Study on full-scale pores characterization and heterogeneity of coal based on low-temperature nitrogen adsorption and low-field nuclear magnetic resonance experiments.

The characteristics and heterogeneity of coal pores are crucial for understanding the production mechanism of coalbed methane (CBM). In this study, coal samples with varying degrees of metamorphism (0.58%  ≤ R, ≤ 3.44%) were collected. The characteristics of pore development and the heterogeneous properties of pores were revealed through low-temperature nitrogen adsorption (LTNA) and low-field nuclear magnetic resonance (NMR) experiments. The results indicate that pores with varying diameters exhibit favorable development in low-rank coals, along with favorable pores connectivity. The micropores composition of middle-rank coals was found to be 73.56%, however, the connectivity among transitional, meso, and macropores was observed to be poor. In high-rank coals, the proportion of micropores was 92.74%, with numerous micropores being closed or semi-closed. This resulted in inferior connectivity between micropores and transitional pores. As coal metamorphism progressed, the D (characterizing the roughness of adsorption pores (AP) surface, ranging from 2.13 to 2.45) and D (characterizing the complexity of AP structure, ranging from 2.56 to 2.77) initially decreased and then increased, whereas the D (characterizing the heterogeneity of seepage pores (SP), ranging from 2.92 to 2.95) consistently improved. Furthermore, the roughness of pore surface and the complexity of pore structure in AP increased as the specific surface area and volume of pores increased. On the contrary, as the SP content increased, the uniformity of the pore structure improved. When the volume of SP remained constant, the complexity of the pore structure decreased due to increased pore connectivity.