Research on the Performance and Influencing Factors of Gas Displacement Measures in Shallow Buried Tunnel Strata

In response to the unsatisfactory treatment effect and high construction risk of shallow buried tunnels crossing high gas formations, based on the theory of fluid porous media, the coal seam gas displacement and extraction technology is applied to the construction treatment of shallow buried tunnels in high gas formations. The performance of gas displacement in the formation was analyzed using theoretical analysis and numerical simulation methods. The influence range and time of gas displacement were determined. Four factors that may affect displacement performance, including negative pressure of drainage, gas injection pressure, formation permeability, and borehole diameter, were studied. Finally, the numerical simulation results of gas displacement in the formation were demonstrated through on-site experiments. The results show that under the condition of negative pressure P_C = 50 kPa and injection pressure P_Z = 2.0 MPa, when the distance between injection and extraction holes is 12 m, gas injection for 40 min is used, and the displacement effect achieved by pumping for 90 min is better. The smaller the suction negative pressure, the better the displacement performance, but the impact of suction negative pressure on the displacement process is relatively small; The increase of gas injection pressure can significantly increase gas drainage capacity and reduce displacement time; There is a nearly linear positive correlation between the decrease in formation gas, gas flow rate from drainage holes, and permeability; For the commonly used borehole diameter, increasing the borehole diameter has little effect on the effectiveness of gas displacement control in the formation. Due to factors such as geological conditions, equipment limitations, and construction disturbances, there are certain differences between on-site experiments and numerical simulations of gas content monitoring data. However, the two have a high degree of consistency in terms of displacement effect and time progress.