Injection of CO2 into a depleted gas field, Otway basin, Australia, is expected to create very subtle changes in elastic properties of the reservoir. This is a serious challenge for the monitoring program at this site. Here, we perform a series of numerical experiments to evaluate the likelihood of detecting a weak 4D signal caused by CO2 injection. We simulate seismic response changes due to variable near surface conditions. We also take into account the expected ambient noise level. To come to realistic input parameters a detailed analysis of borehole seismic data (several Vertical Seismic Profile, or VSP surveys) is performed. We then analyze the possibility of extracting 4D seismic signatures of CO2 from the simulated low repeatability seismic data.
CO2 capture and storage is one of the most promising techniques to reduce CO2 emissions into the atmosphere. A feasibility study, onshore Otway basin, Australia, aims at evaluating the viability of CO2 storage into a depleted gas field. Of particular interest for this study is the assessment of monitoring methodologies: injection of CO2 is expected to cause very subtle changes in elastic properties of the reservoir rock. Such conditions present a serious challenge for the application of time-lapse seismic monitoring technologies. However, poor repeatability caused by significant changes in weathering properties (ground saturation level, variations in the composition of the near surface) and ambient noise (wind, machinery at work) could easily overcome predicted 4D seismic response changes (~5%) caused by CO2 injection (Li et al., 2005). In this framework, an extensive VSP monitoring program has been planned at the Otway site. To investigate the applicability of VSP surveys, we numerically simulate seismic response for in situ conditions. Subsequently, we try to extract a weak CO2-related 4D seismic signature from the data.
We use a finite difference elastic code to model the seismic response from repeat walkaway VSP surveys, which will be acquired several months after CO2 injection starts (April 2008). To get a set of realistic parameters for the modeling study we first analyze available field data.
The first pilot VSP study has been conducted in May 2006. Zero offset VSP (ZVSP), offset VSP (OVSP) and Walkaway VSP (WVSP) data were acquired in the monitoring Naylor-1 well, using high precision threecomponent accelerometers. A compressional slowness log, from 490 to 2129 m MD, and a density log from 1927 to 2143 m MD have also been acquired. This survey aimed at testing VSP methods for CO2 injection monitoring at Otway. The signal strength, frequency content and processed image quality were of primary interest. Comparative analysis between pilot VSP and 2D surface seismic data (Urosevic et al, 2007) showed that VSP methods are preferred at this site because they preserve higher frequencies. Subsequently, in 2007, in the newly drilled injection well (CRC-1), ZVSP, OVSP, and the first 3DVSP in Australia, have been acquired. Logs of compressional slowness (128 to 2224 m MD), shear slowness (460 to 2220 m MD) and density (two intervals from 808 to 1543 m MD and from 2002 to 2190 m MD) are also available.
