Mechanism And Model Of Gas Migration In Micro And Nano Scale Pore

Coal body is a porous medium with complex pore structure.There are both micro and nano scale pores and millimeter scale fractures in the coal body.Among them,the number and structure of nano scale pores are abundant.Due to the influence of nano pore scale,gas pressure and diffusion effect of free molecules,there are still many gas transport mechanisms in micro and nano pores.At present,the research on gas migration in continuous flow zone and slip flow zone is well-researched,but the research on gas flow in transition flow area is less,the understanding is unclear,and the consideration of key factors is insufficient,which makes many theoretical models difficult to be applied in engineering practice,and there is a big difference between theoretical calculation and actual situation.Therefore,it is particularly urgent to study the transport mechanism of transition flow in micro and nano scale pores.Based on the limitation of micro boundary,the effective average free path model of gas molecules in micro and nano scale channels is established in this paper: = 1 + 9)?.Based on the transition flow gas transport model proposed by beskokkarniadakis,combined with the free molecular transport model in the channel and the modified average free path of gas molecules,a nanopore gas flow model considering the micro boundary restriction is established,and the equivalent apparent permeability model of gas flow in the transition flow area is derived;the customized pore structure of anodized alumina membrane is study by SEM.Based on the maximum stable extremum region algorithm of MATLAB,the equivalent diameter and porosity of the membrane were used to quantitatively characterize the nano pores in the scanning electron microscope image of the membrane.The penetration experiments of the anodic membrane under different rarefied gases were carried out by using the pore size analyzer(PMI),and the gas flow model of the nano pores considering the micro boundary restriction was verified.The results show that: the average free path of molecules in micro nano scale pores without considering the micro boundary restriction effect is too large,which directly leads to the value of Knudsen's number is too large;the difference between the theoretical radius and the equivalent radius of the custom-made anodized alumina membrane nanopores is larger,and the MATLAB algorithm of the maximum stable extremum region is more accurate and reliable;in the pressure range of 0.3MPa ? 0.7MPa,the smaller the pore size,the larger the flow increase rate will be;the characteristic size of micro and nano scale pores is usually the same order of magnitude as the average molecular free path,and the pore size has a greater impact on the average molecular free path,so it is necessary to consider the impact of micro boundary restrictions on the flow of nano pore gas;the calculation results of traditional gas flow model are quite different from the experimental results,and the results of the newly-built nanopore gas transmission model are more similar to the experimental results,and more accurate.The new model can describe the gas flow law of different rarefied degree.The research results can provide theoretical support for efficient gas drainage and development of coalbed methane in low permeability coal seam.