Shore hardness measurements of sub-bituminous coal microlithotypes

This paper presents the results of the surface hardness of coal, estimated with the Shore Scleroscope Rebound Hardness tester. The Shore hardness can be related to fracture toughness and tensile strength, which are important input parameters for fracture stimulation design of coal seam gas reservoirs. The Shore hardness experiments were performed on low rank coal specimens from an open cut mine from the Surat-Clarence-Moreton Basin, and a borehole from the Surat Basin, Australia. All measurements were conducted on polished coal surface confined within epoxy blocks following similar preparation required for coal petrographic analyses. The resulting indentations and microlithotypes were analysed under the microscope, then associated to the Shore hardness number. Three common microlithotypes were identified for the sub-bituminous specimens tested: vitrite, clarite and carbominerite. Results indicate the hardest microlithotype is clarite, followed by vitrite, and then carbominerite. Quantification of the micromechanical properties is possible using simple Shore hardness tests, and opportunities to improve fracture propagation may be further explored from this work.