Experimental Investigation of CO 2 /CH 4 Competitive Adsorption Behaviors in Shale under Different Feed Gas Injection Methods

Understanding the CO2 and CH4 competitive adsorption behavior is crucial for the engineering optimization of the CO2-enhanced shale gas recovery (CO2-ESGR) process. Most studies evaluate CO2 and CH4 competitive adsorption behaviors by simultaneously injecting their mixed gases, which does not reflect real conditions, as CH4 initially exists in the reservoir and CO2 is injected at a later stage. Whether premixed experiments can adequately represent this sequential process therefore remains unclear. In this study, a comparison of different competitive adsorption measurement methods, including premixed CO2 and CH4 (CO2 and CH4 coinjection) and CO2 huff-and-puff (CO2 injection into a CH4-preadsorbed shale sample), was conducted under different temperature and pressure conditions. The results show that that competitive adsorption is strongly dependent on the injection pathway. With increasing pressure, huff-and-puff showed a steeper rise in CO2 uptake and a decline in residual CH4, whereas premixed adsorption showed a slower increase in CO2 uptake and a slight increase in CH4 uptake. Under the same equilibrium conditions, huff-and-puff retained higher adsorption capacity for both gases. In both pathways, shale preferentially adsorbed CO2 over CH4, and the adsorption selectivity coefficient of CO2 over CH4 (SCO2/CH4) decreased with the increase of pressure. Nevertheless, huff-and-puff maintained higher adsorption selectivity over comparable pressure ranges and increased with the initial pressure of CH4. By accounting for the sequential injection history, huff-and-puff adsorption provides a more reliable basis for evaluating CO2 storage potential and CH4 recovery in CO2-ESGR.