changes in stress conditions, as long as the hydraulic aperture was changing. Three-Element rheological and Power-Law models successfully predicted the time-dependent fracture displacement, with the former being more accurate at higher levels of pore pressures.