Regulatory mechanisms of multiscale microstructures on the nanomechanical properties of coal: Synergistic effects of mineral filling, aromatic condensation, and pore competition

Abstract

The nanomechanical properties of coal and the response mechanisms between these properties and its multiscale microstructure are crucial for optimising natural gas extraction technologies and enhancing hydrogen storage capacity and geological stability. Nanoindentation (DSI), X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), and fluid intrusion experiments (LP-N₂, LP-CO₂, and MIP) were combined with coals from different mining areas as research objects to investigate the mechanisms of nanomechanical properties regulation by mineral components, microcrystalline structures, and pore structures in coals, and to reveal the main controlling factors of the nanomechanical properties under the effect of multiphase coupling. The elastic modulus (E) and hardness (H) of the coal samples of this study (R_{0, max} = 0.79–2.77 %) were 4.939–5.562 GPa and 0.389–0.467 GPa, respectively. Within the 5–10 mN peak load interval, the mechanical parameters of coal exhibited minimal fluctuation. The higher the coal rank, the lower E and the higher H were. The microcrystalline structure was the intrinsic core and decisive factor controlling their mechanical properties. The reduction in aromatic layer spacing (d₀₀₂) and the increase in degree of graphitisation (G_d) elevated E. Conversely, H exhibited a positive correlation with the number of aromatic layers stacked (N), microcrystalline stacking height (L_c), and degree of aromatisation (f_a). Mineral components, acting as the 'filling phase', either enhanced or weakened H. In clay minerals (70.8–78.7 %), the synergistic effect of kaolinite's extensive filling and chlorite reinforcement enhanced H. Carbonate minerals (19.7–29.2 %), as key mineral constituents, strengthen E. The control exerted by pore structure over coal rock mechanics primarily originates from micropores. Micropore volume exhibited a positive correlation with H, yet a negative correlation with E. An increased mesopore volume weakened H and was a secondary porosity indicator for predicting mechanical behaviour. This result establishes a multiscale analytical framework for coal body nanomechanics, providing a theoretical basis for designing targeted CBM fracturing and assessing underground hydrogen storage site selection.