Optimal designs of time-lapse seismic surveys for monitoring CO2 leakage through fault zones

Abstract

Cost-effective time-lapse seismic surveys are crucial for long-term monitoring of geologic carbon sequestration. In this paper, we numerically model time-lapse seismic surveys for monitoring CO2 leakage through fault zones, and design optimal surveys for time-lapse seismic data acquisition using elastic-wave sensitivity analysis. Two stages of CO2 leakage through fault zones are simulated. In the early stage of leakage, when CO2 is confined in a relatively deep region, and our results show that the most desired location for receivers at the surface is at the hanging-wall side of the fault zone, of normal faults (with large dip angles) or reverse faults. The most sensitive places at the surface to the change of different elastic parameters (e.g. P- and S-wave velocity and density) are similar to one another, and are often not sensitive to the source location. In a late stage of leakage, in which CO2 migrates close to the surface, our modeling demonstrates that the best region at the surface for time-lapse seismic surveys is very sensitive to the source location and the elastic parameter to be monitored. For time-lapse vertical seismic profiling (VSP) surveys, our modeling suggests that it is important to account for the effect of free-surface reflections, especially for large offset VSP surveys.