A Model For Multi-mechanisms Transport Of Gas In Coal Micro/Nano-pores And Its Application

The gas flow in coal is extremely complicated as a combined results of multiple flow mechanisms and complex pore structures.Comprehensive understanding of gas flow behaviors in coal is of great importance for the prediction of coalbed methane(CBM)production and the elimination of gas-induced disasters in coal mines.Micro/nano-pores are the main reservoir of methane in coal seams,and it is difficult to investigate the flow mechanisms of methane in micro/nano-pores by experimental methods.Based on the lattice Boltzmann method(LBM),this study constructs a LB model considering surface adsorption and diffusion effects,which involves multiple mechanisms of gas flow,including viscous flow,enhanced gas slippage and surface diffusion.Based on this LB model,a coupled gas-solid model of gas transport in coal seam with multiple flow mechanisms was developed,and the dynamic evolution of the apparent permeability model under fixed confining stress and uniaxial compression conditions and the effects of multiple flow mechanisms on gas extraction in deep coal seams were analyzed.The main conclusions are as follows:1)The effect of surface diffusion on gas flow in the bulk phase was investigated by the lattice Boltzmann model of gas flow in micro/nano-pores,and the results show that the surface diffusion rate of adsorbed gas in micro/nano-pores gradually decreases with increasing pressure,and the normalized surface diffusion rates at pore pressures of 1 MPa and 5 MPa exceed 10% and 3.7% respectively for pore widths less than 10 nm.The surface diffusion rate of adsorbed gas is not negligible compared to the free gas flow rate.The adsorption and surface diffusion of adsorbed gas enhance and weaken the bulk phase gas flow rate at low and high pressures,respectively.2)The contribution of the permeability of multiple flow mechanisms to the apparent permeability was investigated by the calculation of the permeability for viscous flow,enhanced gas slippage and surface diffusion according to Darcy law,and the results show viscous flow permeability,enhanced gas slippage permeability and surface diffusion permeability all decrease with increasing pore pressure,with surface diffusion permeability accounting for more than 82% and 64% of apparent permeability at gas pressures of 1 MPa and 5 MPa in the micropores,respectively,and viscous flow being negligible.As the pore size increases,the contribution of surface diffusion decreases rapidly,while the contribution of enhanced gas slippage increases and then decreases,and the contribution of viscous flow increases slowly and eventually becomes prominent in gas flow.3)Based on the pore shape distribution of the micro/nano-pores,the coal is simplified to a geometric model consisting of matrix and uniform circular micro/nanopores embedded in the matrix.Considering the poroelastic effect of micro/nano-pores,a dynamic apparent permeability model considering multiple flow mechanisms was developed,and the data predicted by the dynamic apparent permeability model is well fitted with the experimental data.The apparent permeability is controlled by the slippage effect and the poroelastic effect respectively as the pressure increases,with the slippage effect becoming more dominant at lower pressures.4)Based on the contribution of different gas flow mechanisms and the gas-solid coupling model for gas transport in the micro/nano-pores,the results are obtained that in the early stage of extraction,the CBM extraction yield is controlled by Darcy seepage in fractures;in the middle stage,it is controlled by viscous flow in matrix macropores;in the late stage,it is controlled by gas diffusion induced by the superposition of surface diffusion and enhanced gas slippage in matrix micropores and mesopores.The study found that the influence of the slippage effect on the gas extraction rate shows three stages,namely a rapid increase in the initial stage,a slow decrease in the middle stage,and a slow increase in the later stage,and the impact of slippage effect on gas extraction in deep coal seams needs to be highlighted.There are 79 figures,12 tables and 120 references in this thesis.