We have to monitor underground CO2 distribution and saturation in CO2 geological sequestration. In monitoring techniques, geophysical surveys play an important role, however appropriate interpretation is required. When we interpret seismic velocity data and estimate property of partially saturated rocks using Gassmann fluid substitution equation, consideration about different CO2 saturation state is very important. We have studied sonic log and neutron log obtained at OB-2 well in Nagaoka CO2 geological sequestration project and the CO2 saturation state is considered as patchy saturation. Furthermore existence of critical saturation is expected from the logging data. We estimate CO2 saturation for each saturation state, uniform, patchy, and based on Brie’s equation. Although uniform saturation gives much smaller value than actual saturation, patchy saturation and Brie’s equation gives reasonable value and the result shows good agreement with CO2 saturation calculated from neutron log. Therefore we conclude that CO2 saturation state at Nagaoka is expressed as patchy saturation.


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