Layout Optimization of High-Level Directional Boreholes to Prevent Downward Invasion of Carbon Dioxide from an Overlying Coal Mine Goaf

For adjacent coal seams, the downward invasion of harmful gases from an overlying coal mine goaf to the lower mining face could occur, and a high-level drainage tunnel is usually adopted for disaster prevention. Due to the high cost, instead of a high-level drainage tunnel, the high-level directional boreholes are widely adopted. In this study, the effect of a high-level drainage tunnel to prevent the downward invasion of carbon dioxide from the overlying coal mine goaf is analysed by applying a flow model in the numerical simulation. Then, the high-level directional boreholes are analysed to investigate the possibility of taking the place of the high-level drainage tunnel. The research results show that (1) for close adjacent coal seams, the downward invasion range of harmful carbon dioxide from the overlying coal mine goaf reaches one-third of the mining face, around 60 m wide; (2) a high-level drainage tunnel can effectively prevent the downward invasion of carbon dioxide from the overlying coal mine goaf by reducing carbon dioxide concentration within 0.3%; (3) the nine high-level directional boreholes with a careful layout can reduce the downward invasion of carbon dioxide from the overlying coal mine goaf to keep carbon dioxide concentration below 0.8% at the mining area, ensuring the coal mining safety.