N2/CO2驱替煤层CH4非线性渗流特性实验研究OA

[Background]Coal seams in China are generally characterized by low porosity(＜5％)and low permeability(0.001 × 10-3μm2),leading to low coalbed methane(CBM,dominated by CH4)drainage efficiency.Gas flooding-en-hanced CBM drainage technology plays a significant role in surface CBM drainage,underground CBM pre-drainage,and deep geologic CO2 storage.[Methods]This study investigated four common types of gases:N2 and CO2 for flood-ing,CH4 to be displaced,and He for blank control.Based on the physical properties of these gases and employing meth-ods including the quasi-static method,the gas flow rate method,and the Reynolds number,this study explored the seep-age behavior and characteristics of the four types of gases in coal cores measuring 100 mm,200 mm,and 300 mm in length.Furthermore,it analyzed the impacts of the threshold pressure gradient,viscous resistance,and adsorption force on the seepage characteristics of He,N2,CO2,and CH4.[Results and Conclusions]The results indicate that the resist-ance to the migration of the four types of gases in coal cores decreased in the order of FHe,FCO2,FCH4,and FN2.The mag-nitude of the resistance was associated with the average effective diameter of gas molecules,the dynamic viscosity of gas,and gas phase change.The increases in the density and viscous resistance of supercritical gas led to significantly el-evated resistance to gas migration.The threshold pressure gradients of the four types of gases decreased in the order of λHe,λCO2,and λCH4(approximately equal to λN2),and they were inversely proportional to the coal core length.The threshold pressure gradients were affected by the dynamic viscosity of gases,the pore characteristics of coal cores,and adsorptivity.The viscous resistance was generated by the interactions between the gases and pore walls and adsorption layers.The Reynolds numbers for the four types of gases decreased in the order of ReCO2,ReN2,ReCH4,and ReHe,increas-ing with the injection pressure and pore size.The adsorption of coal matrix for gas significantly affected the permeabil-ity and seepage velocity.N2 exhibited a high permeability due to the weak adsorption of coals for it and small pore changes,with gas permeability in coal cores decreasing in the order of kHe,kN2,kCO2,and kCH4.Due to the strong adsorp-tion of CO2,it will cause the expansion of coal matrix,resulting in low permeability of coal core.There existed a critical pressure for gas migration in coal cores.When the injection pressure was less than the critical pressure,gas seepage ex-hibited nonlinear characteristics under the influence of adsorption and slippage effects.Otherwise,the gas seepage ten-ded to be stable and gradually approached linear flow.The results of this study provide a theoretical basis for the pro-cess parameter optimization,efficiency enhancement,and engineering applications of CH4 displacement through gas flooding.