Study on nonlinear seepage characteristics of N2/CO2 displacing CH4 in coal seam at low and medium pressure

The geological structure of coal seams in China is complex, with high degree of metamorphism and low permeability, making it difficult to extract gas from the original coal seams. As an effective method to improve gas extraction efficiency, the enhanced gas extraction technology through gas injection and displacement is a typical process of multiple gas seepage. In order to reveal the nonlinear seepage mechanism of single-component CH4, N2, CO2 and He gases (control groups), this study takes the mass transfer process of single-component CH4, N2, CO2 and He gases in the coal body as the object of study, introduces the average effective diameter of the gas molecules, the coefficient of kinetic viscosity, the Reynolds number and other important parameters, and analyzes the transport characteristics of single-component gases under different pressures from different perspectives, and explores the adsorption and spatial distribution of pore gases. The mathematical characterization relationship between pressure gradient and outlet flow rate is constructed by combining the parameters of physical properties of gases. The results show that: single-component CH4, N2, CO2 and He gases present nonlinear seepage characteristics at the stage of low pressure gradient, and gradually tend to linear seepage characteristics with the increase of pressure; the more compressible the gas is, the higher the starting pressure gradient is; the exit flow rate with the change of injection pressure can be divided into two phases of nonlinear and linear, and the critical pressure point of the two is located near the vicinity of 3.25-3.5 MPa; the interaction with the pore medium can be divided into two phases of nonlinear and linear, and the critical pressure point of both is located near the vicinity of 3.25-3.50 MPa. The critical pressure point of the two is basically located near 3.25-3.50 MPa; the gas with relatively stable flow characteristics and weak interaction with the pore medium has a lower start-up pressure gradient; the stronger the adsorption of coal matrix for gas molecules, the denser the adsorption layer is formed, and the more the viscous resistance of gas molecules, leads to a high deviation of the pressure gradient from the flow curve in a nonlinear manner.