Abstract
In this paper, an experimental study of the variation of resistivity of coal samples in different bedding directions at 1 MHz frequency was performed by establishing an experimental system for resistivity testing of coal under triaxial stress. The low‐pressure nitrogen gas adsorption (LP‐N2GA) experiment and scanning electron microscopy (SEM) were obtained to analyze the pore‐fracture structural characteristics of coal samples and the influence on resistivity anisotropy. Furthermore, the fundamental cause of anisotropy of coal resistivity is expounded systematically. The results show that the resistivity of loaded coal decreased first before increasing. The ionic conductance and the high degree of metamorphism slow down the decrease of resistivity. The distribution of pore and fracture structures is anisotropic. The connected pores and fractures are mainly distributed along the parallel bedding direction. The weak plane of bedding, diagenetic fractures, and plane fracture structures of parallel bedding result in the increase of fractures in the direction of vertical bedding, so increasing the potential barrier. Therefore, the resistivity in the vertical bedding direction is higher than that of the parallel bedding. Loading coal resistivity anisotropy degree is a dynamic change trend; the load increases anisotropy significantly under axial pressure, and the degree of anisotropy has a higher discreteness under confining pressure. It is mainly the randomness of the internal pore‐fracture compaction, closure, and development of the heterogeneous coal under the confining pressure; the more rapid the decline in this stage, the larger the stress damage degree.
Highlights
In recent years, China’s demand for coal energy keeps increasing, which leads to the stage of deep mining. e increase of mining depth and intensity increases the possibility of coal-rock composite dynamic disasters such as rock burst and coal and gas outburst [1, 2]
Distribution, and connectivity lead to distinct resistivity, and pore structure has a significant effect on resistivity [8]. e change of electrical parameters of loaded coal is obviously consistent with the stress. e electrical parameters can better reflect the formation, development, and penetration of microfracture, as well as the characteristics of changes in the internal structure in the failure process of coal and rock [9, 10]. e direction of the anisotropic principal axis of the rock with the largest change in the apparent resistivity is basically the same as the direction of the main fracture of the rock. ere are significant anisotropy of measured points in cracks and fractured zones
Experimental Results on Resistivity Anisotropy of Loaded Coal Samples e variation of the resistivity of coal samples with different bedding direction was measured under axial pressure and confining pressure during 1 MHz frequency. e temperature of the coal samples was controlled at 30°C to reduce the interference of temperature on resistivity changes
Summary
China’s demand for coal energy keeps increasing, which leads to the stage of deep mining. e increase of mining depth and intensity increases the possibility of coal-rock composite dynamic disasters such as rock burst and coal and gas outburst [1, 2]. Research on electrical parameter changes based on the electrical and physical properties of coal is the physical property prerequisite for conducting electrical exploration in underground coal mines [3, 4]. E response position of abnormal high resistance of loaded coal is consistent with the fracture development position [6]. By monitoring the local resistivity and its anisotropy change, the coal and gas outburst can be predicted and forewarned [7]. E change of electrical parameters of loaded coal is obviously consistent with the stress. E electrical parameters can better reflect the formation, development, and penetration of microfracture, as well as the characteristics of changes in the internal structure in the failure process of coal and rock [9, 10]. It was found that the apparent resistivity of the perpendicular formation is greater than that of the parallel formation [15]
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