This paper presents CO2 saturation type transition interpreted from time-lapse sonic and induction log data, acquired in an observation well in the Nagaoka CO2 aquifer storage project. Approximately 10,000 ton of CO2 was injected into a sand stone aquifer formation at a depth of 1100 m as a pilot CO2 sequestration test.

During the 19-month injection period and 6-year post injection period, 39 time-lapse well logs were conducted for monitoring purpose. Resistivity and P-wave velocity were observed and the change was interpreted as signatures of CO2 breakthrough and saturation type.

The relation between CO2 saturation and P-wave velocity had become linear in the injection-period after CO2 breakthrough. It was then shifted toward non-liner at the deeper part of the formation in the post-injection period. The linear and non-linear relations can be explained by patchy and uniform CO2 saturation respectively. The transition at the deeper part can be interpreted that patchy distributed pore-filled CO2 was replaced CO2 saturated water after finishing injection since CO2 moved to shallower formation by buoyancy.

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