Mineral Erosion in Different Microstructures under ScCO₂-Water-Coal Interactions: Implications for CO₂ Sequestration in Coalbeds

Unmineable coal seams are ideal formations for CO2 sequestration. When supercritical CO2 (ScCO2) is injected into these coal seams, it involves a complex ScCO2-water-coal system, which alters the microstructure and mechanical properties of the coal seam, impacting the reliability of the CO2 sequestration. Currently, there is a lack of methods to quantitatively characterize the degree of CO2-water-coal interactions, making it difficult to accurately assess the safety of the sequestration of CO2 in coal seams. In this study, the changes in mineral composition and microstructure of coal under different fluid interactions are investigated using X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). Based on three-dimensional laser scanning tests, a new method is proposed to calculate the erosion volume of coal at microscales before and after ScCO2-water-coal interaction. The impacts of treatment conditions, mineral composition, and microstructure on the reaction degree of coal surface were quantitatively analyzed. The research results indicated that (1) compared to pure ScCO2 treatment, the erosion in the carbonate mineral areas of coal samples was the most severe under the treatments of ScCO2-water or ScCO2-brine. This indicates that the presence of groundwater can significantly affect the safety of CO2 sequestration in deep unmineable coal seams. (2) During the ScCO2-water-coal interaction, no significant alteration was observed in organic matrix areas under any treatment conditions. However, the erosion degree of different minerals exhibited dramatic variability. Carbonate minerals such as calcite, melanterite, and dolomite were the primary reactive minerals, while quartz, kaolinite, and pyrite did not participate in the reaction. (3) The same mineral with different microstructures demonstrated variable degrees of erosion in the ScCO2-water-coal interaction. It may be because of the different contact areas between fluid and solid. However, owing to different mineral types in the same microstructure, the effect of microstructures on mineral erosion was disturbed. (4) The presence of SO42– promotes the dissolution of carbonate minerals, while the presence of HCO3–, Ca2+, and Mg2+ inhibits their dissolution. Therefore, when injecting ScCO2 into deep unmineable coal seams for sequestration, it is advisable to select reservoirs where the groundwater contains high levels of HCO3–, Ca2+, and Mg2+ but low levels of SO42–, and where the reservoirs have high organic content and low content of carbonate minerals.