Source Repeatability of Time-Lapse Offset VSP Surveys For Monitoring CO2 Injection

The sources of time-lapse offset vertical seismic profiling (VSP) surveys should be located exactly at the same positions to reliably monitor reservoir changes due to CO2 injection. However, there is often some uncertainty in source locations during time-lapse data acquisitions. We use double-difference tomography and downgoing waves of time-lapse offset VSP data to invert for the source locations and the velocity structures simultaneously. We use synthetic data for validation and apply doubledifference tomography to time-lapse offset VSP data acquired at the Aneth oil field in Utah for monitoring CO2 injection. Our synthetic studies demonstrate that doubledifference tomography can accurately invert for VSP source locations. Our results of Aneth field data show that real source locations of time-lapse offset VSP surveys are separated up to a few tens of meters. Accounting for these source location differences can improve reliability of timelapse VSP monitoring.