Dynamic pore-fracture characteristic and evolution influenced by the underground mining considering the in-situ stress

Abstract

The in-situ pore-fracture development is crucial for the methane drainage during underground mining. In this study, the dynamic characteristic and evolution mechanism of the internal pore-fracture structure located at the mining face were investigated by a self-developed true triaxial coupling nuclear magnetic resonance (NMR) experiment system, considering the mining-induced in-situ stress. The mechanical response characteristics, pore-fracture distribution, morphology and connectivity of coal were analyzed through the T2 spectrum and magnetic resonance imaging (MRI). The result shows that the pore-fracture of the coal is composed of adsorption pore (58%–67 %, AP＜1.5 ms), seepage pore (28%–43 %, 1.5 ms＜SP＜98 ms) and migration pore (2.3%–7.5 %, 98 ms＜MP). The compression, pore-dilation, and crack and fracture creation are successively occurred responding to the stress loading. The dynamic drop, slight increase and stabilization were sequentially presented in pore volume, while pore distribution gradually approached the homogeneity, which was mainly contributed by the evolution of SP and MP. The development-, transformation- and mixture-type pore-fracture, occurred at lower in-situ stress (LISS), middle in-situ stress (MISS), and high in-situ stress (HISS), was dominated by the SP-MP, AP-SP-MP, and SP-MP respectively. The fractal dimension experienced the sharp drop, dynamic increase and dramatic decrease with the dynamic- and slight-variation amplitude were observed at LISS, HISS and MISS, respectively. A slight increase of the number in connected channels was formed by the transformation-dependent AP and mixture-dependent AP and MP for LISS and HISS, while the dynamic increase of connective channel was created by the developed crack and fracture. The optimal methane channels were presented the elastic-plastic period with mature SP-MP pore-fracture in HISS, AP in MISS, and SP-MP in LISS. The finding provides insight into the co-mining of coal and methane.