Gas Seepage Characteristics And CO₂ Permeability-Enhancing Technology For Low Permeability Coal Under Complex Mining Conditions

In this paper,taking the No.7502 working face of No.K₂（5#）coal seam in Nantong Coal Mine as the research object,by combining laboratory experiment,theoretical analysis,numerical simulation and field experiments to discuss the movement law of overburden rock in the mining process of multiple coal seams,and analyzes the deformation and seepage characteristics of low-permeability coal rock and"coal rock with gangue"samples under complex mining conditions such as complex mining stress,dynamically changing gas pressure,and gangue mixed coal.On this basis,a mining intrinsically safe liquid CO₂ phase change fracturing and permeability increasing system for low permeability coal mass permeability increasing and gas displacement is designed and developed,which has been applied in the No.7502working face of Nantong Coal Mine and No.S3012 working face of Shanmushu Coal Mine.The main research results are as follows:（1）According to the acoustic emission in-situ stress test method,complete coal cores were drilled at different dip angles in the No.5#coal seam and roof of Nantong Coal Mine.Through uniaxial compression mechanics and acoustic emission tests,the evolution laws of mechanical,deformation and energy characteristics of coal and rock under different loads were studied.The"Kaiser"effect is used to calculate the in-situ stress,and the magnitude and direction of in-situ stress in No.7502 working face of Nantong Coal Mine are determined,σ₁=26.62 MPa,σ₂=18.73 MPa,σ₃=12.16 MPa.（2）Based on the research results of in-situ stress distribution characteristics of No.7502 working face in Nantong Coal Mine,a three-dimensional numerical model was established by using the finite difference method,the in-situ stress value was used as the boundary condition of the three-dimensional model,and 6#,5#and 4#under complex conditions were studied.The evolution characteristics of overburden stress field,displacement field and plastic zone during the step-by-step mining of multiple coal seams provide stress boundary conditions for the subsequent study of gas seepage in coal and rock mass.（3）Based on the evolution characteristics of the stress field in the overburden of the stope during the mining process of multiple coal seams,a three-dimensional stress loading and unloading experimental scheme is designed.The"gas bearing coal thermal fluid solid coupling triaxial servo seepage device"was used to carry out the coal rock deformation and seepage characteristics experiments of the gas bearing raw coal and coal rock gangue samples under the complex mining stress conditions during the normal loading,loading and unloading process.The volume expansion and compression deformation of gangue mixed coal and rock were expressed by fracture volume strain,and the deformation failure mechanism and seepage characteristics of raw coal and gangue mixed coal and rock samples under load are discussed.The research results show that during the triaxial compression test,the permeability and principal stress difference show an oblique"V"shape change trend,that is,the permeability of the coal sample first decreases and then increases with the increase of the principal stress difference,and the permeability of the sample increases obviously after the failure,and the growth rate of the permeability of the coal sample after the failure is greater than that of the initial stage.The fitting of the permeability and principal stress difference of the coal sample before the failure conforms to the negative exponential function distribution during the loading process.（4）In combination with the research results of the distribution characteristics of in-situ stress in Chapter 2 and the evolution law of mining stress in Chapter 3,experiments on deformation and permeability characteristics of gas-bearing coal under two true triaxial loading conditions were carried out（Path 1:fixed stress in X and Y directions,Load the stress in Z direction until the sample fails;Path 2:Loading in the Z direction and unloading in the X and Y directions at the same time）,the coupling change law between the strength,volumetric strain and seepage characteristics of the raw coal sample under the condition of true triaxial loading is studied.And CT scan was used to analyze the three-dimensional fracture distribution of raw coal samples under the condition of true triaxial loading.（5）In order to study the mechanism of permeability enhancement of coal and rock under complex conditions,a pressure chamber for permeability measurement of coal and rock samples was developed.Based on the evolution characteristics of complex mining stress in multiple coal seams,the permeability experiments of single CO₂ gas,CH₄ gas and different volumes of CO₂ and CH₄ mixed gas in coal were carried out.Based on the fact that coal adsorbs CO₂ more strongly than CH₄,experiments on competitive adsorption and permeability of CO₂ and CH₄ mixtures with different volume ratios in raw coal were carried out.The research results can provide a theoretical basis for the design of liquid CO₂ phase change cracking scheme.（6）Based on the competitive adsorption and permeability characteristics of CO₂ and CH₄ mixed gas with different volume ratios in raw coal,the principle of liquid CO₂phase change fracturing and permeability enhancement was analyzed,combined with the evolution law of high gas and low permeability of No.5#coal seam in Nantong Coal Mine.And a mine-used intrinsically safe liquid CO₂ phase transition fracturing system was developed for gas permeation and gas displacement by low permeability coal.The experiments of liquid CO₂ phase change fracturing and CO₂ displacement of CH₄ were carried out in the No.7502 working face of Nantong Coal Mine and the No.S3012working face of Shanmushu Coal Mine,respectively.After the fracturing experiment of Nantong Coal Mine,the gas concentration in the extraction hole showed a significant increase,and the extraction concentration reached more than 70%.After the fracture experiment in Shanmushu Coal Mine,the occurrence of gas overrun was effectively solved,and the"zero overrun"of gas in the working face was realized.The engineering application results of the two mines show that the liquid CO2 phase change fracturing technology has important engineering practical value for coal mine gas disaster control.