Pore–Fracture Structure Characteristics of Low-Medium Rank Coals from Eastern Surat Basin by FE-SEM and NMR Experiments

The Middle Jurassic Walloon Subgroup coals in Australia’s Surat Basin are extremely rich in low-medium rank coal seam gas (CSG) resources, making it one of the world’s most productive CSG development basins. The desorption, diffusion and seepage behaviors of CSG are remarkably influenced by the pore–fracture structure characteristics of coals; therefore, their detailed characterizations are greatly significant for CSG exploration and development. There are, however, currently few researches on the pore–fracture structure characteristics of Surat Basin coals. Thus, 12 low-medium rank coals ( R ¯ r: 0.42–0.60%) from the Walloon subgroup of Eastern Surat Basin were obtained for this study, and then coal petrology analyses, field emission scanning electron microscopy and nuclear magnetic resonance (NMR) experiments were performed on these samples. The results show that the organic macerals of Surat coals are dominated by vitrinite, followed by liptinite, and the inertinite is generally rare. The average porosity, movable porosity and movable water saturation of the coal samples were 5.08%, 1.01% and 22.47%, respectively. The coal samples’ pore–fracture structure was characterized by an overwhelming predominance of MIT (micropores and transition pore) and relatively less developed ME (mesopore) and MAF (macropore and fracture), with average volume proportions of 74.58%, 14.06% and 11.36%, respectively. The movable porosities of different pore–fracture structure types were obtained, and the average values were presented as MAF > ME > MIT. The NMR experiments showed that the average movable spaces in the MIT, ME and MAF of the coal samples were 6.6%, 41.8% and 97.9%, respectively. The pore–fracture structure of the sampled coals was influenced by coal maturity, as well as the coal macerals. The coal facies have some impacts on the porosities of coal samples, and the coals with higher texture preservation index and vegetation index, and lower gelation index overall have higher porosities.