Time-lapse crosswell seismic tomography for monitoring injected CO_2 in an onshore aquifer, Nagaoka, Japan

摘要

Japan's first pilot-scale CO_2 sequestration experiment has been conducted in Nagaoka, where 10 400 t of CO_2 have been injected in an onshore aquifer at a depth of about 1100 m. Among various measurements conducted at the site for monitoring the injected CO_2 we conducted time-lapse crosswell seismic tomography between two observation wells to determine the distribution of CO_2 in the aquifer by the change of P-wave velocities. This paper reports the results of the crosswell seismic tomography conducted at the site. The crosswell seismic tomography measurements were carried out three times; once before the injection as a baseline survey, and twice during the injection as monitoring surveys. The velocity tomograms resulting from the monitoring surveys were compared to the baseline survey tomogram, and velocity difference tomograms were generated. The velocity difference tomograms showed that velocity had decreased in a part of the aquifer around the injection well, where the injected CO_2 was supposed to be distributed. We also found that the area in which velocity had decreased was expanding in the formation up-dip direction, as increasing amounts of CO_2 were injected. The maximum velocity reductions observed were 3.0 percent after 3200 t of CO_2 had been injected, and 3.5 percent after injection of 6200 t of CO_2.