Simulation Approach for the Time-lapse Imaging of CO2 Injected Zones Using a Few Seismic Sources and a Geophone Array

The time-lapse study for the CO2 injection or CO2-EOR requires the excellent repeatability of the measurement system. Many factors control the repeatability. The overburden effects from a source to receivers can be reduced by fix source location. To obtain excellent repeatability, we assume single or a few ACROSS (Accurately Controlled and Routinely Operated Signal System) seismic source(s) which have such ultra-stable repeatability for the operation of long duration. Using residual waveforms before and after the injection of vapor or supercritical CO2 to reservoirs, we can image the time lapse of reservoir. We carried out simulations for a reservoir at 2 km depth. We assumed only one seismic source and a geophone array of variable spacings at the ground surface. The result of simulation gave a precise retrieval of image. For the simulated case, 200 m spacing is enough to resolve the target. In addition, we demonstrate the effect of physical property change in the nearsurface layer which causes interference in imaging of the deep target. Truncating surface-wave components in the residual waveforms could reduce the effect. The applications of our method to the real fields might greatly contribute to reduce the installation and time-lapse monitoring costs.