Characterization of the physical properties of coal reservoirs in the western Guizhou and eastern Yunnan by X-ray computed tomography

The western Guizhou and eastern Yunnan are potential exploration areas for coal bed methane (CBM) in South China, and their favorable CBM potential has been demonstrated by CBM exploration and pilot tests. In this paper, the physical properties of coal reservoirs in the study area were systematically studied by means of X-ray computed tomography (X-CT). The research showed that X-CT could identify 4 kinds of media: mineral, durain, vitrain and pore. The CT number of mineral, durain, vitrain and pore were approximately >1 800, 1 500-1 800, 1 000-1 500 and <1 000 Hounsfield unit (HU), respectively. Their specific CT number varied from sample to sample. The estimated porosity values of coals ranged from 3.33% to 7.14%, and mineral contents ranged from 0.11% to 89.03% by means of X-CT, which correlated well with the porosities determined using the helium gas method and proximate analysis. X-CT scans showed that under the influence of stress, undeformed coal has the highest homogeneity, mylonitic coal has the highest heterogeneity, and proto-cataclastic and cataclastic coals were between them. If coals were not affected by stress, their composition heterogeneity in the axial direction was relatively good. The later stress action caused the uneven distribution of coal components, pores, fractures and minerals, which enhanced the heterogeneity of coals. A three-dimensional model of coals was built to show the quantification and 3D visualization of the spatial disposition of minerals, pores and fractures of coals in these areas.