Quantifying CO₂ Saturation from Time-Lapse Well Logging in an Onshore Saline Aquifer, Nagaoka, Japan

Ziqiu Xue, CO2 Sequestration Research Group, Research Institute of Innovative technology for the Earth, 9-2 Kizugawadai, Kizu-cho, soraku-gun, Kyoto, 619 0292, Japan, phone: 81 774 75 2312, fax: 81 774 75 2313, xue@rite.or.jp, Jiro Watanabe, Geophysical Surveying Co. Ltd, and Daiji Tanase, Engineering Advancement Association of Japan.

Injection of carbon dioxide (CO₂) into saline aquifers has been proposed as a means to reduce greenhouse gas emissions. In geological carbon sequestration, monitoring is necessary to confirm the containment of CO₂, assess leakage paths, and gain understanding into interactions between CO₂, the rock-forming minerals and formation fluids. Monitoring is also necessary to quantify the net quantity of CO₂ that has been sequestrated within the reservoirs. The major challenge is how to correlate wave velocity or electrical conductivity with fluid (CO₂ or formation water) saturations in partially saturated porous rocks. A series of time-lapse geophysical loggings consisted of dual induction, neutron, sonic and gamma-ray has been performed frequently at the Nagaoka pilot-scale sequestration site. The pilot-scale CO₂ injection site is located at Minami-Nagaoka oil and gas fields, and one injection well (IW-1) and three observation wells (OB-2,-3,-4) were drilled at the pilot site. The goal of this paper is to interpret and analyze time-lapse sonic log data quantitatively obtained from observation wells OB-2 and OB-4, to better understand fluid saturations around the two wells. The CO₂ breakthrough was clearly confirmed from the sonic velocity reduction up to 20%. This significant velocity reduction agreed fairly well with the laboratory experiment result on drilled cores. Based on the rock-properties model and Gassmann's equation, we successfully matched the sonic velocity change due to presence of injected CO₂ at observation wells. The CO₂ saturation around the observation wells ranged from 10 to 40%. From the history matching result we also found that there is no CO₂ leakage from the reservoir, even a huge earthquake hit the Nagaoka area. Our results strongly suggested the capability for quantifying CO₂ concentration from seismic wave data with Gassmann's equation.

CO2 Sequestration – Concepts and Future Plans (EMD/DEG)
2006 AAPG International Conference and Exhibition, (November 5-8, 2006) Technical Program