Abstract
Practice shows that CO2/N2-ECBM is an effective technology to enhance coalbed methane. However, there are few field tests in which the technology is applied to enhance the gas drainage in underground coal mines, and the effect is uncertain. In this study, firstly, the reasons for the decrease of gas drainage efficiency in the exhaustion period were analyzed based on the theory of fluid mechanics. Secondly, the mechanism of N2 injection to enhance coal seam gas drainage (N2-ECGD) was discussed: with the gradual decrease of gas pressure in the drainage process, coal seam gas enters a low-pressure state, the driving force of flow is insufficient, and the drainage enters the exhaustion period. The nitrogen injection technology has triple effects of “promoting flow”, “increasing permeability” and “replacing”. Thirdly, the numerical simulations of the nitrogen pressure on drainage effect were carried out based on the fully coupled model. The results show that the higher the nitrogen pressure, the greater the displacement effect between injection and drainage boreholes, the larger the effective range. Finally, a field test of N2-ECGD was carried out in the Liu Zhuang coal mine in Huainan Coalfield, China. The results show that N2 injection can significantly enhance the gas flow rate and CH4 flow rate in the drainage boreholes, and the coal seam gas content decreased 39.73% during N2 injection, which is about 2.6−3.3 times that of the conventional drainage. The research results provide an important guidance for promoting the application of N2-ECGD in underground coal mines.