Triple Porosity Model and Its Application for Coal Seam Gas Injection Displacement of CH 4

ABSTRACT In order to accurately describe the time‐varying characteristics of coal seam gas injection displacement of CH 4 , a triple porosity model and coal seam gas numerical simulation software SIMED were used to numerically simulate gas flow time history curve. Both the variation trend and quantitative results were consistent with the measured results. The simulation results show that before the gas penetrated the coal sample, the flow rates of both N 2 and CO 2 are zero. After the penetration, both increased rapidly and stabilized. However, there is a significant difference in the dynamic behaviors. Although N 2 injection could quickly penetrate, the subsequent methane replacement decay process is relatively slow; while CO 2 injection, although with a delayed penetration time, could complete the entire replacement process at a faster rate. This asymmetric displacement characteristic is precisely the key advantage that the three‐pore model can accurately capture, and it also reflects the differences in the adsorption‐diffusion‐permeation coupling mechanism of different gases in coal. Moreover, as the injection pressure increases, the amount of methane that is initially replaced is relatively large, and then decreases rapidly; for gas injection, the methane flow rate decreases rapidly over time and approaches zero, while the flow rates of N 2 and CO 2 increase and stabilize after the penetration. The higher the injection pressure for gas injection, the larger the initial methane flow rate, and the faster the decrease rate.