Pore structure and methane adsorption of coal reservoir are closely correlated to the coalbed gas occurrence and outburst. Full-scale pore structure and its fractal heterogeneity of coal samples were quantitatively characterized using low-pressure N2 gas adsorption (LP-N2GA) and high-pressure mercury intrusion porosimetry (HP-MIP). Fractal pore structure and adsorption capacities between outburst and nonoutburst coals were compared, and their geological significance to gas occurrence and outburst was discussed. The results show that pore volume (PV) is mainly contributed by macropores (>1000 nm) and mesopores (100–1000 nm), while specific surface area (SSA) is dominated by micropores (<10 nm) and transition pores (10–100 nm). On average, the PV and SSA of outburst coal samples are 4.56 times and 5.77 times those of nonoutburst coal samples, respectively, which provide sufficient place for gas adsorption and storage. The pore shape is dominated by semiclosed pores in the nonoutburst coal, whereas open pores and inkbottle pores are prevailing in the outburst coal. The pore size is widely distributed in the outburst coal, in which not only micropores are dominant, but also, transition pores and mesopores are developed to a certain extent. Based on the data from HP-MIP and LP-N2GA, pore spatial structure and surface are of fractal characteristics with fractal dimensions D m 1 (2.81–2.97) and D n (2.50–2.73) calculated by Menger model and Frenkel–Halsey–Hill (FHH) model, respectively. The pore structure in the outburst coal is more heterogeneous as its D n and D m 1 are generally larger than those of the nonoutburst coal. The maximum methane adsorption capacities ( V L : 15.34–20.86 cm3/g) of the outburst coal are larger than those of the nonoutburst coal ( V L : 9.97–13.51cm3/g). The adsorptivity of coal samples is governed by the micropores, transition pores, and D n because they are positively correlated with the SSA. The outburst coal belongs to tectonically deformed coal (TDC) characterized by weak strength, rich microporosity, complex pore structure, strong adsorption capacity, but poor pore connectivity because of inkbottle pores. Therefore, the area of TDC is at high risk for gas outburst as there is a high-pressure gas sealing zone with abundant gas enrichment but limited gas migration and extraction.

Low-rank coal samples from the Xishanyao Formation in the southern Junggar basin of Xinjiang were processed under different negative pressures in order to examine the microstructural characteristics of low-rank coal reservoirs. The pore structures of low-rank coal under different negative pressures were tested using scanning electron microscopy, low-temperature nitrogen adsorption–desorption, and water saturation and centrifugal low-field NMR experiments. The results showed that for the low-rank coal samples from the study area, a high portion of the porosity and surface area came from micropores and small pores; the fractal dimension of the adsorption pores of the low-rank coal samples was divided into surface fractal dimension D1 and structural fractal dimension D2, which showed that the microstructure of the low-rank coal from the study area was complex. The transverse relaxation times T2 of the low-rank coal samples in the test were approximately 0.1~2.5, approximately 10, and greater than 100 ms; the T2 spectrum had basically three peak types. By combining scanning electron microscopy and nuclear magnetic resonance tests, it was concluded that the pore connectivity of the low-rank coal reservoirs in the study area was poor and the effective porosity was relatively low, which may be unfavorable for the exploration and development of coalbed methane.