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.