Influencing mechanisms of multi-scale pore-fracture responses of coals on their macro/micromechanical behaviors under ScCO₂ injection

To decipher the mechanical response mechanisms of coal seams with multi-scale pore-fracture structure to supercritical CO₂ (ScCO₂) injection, two coal samples from different mines of the Ordos Basin, North China, were first selected for conducting ScCO₂-water-coal reaction experiments. Subsequently, the pore-fracture structure, macroscopic and microscopic mechanical behaviors of samples with different reaction times were analyzed, and the evolution patterns of pore-fracture parameters and relationships between the macroscopic and microscopic mechanical parameters were finally elucidated. The results showed that the ScCO₂-water-coal reaction modifies the pore-fracture structure in coal. Originally filled fractures re-open, original micro-fractures expand, new fractures form, and pores evolve from small- to large-sized. After the ScCO₂-water-coal reaction, the evolution of the compaction stage, the macroscopic mechanical parameters and the energy dissipation during loading corroborate the weakening effect of the ScCO₂-water-coal reaction on coal. The changes observed in the microscopic mechanical parameters align with those in the macroscopic mechanical parameters; however, due to the strong heterogeneity of coal and the inability of microscopic parameters to reflect the component and pore-fracture distribution, certain characteristics of the change amplitude of macroscopic and microscopic mechanical parameters of coal are inconsistent. The ScCO₂ extraction effect, the chemical dissolution, the different-sized pore-fracture evolution, the coupling effect of geostress, reservoir pressure, and swelling stress are the main factors to consider during the process of ScCO₂ sequestration in deep coal seams at the micro-, meso- and macro-scales, as they are responsible for potential safety issues.