[HSC07-P01] On the relationship between Vp and CO2 saturation in the logging data at the Nagaoka site
Keywords:Geological CO2 storage, Sonic logging, P-wave velocity, Histeresis, Nagaoka site
The monitoring data at a down-dip direction showed that CO2 saturation increased at the injection period, became maximum one year after the end of the injection, and was gradually approaching at a certain value. The latest data is thought to be the residual saturation state. Vp change in the sonic logging data represented that the Vp decreased uniformly during the drainage process but the trajectories during the imbibition process have facies dependencies. This facies dependency means that there is hysteresis in the relationship between Vp and CO2 saturation, the hysteresis is stronger in the silty stone than the sandy stone at the Nagaoka site.
Physical meanings of these behaviors were considered. The relationship at the drainage process is thought to be modified patchy saturation as Azuma et al., (2013) pointed out. However, during the imbibition process, it became more uniform relationship. The reason of this difference is thought to be come from that injected CO2 would be invading from the large pore space, while the residual CO2 would exist only in small pore space, which is thought to be uniformly spread out than large pore space. This supports that the silty layer worked like a uniform medium at the imbibition process.
The results of these analyses indicate that the evaluation of CO2 saturation from seismic methods needs to be more careful at the post-injection period.