Seismic Attenuation in Partially Saturated Porous Rocks

We provide an overview on recent developments on the acoustics of partially saturated porous rocks. The focus is on the mesoscopic flow induced by seismic waves and leading to wave attenuation and dispersion. At the laboratory scale recent core plug imbibition experiments with simultaneous acquisition of X-ray CT and ultrasonic waveforms allow us to retrieve the saturation dependence of velocities and attenation. Fluid patches and their evolution at the milimetre scale are observed. To model these relations in a consistent manner we invoke the concept of fluid patch membrane stiffness. The latter accounts for the net effect of capillary forces at the macroscale. We further extract the velocity saturation relation from time-lapse sonic logs aquired during CO2 injection into a sandstone formation. It is shown that this velocity-saturation relation can be also modelled using the mesoscopic wave-induced flow effect. Simulation results give further support that mescopic fluid patches on the centimetre scale have a first-order effect on seismic amplitudes provided that the fluid bulk modulus contrast across the patch boundaries is suffiently large. This is typically the case in the presence of a gas phase. We conclude that fluid patches on the milimetre-to-centimetre scale have important implications for attenuation estimates.