Molecular dynamics study of CO2 sorption and transport properties in coal

• CO2 sorption-induced coal swelling negatively correlates with CO2 self-diffusivity.
• CO2 – coal interaction controls the behaviors of coal swelling/CO2 self-diffusivity.
• Swelling/self-diffusivity are separated into two opposing stages at around 8 MPa.
• Transport diffusivity exceeds self-diffusivity as thermodynamic factor >1.
• CO2 permeability exponentially increases with decreasing reservoir gas pressure.

An understanding of gas transport in nano-scale porous media is crucial for many industrial applications, for example, processes associated with CO2 injection, storage and enhanced coalbed methane (ECBM) production. In this study, we carried out combined molecular dynamics (MD) and Grand Canonical Monte Carlo (GCMC) simulations on the transport properties (i.e. self- and transport diffusivities and permeability) of CO2, in a realistic intermediate rank bituminous coal (flexible coal model) at a temperature of 328 K (55 °C) and a range of pressures up to 25 MPa. Self-diffusivity and sorption isotherms of CO2 are obtained directly from the MD and GCMC simulations. The Maxwell–Stefan diffusion model was then applied to correlate the self- and transport diffusivities. The permeability was computed through an integration of the transport diffusivity over the sorption concentration obtained from the simulations. The results show that CO2 self-diffusivity decreases with increasing reservoir gas pressure up to 8 MPa, then increases with pressure due to the interaction between coal and CO2. The transport diffusivity increases with the reservoir gas pressure as a result of an enhanced thermodynamic factor. The simulation results reveal a negative correlation between the sorption-induced coal swelling and CO2 self-diffusivity due to the interaction between CO2 and coal. Rigorous modeling of gas recovery and production thus requires consideration of specific interaction of the gas and coal matrix. Permeability of CO2 exponentially increases with the decreasing reservoir gas pressure, which is comparable with published field data.