Digital rock characterization and CO2 flow simulation of high-volatile bituminous coal: An application to carbon geosequestration

The permeability of CO2 sequestration is influenced by the characteristics of pore-fracture structures in coal, yet the flow simulation via the pore network model (PNM) and the controls of topology and morphology on permeability remain to be understood. First, the pore-fracture structure of high-volatile bituminous coal (HVBC) is characterized using micro-computed tomography, and the flow characteristics of single-phase CO2 in the connected pore-fracture network are simulated by the PNM. Second, the surface roughness of the connected pore-fracture is extracted by fractal characterization, and morphological algorithms are applied to accurately present the pore-fracture network structure. Finally, the implications of structural parameters (pore or throat diameter, coordination number, tortuosity, and sphericity) on CO2 transport are discussed, and the mechanism of pore-fracture structure response due to mineral dissolution in CO2 injection is analyzed. The results shows that the HVBC sample provides significant pore-fracture space (porosity of 10.87%) and flow path (connectivity of 66.50%) for hydrogen and carbon storage. CO2 flow simulation results demonstrate anisotropic flow, with higher CO2 permeability observed in the Z-axis direction (0.061 x 10-3 mu m2) compared to the X-and Y-axis directions (0.044 x 10-3 mu m2 and 0.059 x 10-3 mu m2, respectively). Moreover, the fractures perpendicular to the coal bedding plane in the coal seam with a large tectonic dip strongly influence the flow of CO2 injection. Fractal analysis reveals a positive correlation between fractal dimension and porosity, indicating that structures with higher surface roughness are not convenient for CO2 transport. Significant pore angle characteristics (sphericity average of 0.58), high pore-throat connectivity (coordination number average of 4.31), and low capillary resistance (tortuosity average of 1.19) collectively affect the flow of CO2. Overall, the strongly anisotropic pore-fracture structure contributes to the inhomogeneous flow pattern in CO2 geosequestration. Changes in pore-fracture structure resulting from mineral dissolution during the early stage of CO2-coal matrix interaction in the HVBC reservoir can significantly enhance storage potential. This study contributes to the existing un-derstanding of flow characteristics and provides insights for optimizing CO2 injection efficiency in carbon geosequestration.