Adsorption Behavior of CH₄, C₂H₆, and CO₂ on Moisture-Equilibrated Shale

Abstract

Investigation on the influence of CO2 on adsorption behavior of CH4 and C2H6 in moisture-equilibrated shale is important for understanding the mechanism of CO2 injection for shale gas recovery. In this study, the adsorption capacities of CH4, C2H6, and CO2 on shale samples are first compared using the thermogravimetric method. The low-field nuclear magnetic resonance technique is then used to investigate the influence of CO2 on the behavior of adsorbed CH4 and C2H6 on shale, and the performance of CO2 introduction for CH4 and C2H6 recovery is thus evaluated. Test results show that the adsorption capacity is measured in sequence of C2H6 > CO2 > CH4 for dry shale; interestingly, the adsorption capacity of CO2 can be stronger than C2H6 on the moisture-equilibrated shale, indicating the potential of CO2 for enhancing C2H6 recovery. On the basis of the NMR results, the T 2 spectrum decreases in the adsorbed CH4 region for the dry and moisture-equilibrated shale samples after injecting CO2. On the contrary, the T 2 spectrum of the adsorbed C2H6 is almost unchanged for the dry shale, while it decreases remarkably for the moisture-equilibrated sample, indicating the potential of CO2 for enhancing C2H6 recovery for water saturated shale gas reservoirs. It is found that the recovery of CH4 and C2H6 from the dry cores is always lower than that for the moisture-equilibrated shale cores, suggesting that the moisture content in shale is beneficial for shale gas recovery during CO2 injection. This study inspires new understanding of the fundamental mechanisms of applying a CO2 injection method for shale gas development; more importantly, it provides new strategies that could be employed for shale resource production.