CO2-ECBM in coal nanostructure: Modelling and simulation

Coal is a porous media, and most pores in coal are nanopores (pores at the nanoscale). In order to investigate CO2-ECBM based on the nanostructure obtained by synchrotron radiation nano-CT at the Beijing Synchrotron Radiation Facility (BSRF), a nanopore structure-based CO2-ECBM model was developed, in which extended Langmuir Equation was applied to formulate the competitive adsorption mechanism of binary gas system of CO2-CH4 and gas ad/de-sorption and diffusion were coupled. CH4 and CO2 distribution in the coal nanostructure during the CO2-ECBM was visualized and the effect of CO2 injection pressure on methane recovery and CO2 sequestration was investigated. The increase of CO2 injection pressure enhances methane recovery and CO2 injection, but the contribution of CO2 injection pressure on methane recovery rate and CO2 injection rate becomes weaker with the increase of CO2 injection pressure. Because prior researches have shown that gas diffusivity varies with gas pressure, the impact of CO2 diffusion coefficient which varies with CO2 injection pressure on methane recovery and CO2 injection was investigated, and results show the variation of CO2 diffusion coefficient caused by the variation of CO2 injection pressure can explain why the increase of CO2 injection pressure can enhance CH4 recovery and CO2 injection. The difference in CO2 injection rate and CH4 recovery rate between different coal samples is related to the difference in gas diffusivity in different coal samples. CO2 injection rate and CH4 recovery rate is relatively higher in coal with higher gas diffusivity. The higher diffusion coefficient of CO2 caused by the lower kinetic diameter of CO2 can also help to explain why CO2 injection rate is larger than CH4 recovery rate. This study provides a new option to explore the CO2-ECBM at the nanopore scale.