Feasibility of CO2 plume detection using 4D seismic: CO2CRC Otway Project case study - Part 2: Detectability analysis

A key objective of stage 2 of the Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) Otway Project is to explore the ability of the seismic reflection method to detect and monitor injection of a small amount of greenhouse gas into a saline formation. Development of a seismic monitoring program requires an understanding of expected time-lapse (TL) seismic signals. Hence, before such an injection experiment is undertaken, we assessed the feasibility of seismic monitoring in a modeling study. Considering realistic gas distributions inferred from reservoir simulations, we analyzed the influence of various factors (injection volume, time after injection, and realizations of the reservoir flow model) on the TL seismic signal. However, the applicability of seismic monitoring depends not only on the strength of the TL seismic signal but also on the noise level of the seismic data. Hence, to estimate the detectability of gas in the subsurface, we have developed a workflow that integrated actual data repeatability observed at the Otway test site into the seismic feasibility study. Although we observed differences between the considered scenarios, all of the scenarios indicated a high likelihood of successful plume detection with the observed noise level and surface 4D seismic acquisition geometry used in stage 1 of the CO2CRC Otway Project at the same site. However, a thin layer of gas spreading out from the edges of the main plume below the seal in all scenarios would be a challenge for surface seismic monitoring.