Potential Effect of Intra-Fracture Morphology on the Mode of Choked Flow for Injected CO2

In present, the carbon neutrality to cope with the risks caused by climate change, global warming, and human activities is emerging worldwide. CO2 sequestration has been studied as a notable technique for carbon neutrality. Practically, most CO2 is injected in the super-critical phase and flows, however, phase transition from super-critical to gas phase occurred based on surrounding conditions including pressure, temperature, and the existence of brine or salt. Under such conditions, choked flow can be triggered by the gas phase CO2 flowing through fractures nearby. The choked flow can incur large-scale variation on thermodynamic properties, and even a shock wave. In this study, the choked flow while the CO2 is flowing through the fractures was simulated with the ANSYS Fluent. Fractures of mm scale were assumed to be ideal converging-diverging (C-D) nozzles. The C-D fracture is composed of two cylindrical segments that have a constant cross-sectional area, and a converging-diverging segment that has a narrower cross-section between the two cylindrical segments, the throat. Particularly, this study focused on the morphology of those C-D fractures. The diameters of C-D fractures inlet (Dfin) and outlet (Dfout) were discretely changed from 3 mm to 10 mm. For the diameter of the throat (Dt), it increased from 1 mm to 5 mm. The length of the cylindrical segments (Lf) and C-D segment (Lt) were linearly changed from 8 mm to 12 mm and 10 mm to 18 mm, respectively. For all designed C-D fractures, the gas phase CO2 was injected at the inlet with the constant mass flow rate, 0.1 kg/s, and the outlet of C-D fractures was considered as a constant pressure boundary (6.5 MPa). In result, The length of the cylindrical segment did not affect to the occurrence of choked flow and the variation of the thermodynamic properties. In the various cases of converging-diverging segment lengths, the constant ratio of the length to the position of maximum Ma was observed. However, Dfin, Dfout and Dt affected both the choked flow occurrence and the maximum Ma. Especially, the fractures which have sharply decreased cross-sectional area in converging segment while was decreasing, and the Ma and thermodynamic properties showed a wide range of its variation. Finally, the shock wave was observed in the diverging segment when the choked flow condition was achieved.