Time-lapse well logging to monitor the injected CO_2 in an onshore aquifer, Nagaoka, Japan

摘要

A pilot-scale sequestration of CO_2 into an onshore aquifer has been conducted by Research Institute of Innovative Technology for the Earth (RITE) in cooperation with Engineering Advancement Association of Japan (ENAA). The CO_2 injection site is located at Minami-Nagaoka oil and gas field, Nagaoka city, Niigata prefecture, Japan. One injection well (IW-1) and three observation wells (OB-2, OB-3, OB-4) were drilled. CO_2 was injected into a thin permeable zone of the reservoir at 20-40 tonnes per day. The CO_2 injection started on 7 July 2003, and ended on 11 January 2005 with the total amount of 10,400 tonnes. The pilot-scale demonstration allowed an improved understanding of the CO_2 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. CO_2 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 CO_2 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 CO_2 saturation on resistivity resulted in small changes in induction logs when the reservoir was partially saturated. We also found that CO_2 saturation in the CO_2-bearing zone responded to suspension of CO_2 injection.