Advanced characterization of physical properties of coals with different coal structures by nuclear magnetic resonance and X-ray computed tomography

In order to understand the correlation between coal structure and physical property of coal, samples with different coal structures were collected from the Late Permian period coal seams in the Laochang area, Yunnan Province, China. A set of experiments were carried out to quantitatively characterize the physical properties of coals with different coal structures using advanced and nondestructive low-field nuclear magnetic resonance (NMR) and X-ray computed tomography (X-CT). The experimental results gave us confidence to conclude that the evolution of the coal structures can be divided into five stages with stress increasing: the fractures closing stage, the microfractures development stage, the cracks development stage, the shear deformation stage, and the plastic deformation stage. Each stage corresponds to a different coal structure with unique physical characteristics. The undeformed coal is dominated with pores and a small amount of poorly connected fractures. In the proto-cataclastic stage, the volume of the mesopores, macropores and fractures sharply decreases with stress increasing. The coal rock becomes more compacted. Additionally, the connectivity between fractures and pores becomes worse. The cataclastic coal has well-developed mesopores, macropores and fractures but few micropores and transition pores. The connectivity between fractures and pores is most conducive to the exploitation of coalbed methane. In the mylonitic coal stage, the plastic deformation occurs, resulting in the reduction and discontinuity of mesopores, macropores, and fractures. Moreover, the undeformed coal has the best homogeneity, and the mylonitic coal has the highest heterogeneity, resulted from the uneven distribution of the maceral, pores, fractures, and minerals caused by later stress effect. Furthermore, the CT porosities have a good positive correlation with the permeability; the average CT number, the standard deviation of CT number have a negative correlation with the permeability.