Numerical modeling and laboratory measurements of seismic attenuation in partially saturated rock

To better understand the effects of fluid saturation on seismic attenuation, we combined numerical modeling in poroelastic media and laboratory measurements of seismic attenuation in partially saturated Berea sandstone samples. Although in laboratory experiments many physical mechanisms for seismic attenuation take place simultaneously, with numerical modeling we separately studied the effect of a single physical mechanism: wave-induced fluid flow on the mesoscopic scale. Using the finite-element method, we solved Biot’s equations of consolidation by performing a quasistatic creep test on a 3D poroelastic model. This model represents a partially saturated rock sample. We obtained the stress-strain relation, from which we calculated frequency-dependent attenuation. In the laboratory, we measured attenuation in extensional mode for dry and partially water-saturated Berea sandstone samples in the frequency range from 0.1 to 100 Hz. All the measurements were performed at room pressure and temperature conditions. From numerical simulations, we found that attenuation varies significantly with fluid distribution within the model. In addition to binary distributions, we used spatially continuous distributions of fluid saturation for the numerical models. Such continuous saturation distribution was implemented using properties of an effective single-phase fluid. By taking into account the matrix anelasticity, we found that wave-induced fluid flow on the mesoscopic scale due to a continuous distribution of fluid saturation can reproduce seismic attenuation data measured in a partially saturated sample. The matrix anelasticity was the attenuation measured in the room-condition dry sample.