Investigation Of Multi Scale Transport Model And Experiments For Gas Through Micro Nano Pores In Coal

In terms of the problem of large amount of gas extraction in the early stage,rapid attenuation in the process of gas extraction,and extract difficult during the later stage in low permeability coal seam.this paper carried out the analysis and experimental research on gas migration mechanism in low permeability coal seam,considering the influence of stress load and multi-scale characteristics of pore structure on gas migration mechanism.Firstly,the unidirectional multi-scale dynamic apparent diffusion model of columnar coal core is established.Secondly,the measurement method of multi-scale apparent dynamic permeability is proposed further through the conversion expression between apparent diffusion coefficient and apparent permeability.Furthermore,a multi-scale dynamic comprehensive apparent permeability model describing the gas flow mechanism is established by considering the stress,flow pattern and the multi-scale series theory of pore structure.The permeability of CH₄ and He decreases first and then increases（U-shape）with the decrease of gas pressure when the coal body is under stress loading and non stress loading.It is proved that the reason for this law is the joint action of effective stress and slip effective,and the permeability under stress loading is two orders of magnitude smaller than that under non stress loading.The denser the coal mass is（ultra-low permeability）and the lower the pore pressure is,the more significant the slip effect is,and the greater the influence on the variation of gas permeability in the coal mass is.After the destruction of coal,it is found that it can effectively increase the permeability of large pore fractures,but the effect on increasing the permeability of matrix micro nano pore fractures is limited.It is also shows that the destroyed coal only changes the structure of larger pores and fracutres,but does not change the structure of micro nano pores.Measuring the permeability of coal by using different experimental methods,it is found that the steady-state method and pulse attenuation method（traditional method）can measure the permeability in large pores and fractures of coal,but can not measure the multi-scale permeability in micro nano pores of low permeability coal matrix.The new method can measure multi-scale dynamic permeability in micro nano pores,and the numerical value of permeability is three orders of magnitude smaller than the traditional method.It shows that the traditional permeability measurement method may cause the overall permeability to be larger,and it is difficult to measure the intrinsic permeability without loading.By substituting different permeabilities into the model to simulate the change of gas drainage pressure,it is found that the numerical simulation of gas drainage effect by using the permeability measured by the steady-state method will lead to an over optimistic result.The different permeability attenuation coefficient is substituted into the model to simulate the gas extraction effect.It is found that the smaller the attenuation coefficient is,the greater the change of the pressure of the gas extraction is,the better the gas extraction effect is.The multi-scale property of coal porosity determines the multi-scale property of permeability and attenuation coefficient,which explains the main reason why the effect of pumping is good in the early stage,but it decays rapidly,and it is difficult to pump in the later stage.