A laboratory study of the dependence of elastic wave velocities on pore scale fluid distribution

Laboratory data have been collected during a continuous imbibition/drainage experiment that show a clear dependence of elastic wave velocities on the details of the pore scale distribution of water and air in a sandstone. Compressional wave velocity (Vp) was measured at a frequency of 1 MHz; shear wave velocity (Vs) was measured at a frequency of 600 kHz. During the experiment, Vp showed little variation with the level of water saturation (Sw) during increasing Sw through imbibition until Sw = 0.80, at which point Vp increased dramatically. When Sw was decreased, pronounced saturation-induced hysteresis was observed in the region 03 0.4. As a simple model, we consider the imbibition process as producing a partially saturated state in all pores; i.e. all pores contain both air and water. The drainage process, in contrast, favors the existence of either air-filled or water-filled pores. As elastic wave velocities are very sensitive to the saturation state in the smaller, “crack-like” pores, these variations in fluid distribution cause related variations in velocities.

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