Selective Adsorption and Transport Diffusion of CO2-CH4 Binary Mixture in Carbon-Based Organic Nanopores

Abstract

Abstract Production from shale gas reservoirs provided a unique opportunity for underground storage of the CO2. Because of the higher affinity of CO2 to the organic matter of shale compared to CH4, injecting CO2 into these resources can cause the adsorbed CH4 molecules to be desorbed and then replaced by the CO2 molecules. In this work, we perform molecular dynamics simulations to calculate the Onsager and Fickian diffusion coefficients of CH4 and CO2. Onsager coefficients of the pure species increase as its partial pressure increases and then level off at higher pressures. The off-diagonal Onsager coefficients increase as CH4 pressure increases up to a CO2 pressure of approximately 100 atm and followed by a reduction in the coefficients. The Onsager diffusion coefficients of CO2-CO2 are the highest compared to CH4-CH4 and CO2-CH4 diffusion coefficients. The Onsager diffusion coefficients for both species are a function of their occupancies. As concentration of a species increases, its Onsager diffusion coefficient increases as well. The results also demonstrated that the off-diagonal terms in the Onsager and Fickian diffusion coefficients matrix have the same order of magnitude as the diagonal terms and therefore cannot be ignored in modeling of CO2 sequestration in shale.