Micro-mechanism of the effect of multi-component gas injection on methane recovery

Abstract

To investigate how the micro-mechanism of multi-component gas injection affects the process of retrieving methane, a combination of density functional theory and molecular dynamics simulation method has been conducted under different gas injection ratios of CO2 and N2. The results of DFT demonstrate that the adsorption energy of the gas molecules over bituminous coal fragment are in order of CO2 > CH4 > N2. Independent gradient model analyses show that the Van der Waals interactions are the dominant force contributing to these three adsorption behaviors. To gain further insight into the influence of functional groups on CH4 adsorption, the radial distribution function analyses have been performed. It was found that oxygen-containing functional groups have a substantial impact on the adsorption of CH4 on coal molecules. Among these, CH4 preferentially adsorb around the hydroxyl group of coal molecule. Moreover, density profiles of the three gas molecules within nanopores demonstrate that CO2 molecules cause CH4 molecules to desorb by molecular swapping. In contrast, N2 slows down CH4 adsorption spontaneity by lowering CH4 partial pressure. Furthermore, it should be noted that increasing the proportion of CO2 in the gas mixture system can significantly improve the displacement efficiency of CH4. Conversely, the diffusion coefficient becomes small. The microscopic mechanism revealed in the present work helps us gain a better understanding of gas displacement processes, which provides new insights for optimizing gas injection compositions for field applications.