Microstructure Analysis on the Fracture Network in High-Rank Coals

Abstract

Microfractures in the coal matrix have complex spatial structure because of the joint effects of coalification and tectonism. The accurate modeling and characterization of microfractures directly affect mechanism analysis of coal rock fracturing and mining. In this study, taking the high-rank coal in the north of Qinshui Basin, structural characteristics of microfracture in intact and tectonic coals were analyzed systematically using 2D thin section, 3D micro-CT modeling and mercury intrusion based on digital image analysis and fractal theory. The result indicates that the response sensitivity of fractal parameters obtained from different methods to microfractures from two types of coal structures is different greatly. Single fractal dimension (Df) in tectonic coal is higher than that of intact coal. Specially, for 2D thin sections, the indicative effect on the Df of manually corrected images is better than that of automatically extracted and linear ones. Df has a positive correlation with the pixel-based microfracture porosity, as well as generalized fractal parameters. Moreover, microfracture permeability based on the Cubic-Law increases with the increase of Df. For 3D micro-CT modeling, the classical Watershed algorithm cannot segment the spatial microfracture network, which, however, can be segmented and characterized by the self-developed FracSC3D program. Spatial microstructural parameters including volume, surface and length at three-axis directions meet bifractal characteristics. Specially, for 2D micro-CT slices, with the increase of porosity, the multifractal spectrum width Δα also increased. Besides, the fractal parameter based on Menger model from mercury intrusion experiment can act as a good indicator for identifying microfractures, flow pores and diffusion pores.