VP/VS monitoring of CO2 sequestration using virtual horizontal crosswell tomography

Abstract

Time-lapse crosswell seismic monitoring with virtual sources redatumed to the positions of the receivers installed in a horizontal well is an efficient method of monitoring the migration of the CO2 plume or lithological changes. Two horizontal wells and virtual sources can minimize the problems caused by the near-surface complexity and the variation in source position during monitoring. In traditional monitoring using P-waves, the change in the P-wave velocity (VP) is affected not only by the injected CO2 but also by geological changes. Conversely, the S-wave velocity (VS) is affected only by the geological changes in the reservoir because the S-wave does not propagate through the pore fluid. Consequently, monitoring using both the P- and S-waves gives more accurate and useful information on the target layers. In this study, we developed an effective monitoring method that not only detects the boundary of CO2 plumes but also identifies the cause of the velocity change; this method uses two horizontal wells and virtual sources. The VP/VS ratio obtained from the P- and S-wave velocity tomograms from the synthesized data clearly distinguished the cause of the velocity change and the velocity tomograms sets successfully described the velocity change due to CO2 plume injection or lithological changes. This method can also be used for enhanced oil recovery monitoring and, if a mode converted S-wave is used, it can be applied to marine surveys.