Enhancing the Efficiency of Coal Bed Methane Recovery by Injecting Carbon Dioxide Based on an Anthracite Coal Macromolecular Model and Simulation Methods

Enhancing the efficiency of coal bed methane (CBM) recovery by injecting carbon dioxide (CO2) is regarded as an effective method to exploit CBM, and CO2 geological sequestration also mitigates greenhouse gas emissions. In this study, C-13 nuclear magnetic resonance (C-13 NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) experiments are employed to construct a two-dimensional (2D) chemical molecular structure. The 2D chemical molecular structure is annealed and geometrically optimized to construct a three-dimensional (3D) anthracite molecular model by molecular simulations. The CH4/CO2 mixed gas competitive adsorption and displacement of CH4 by injecting CO2 in an anthracite coal molecular model is investigated by Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. The simulation results show that CO2 is easier to adsorb than CH4, which is conducive for CO2 to have an advantage in competitive adsorption at a temperature of 303.15 K and a pressure of 10 MPa. The total amount of CH4/CO2 mixed gas is higher in small pores than in large pores at a pressure of less than 1 MPa. Simultaneously, the MD simulation result reveals that the displacement efficiency of CH4 recovery by injecting 10 MPa CO2 into a 1 nm pore was enhanced by about 73.08% at the temperature of 303.15 K. These research results will help to realize CO2-enhanced coal bed methane (ECBM) recovery at the microscopic level and geological CO2 sequestration to reduce environmental pollution.