Microfracture characteristics and propagation mechanisms in inertinite-rich coal controlled by macerals

To elucidate the controlling mechanisms of macerals in coal on the development and propagation of microfractures in coal, a systematic combined experimental and numerical simulation study was conducted on coal seams from the Yan'an Formation in the Ordos Basin. Fracture density distribution and propagation characteristics within different macerals were quantitatively analyzed using optical microscopy and scanning electron microscopy. Uniaxial compression tests integrated with numerical simulations were employed to reveal the evolutionary patterns of fractures under stress loading and the associated control mechanisms of macerals in coal. The results show that non-through-going microfractures are almost exclusively developed in vitrinite, whereas through-going fractures exhibit more complex distribution patterns. Fracture density in vitrinite is relatively high and increases with decreasing band thickness, while in inertinite, it shows a weak correlation with band thickness due to the higher plasticity of inertinite bands. An increase in the average thickness of maceral in coal bands facilitates fracture propagation, whereas a higher inertinite content suppresses the propagation of through-going fractures. Under stress loading, fracture evolution follows a stage-wise sequence of "initiation within vitrinite→propagation within inertinite→fracture coalescence". During the initiation stage, vitrinite is predominantly subject to tensile failure; in the propagation-coalescence stage, vitrinite transitions to compression-shear failure, whereas inertinite remains dominated by tensile failure. This study reveals the distinct mechanical responses of different macerals in coal across the stages of fracture evolution, providing a theoretical basis for coalbed methane reservoir evaluation and gas disaster prevention.