Determining the Correlation Between Vicker's Microhardness and Young's Modulus of Coal

Coal mechanical properties are very important in assessing coal seam gas (CSG) borehole stability and reservoir permeability response to depletion. Obtaining coal directional mechanical properties can be challenging due to the lack of access to cores and the technical hurdle in making standard coal samples from cleated coal. Although there are reasonable amounts of work available to indirectly estimate material (e.g., sandstone rock) mechanical properties from the hardness by applying certain fitted correlations, few attempts have been carried out to confirm if such correlation exists for coal, which is highly anisotropic, heterogeneous, and fractured. This work explored the possibility of estimating the mechanical properties indirectly from the Vickers microhardness test results. A total of 24 cubic samples with an average dimension of 28mm× 28mm × 28mm (W×L×H) divided into four different groups are tested in this study. The Vickers microhardness measurements on three different indentation loads were carried out first, followed by the uniaxial compression test. Comparison between Vickers hardness and Young's modulus was conducted for each of the three directions orthogonal to cleat orientation. Our results show that hardness and Young's modulus follows the same trend, but no strong correlation is demonstrated, meaning that indirect estimation of coal mechanical properties from hardness does not seem reliable. Coal samples show noticeable anisotropic Young's modulus and Vickers hardness values for all tested coal locations. The comprehensive laboratory data from this work provides useful information for directing future work on exploring alternative methods of estimating coal mechanical properties.