Acoustic measures of partial gas saturation in tight sandstones

The acoustic properties of tight sandstones are very sensitive to partial gas/water saturation. Measurements at 5 kHz show that compressional velocity drops sharply with undersaturation. Extensional and shear losses at 5 kHz are large in both fully and partially saturated sands. The shear loss Q s −1 declines linearly with increasing gas content. The extensional loss Q e −1 peaks at 0.80 water saturation, Q −1 is frequency dependent. The attenuation is well explained by hydrodynamic relaxation in the grain contacts. The measurements indicate velocity dispersions consistent with this mechanism. Ultrasonic velocities at 500 kHz are roughly 10–25% higher than acoustic velocities at 5 kHz. The ultrasonic velocities exhibit a weak dependence on saturation. Frame moduli are unrelaxed at these frequencies.