Experimental evaluation of CO₂ enhanced recovery of adsorbed-gas from shale

Existing methods for shale reservoir stimulation (e.g., hydraulic fracturing) focus mainly on improving the physical recovery of free phase gases from shales. Few studies explore the enhanced recovery of adsorbed methane from shale due to the difficulty in simultaneously separating the recovery of both the adsorbed and free phase gas. In this study, we use an NMR-based methodology to separately evaluate the change in adsorbed and non-adsorbed (free-phase) methane during the injection of CO₂. We evaluate mechanisms contributing to the enhanced recovery of adsorbed-gas with CO₂ injection (Longmaxi shale, Jiaoshiba field, SW China). Results show that the injection of CO₂ can improve the efficiency of recovery of residual gas in the adsorbed phase by an additional ~ 25% and can enhance the percent recovery per hour of adsorbed methane by ~ 11% at ambient pressure and ~ 4% at abandonment pressure. The experiments also demonstrate that a higher concentration ratio of CO₂/CH₄ is more suitable for CO₂-CH₄ displacement, and the CO₂-CH₄ displacement rate is relatively rapid during the early phase of interaction, immediately after injection. Despite inherent uncertainty, these experiments demonstrate the capability of enhancing adsorbed-gas recovery by injection of CO₂ into shale.