Experimental Study On Fracture Evolution And Gas Seepage Law Of Coal Micro-Pore

With the continuous increase of coal mining depth,the stope stress of deep coal seam increases,and the spatial distribution is not uniform.The fracture of coal body presents a nonlinear development,and the permeability of coal seam changes,which leads to abnormal gas emission in the mining face and frequent gas concentration overrun,seriously affecting the efficient extraction of gas.The prevention and control of mine gas disaster accidents has become one of the key issues of coal and gas safety co-mining.The fracture propagation of coal body is a process from the microscopic structure change caused by stress to the microscopic structure development.It is a multi-scale pore fracture evolution process.The seepage mode of gas in different scale pore fractures is essentially different.Therefore,it is of certain guiding significance for scientific and reasonable prevention and control of gas disasters to clarify the evolution of micro-pore cracks in coal and the law of gas seepage.Through the mercury intrusion experiment,nuclear magnetic resonance experiment and X-ray CT scanning experiment,the pore characteristic parameters of coal body were obtained.It is concluded that the overall pore structure of the experimental coal body is developed,in which the micro pores are more developed,and the connectivity between the micro pores and the large pores is poor.Using fractal theory,the pore structure of coal is quantitatively characterized by the pore size section,and the complexity of pore structure in coal is analyzed.It is shown that the fractal dimension of coal pores is more than 2.7,and the overall complexity of pore structure is high and the heterogeneity is strong.Combined with mercury injection experiment,nuclear magnetic resonance experiment and X-ray CT scanning experiment,the full pore size distribution curve of coal pores was established,indicating that the development of micropores and macropores in coal pores is conducive to gas seepage.The evolution characteristics of surface cracks and internal damage during the loading process of coal samples were analyzed by optical speckle technology and acoustic emission technology.It is concluded that the surface displacement and deformation of coal samples decrease with the increase of loading rate,and the failure mode changes from tensile and shear failure to tensile and shear composite failure.The acoustic emission count and energy increase with the increase of loading rate.The spatial distribution of acoustic emission events can better reflect the evolution characteristics of cracks in coal body.X-ray CT was used to scan the coal specimen before and after fracture.The pore-fracture structure characteristics inside the coal were quantitatively characterized by AVIZO software.The three-dimensional visualization model before and after the fracture of the coal was reconstructed,and the equivalent pore network model was extracted.The equivalent pore radius,pore throat radius,coordination number,surface porosity and other parameters were counted,and the change characteristics of pore-fracture structure before and after the fracture of the coal were analyzed.The absolute permeability model in AVIZO software is used to simulate gas seepage.It is concluded that the greater the flow velocity of the seepage of the pore and fissure in the coal body,the smaller the seepage pressure,the more developed the pore and fissure of the coal body,and the better the permeability.On the contrary,the smaller the flow velocity of the seepage,the greater the seepage pressure,the pore and fissure of the coal body is not developed and the permeability is poor.Compared with the seepage of pores and fissures before and after coal fracture,the seepage velocity of pores and fissures after coal fracture is larger,the permeability is better,and the pores and fissures in coal are more developed,which is conducive to the seepage of gas.The research results provide a theoretical basis for exploring the law of coal seam gas seepage and formulating measures for preventing coal and gas mining accidents.