Evaluation of Relationship Between Post-injection Plume Stability and Leakage Risks

Abstract

One of the critical questions for geologic CO2 storage is how long should a storage site be monitored after the injection is stopped. A storage site operator is required to demonstrate that the storage site is evolving towards long-term stability and the injected CO2 as well as reservoir pressure do not pose any threat including to the groundwater. Migration and evolution of injected CO2 plume depend on site-specific geologic structure, storage reservoir properties as well as operational conditions. While injected CO2 plume may continue to evolve after injection is stopped, its dynamic evolution does not directly imply imminent leakage risk. Leakage risks depend on multiple factors including not only CO2 presence but also reservoir pressure and the characteristics of potential leakage pathways. We performed a modeling based study to evaluate the relationship between post-injection plume migration and leakage risks. Our hypothetical case study is based on the Rock Springs Uplift in southwestern Wyoming in USA. We used a numerical model to perform multiple sets of reservoir simulations each with 29 equi-probable realizations of reservoir permeability heterogeneity, simulating different injection scenarios. We applied newly developed moment-based plume mobility metrics to characterize the migration and evolution of injected CO2 plume. The plume mobility metrics provided detailed analyses of the effect of reservoir permeability heterogeneity and CO2 injection rate on spatial and dynamic evolution of CO2 and overpressure plume. Next, we assessed the potential leakage risks using the integrated assessment modelling approach developed by US DOE’s National Risk Assessment Partnership (NRAP). The combined leakage risk assessment and plume mobility analyses results indicate that lack of CO2 plume stability (or non-zero plume mobility) may not directly imply groundwater aquifer endangerment and the leakage risks are dependent on multiple factors, including presence of wells and their locations and types. This paper is a condensed version of a peer-reviewed paper by Pawar et al [1] published in the International Journal of Greenhouse Gas Control.