Influence of different CO2 phase states on fluid flow pathways in coal: insights from image reconstruction and fractal study

The CO₂ phase state changes during the long-term interaction with coal seams. A proper understanding of the influence of different CO₂ phase states in water-bearing coal seams is of great significance in evaluating the leakage risk during CO₂ sequestration. The evolution of fluid flow pathways in water-bearing coal subjected to the treatment of different CO₂ phase states was investigated by using X-ray computed tomography (CT) technology, fractal dimension, and three-dimensional (3D) pore-scale flow modeling. The results show that the supercritical CO₂ (ScCO₂) treatment reduced the coal heterogeneity by 12.69% and increased the absolute permeability by 58.75%. Conversely, only 0.71% of coal heterogeneity reduction occurred after subcritical CO₂ (SubCO₂) treatment, increasing the absolute permeability by 24.91%. The reduction in the tortuosity fractal dimension after ScCO₂ treatment was 2.69% larger than that of SubCO₂ treatment, indicating that ScCO₂ treatment was more favorable for improving the transport capacity of flow pathways. The pressure field distributions in the pore network model (PNM) were determined by coal heterogeneity and influenced by the number of flow pathways. The effect of ScCO₂ treatment on the size, quantity, and location of preferential flow pathways in coal is more significant than that of SubCO₂ treatment. Moreover, the proportion of preferential flow pathways in ScCO₂-treated coal was less than 50%, much lower than that of the throats in PNMs. In contrast, the proportion of preferential flow pathways in SubCO₂-treated coal was approximately 30% higher than that in ScCO₂-treated coal. The distribution of preferential flow pathways also indicated that not all pores with larger radii participated in preferential flow. Furthermore, more than 50% of the pathways in the ScCO₂-treated coal did not contribute to fluid flow, and could affect the stability of the coal seam.