Simulation of an enhanced gas recovery field trial for coal mine gas management

Coal mine gas management has evolved from being predominantly dependant on mine ventilation systems to utilising sophisticated surface based directional drilling for pre-drainage of coal seams. However the advent of enhanced gas recovery techniques in the coalbed methane industry has provided an opportunity to address gas management objectives hitherto impractical. Specifically: achieving very low residual gas contents to mitigate against frictional ignitions and fugitive emissions; the means to accelerate gas drainage to accommodate mine schedule changes; and to enable pre-drainage of coal reserves with very low permeability. This article examines a possible enhanced gas recovery field trial at an Australian mine site. Production data from four surface to inseam medium radius gas drainage boreholes was modelled and history matched. The resulting reservoir characteristics were then used to model the performance of the boreholes using an enhanced recovery technique. One of the boreholes is modelled as an (nitrogen) injection well and two flanking wells are modelled as production wells.The model results suggest that accelerated gas flow rates as well as very low residual gas contents are achievable using typical coal mine gas drainage infrastructure and goaf inertisation systems.

Research highlights
► It is feasible to design an enhanced gas recovery system using a coal mine gas drainage system and coal mine goaf inertisation system.
► Modelling of nitrogen injection into a depleted coal seam gas reservoir indicates that accelerated gas drainage may be achieved.
► Furthermore, modelling of nitrogen injection into a depleted coal seam gas reservoir also indicates that very low residual gas contents may be achievable which in turn may find application for mitigation of frictional ignitions and fugitive emissions in coal mines.