CO2 storage potential of South African coals and gas entrapment enhancement due to igneous intrusions

Methane outburst events have occurred in some underground coal mines of South Africa such as those in Twistdraai Mine in the Highveld Coalfield in the early 1990s. Occurrences of methane outbursts suggest that high gas content in adsorbed and free phase might be present in some sections of the coal seams. Coal seams might also be friable in those sections, allowing pulverisation during outburst. The survey of gas outburst events indicates that the locations of high gas emissions were mainly in the vicinity of the igneous intrusions (mainly dolerite intrusions). These intrusions are a result of post-depositional volcanism into coal seams in some major coalfields of South Africa such as the Highveld Coalfield. This study was undertaken, firstly, to investigate potential locations for the occurrence of coal seam gas in South African coal seams with respect to the igneous intrusions and their enhancement effect on gas release and storage of these coals. Secondly, it was thought that as the metamorphosed coals with high ash content were not economically viable for mining, they might be potentially suitable for CO2 disposal. Measurements on 14 coal samples, taken in an underground coal mine at various distances from a dolerite dyke, show that the adsorption and porosity properties of metamorphosed coals have been enhanced thus increasing their gas storage capacity. Diffusivity results show that dolerite intrusions would act as enclosures and gas traps for storage of gas. Gas diffusivity was greater in coals near the dyke by a factor of two. In general, it is expected that the CO2 storage and gas flow properties of such coals be enhanced due to the heating effect of intrusions, making the coals more appealing for possible future sequestration of CO2 from anthropogenic sources.