Objective: This study aims to investigate the characteristics and distribution of CH₄-CO₂ occurrence in shale kerogen at different depths, and subsequently simulate and analyze the diffusion characteristics of CH₄-CO₂. The findings will provide an evaluation for the displacement extraction of CO₂ in shale gas exploitation and geological CO₂ sequestration.
Methods: Micro-pore development in shale kerogen was characterized through experiments, and a micro-pore model corresponding to the experimental results was established based on the macromolecular structure model of kerogen. Furthermore, the competitive adsorption characteristics of CH₄-CO₂ were simulated and analyzed using the Monte Carlo method, and the molecular motion characteristics of gases were analyzed through molecular dynamics simulation, aiming to provide theoretical guidance for shale gas exploitation and CO₂ sequestration.
Results: A micro-pore model for kerogen was established, and the adsorption characteristics of CH₄-CO₂ at different formation pressures were simulated. With increasing burial depth, at depths of 0-1 km, the adsorption capacity increased rapidly, followed by a slow and plateau-like increase. The number of CO₂ adsorption sites was higher than that of CH₄. When combined adsorption occurred, the number of adsorption sites for both CO₂ and CH₄ increased or decreased. The decrease in CH₄ adsorption sites was more significant, while the change of CO₂ adsorption sites was relatively balanced. This is because the total adsorption amount increases during combined adsorption, and CO₂ molecules explore the space more thoroughly.
Conclusion: Shale kerogen possesses interconnected pore networks formed by the development of fissure-like pores and the connection of micro-pores to micro-fractures. CO₂ can enter smaller volume micro-pores, increasing its adsorption capacity. CH₄ exhibits stronger diffusion characteristics, making it more efficient to utilize CO₂ displacement for methane extraction. On the other hand. CO₂ has poor diffusion characteristics, indicating its potential effectiveness for geological sequestration.