Study On The Multi-mechanism Model Of Gas Transport In Low Permeability Coal Seams

The coal seam is a typical low permeability porous medium with complex pore system.The pore structure of the internal pore structure has a wide range of pore size,the range of from nanoscale pores to millimeter level fractures.The different sizes of pores correspond to different gases flow mechanism,coal seam gas flow of the multi-mechanism coexistence phenomenon is obvious.It is of great significance to study the flow law of coal seam gas multi-mechanism in coal seam gas drainage,coalbed methane development,coal mine gas disaster control and energy consumption structure adjustment in china.In order to reveal the flow law of coal gas multi-mechanism,this paper has carried on the related research work from the following:Using the self-developed gas multi-mechanism flow experiment system,the gas permeability test of alumina film under different thinness conditions was carried out,based on the experimental results and theoretical analysis,a new gas coal flow mechanism with slip boundary condition is proposed.The fractal theory is used to characterize the pore structure distribution of the coal seam,and the influence of the pore distribution on the gas diffusion capacity is analyzed.Based on the assumption of nanotube bundles in coal seam pore structure,the flow model of coal seam gas fractal is established by using the fractal characteristics of coal pore structure and the flow equation of coal seam multi-mechanism.A reasonable model is proposed to solve the difference equation.The corresponding calculation procedure is compiled by MATLAB software,and the validity of the new model is verified by comparing the accuracy of the model of the gas fractal multi-mechanism with the traditional classical diffusion.The results show that the gas flow in the coal seam has obvious multi-mechanism flow characteristics.When the Kn<0.001,the gas flow is Darcy flow;when 0.001?Kn<0.01,the gas flow belongs to slip flow;When 0.01?Kn<10,the gas flow belongs to the transition flow;When the Kn?10,the gas flow belongs to the free molecular flow.The gas flow mechanism of the low permeability coal seam with the new slip boundary condition can effectively describe the gas flow characteristics of coal seam coexisting with various flow mechanisms.And the flow behavior of the seepage hole in the coal seam is consistent with the random fractal distribution law,which can be described by the stochastic fractal model,but the diffusion hole does not satisfy the random fractal distribution rule.There is no direct relationship between the random fractal dimension and the diffusion coefficient.It is found that the fractal dimension D_t is positively related to the gas diffusion coefficient,and the fractal dimension D_f is negatively correlated with the gas diffusion coefficient.The correlation coefficient between the fitting result and the experimental data is 0.927,and the maximum correlation coefficient between the fitting result and the experimental data is 0.914.Therefore,The multi-mechanism flow model of coal seam in this paper is fractured and reasonable,and it can be used to describe the flow process of coal gas.In this paper,a flow equation with a new slip boundary condition is proposed by studying the flow model of the gas in the low permeability coal seam.The flow model of coal seam gas fractal is established,the study which based on the model also can provide theoretical support for the efficient drainage of low permeability coal seam and provide useful reference for the efficient economic exploitation of coalbed methane mining.