Fracture Morphology Influencing Supersonic CO₂ Transport: Application in Geologic CO₂ Sequestration

Geologic carbon sequestration requires CO₂ injection into the storage formation at high-injecting pressure. Such high pressure could induce choked flow accompanying huge variations in thermodynamic properties of CO₂ at a converging-diverging (CD) fractures in the storage formation. In this study, high-velocity CO₂ transport through CD fractures was investigated to quantify the effect of fracture morphology on occurrence of both choked flow and shockwave that constrain the mass flow rate of fluid. In addition, variations in thermodynamic CO₂ properties including Mach Number (Ma), defined by the ratio of fluid velocity to sound speed, and shock properties were investigated. The morphological characteristics of CD fractures were determined by nine properties, such as throat diameter, throat length, inlet diameter, outlet diameter, throat diameters of two-connected CD fractures, fracture wall curvature, roughness amplitude, and frequency. As a result, the throat diameter crucially affected choked flow occurrence and maximum Ma. When the inlet and outlet diameters varied, the profiles for Ma variation were consistent in the converging and diverging segments, respectively. In addition, regardless of change in the throat length, the position of maximum Ma was nearly constant with showing the position length ratio of 0.13–0.14. However, roughness of fracture wall significantly influenced the Ma variation and occurrence of shock. In particular, backflows segregated from the main CO₂ flow were observed near the wall roughness.