7. Spatial/Time-lapse Data Management
Time-lapse Crosswell Seismic Tomography for Monitoring the CO2 Injected Into an Onshore Aquifer, Nagaoka, Japan
Hideki Saito(1), Dai Nobuoka(1), Hiroyuki Azuma(1), Daiji Tanase(2) and Ziqiu Xue(3)
(1) Oyo Corporation, Japan. (2) Engineering Advancement Association of Japan, Japan. (3) Research Institute of Innovative Technology for the Earth, Japan.
Abstract
Japan”Ēs first pilot-scale CO2 sequestration experiment has been conducted at the Minami-Nagaoka gas and oil field, where Teikoku Oil Co., Ltd. produces natural gas from the deep reservoir (4,700 m). One injection well (IW-1) and three observation wells (OB-2, OB-3 and OB-4) were drilled at the pilot site. Purchased 99.9% pure CO2 was injected from IW-1 at 20-40 tonnes per day. The depth of the reservoir consisting of a 60 m-thick sandstone bed of the Haizume Formation is about 1,100 m below the ground surface. A thin permeable layer confirmed from the core samples, well logging, and well pumping test results, having a thickness of 12 m within the reservoir was selected for injection of CO2. The CO2 injection started on July 2,003 and ended on January 2,005. The total amount of injected CO2 was 10,400 tonnes. Among various measurements conducted at the site for monitoring the injected CO2, we conducted time-lapse crosswell seismic tomography between two observation wells OB-2 and OB-3 to determine the distribution of CO2 in the aquifer by the change of seismic P-wave 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 CO2 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 CO2 injected. The maximum velocity reductions observed were 3.5% after 10,400 t of CO2 had been injected. Although the CO2 was injected into a thin permeable sandstone layer, a part of the area of velocity decrease seems to extend into the cap rock and the lower layers. We have already conducted some numerical experiments simulating these phenomena, and concluded as follows. The velocity anomaly extending into the cap rock is an artifact or ghost caused by the source-receiver geometry of this particular experiment. Because of the apparent expansion of velocity decreasing area by the artifacts, the velocity reduction values were not reconstructed correctly. Further studies are needed to solve the problem by some sophisticated inversion schemes including adequate constraints.
Last modified: Thu May 25 11:38:22 2006