Molecular simulation study on influence of water content on CO2 displacement of CH4 in deep coal seams

Abstract

To reveal the influence of water content on CO2 injection-enhanced coalbed methane recovery (CO2-ECBM) (CO2 displacement of CH4) in deep coal seams, this study used Fukang deep coal as the research object. Experiments such as 13C nuclear magnetic resonance spectroscopy (13C-NMR) and X-ray photoelectron spectroscopy (XPS) were conducted. A coal matrix model was constructed using molecular simulation software, and the microscopic mechanism of water’s effect on the CO2-ECBM process in deep coal seams was studied using molecular simulation methods. The results showed that after coal matrix adsorbed gas, it underwent significant expansion, and the pore volume significantly decreased. When saturated with adsorbed CH4, the coal matrix porosity decreased by 72.2% compared to the initial value. When the molar ratio of CO2 to CH4 ( xCO2/xCH4 <?fx-imagestate width="10.24466610" height="3.30200005"?> <?fx-imagestate width="10.24466610" height="3.30200005"?> ) was 2, the permeability of coal matrix decreased by 83.8%. Increasing water content significantly inhibited coal reservoir performance. Compared to dry coal, the permeability of the coal matrices with 1%, 3%, and 5% water contents decreased by 50.9%, 94.9%, and 99.6%, respectively, indicating that water strongly hindered gas flow. Competitive adsorption characteristics showed that as xCO2/xCH4 <?fx-imagestate width="10.24466610" height="3.30200005"?> <?fx-imagestate width="10.24466610" height="3.30200005"?> increased, CO2 adsorption amount increased, while CH4 adsorption amount rapidly decreased and was displaced. When xCO2/xCH4 <?fx-imagestate width="10.24466610" height="3.30200005"?> <?fx-imagestate width="10.24466610" height="3.30200005"?> ≥ 1.2, the displacement rate tended to be stable. Increasing water content reduced the absolute adsorption amounts of both CO2 and CH4, as well as the CO2 injection displacement ratio, but had a smaller impact on the relative displacement rate of CH4. The adsorption heat of CO2 was higher than that of CH4, indicating that CO2 had a stronger affinity for coal. Increasing water content enhanced the adsorption heat of both gases, but both values remained below 42 kJ/mol, indicating that the adsorption process was physical adsorption. The interaction energies of coal-CO2, coal-CH4, and CO2-CH4 followed the order of ECoal‒CO2 <?fx-imagestate width="8.97466660" height="3.30200005"?> <?fx-imagestate width="8.97466660" height="3.30200005"?> > ECoal‒CH4 <?fx-imagestate width="9.05933285" height="3.30200005"?> <?fx-imagestate width="9.05933285" height="3.30200005"?> > ECO2‒CH4 <?fx-imagestate width="8.97466660" height="3.30200005"?> <?fx-imagestate width="8.97466660" height="3.30200005"?> , and CO2 maintained an advantage in competitive adsorption. The diffusion coefficients of CO2 and CH4 decreased significantly with increasing water content and xCO2/xCH4 <?fx-imagestate width="10.24466610" height="3.30200005"?> <?fx-imagestate width="10.24466610" height="3.30200005"?> , and the decrease in CH4 was greater than that in CO2, indicating that CH4 diffusion was more sensitive to water. The study reveals the microscopic mechanism of CO2 displacement of CH4 in water-containing coal seams and provides a theoretical basis for the efficient development of CBMe and the engineering practices of CO2 geological storage.