Effect of Fluid Saturation on Acoustic Wave Velocity for Sandstone Reservoirs

Abstract

Compressional and shear velocities were measured at room temperature and ambient pressure on cylindrical samples. The p-wave and s-wave velocities have been measured at ultrasonic frequencies of 63 kHz and 33 kHz, respectively. Compressional and shear waves have different behaviour in rocks depending on difference in porosity, fluid saturation, fluid viscosity, rock density, lithologic laminations, fracturing, clay content, mineralogy, compaction and pore space framework. The velocity ratio of compressional and shear waves (Vp/Vs) varies in crystalline and metamorphic rocks within a very narrow range (from 1.7 to 1.9). In sedimentary rocks, it varies in a wider range from 1.5 to 14.0 due to the very low shear strength of highly porous rocks (Φ > 25%). The technique used to measure acoustic wave velocity is the pulse first arrival technique, in which the travel times are determined for a pulse of compressional and/or shear waves to pass a known measured thickness of the rock (sample length). P-wave and s-wave velocities values (at different saturation) have been determined on a subset of 67 sandstone core samples. Only 26 sandstone samples are belonging to the upper Cretaceous in age (Bahariya Fm, Egypt) and the rest of samples belongs to the Miocene, Lower Pliocene and Upper Pliocene in age (Endrod gas field, Hungary). The p-wave and s-wave velocities were measured with the sample fully saturated with air (dry, Sw = 0) and partially saturated with brine water (Sw = 25%, 50% and 75%) and fully saturated with brine water (Sw = 100%). The effect of sandstone sample water saturation either partial or full water saturation on acoustic parameters were investigated. Some empirical approaches in obtained data analysis are developed based on both Wyllie and Raymer equations in order to predict seismic velocities (Vp and Vs) and Poisson’s ratio for either dry or saturated sandstones.