Coupling x-ray computed tomography and gray-level co-occurrence matrix to assess the evolution of microstructural damage in coal treated with supercritical CO2 fluid

The reliability of segmentation of x-ray computed tomography (CT) images based on user-selected thresholds is important for analyzing supercritical CO2 (ScCO2)-water-induced microstructural damage in coal. In this study, a novel approach is introduced by integrating the gray-level co-occurrence matrix (GLCM) method with CT to analyze the evolution of microstructural damage in coal treated by ScCO2-water. The factors influencing GLCM calculation are also examined. GLCM heat maps and statistics were employed to visualize and quantitatively analyze the evolution of microstructural damage. The results show that the application of image grayscale registration and noise reduction to GLCM statistics did not produce directional bias, with noise reduction having a more significant impact. The pixel pairings depicted in GLCM heat maps provide insights into the spatial distribution of pore-fracture (in the low-density quadrant) and mineral components (in the high-density quadrant) within CT images, elucidating the evolution of micro-damage in coal samples. After the ScCO2-water treatment, a conspicuous rise in pixel pair counts within the low-density quadrant can be observed, accompanied by significant dissolution of the highest-density minerals, leading to a maximal reduction in pixel values within the high-density quadrant. Moreover, GLCM statistics show that the maximum damage location after ScCO2-water treatment does not completely match the initial coal. An accurate description of the evolution of GLCM statistics in the structural damage behavior induced by CO2 injection into deep coal-rock reservoirs is crucial, suggesting that these GLCM statistics have the potential to predict structural damage and reservoir instability during the geological sequestration of CO2.