Quantitative characterization of CO₂-brine-coal interaction at the microscopic scale: Insight from surface morphology and mechanical properties

Carbon dioxide (CO 2 ) sequestration represents a viable approach to mitigate anthropogenic CO 2 emissions and combat climate change. Injecting supercritical CO 2 (SCCO 2 ) into unmineable coal seams induces complex ScCO 2 -brine-coal interactions, significantly altering the physicochemical properties of the coal matrix. However, these interactions occur at slow rates and on microscopic scales, presenting challenges for quantitative characterization. This study utilized laser confocal microscopy and nanoindentation techniques to analyze the microstructure of coal. The effects of these interactions were quantified by examining changes in coal surface morphology and micromechanical properties before and after interactions. Key findings include: (1) A 3D visual model was developed using point cloud coordinates, and a novel algorithm was applied to calculate the interaction volume, quantifying the extent of ScCO 2 -brine-coal interactions. (2) ScCO 2 -brine-coal interactions caused minimal changes to the organic matrix but significantly altered the interaction volume of mineral structures. With brine alone, the average interaction volume of mineral structures was 4.85 × 10 3 μm 3 while with ScCO 2 alone, it decreased to 1.06 × 10 3 μm 3 . Under ScCO 2 + brine treatment, the interaction volumes of mineral structures increased to 6.517 × 10 3 μm 3 respectively, with extensive pore formation on the surface. (3) The effects of these interactions on the micromechanical properties of coal microstructures varied significantly. These interactions had negligible effects on the micromechanical properties of the organic matrix. With brine or ScCO 2 alone, the effects on the micromechanical properties of mineral structures were minimal. Under ScCO 2 + brine treatment, ScCO 2 -brine-coal interactions significantly reduced the micromechanical properties of the mineral structures in coal. Therefore, the erosion depth caused by ScCO 2 + brine treatment in coal is significantly greater than that caused by brine or ScCO 2 alone. This made it a critical factor affecting the stability of CO 2 geological storage.