Numerical modelling of coal and gas outburst initiation using energy balance principles

Coal and gas outburst is a common mining hazard as it cannot be readily predicted due to complex processes controlling its occurrence. Understanding these complex processes such as gas flow, mechanical properties and behaviour, stress regimes, and geological structures, is required to enable mitigating the outbursts. The most current thresholds proposed for outburst prediction are empirically based thus ignoring the physical processes involved. While numerical modelling is a powerful tool to simulate the involved physical mechanism in coal and gas outbursts, the use of thresholds for outburst initiation in numerical models has not attracted enough attention to date. In this study, the law of conservation of energy is used to determine the moment of the outburst occurrence; the energy of which is obtained through numerical simulation. Different sources of energy including elastic energy and, free and desorbed gas energies linked to the complex physical processes are modelled and discussed. Different mechanical properties of coal are assigned to discuss the distinction of energy values between normal coal and coal affected by geological structures (known as deformed coal). A 30 m long roadway excavation that penetrates through both normal and deformed coals is simulated to assess the effect of induced stress on outburst initiation during roadway development. In addition, the effects of initial gas pressure and damaged area on the propensity of outburst events are analysed. The numerical model can successfully simulate gas outbursts caused by deformed coal, which poses a higher outburst risk. In addition, the modelling results demonstrate that by increasing the gas pressure in the coal seam, outburst happens in the earlier stages of the roadway development, and more intense outburst can be expected.