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

Abstract

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 (V p ) was measured at a frequency of 1 MHz; shear wave velocity (Vs) was measured at a frequency of 600 kHz. During the experiment, V p showed little variation with the level of water saturation (Sw) during increasing S w through imbibition until S w = 0.80, at which point V p increased dramatically. When S w was decreased, pronounced saturation‐induced hysteresis was observed in the region 03 < S w < 0.9, with V p measured during drainage greater than V p measured during imbibition. Similar results were obtained for V s versus S w , with V s during drainage greater that V s during imbibition in the saturation range S w > 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.