LBM Simulation Study On The Influence Of Random Pore Structure Distribution On Gas Micro-seepage In Coal

The pore structure of coal directly affect the gas seepage-behavior inside the pores.It was very important to explore the influence of pore and fissure structure distribution on gas mesoscopic seepage theory in coal,so the microscopic seepage characteristics of gas need further study.With the deepening of research on coal seam gas seepage,it was difficult to fully reveal its inherent law mechanism only by macroscopic method,while the microscopic simulation method was limited to the interaction between gas molecules or coal in a single pore space,and lacks the connection between local and whole.Therefore,it was easier to find and study the influence of microscopic pore structure of coal on gas seepage from the mesoscopic perspective.In this paper,the Quartet Structure Generation Set(QSGS)method was used to restructure random pore structure of coal,and the image recognition and analysis software on pore structure of coal was developed to analyze the characteristics of coal pore structure.On this basis,the lattice Boltzmann numerical method was used to simulate gas seepage,and the influence of different porosity,different pore distribution with the same porosity and pressure on gas seepage was discussed.The main conclusions are as follows:(1)The image recognition analysis results of the reconstructed random pore structure of coal showed that,with the increase of porosity,the number of pores,the average pore size and the proportion of open pores will increase.Under the same porosity,the less the number of pores,the larger the pore size,and the smaller the fractal dimension with the smaller the probability of core distribution(Pc),and the "through" pores would appear.The pore size distribution showed that the number of macropores was less,and the number of mesopores accounted for more,the number of mesopores accounted for 75%?90%.(2)The LBM seepage model at REV scale was established to explore the local characteristics of gas seepage in coal micro pores.The results showed that the larger the porosity was,the greater the gas seepage velocity and permeability were.When the porosity is greater than 0.18,the increase in permeability of pore structure was much larger than that of pore structure below 0.18,reaching more than ten-fold.Under the same pressure condition,the permeability of pore structure with porosity of 0.32 was as high as two orders of magnitude higher than that of pore structure with porosity of 0.12.The main reason was that the proportion of open pores was relatively more and the average pore size was relatively larger,which leads to better connectivity of coal.(3)By comparing the gas seepage laws in different structures with the same porosity,it could be found that gas seepage velocity and permeability were much higher in the pore structure with through-hole than those without through-hole.The average seepage velocity of gas was more than three-fold that of other structures,and the permeability was more than ten-fold that of other structures.The through-hole provided advantageous seepage channels for gas seepage in coal,leading to uneven distribution of gas seepage velocity in coal pore structure.Gas seepage velocity was larger in the well connected pore area,and very small or even stagnant in the closed pore area.In engineering practice,more attention should be paid to influence of this kind of pore distribution characteristics on gas seepage.(4)The gas seepage characteristic was vary with different pore inclination angles.The average gas seepage velocity and permeability decreased with the increase of pore inclination angle.The average gas seepage velocity in the pore structure with 0° pore angle was more than three times higher than that in the pore structure with 45° and 90° pore angles.Therefore,in practice,more attention should be paid to the influence of pore-fracture distribution in the same direction as gas seepage on seepage.The average seepage velocity of gas increased with the increase of pressure difference.The greater the porosity of coal was,the greater the influence of pressure difference on the average seepage velocity was.In this paper,the influence of random pore structure of coal on gas microscopic seepage was explored by lattice Boltzmann method.The research results supplemented and improved the microscopic seepage of gas,which was helpful to further understand the law of gas seepage and provided theoretical basis for the efficient exploitation of gas in coal seam.