Assessment of factors impacting coal seam gas production

Much research has been undertaken to better understand the impact of a variety of factors on coal seam gas production. The bulk of this research has been undertaken in laboratories using small samples of coal recovered from exploration drilling or operating mines. Given the structural change and potential degradation of coal samples during extraction, transport, storage and preparation for testing, the results of laboratory testing potentially lack transferability and may not be representative of the behaviour of gas flow and emission from intact coal seams. The results of a mine based investigation undertaken at an operating coal mine working the gassy Bulli seam of Australia’s Illawarra coal measures is discussed. Gas production data from 279 inseam gas drainage boreholes was gathered and used to evaluate the impact of a variety of geological properties and mine operational factors on subsequent gas production performance. While the total production life of the borehole was found to directly relate to total production it was the geological properties of the coal seam that had most impact on gas production. Properties such as coal rank and type, ash content, gas content, seam thickness and gas composition were all found to impact gas production, however these properties were all found to coincide directly with the total gas in place and degree of saturation. From this analysis the degree of saturation is considered to have the most significant impact on coal seam gas production performance. INTRODUCTION Many Australian underground coal mines have experienced, or are working toward, areas of increased gas content which are difficult to drain. Such conditions are common in the Bulli seam, located in the southern Sydney Basin of NSW, Australia, and the impact on mine operations include increased gas drainage drilling expenditure and coal production delays, and in the extreme cases, loss of coal reserves. Intensive coal seam gas drainage is used in the gassy Bulli seam mines to reduce gas content below prescribed threshold limit values ahead of mining. Figure 1(A) shows the typical layout of underground to inseam (UIS) boreholes relative to mine workings, drilled in a fan pattern from developed roadways. Gas production data was regularly collected from the producing UIS boreholes. Of the many hundred UIS boreholes drilled throughout the mine 279 were deemed appropriate for inclusion in this study. Boreholes that had obvious interaction with adjacent boreholes were excluded from the dataset along with boreholes reported to have experienced problems, such as borehole 1 Pacific Mining and Gas Management (PacificMGM), www.pacificmgm.com.au