Mechanical damage evolution mechanism of coal injected with supercritical CO₂ using Nanoindentation and XRD-EPMA technology

CO₂ geological storage technology is one of the effective technologies for achieving carbon reduction and the reuse of abandoned mine resources. In order to reveal the interaction mechanism between supercritical CO₂ and coal during injection and its impact on the mechanical properties of coal, comprehensive characterization techniques such as nanoindentation, XRD, SPM, and EPMA were used to study the changes in coal surface morphology, mineral composition, and micro mechanical properties with leaching time under ScCO₂ storage conditions in deep coal seams. The correlation mechanism between coal mechanical damage and CO₂ mineralization reaction was analyzed. The results indicate that ScCO₂ has solvent properties and can undergo dissolution with coal. Carbonates and sulfates have higher solubility and dissolution rates compared to silicates. As the soaking time increases, the content of carbonate and sulfate minerals in coal slowly and continuously decreases. ScCO₂ leaching significantly changed the micro mechanical properties of coal, with both elastic modulus and hardness showing an exponential decrease trend with increasing ScCO₂ leaching time. The elastic modulus decreased to 85.71%−90.33% of the original coal, and the hardness decreased to 76.07%−84.70%. Among them, after 2 days of leaching, the elastic modulus and hardness values of coal decreased the most significantly, with a decrease of over 86.9% and 81.2%, respectively. Due to the dissolution of some minerals during the leaching process, the types and distribution of minerals in the coal body are relatively uniform and stable, and the mechanical properties of the coal sample are more homogeneous. In the process of ScCO₂ solution leaching, the coal gradually changes from elastic deformation to plastic deformation, the nano indentation crack expands rapidly, the micro morphology of the coal surface is seriously damaged, and the creep deformation increases linearly. The dissolution reaction changes the pore structure and elastic-plastic transformation of coal, enhancing the discreteness of the nanoindentation load curve and causing an increase in the frequency of popping events. The study can provide scientific basis for the development of coalbed methane production and the safety assessment of CO₂ storage in deep coal seams.