Repeatability analysis of land time-lapse seismic data: CO2CRC Otway pilot project case study.

Abstract

Time-lapse seismics is the methodology of choice for remotely monitoring changes in oil/gas reservoir depletion, reservoir stimulation or CO2 sequestration, due to good sensitivity and resolving power at depths up to several kilometres. This method is now routinely applied offshore, however, the use of time-lapse methodology onshore is relatively rare. The main reason for this is the relatively high cost of commercial seismic acquisition on land. A widespread belief of a relatively poor repeatability of land seismic data prevents rapid growth in the number of land time-lapse surveys. Considering that CO2 sequestration on land is becoming a necessity, there is a great need to evaluate the feasibility of time-lapse seismics for monitoring. Therefore, an understanding of the factors influencing repeatability of land seismics and evaluating limitations of the method is crucially important for its application in many CO2 sequestration projects. We analyse several repeated 2D and 3D surveys acquired within the Otway CO2 sequestration pilot project (operated by the Cooperative Research Centre for Greenhouse Technologies, CO2CRC) in Australia, in order to determine the principal limitations of land time-lapse seismic repeatability and investigate the influence of the main factors affecting it. Our findings are that the intrinsic signal-to-noise ratio (S/N, signal to coherent and background noise levels) and the normalized-root-mean-square (NRMS) difference are controlled by the source strength and source type. However, the post-stack S/N ratio and corresponding NRMS residuals are controlled mainly by the data fold. For very high-fold data, the source strength and source type are less critical.