Simulation to Enhance Shale Gas Recovery Using Carbon Dioxide in Silica Nanopores with Different Sizes

An understanding of molecular behavior on a microscopic level is always important not only to find out the natural principles but also to decide how to solve problems in applications. In this study, the grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulation methods were employed to investigate the adsorption and diffusion properties of CH4 and CO2 in silica nanopores with different sizes, and the displacement of residual CH4 in silica nanopores by CO2 at a constant temperature of 323 K and various pressures was studied. The microscopic molecular states of the adsorbed CH4 and CO2 in nanopores of various sizes are different. The competitive adsorption of CO2 over CH4 occurs broadly because of the different intensity of interactions between the gases molecules and the pore surface, of which the degree decreases with the increase of the pore size. An effective displacement process of residual adsorbed CH4 by CO2 was performed, and it is found that the displacement is enhanced with the increase of the CO2 bulk pressure and that the pore size has a significant influence on the displacement. According to the results, CO2 capture and storage (CCS) and the enhancement of CH4 recovery could be achieved at the same time in silica nanopores. This work provides microscopic information on the molecular behavior of CH4 and CO2 in silica nanopores and testifies to the efficiency of the displacement of CH4 by CO2 in silica nanopores with various sizes to provide useful guidance for applications in the enhancement of shale gas recovery by CO2.