Changes in the three-dimensional molecular structure of coal during methane adsorption induced swelling

Methane (CH4) adsorption-induced swelling is one of the critical factors controlling the permeability of coalbed methane (CBM). CH4 adsorption alters the molecular structure of coal so as to induce coal swelling, and many uncertainties still exist in the process. In this study, the change in the molecular structures of different chemical structures by CH4 adsorption was investigated using the Grand Canonical Monte Carlo method to simulate the alteration of bond lengths and bond angles during swelling. The results demonstrate that the alteration of chemical structure is more extensive than a chemical bond, which is the critical factor causing the swelling behavior. Owing to the complex molecular structure of coal, among the different types of chemical structures, the C-O-C (-O-) chemical structure showed the most significant change in bond angle, with the largest degree of change is 12.89%. Compared with other chemical structures, the C-C-C (aromatic -C-) chemical structures are more stable and the largest degree of change is 0.65%. For the different types of chemical bonds, the C-C chemical bonds showed the most significant change in bond lengths, with the largest degree of change is 2.94%. And the O-H chemical bond showed the smallest change, with the largest degree of change is 0.79%. Considering the structure evolution of coal, the C-O-C (-O-) chemical structure decreases with increasing maturity and changes to the greatest after the adsorption of methane. The aromatic structure increases and the degree of deformation decreases, which is consistent with the previous experimental values for swelling. These results reveal the details of different types of chemical group deformation, providing a molecular-level insight into adsorption swelling and permeability changes.