Ultrasonic velocity and attenuation during CO2 injection into water-saturated porous sandstone: Measurements using difference seismic tomography

We undertook laboratory-based seismic measurements with dense sensor array at ultrasonic frequencies during the injection of CO2 into a water-saturated sandstone specimen. The resulting high-quality seismic data enabled detailed determination of the relative velocity and attenuation coefficient of the compressional wave using difference seismic tomography, which directly inverses time-lapse changes in rock properties from time-lapse changes in observed data. CO2 migration and water displacement were clearly mapped using tomographic images of relative velocity and the attenuation coefficient. The final and largely stabilised volume fraction of CO2 in the pore space of the sample is about 30-40%. On average, the P-velocity fell by 7.5, 12, and 14.5% and the attenuation coefficient Q−1 increased by factors of 3.3, 2.7, and 3.7 as a result of the replacement of water with CO2 during the injection of gaseous, liquid, and supercritical CO2, respectively. As a function of gas saturation, both the velocity and attenuation data are in good agreement with results obtained using the White and Dutta-Odé model for partial saturation, indicating that viscous losses due to fluid diffusion are of significant importance for compressional waves travelling at ultrasonic frequencies in porous rocks.