Estimating total gas content from early stage gas emission data

The measurement of gas content plays an important role in mine safety and resource evaluation for coal and coalbed methane gas recovery. Gas content analysis data was gathered from eight Australian coal mines, representing the three major coal producing regions of the north-west and southern Sydney Basin and the Bowen Basin. The combined dataset, comprising more than 4 700 samples, is considered representative of the variable conditions that may be encountered in Australian coal mines, which among other properties include variable gas content, gas composition, permeability, rank, type and structure. Analysis of the data has identified significant and consistent relationships between the three gas content components, Q1, Q2 and Q3, as determined through fast desorption gas content testing. These relationships and the impact of gas composition are discussed. Several new methods to estimate total gas content are also presented. These methods, based on initial gas emission data, enable values of average and maximum likely total gas content to be provided to operators and planners upon completion of the initial field desorption testing, following core recovery, prior to the coal sample being dispatched to the gas analysis laboratory. The rapid availability of such data is particularly useful when required for operational decision making such as in mines with coal and gas outburst risk. INTRODUCTION Coal mines operating in seams with moderate to high gas content employ routine gas drainage drilling ahead of mining to reduce gas content for two purposes; compliance with prescribed outburst threshold limits, and compliance with prescribed mine ventilation air (MVA) gas concentrations limits. Core samples are regularly collected during exploration and gas drainage drilling and tested in accordance with the fast desorption method, described in AS3980 (1999), to determine the content and composition of the seam gas. Gas analysis is an integral part of the mine’s ability to effectively manage the ventilation, gas and outburst risk. Fast desorption gas content analysis typically produces results within 24 hours. Figure 1 shows a typical underground-to-inseam (UIS) gas drainage drilling pattern employed at gassy Australian underground coal mines. Where gas content testing confirms an area remains above the prescribed limit additional boreholes are drilled in an attempt to rapidly reduce the gas content and avoid mine production delays. 1 Pacific Mining and Gas Management (PacificMGM), www.pacificmgm.com.au