The potential of Vs and Vp-Vs relation for the monitoring of the change of CO2-saturation in porous sandstone

In this study, we conduct a simultaneous measurement of Vp and Vs of porous sandstone during drainage and imbibition to monitor the change of CO2 saturation (SCO2) in porous sandstone under in situ pressure and temperature conditions. We estimate the SCO2 from the change in Vs. The relationship between measured Vp and SCO2 indicates two interesting results. First, we can find that Vp does not recover to pre-drainage levels after the end of the imbibition process. This unrecovered Vp is considered to be the effect of residual trapped CO2. Second, there is a hysteresis between drainage and imbibition. Finally, we investigate the Vp-Vs relation and find clear differences between water-saturation and irreducible water-saturation. From the Vp-Vs relation, we also identify the un-recovered Vp-changes between drainage and imbibition. These results indicate the high-potential of monitoring the change of SCO2 from Vp and Vs.