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Study on the evolution mechanism of permeability and porosity of coal under repeated mining stress path

• 1、School of esources and Geoscience，China University of Mining ＆Technology，Xuzhou 221116
• 2、Key Laboratory of Coalbed Methane Resource＆Reservoir Formation Process，Ministry of Education(China University of Mining＆Technology)，Xuzhou 221116

Abstract：Coal rock permeability plays a critical role in the synergistic mining of coal and gas, and the permeability is closely related to the internal microstructure of coal rock. In order to veStudy on the evolution mechanism of permeability and porosity of coal under repeated mining pathrify the influence of repeated unloading stress paths on the permeability of coal rocks, this paper uses the 1000-50VHT rock high temperature triaxial rheological experiment system to conduct triaxial permeability experiments on coal blocks by repeatedly unloading surrounding and axial pressures at rated rates, and analyzes the evolution of the permeability of coal blocks from 0, 15, 30, 40 to 58 MPa by repeatedly unloading partial stresses at 15 MPa surrounding pressure. In order to study the influence of internal microstructure on permeability of coal blocks, the internal pore characteristics of coal samples after each unloading stage were observed by NMR T2 spectroscopy using NMR instruments, and the comparison of the changes of internal microstructure of coal blocks before and after each stage of repeated unloading was completed. The results showed that the permeability change pattern of the coal samples during repeated unloading coincided with the pore size change pattern and was positively correlated, and the five repeated unloading peritectic and bias pressure experiments resulted in the increase of adsorption pore volume by 50.79%, increase of percolation pore by 17.68%, decrease of fracture by 46.15%, and increase of porosity by 36.02%; the damage of coal samples under the repeated unloading stress path could be The damage of coal samples under repeated loading and unloading stress paths can be divided into two stages: internal structural damage and complete shear damage, in the internal structural damage, the permeability of coal samples was 7.58728E-18m2, the intensity of fracture signal was 277.78, the permeability and the amount of fracture development reached the maximum, in the complete shear damage, large cracks appeared in coal samples, the internal pores and fractures were secondarily compressed, the permeability and the number of fractures were reduced; the initial permeability of coal samples during cyclic loading partial stress The initial permeability of the coal samples during the process of cyclic loading of partial stresses changed less compared with the initial permeability of the previous stage, and the maximum damage rate was only 9.01%, indicating that the repeated loading and unloading of partial stresses under the action of surrounding pressure caused less influence on its internal microstructure. The research results have theoretical significance and reference value for the synergistic mining of coal and coalbed methane.

Keywords： Lignite mining stress path permeability pore structure