Time-lapse crosswell seismic tomography for monitoring the pilot CO_2 injection into an onshore aquifer, Nagaoka, Japan

摘要

Japan's first pilot-scale CO_2 sequestration experiment has been conducted at the Nagaoka test site, where 10400 t of CO_2 have been injected into 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 seismic velocities. The crosswell seismic tomography measurements were carried out five times; once before the injection as a baseline survey, and four times during and after the injection as monitoring surveys. The velocity tomograms obtained 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 injected. The maximum velocity reductions observed were 3.5% after 104001 of CO_2 had been injected.