Characterizing competitive adsorption and diffusion of methane and carbon dioxide in kerogen type-III slit model

Abstract

Clarifying methane’s adsorption and diffusion properties in kerogen contributes to efficiently exploiting shale gas reservoirs. We refined the kerogen III-series model to construct the kerogen III-B and kerogen III-C molecular structures. In contrast to the traditional simplified slit model containing small organic matter, a mixed kerogen-quartz slit model was further proposed. These inorganic-organic models, including kerogen II-D, kerogen III-B, and kerogen III-C, provide a realistic reservoir environment for the study of the shale gas adsorption and diffusion characteristics in shale. Based on these models, we investigated the competitive adsorption of methane and carbon dioxide using the grand canonical Monte Carlo (GCMC) method. We then studied the diffusion characteristics of methane molecules throughout the model area and in the different adsorption blocks classified as the inner slit zone, surface zone, and matrix zone using the molecular dynamics (MD) method. The results showed that carbon dioxide gradually replaces methane molecules as the injection pressure of carbon dioxide increases, causing desorption and diffusion of methane. The order of the overall diffusion capability of methane in the kerogen slit models is kerogen II-D >kerogen III-C >kerogen III-B. In addition, the diffusion capability of methane molecules in the different zones is ordered as inner slit zone >surface zone >matrix zone. This work is a step towards more effective exploitation of shale gas.