Micro-Nano Fine Characterization of Coal Fracture Evolution During the Triaxial Compression Creep

The production and evolution of fractures during coal creep will directly affect the occurrence, extraction and flow law of gas in a coal seam. The coal fracture evolution under creep conditions was studied by qualitative analysis and quantitative characterization. At a room temperature of 24 °C, triaxial compression creep tests of coal samples from the Zhaogu No. 2 coal mine in Jiaozuo were carried out under different loading conditions (0 MPa, 6 MPa, 9 MPa and 12 MPa), and low field nuclear magnetic resonance technique tests and industrial CT scanning experiments were performed. The obtained CT images were analyzed with the MATLAB software for equalization and binary image processing. The development and distribution of fractures in coal samples under different loading conditions were studied. The results show that the internal fractures are unevenly distributed and controlled by the main fracture, and the expansion direction of fractures is parallel to the direction of the maximum effective compressive stress. The number of fractures shows an increasing trend with the increase of axial stress, and the pace of growth of new fractures accelerates. The primary fractures in the coal body expand and generate new fractures, which improves the connectivity of the fractures in the coal body. The research results can provide a basis for studying the gas flow rule around the borehole and determining the influence range of the borehole.