Classification of Pore-fracture Combination Types in Tectonic Coal Based on Mercury Intrusion Porosimetry and Nuclear Magnetic Resonance

Differences in content, distribution, and connectivity of pores and fractures with different sizes in coal lead to different modes of gas migration. An accurate classification of pore-fracture combination types in coal can lay a foundation for studying gas migration. High-pressure mercury intrusion and nuclear magnetic resonance (NMR) experiments were conducted on coal samples collected from the Changping coal mine in Jincheng City, Shanxi Province, and Pingdingshan no. 4 mine in Pingdingshan City, Henan Province, China. The fractal dimensions of pores with different sizes were calculated using the Menger model. By combining the data with T-2 spectra obtained by NMR, critical values for distinguishing diffusion pores from seepage pores-microfractures were determined. In addition, the main parameters affecting development of diffusion pores and seepage pores-microfractures and pore-fracture connectivity were analyzed, and a comprehensive evaluation index system for pores and fractures was established by selecting eight indices. Based on the method combining the analytical hierarchy process with multiparameter superposition, a method for determining critical values, establishing the evaluation index system, and classifying pore-fracture combination types was formed. The pore-fracture combination types in the test coal samples were classified according to the experimental data. The results indicate that the critical values for distinguishing diffusion pores from seepage pores-microfractures based on fractal dimensions obtained through mercury intrusion porosimetry and T-2 spectra obtained by NMR are 72 nm and 2.5 ms, respectively. The studied coal samples can be classified into three combination types, separately characterized by high diffusivity and permeability and poor pore-fracture connectivity; low diffusivity, high permeability, and good pore-fracture connectivity; and low diffusivity and permeability and good pore-fracture connectivity. In the coal samples from the Changping coal mine, diffusion pores and seepage pores-microfractures are developed, while the connectivity between pores and fractures is poor. The coal samples from Pingdingshan no. 4 mine have undeveloped diffusion pores and seepage pores- microfractures but good connectivity between pores and fractures. The research results provide a method for classifying pore- fracture combination types in coal samples taken from different regions.