Seismic Signatures of Fractured Porous Rocks: The Partially Saturated Case

Seismic attenuation and phase velocity dispersion due to mesoscopic fluid pressure diffusion (FPD) have received increasing attention due to their inherent sensitivity to the hydromechanical properties of monosaturated fractured porous media. While FPD processes are directly affected by key macroscopic properties of fractured rocks, such as fracture density and fracture connectivity, there is, as of yet, a lack of comprehension of the associated characteristics when multiple immiscible phases saturate the probed fractured medium. In this work, we analyze the variations experienced by P and S wave attenuation and phase velocity dispersion when CO2 percolates into an initially brine-saturated fractured porous rock. We study such variations considering a simple model of a porous rock containing intersecting orthogonal fractures as well as a more complex model comprising a fracture network. In the latter, we simulate the flow of a CO2 plume into the medium using an invasion percolation procedure. Representative samples are subjected to numerical upscaling experiments, consisting of compression and shear tests, prior to and after the CO2 invasion process. Results show that fracture-to-background FPD is only sensitive to the presence of CO2, which decreases its effects. However, fracture-to-fracture FPD depends on both the overall CO2 saturation and the fluid distribution within the fracture network. While the former modulates the magnitude of the dissipation, the latter can give rise to a novel FPD process occurring between CO2-saturated and brine-saturated regions of the fracture network.