Modelling the interaction between underground coal fires and their roof rocks

The behavior of underground coal fires is determined by a complex interaction between geomechanical effects, related overburden permeability and reactive natural convection. We are building on previous work, where natural convection flow and heat flow in response of a given temperature of the burning coal face were considered. A two-dimensional model is developed, combining both geomechanical and reactive flow effects. For an initial uniform permeability field the reactive free convection model is used to obtain temperature distribution, which is used in a compaction model, from which a new permeability distribution is derived. This final permeability distribution is used in the reactive free convection model to obtain the velocity distribution, the oxygen concentration profile and the temperature distribution. The temperature profile and oxygen concentration profile, within the permeability range of interest, depend on the permeability distribution, but is relatively insensitive to the average permeability value. However, the oxygen consumption by combustion is approximately proportional to the square root of the average permeability. The rubble zone and the fault closest to the burning coal front are the main path ways to a deep coal fire, as these highly permeable zones act respectively as an air intake and exhaust under practical heterogeneous permeability conditions. For shallow coal fires the main inflow of oxygen is through faults and overburden, while heat loss effects are more important than for deep coal fires.