Study On Non-Darcy Seepage Law Of Low Permeability Coalbed Methane Reservoir

Based on the increasing demand for coalbed methane in China and the decreasing of coalbed methane resources under medium and high permeability conditions,the development of coalbed methane under low permeability conditions has become an inevitable trend.But under the condition of low permeability,the permeability of CBM no longer conforms to Darcy's law,which makes it more difficult to exploit CBM.In this paper,the non-Darcy percolation law of coalbed methane reservoirs under low permeability conditions is studied by means of experimental investigation,theoretical derivation and numerical simulation.Firstly,under low permeability conditions,the main characteristics of non-Darcy seepage are that the reverse extension line of the straight line section of the relationship between seepage velocity and pressure gradient does not pass the coordinate origin;the main causes of non-Darcy seepage are the start-up pressure effect,slippage effect and boundary layer effect of gas permeation;the main methods to determine non-Darcy seepage are Reynolds number discrimination and pressure number discrimination.Law.Secondly,the permeability experiments of coal under different confining pressures,temperatures and water content were carried out through laboratory experiments.It is concluded that the permeability of coalbed gas under different conditions is in the range of 10^-3?m² and belongs to low permeability coal seam.With the increase of confining pressure,the threshold pressure gradient and the critical pressure gradient of non-Darcy seepage increase,while the permeability of gas decreases with the increase of confining pressure,and when confining pressure changes in a lower range,the permeability changes with temperature are more sensitive.The higher the temperature,the weaker the slippage effect and the greater the threshold pressure gradient.With the increase of water content,the permeability tends to decrease.The threshold pressure and critical pressure gradient increase,and the slippage effect decreases.The whole experimental law conforms to the main characteristics of non-Darcy seepage.Then the relationship between flow rate and pressure gradient is fitted exponentially by the experimental data,and the critical points of Darcy seepage and non-Darcy seepage are obtained by Monte Carlo numerical solution,and the mathematical expressions of non-Darcy seepage under different conditions are obtained.The non-Darcy seepage equation considering the effect of boundary layer is established,and a non-Darcy seepage equation based on the radius of pore throat is proposed to determine non-Darcy seepage.A new method of West seepage flow is proposed.The more the thickness of boundary layer is,the smaller the average pore throat radius is,the more obvious the non-Darcy seepage phenomenon is.The non-Darcy seepage law of coalbed methane in Fukun mining area is determined.Finally,through the establishment of non-Darcy fluid-solid coupling seepage field equation,the seepage law of coal seam in low permeability gas reservoir is studied by numerical simulation of gas drilling and extraction,which confirms the inevitability of starting pressure gradient in coal seam gas seepage under low permeability condition.Through the analysis of permeability variation law,it is concluded that the gas seepage law under low permeability condition conforms to the established non-Darcy seepage mathematical model.