LAUSR

Abstract The performance of pre-gas drainage in low permeable seams especially in CO 2 abundant coal seams is not satisfactory. Due to the high affinity of coal to CO 2 , much lead time is needed for gas drainage boreholes to reduce the coal seam gas content below the threshold limit value (TLV), thus meeting the demand of the scheduled mining activities. In this study, simulations of N 2 injection enhanced gas drainage were carried out for CO 2 -rich coal seams. A binary gas migration model was developed to illustrate gas migration in coal seam. The simulation results matched well with the laboratory test results, indicating that this model could be used to investigate the mechanism of N 2 flushing coal seam gas. The parameters affecting the flushing performance were then studied using this model, including coal seam permeability, gas diffusion coefficient, N 2 injection pressure andin-situ gas content. These results suggest that coal seam permeability and CO 2 diffusion coefficient are the key parameters having significant impacts on the drainage performance whilst N 2 diffusion coefficient only has negligible effect. A critical parameter theory was developed to describe the correlation between these parameters (especially coal seam permeability and the diffusion coefficient of coal seam gas). Specifically, in low permeable coal seams, permeability was the critical parameter and the effects of other parameters were insignificant. Similarly, if the diffusion coefficient of coal seam gas was very low, it could become a critical parameter.