| Coal seam gas is one of the main hazards that threaten the safety of mine production,and its extraction efficiency is largely determined by the characteristics of the seepage flow.Practice shows that the long-term low permeability oil reservoir development,the fluid in low permeability porous medium flow with the starting pressure gradient of low velocity non-Darcy flow However,the current gas pumping mining technology is mostly follow a linear seepage theory,often ignoring the low permeability coal seams in the low-speed nonlinear seepage and diffusion phenomenon,resulting in drainage evaluation of difficulty and smoke production process selection bias According to different coal seam,it is urgent to solve the problem of how to quickly and accurately evaluate the degree of extraction and the selection of appropriate extraction technology.The premise and key to solve this problem is to grasp the flow state of gas in coal seam with different coal structure.For this purpose,this paper studies the characteristics of non-Darcy percolation in low permeability coal seam.Based on the results of coal sample pore structure analysis,discussed the relationship between the starting pressure gradient and permeability,used the test system of starting pressure gradient,the effects of confining pressure,gas properties and water content on the starting pressure gradient of coal samples with different coal structure were studied.Research results are:(1)the relationship between the permeability and the starting pressure gradient are exponential;(2)there is a big difference between the starting pressure gradient of different types of coal,performance as: the maximum pressure gradient of the starting pressure is the I type of coal and the ? type of coal,the minimum pressure gradient of the starting pressure is the ? type of coal.The starting pressure gradient of the III type coal is in the middle.(3)For each type of coal,the greater the confining pressure is,the greater the starting pressure gradient is.At the same time,the starting pressure gradient of the four kinds of coal is different in the low pressure.When the confining pressure is relatively large,the starting pressure gradient of the four kinds of coal starts to approach gradually.(4)For each type of the I type of coal,the relationship betweenthe starting pressure gradient of the three kinds of gas flow is N2 > CH4 > CO2;(5)For each type of the I type of coal,when the moisture content is in the range of 1%~7%,the starting pressure gradient increases gradually with the increase of water content.Based on the deformation control equation and the gas solid coupling seepage equation,the influence of confining pressure,different gas and air inlet pressure on the starting pressure gradient was discussed by means of COMSOL-Multiphysics software.Simulation results are(1)In a certain range,with the increase of confining pressure,the starting pressure gradient increases gradually.(3)Without taking into account the effect of adsorption,the relationship between the start pressure gradient of the three kinds of gas flow is when considering adsorptioned.The relationship between the starting pressure gradient of the three kinds of gas flow is CO2 > CH4 >N2.The reason of this phenomenon is that the adsorption function affects the stress sensitivity coefficient of the specimen,which leads to the change of the strength of the specimen.(3)In the test piece reference line L1 and the reference point B,the inlet pressure is the lower the specimen outlet flow is larger,the greater the permeability,the starting pressure gradient is small.Based on the GSI coal classification system,quantitative characterization of coal samples with different structure of coal is carried out.The relationship between the GSI and the starting pressure gradient is discussed.On the basis of this,the method of identifying the gas flow in low permeability coal seam based on the starting pressure gradient is put forward.Research results are that when GSI is equal to 46.58,it is?=0,gas flow regime is linear flow,when GSI is less than 46.58,?=0.00189(46.58-GSI)1.49665,when GSI is greater than 46.58,=0.00189(GSI-46.58)1.50559,? increases with the increase of GSI.The GSI is not equal to 46.58,?p/L is greater than or equal to?.Gas flow regime is low velocity nonlinear flow.?p/L is less than ?.Gas flow regime is diffusion. |
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