Sensitivity of geophysical techniques for monitoring secondary CO₂ storage plumes

For geologic carbon storage, the ability to detect secondary CO₂ plumes—defined as those CO₂ plumes accumulating outside the intended storage reservoir—is fundamental to preventing unexpected CO₂ migration into groundwater resources and for risk and liability management. Understanding the sensitivity of various geophysical methods to secondary plumes is crucial for designing cost-effective monitoring schemes. We use several modeling scenarios to demonstrate the process of assessing sensitivities and detection thresholds of three primary geophysical techniques—surface seismic, borehole-to-surface electromagnetic (EM), and surface and borehole gravity—for early detection of secondary CO₂ plumes in the post-injection phase. While seismic reflection methods are often considered in monitoring strategies to track the evolution of CO₂ plumes, they are also the most expensive. Due to cost considerations, especially for long-term post-injection monitoring, other techniques complement seismic monitoring when designing an adaptive monitoring network. Borehole-to-surface EM or surface gravity surveys are feasible for time-lapse monitoring of deep secondary CO₂ plumes. These surveys could be carried at intervals defined by site-specific conditions. If time-lapse EM and/or gravity surveys detect any signal responses beyond the expected change, it would trigger a need for the higher resolution seismic survey.