FIB-SEM and X-ray CT characterization of interconnected pores in high-rank coal formed from regional metamorphism

Pores in coal and their connectivity are important properties of coal, providing network or channels for gas storage and migration within coal, e.g. during the coalbed methane (CBM) recovery. To investigate the growth characteristics and genetic types of pores in coal in terms of macropore and mesopore, the pores of a high-rank coal were measured by various techniques such as the mercury intrusion method, nitrogen adsorption, focused ion beam scanning electron microscopy (FIB-SEM), and X-ray micro-CT (Computed Tomography). Two high-rank coals formed from regional metamorphism collected from the southern Qinshui basin were selected. The FIB-SEM and X-ray micro-CT provides detailed experimental information for development of a three dimensional (3D) pore network model, which was further used to characterize the pore connectivity. Volume percent of pores of these high-rank coals are dominated by mesopores of approximately 10–50 nm in width, and then followed by micropores, along with the smallest volume percent of macropores. The connectivity within this high-rank coal was mesopore-dominated pore network. Electron microscopy observations further revealed there are coalification-related pores and mineral-related pores in the high-rank coal. The coalification-related pores can be classified as secondary gas pores in organic matter and shrinkage-induced pores around quartz and clay minerals; and the mineral-related pores are developed within minerals, and can be classified as dissolution-created pores and intercrystalline pores. The secondary gas pores are macropores and have poor connectivity. The mineral-related pores can be both macropores and mesopores, and have little influence on pore connectivity due to small content of carbonate minerals in these samples. Under electron microscopy, the shrinkage-induced pores are mainly mesopores. The regional metamorphism, with a high abnormal old thermal field in the research area, is the precondition of the formation of the shrinkage-induced pores. The quartz and clay minerals in the coal provide different formation conditions and hence form different shapes of the shrinkage-induced pores. The coal samples include a large number of shrinkage-induced pores that act as the interconnected pores in the coal and exhibit good connectivity. The quartz and clay minerals play a significant role in developing the interconnected pores in the high-rank coal formed from regional metamorphism.

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