Confinement effect on transport diffusivity of adsorbed CO2–CH4 mixture in coal nanopores for CO2 sequestration and enhanced CH4 recovery

Abstract

The transport of CO2–CH4 mixture in coal nanopores is crucial to CO2-enhanced coalbed (CO2-ECBM) recovery, but is highly challenging on adsorption-diffusion dominated nanoscale due to the gas-surface and intermolecular interactions. Using Monte Carlo and molecular dynamics simulation, the confinement effect of nanopores with two slit widths (2 and 6 nm) on the CO2–CH4 binary diffusion in the adsorption layer are investigated. A modified computational method for multi-component transport diffusivity in the adsorption layer is proposed by incorporating the survival probability function. The results show that the transport diffusivity in the semi-confined adsorption layer shows a general decreasing tendency with the increase of pressure, while it slightly decreases when the pressure <6.4 MPa in the confined adsorption layer, following an inverse parabola trend with the maximum value at around 12 MPa. Three mechanisms are classified regarding this pressure-dependent evolution. Firstly, the collision between molecules of weaker attraction and surface dominates the transport capacity, and this collisional frequency reduces at the low-pressure region. Accompanying the pressure exceeds the critical threshold, surface diffusion becomes the primary contribution. Lastly, significant intermolecular interactions appear after the adsorption saturation. During the CO2-ECBM process, pressure drawdown is the primary method to release the adsorbed CH4, and the largest recovery ratio can reach 25.9% in 2 nm micropore and 30.1% in 6 nm mesopore. After the early CO2 injection, the transport diffusion of CH4 exceeds that of CO2, and the selectivity of CH4 over CO2 increases in the subsequent CO2 huff-n-puff cycles. The CO2–CH4 mixture under greater confinement effect manifests higher diffusion selectivity, which benefits the separation of the CO2–CH4 mixture and further contributes to the CH4 recovery.