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The mechanisms of CO 2 /CH 4 adsorption in coal are the theoretical foundation for CO 2 sequestration in coal seams targeted for enhanced coalbed methane recovery. Herein, by changing the model (low rank coal: WMC, middle rank coal: XM and high rank coal: CZ) with plenty of side aliphatic chains and functional groups established in the literature, the influence and mechanism of pore parameters and functional groups(−CH 3 , − OH, −C 2 O, −C=O) on the adsorption of CO 2 and CH 4 in different rank coals are systematically studied. Using the Connolly surface algorithm to calculate the pore volume (V F ) and the specific surface area (S SA ) of coal with different functional groups, it can be seen that the influence of the functional group change on the pore structure is related to the coal rank. Changing the various functional groups in the original coal structure to a unified functional group (−CH 3 , −OH, −C 2 O, or −CO) will increase the accessible pore volume (V F ) and the specific surface area (S SA ), except in low-rank and middle-rank coal, where the ordered arrangement of −CO will decrease V F and S SA . The adsorption capacities of different pore parameters and functional groups were calculated by Grand Canonical Monte Carlo simulation and density functional theory. On pure adsorption, the pore parameters exert greater influence than the functional groups. By comparing the adsorption energy of the original pore structure containing functional groups and that of modified pores without functional groups, the contributions of the pore structure and original functional groups on CO 2 /CH 4 adsorption are 71 and 29% and 83 and 17%, respectively. Small-diameter pores and −C 2 O have a strong adsorption capacity. In terms of competitive adsorption, the −C O functional groups and pore diameters ranging from 1.0 to 2.0 nm can significantly enhance the selectivity of CO 2 over CH 4 . The CH 4 and CO 2 adsorption does not occur via rigorous monolayer adsorption; multilayer adsorption can occur for CH 4 and CO 2 with pore diameters of 1.0−2.0 and 1.0−2.2 nm, respectively, thus causing micropore filling. These quantitative results establish a foundation for the development of adsorption theory for CO 2 /CH 4 in coal.

In CO 2 -enhanced coalbed methane (CO 2 -ECBM) engineering, accurate knowledge of the interaction mechanism of CO 2 and coal matrix is crucial for improving the recovery of CH 4 and contributing to the geological sequestration of CO 2 . This study is performed to prove the accuracy of molecular simulation and calculate the variation characteristics of pore structure, volumetric strain, mechanical properties, Fourier transform infrared (FT-IR) spectra, and the system free energy by molecular dynamics (MD) and grand canonical Monte Carlo (GCMC) methods. According to the obtained results, a relationship between pore structure, swelling strain, mechanical properties, chemical structure, and surface free energy was established. Then, the correlation of various coal change characteristics was analyzed to elucidate the interaction mechanism between CO 2 and coal. The results showed that (1) the molecular simulation method was able to estimate the swelling mechanism of CO 2 and coal. However, because the adsorption capacity of the molecular simulate is greater than that of the experiment and the raw coal is softer than the macromolecular structure, the molecular results are slightly better than the experimental results. (2) As pressure increased from 0 to 4 MPa, the intramolecular pores and sorption-induced strain changed significantly, whereas when the pressure increased from 4 to 8 MPa (especially at 6–8 Mpa), there was an increase of the intermolecular pores and mechanical properties and transition from elastic to plastic. In addition, when the pressure was >8 MPa, the coal matrix changed slightly. ScCO 2 with a higher adsorption capacity results in greater damage and causes larger alterations of coal mechanical properties. (3) The change of the coal matrix is essentially controlled by the surface free energy of the molecular system. E valence affects the aromatic structure and changes the volume of the intramolecular pores, thus affecting the sorption-induced strain change rate. E non affects the length of side chains and the disorder degree of coal molecules and changes the volume of the intramolecular pores, thus affecting the mechanical property change rate. Our findings shed light on the dynamic process of coal swelling and provide a theoretical basis for CO 2 enhancing the recovery of CH 4 gas in coal.

Bituminous coal reservoirs exhibit pronounced heterogeneity, which significantly impedes the production capacity of coalbed methane. Therefore, obtaining a thorough comprehension of the pore characteristics of bituminous coal reservoirs is essential for understanding the dynamic interaction between gas and coal, as well as ensuring the safety and efficiency of coal mine production. In this study, we conducted a comprehensive analysis of the pore structure and surface roughness of six bituminous coal samples (1.19% < Ro,max < 2.55%) using various atomic force microscopy (AFM) techniques. Firstly, we compared the microscopic morphology obtained through low-pressure nitrogen gas adsorption (LP-N2-GA) and AFM. It was observed that LP-N2-GA provides a comprehensive depiction of various pore structures, whereas AFM only allows the observation of V-shaped and wedge-shaped pores. Subsequently, the pore structure analysis of the coal samples was performed using Threshold and Chen’s algorithms at ×200 and ×4000 magnifications. Our findings indicate that Chen’s algorithm enables the observation of a greater number of pores compared to the Threshold algorithm. Moreover, the porosity obtained through the 3D algorithm is more accurate and closely aligns with the results from LP-N2-GA analysis. Regarding the effect of magnification, it was found that ×4000 magnification yielded a higher number of pores compared to ×200 magnification. The roughness values (Rq and Ra) obtained at ×200 magnification were 5–14 times greater than those at ×4000 magnification. Interestingly, despite the differences in magnification, the difference in porosity between ×200 and ×4000 was not significant. Furthermore, when comparing the results with the HP-CH4-GA experiment, it was observed that an increase in Ra and Rq values positively influenced gas adsorption, while an increase in Rsk and Rku values had an unfavorable effect on gas adsorption. This suggests that surface roughness plays a crucial role in gas adsorption behavior. Overall, the findings highlight the significant influence of different methods on the evaluation of pore structure. The 3D algorithm and ×4000 magnification provide a more accurate description of the pore structure. Additionally, the variation in 3D surface roughness was found to be related to coal rank and had a notable effect on gas adsorption.