Utilization of zeolite as a potential multi-functional proppant for CO2 enhanced shale gas recovery and CO2 sequestration: A molecular simulation study on the competitive adsorption of CH4 and CO2 in zeolite and organic matter

It is well known that CO2 is one of the most effective enhanced hydrocarbon recovery agents due to its thermodynamic characteristics, and extensive research and pilot studies have been conducted in recent years on how to utilize CO2 for enhanced gas recovery in shales. The common delivery method involves injecting CO2 in its liquid or supercritial form into a shale formation. In this paper, we propose a novel approach to shale gas recovery that uses zeolite as a multi-functional proppant and carrier of adsorbed CO2 to enhance shale gas recovery as well as CO2 sequestration and storage. This process involves complex thermodynamic and transport processes, among which the competitive adsorption behaviors of CO2 and CH4 into organic matter and zeolite is the most critical to the success of the proposed approach. In this paper, we carry out a systematic molecular simulation study to investigate the adsorption behaviors of methane and CO2 into organic matter (kerogen) and silica zeolite (silicalite-1). We use grand canonical Monte Carlo simulations to measure single-component adsorption isotherms and calculate the isosteric heat of adsorption at surface temperature and at elevated temperatures of up to 425 K. Moreover, we simulate the competitive adsorption of binary mixtures of CH4 and CO2 with various compositions and investigate the competition between the two gas components in kerogen and silicalite-1. Both silicalite and kerogen show a stronger affinity for CO2 than for CH4. While the adsorption capacity of kerogen is about two times that of silicalite, the isosteric heat of adsorption demonstrates that the kerogen/CO2 interaction is the strongest among all four single-component adsorption systems. These findings demonstrate the great potential of using zeolite as a proppant and CO2 carrier to displace CH4 in shale organic matter under subsurface conditions. This observation is also validated via a competitive adsorption study, in which kerogen preferentially adsorbs CO2 over CH4 under all conditions and silicalite exhibits weaker CO2/CH4 selectivity, especially when the CO2 fraction is very low in the bulk phase. These results suggest the potential applicability of using zeolite as a proppant and CO2 carrier to enhance shale gas recovery. In reservoir conditions, the CO2 desorbed from zeolite can be favorably adsorbed by kerogen due to the increase in temperature and decrease in pressure; in the meantime, it can displace the adsorbed CH4 to enhance gas production.