An experimental study on nonlinear seepage characteristics in coal seams during CH4 displacement through N2 and CO2 flooding

BackgroundCoal 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-enhanced 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 flooding, CH4 to be displaced, and He for blank control. Based on the physical properties of these gases and employing methods including the quasi-static method, the gas flow rate method, and the Reynolds number, this study explored the seepage 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 ConclusionsThe results indicate that the resistance to the migration of the four types of gases in coal cores decreased in the order of FHe, \begin{document}$F_{\mathrm{CO_2}} $\end{document}, \begin{document}$F_{\mathrm{CH_4}} $\end{document}, and \begin{document}$F_{\mathrm{N_2}} $\end{document}. The magnitude of the resistance was associated with the average effective diameter of gas molecules, the dynamic viscosity of gas, and gas phase change. The increases in the density and viscous resistance of supercritical gas led to significantly elevated resistance to gas migration. The threshold pressure gradients of the four types of gases decreased in the order of λHe, \begin{document}$\lambda _{\mathrm{CO_2}} $\end{document}, and \begin{document}$\lambda _{\mathrm{CH_4}} $\end{document} (approximately equal to \begin{document}$\lambda _{\mathrm{N_2}} $\end{document}), and they were inversely proportional to the coal core length. The threshold pressure gradients were affected by the dynamic viscosity of gases, the pore characteristics of coal cores, and adsorptivity. The viscous resistance was generated by the interactions between the gases and pore walls and adsorption layers. The Reynolds numbers for the four types of gases decreased in the order of \begin{document}$Re _{\mathrm{CO_2}} $\end{document}, \begin{document}$Re _{\mathrm{N_2}} $\end{document}, \begin{document}$Re _{\mathrm{CH_4}} $\end{document}, and ReHe, increasing with the injection pressure and pore size. The adsorption of coal matrix for gas significantly affected the permeability and seepage velocity. N2 exhibited a high permeability due to the weak adsorption of coals for it and small pore changes, with gas permeability in coal cores decreasing in the order of kHe, \begin{document}$k _{\mathrm{N_2}} $\end{document}, \begin{document}$k _{\mathrm{CO_2}} $\end{document}, and \begin{document}$k _{\mathrm{CH_4}} $\end{document}. Due to the strong adsorption 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 exhibited nonlinear characteristics under the influence of adsorption and slippage effects. Otherwise, the gas seepage tended to be stable and gradually approached linear flow. The results of this study provide a theoretical basis for the process parameter optimization, efficiency enhancement, and engineering applications of CH4 displacement through gas flooding.