Pore structure evolution of low-rank coal in China

In China, low-rank coalbed methane (CBM) resources are in the early stage of exploration. The evolution of pore structure for these coals around the first coalification jump is incomplete. Here systematic experiments were conducted to analyze the reservoir of low-rank coal (lignite and subbituminous coal) from China's major low-rank basins, thus revealing the pore structure evolution of low-rank coal in China. Scanning electron microscopy (SEM) demonstrates that there are obvious differences in the pore types of different rank coals. Well-developed cell lumina provides a large amount of macropore space in coals with Ro < 0.5%. After coal metamorphism and physical compaction, the volume and number of plant tissue pores are reduced, resulting in a significant reduction in macropores. The Fourier transform infrared spectroscopy (FTIR) spectral characteristics indicate that the evolution of chemical structure also has an effect on the pore structure of coal. Due to the low proportion of aromatic nuclei, the high proportion of functional groups and long side chains, when Ro is <0.5%,the spatial structure of coal reservoir is “loose”, resulting in a large surface area. With increasing coal maturity, the side chains gradually decompose and the length decreases, and the aromatic nuclei increase so that the structure becomes more “compact”. With increasing coal rank, the pore size distribution changes significantly. The peak shape of the NMR T2 spectrum shows a series of complex changes from trimodal to bimodal to trimodal and finally to bimodal, indicating that the pore size distribution undergoes continuous changes. With the continuous coalification and physical compaction, the porosity decreases rapidly with vitrinite reflectance increases from 0.42–0.68%, and the adsorption capacity shows an increasing trend with the increase of coal rank under the control of moisture content, pore type, and specific surface area. Understanding the evolution of pore structure in the early stage of coalification will provide a scientific reference and foundation for exploration and development of low-rank CBM.