Changes in Gas Composition during Coal Seam Drainage and Laboratory Testing

Analysis of gas emissions from underground-to-inseam (UIS) gas drainage boreholes located in the Bulli seam found methane gas liberated from the coal in preference to carbon dioxide gas, particularly in mixed gas (CH4 and CO2) conditions. The rate of emission of the methane gas component from the in situ coal seam was also found to be greater than the rates observed during laboratory gas desorption testing on coal cores recovered during the drilling of the UIS gas drainage boreholes. The nature of changes in the composition of the gas mixture liberated from coal, and the relative differences in gas composition determined by fast and slow desorption testing, and UIS gas production, may have a potentially significant impact on outburst risk assessment and coal seam gas reservoir assessment. Results of gas composition measurement from the three sources are presented and discussed.