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

• Published in: Fuel (Guildford) 2020-04, Vol.266, p.117137, Article 117137
• Main Authors: Liu, Xiao-Qiang, Li, Meijun, Zhang, Chenghua, Fang, Ronghui, Zhong, Ningning, Xue, Ying, Zhou, Yang, Jiang, Weidong, Chen, Xu-Yang
• Format: Article
• Language: English
• Subjects: Adsorbates; Adsorption; Adsorption percentage; Bituminous coal; Carbon dioxide; Carbon sequestration; Coal; Coal mines; Coal mining; Coalbed methane; Competitive adsorption; Computer simulation; Efficiency; Exploitation; Geological depth; Geology; Methane; Moisture; Optimization; Pore size; Pore size distribution; Porosity; Pressure; Pressure effects; Pressure gradients; Recovery; Recovery efficiency; Size distribution; Stress concentration; Sub-bituminous coal model; Temperature
• ISSN: 0016-2361; 1873-7153
• DOI: 10.1016/j.fuel.2020.117137

[Display omitted] 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-N2 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 (δ) and the recovery efficiency (η) were first put forward in terms of theoretical calculations. The results showed that, the δ was determined by pore size distribution of the SCM, adsorption temperature/pressure, moisture-containing in coal and the types of adsorbates. The value of ηCH4 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 ∼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.