Effect of CO2 on P- and S-wave velocities at seismic and ultrasonic frequencies

Time-lapse seismic is a widely used technology for monitoring the geological sequestration of carbon dioxide (CO2), consisting of mapping its movement in the subsurface and of demonstrating that the CO2 is safely stored in the reservoir (Xue and Ohsumi, 2004). In this work, the effect of CO2 on P- and S-wave velocities was investigated. Laboratory measurements were performed with Castlegate sandstone both at seismic frequencies (1-155 Hz), and at ultrasonic frequency (around 500 kHz). Different CO2 saturations between 2% and 10% were obtained by controlled depressurization of CO2-saturated water with which the sandstone sample had been saturated with. At seismic frequencies, the results of the experiments revealed that P-wave velocity is strongly reduced in the presence of free gas CO2 in the pore space, whereas at ultrasonic frequency, the P-wave velocity changed only slightly. Therefore, the presence of free CO2 gas increased significantly the P-wave dispersion between seismic and ultrasonic frequencies. The S-wave velocity, on the other hand, was hardly affected by the pore fluid at seismic frequencies. At seismic frequencies, P- and S-wave velocities were consistent with the Biot-Gassmann model. The P-wave velocity dispersion and corresponding attenuation were simulated by applying the Cole-Cole model. The transition frequency was found around 200 kHz.