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The adsorption gas of coal matrix will cause its swelling, change the fracture structure, and further affect the permeability of the coal seam. In this study, CT image processing technology was used to numerically simulate the swelling of different extents (0.05%, 0.10%, 0.15%, 0.20%, and 0.25%). At the same time, the evolutions of geometric parameters, connectivity, and fractal dimension of fractures under different swelling extents were quantified. The results show that with the increase of the swelling extents, the average length, width, and aperture of fractures decreased by 18.69%, 24.14%, and 52.24%, respectively. The connected porosity of fractures decreased by 44.83%, indicating fractures are closed and the connectivity becomes worse. It is noteworthy that when the swelling extent is less than 0.1%, the average aperture and connected porosity of fractures are significantly reduced by 32.84% and 31.03%, respectively. When it is greater than 0.15%, and the average aperture and width of fractures are clearly reduced by 49.25% and 23.28%, respectively. The fractal dimension increased from 2.132 to 2.211, indicating that the fracture structure became complex. Additionally, using Darcy’s law to determine the change in absolute permeability during swelling, the results show that there was a significant negative exponential correlation between permeability and swelling extents. The decrease of aperture and connected porosity are the main factors of poor permeability.

Coalbed methane (CBM) mainly adsorb in massive pores of coal. The accurate characterization of pores benefits CBM resource evaluation, exploration and exploitation. In this paper, mercury intrusion porosimetry (MIP) and low temperature nitrogen adsorption (N2GA) combined with low field nuclear magnetic resonance (NMR) experiments were conducted to analyze the advantages and differences among different experimental techniques in pore characterization. The results show that the total porosity has a tendency to decrease first and then rise with the increase of coal rank, which is mainly caused by the compaction in early stage and the thermogenic gas produced in middle and late stages of coalification. The comparison between different techniques shows that NMR is superior to the conventional methods in terms of porosity and pore size distribution, which should be favorable for pore characterization. The N2GA pore size measurement, based on BJH model, is only accurate within 10‒100 nm in diameter. There is a peak misalignment between the NMR and MIP results in the pore size comparison. The reason for this phenomenon is that there is a centrifugal error in NMR experiment, which could cause a differential damage to the coal sample, resulting in partial loss of the nuclear magnetic signal.