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.