Nitrogen enhanced drainage of CO2 rich coal seams for mining

Coal seams with high CO2 gas contents can be difficult to drain gas for outburst management. Coal has a high affinity for CO2 with adsorption capacities typically twice that of CH4. This paper presents an analysis of nitrogen injection into coal to enhance drainage of high CO2 gas contents. Core flooding experiments were conducted where nitrogen was injected into coal core samples from two Australian coal mining basins with initial CO2 gas contents and pressures that could be encountered during underground mining. Nitrogen effectively displaced the CO2 with mass balance analysis finding there was only approximately 6%-7% of the original CO2 gas content residual at the end of the core flood. Using a modified version of the SIMED II reservoir simulator, the core flooding experiments were history matched to determine the nitrogen and methane sorption times. It was found that a triple porosity model (a simple extension of the Warren and Root dual porosity model) was required to accurately describe the core flood observations. The estimated model properties were then used in reservoir simulation studies comparing enhanced drainage with conventional drainage with underground in seam boreholes. For the cases considered, underground in seam boreholes were found to provide shorter drainage lead times than enhanced drainage to meet a safe gas content for outburst management.