A study of hardness and fracture propagation in coal

The surface hardness of coal was estimated using a Shore Scleroscope Rebound Hardness (SSRH) tester. SSRH can be related to fracture toughness and strength, and was trialled as a quick field test to illustrate variability in these properties for coal seam gas reservoirs. Therefore, samples were selected to include different coal ranks and lithotypes from a suite of boreholes that intersect Permian coal seams within the Bowen Basin in Queensland, Australia. The tests were conducted unconfined on slabbed coal core and confined on epoxy encased coal blocks used for coal petrographic examination. The test results of the unconfined samples show that the hardness varies with lithotype. It increases with decreasing amount of bright bands, moreso than coal rank or thermal maturity. The test results of the confined samples show little variation with lithotype, but show a parabolic correlation of hardness with rank, similar to the behaviour found with Hardgrove Grindability tests. The resulting fractures of the SSRH test were analysed under the microscope to understand the fracture pattern, which can be scaled up to understand fracture propagation in natural systems and when induced in gas reservoirs.