Estimation of CO2 saturation from time-lapse CO2 well logging in an onshore aquifer, Nagaoka, Japan

The first Japanese pilot-scale CO2 sequestration project has been undertaken in an onshore saline aquifer, near Nagaoka in Niigata prefecture, and time-lapse well logs were carried out in observation wells to detect the arrival of injected CO2 and to evaluate CO2 saturation in the reservoir. CO2 was injected into a thin permeable zone at the depth of 1110 m at a rate of 20-40 tonnes per day. The total amount of injected CO2 was 10 400 tonnes, during the injection period from July 2003 to January 2005. The pilot-scale demonstration allowed an improved understanding of the CO2 movement in a porous sandstone reservoir, by conducting time-lapse geophysical well logs at three observation wells. Comparison between neutron well logging before and after the insertion of fibreglass casing in observation well OB-2 showed good agreement within the target formation, and the higher concentration of shale volume in the reservoir results in a bigger difference between the two well logging results. CO2 breakthrough was identified by induction, sonic, and neutron logs. By sonic logging, we confirmed P-wave velocity reduction that agreed fairly well with a laboratory measurement on drilled core samples from the Nagaoka site. We successfully matched the history changes of sonic P-wave velocity and estimated CO2 saturation after breakthrough in two observation wells out of three. The sonic-velocity history matching result suggested that the sweep efficiency was about 40%. Small effects of CO2 saturation on resistivity resulted in small changes in induction logs when the reservoir was partially saturated. We also found that CO2 saturation in the CO2-bearing zone responded to suspension of CO2 injection.