Meso-structural evolution and failure mechanisms of coal under cyclic hydraulic loading: insights from micro-CT imaging

Cyclic hydraulic pressure has proven effective in enhancing the permeability of deep, low-permeability coal seams. This study investigates the mesostructural evolution and failure mechanisms of coal samples subjected to cyclic hydraulic pressure, utilising Micro-CT scanning to characterise and quantify changes in pore and fracture morphology. Three coal samples were tested to explore how cyclic hydraulic pressure influences internal structure and failure behaviour. The results demonstrate that cyclic hydraulic pressure promotes the expansion of existing pores and fractures while inducing the formation of new defects. Total pore and fracture volume increased by 51.20%, 53.54%, and 69.08%, with surface area increases of 20.84%, 36.44%, and 81.57%. Notably, pre-hydraulic drilling significantly amplified structural changes, enhancing capillary pore connectivity and increasing equivalent pore radii. This study reveals critical mesostructural failure modes in coal and underscores the role of hydraulic coupling in destabilisation. These findings provide valuable insights for optimising permeability enhancement techniques and improving hydraulic pressure-based engineering interventions.