Mechanistic insight into the optimal recovery efficiency of CBM in sub-bituminous coal through molecular simulation

In this study, a newly created Fortran program was used to assist in the constructing of a sub-bituminous coal model (SCM). Followed by grand canonical Monte Carlo simulations, the adsorption behaviors of CBM adsorption on dry-/moisture-SCM systems have been systematically studied at different pressure-temperature conditions. A geological model with typical geothermal and pressure gradients was established to investigate the effect of coupled pressure and temperature on the absorption and recovery. As a result, pure-N-2 injection has been confirmed to paly a negative role on CH4 recovery, whereas CO2 could effectively improve the CO2 sequestration and CH4 displacement. Meanwhile, the adsorption percentage (delta) and the recovery efficiency (eta) were first put forward in terms of theoretical calculations. The results showed that, the delta was determined by pore size distribution of the SCM, adsorption temperature/pressure, moisture-containing in coal and the types of adsorbates. The value of eta(CH4) is 53.07% in dry coal seam, whereas 47.20% is evaluated in moisture coal seam, the corresponding optimum mining depths are 600 m and 800 m, respectively. Remarkably, water-containing in coal seam has an obviously inhibitory effect on simultaneous extraction of coal and gas, and it should be valued in the follow-up studies. Based on this principle, the optimum mining depth for CO2-ECBM was predicted to be similar to 800 m in actual deep coal seams through our geological model. Our results provide a molecular-scale insight into CBM recovery efficiency in coal at different burial depths, and offer useful guidance for realistic exploitation.