Impact of Residual Fluid Composition and Injected Gas Purity on the Storage Capacity of the Depleted Shale Oil Reservoirs in the Presence of Diffusion and Sorption

Abstract Depleted shale oil reservoirs signify a forward-looking solution for CO2 storage that leverages existing resources — It presents an ideal opportunity for CO2 storage, primarily because extensive data collection from prior extraction activities offers a detailed understanding of their geological characteristics. Additionally, the massive number of depleted wells in the US shales makes it a viable option for carbon storage. Shales are known for their high organic content, which enhances CO2 adsorption into the geological formation, which in turn enhances the storage capacity and formation integrity. On the other hand, due to the high CO2 concentration gradient, the diffusion process plays a significant role in CO2 storage. The wealth of information collected throughout the lifetime of those wells, including the adsorption and diffusion rates, ensures precise planning and safe implementation of CO2 sequestration projects. This precise planning for carbon storage contributes to the environmental and energy sustainability goals. In this study, we evaluate the storage performance in shales to evaluate the impact of reservoir fluid composition, diffusion, and adsorption. Furthermore, the impact of the injection gas purity on storage capacity is evaluated. In this study, we assessed the CO2 storage in shales by conducting large-scale numerical simulations for a wide range of reservoir fluid compositions—retrograde-condensate, volatile-oil, black oil—and different purities of injected CO2 by considering diffusion and sorption. The results showed that ignoring the adsorption and diffusion underestimated shale storage capacity while ignoring the diffusion only overestimated the storage capacity. These results emphasize the importance of including adsorption and diffusion when modeling CO2 storage. Furthermore, the purity of the injected CO2 is positively proportional to the storage capacity as the storage capacity is reduced when the purity is reduced due to the increasing competition of the impurities to be adsorbed into the shales.