Impact of goaf gas drainage from surface vertical boreholes on goaf explosive gas zones

Goaf gas drainage is extensively employed in Australian gassy underground coal mines to manage safety and productivity and to mitigate gas emissions. As mining operations reach greater depths and produce higher levels of gas emissions, narrower spacing between adjacent vertical goaf boreholes and higher suction pressure are increasingly being adopted. While this proactive goaf gas drainage design enhances gas extraction efficiency, there is a concern that an increased amount of ventilation air might be drawn back into the deep goaf, potentially resulting in the formation of an explosive gas zone (EGZ) composed of methane-air mixtures. Extensive goaf gas drainage data from various Australian coal mines have undergone detailed analysis in preceding back analysis studies (Wang et al., 2022a, 2023). These findings serve as crucial validation input for a CFD model of the goaf, providing ventilation engineers with visualization of an otherwise inaccessible environment. In this paper, the simulation outcomes of the CFD model were integrated with Coward's triangle to demarcate potential EGZ within the active goaf areas. It indicated that the EGZ was pushed far away from the longwall face under the impact of intensive goaf gas drainage compared to the EGZ without the active goaf boreholes, exhibiting a ‘U-shaped’ distribution. Furthermore, this study delves into the gas drainage factors influencing EGZs in the goaf, emphasising the impact of various gas drainage designs on gas explosion risks within the goaf. Factors such as the number of active boreholes and completion depth are assessed, with the size of EGZ serving as a quantitative evaluation criterion. Therefore, this paper plays a pivotal role in optimising goaf gas drainage efficiency, striving to minimise gas emissions into the atmosphere while upholding the priority of mining and worker safety.