Experimental and Simulation Investigation of N2 Enhanced Gas Drainage in Low Permeable Coal Seam

The performance of pre-gas drainage in low permeable seams especially in CO2 abundant coal seams is not satisfied. Due to the high affinity between coal and CO2, much more time is required for gas drainage work to reduce the coal seam gas content below the threshold limit value, thus impacting the scheduled mining activities. In this study, N2 injection to enhance gas drainage efficiency was investigated in CO2-rich coal seams. Laboratory tests of N2 enhanced gas drainage were carried out under different permeability conditions. A triaxial permeability test rig equipped with a back pressure regulator was used to conduct the test. The variation of gas composition, gas outlet flow rate, N2 flushing efficiency, and permeability variation were analyzed. A binary gas migration model was developed to illustrate gas migration in laboratory scale and this model was imported to COMSOL Multiphysics. The simulation results were matched well with the laboratory results. Based on the test results, a two-stage flushing mechanism was obtained. In the first stage, free phase coal seam gas accounted for the large percentage and this stage last for a shorter time. In the second stage, desorption time governed the flushing efficiency and desorbed gas was the primary gas. Through these laboratory experiments and simulations, we concluded that N2 injection could significantly decrease coal seam CO2 content level and increase gas drainage efficiency.