Abstract
This paper presents the results of an evaluation study of gas drainability in the Bulli seam in the Southern Coalfield of the Sydney Basin, NSW, Australia, where the coal seam gas (CSG) contains a high proportion of carbon dioxide (CO2). Historically the gas drainability in some particular areas of this coal seam was found to be particularly poor, which posed a significant challenge to gas predrainage. As a result, a large volume of greenhouse gases were released to the atmosphere during mining of the coal seam. Furthermore, the high gas content associated with the CO2-rich composition also increased the risks of coal and gas outburst incidents, affecting the safety of mining. After systematic literature review of evaluation factors affecting gas drainability, this evaluation study comprehensively analyzed the main critical factors, including the geology of the area, the coal cleat system, coal microstructure, coal permeability, coal sorption capacity, gas content, and gas composition. Field geology analysis showed geological variations that affected the variations of the coal cleat system and CO2 content in the coal seam. Scanning Electron Microscope (SEM) tests showed the tight and less-porous features in hard-to-drain coal samples. The colliery gas database analysis was carried out to assess the impact of gas content and gas composition on the drainability of the coal seam. Laboratory tests showed that the coal seam had a permeability of less than 1 mD and also showed that the coal seam was highly undersaturated, especially with high CO2 content.
Highlights
Australia has the third largest coal reserves in the world, with 144.8 billion tonnes (Bt) of proved coal resources, including 68.3 Bt anthracite and bituminous coals and 76.5 Bt subbituminous and lignite coals [1]
Due to the mixed CO2/CH4 gas, attention has been drawn to the impact on global climate change from coal seam gas emissions as coal production has powered Australian economic development [4]
Coals that are capable of holding the maximum amount of gas at a certain reservoir pressure and temperature condition generally are referred to as being “saturated” or, otherwise, “undersaturated.” The most successful coal seam gas (CSG) drainage and production occurs in coal seams that are close to fully saturated [15, 22]
Summary
Australia has the third largest coal reserves in the world, with 144.8 billion tonnes (Bt) of proved coal resources, including 68.3 Bt anthracite and bituminous coals and 76.5 Bt subbituminous and lignite coals [1]. Of Australian longwall mines require regular gas drainage to manage coal seam gas emissions [3]. Coal adsorption isotherms describe the maximum gas adsorption capacity of coal, which is one of the key characteristics that affect CSG drainage operations, outburst prevention, and CO2 storage [5, 21,22,23]. Coals that are capable of holding the maximum amount of gas at a certain reservoir pressure and temperature condition generally are referred to as being “saturated” or, otherwise, “undersaturated.” The most successful CSG drainage and production occurs in coal seams that are close to fully saturated [15, 22]. The Metropolitan Colliery, located at New South Wales (NSW) as shown, has exhibited problems in that the absorbed gas is hard to drain in some longwall blocks of the Bulli seam. In order to identify the main factors that would indicate that a deposit would be “hard-to-drain” and give early warning signs of the issue, the methodology for laboratory evaluation and investigation of CSG drainability and its influencing factors are described in the present paper
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