Characterization of Choked Conditions Under Subsonic to Supersonic Flow in Single-Phase (Supercritical to Gaseous CO₂ or Liquid H₂O) and Multiphase (CO₂ and H₂O) Transport

In geologic media, fluids exist in gas, liquid, and supercritical phases, generating multiphase and multicomponent systems. As fluids migrating through geologic fractures reach the speed of sound, choked flow can be developed in microfractures. To elucidate such choked flow, thermodynamic analysis and numerical simulations were conducted with CO₂, H₂O, and CO₂-H₂O mixtures at various phases ranging from supercritical to gaseous CO₂ and liquid H₂O. Compressible CO₂, with a relatively low speed of sound (~225 m/s at 31.1 °C and 7.38 MPa), demonstrated significant changes in thermodynamic properties with small pressure and temperature variations. In contrast, H₂O, having a relatively high speed of sound (1,524 m/s), showed little thermodynamic variation. For CO₂-H₂O mixtures, a small addition of CO₂ (or H₂O) dramatically reduced the speed of sound relative to those for pure H₂O or CO₂. For an idealized converging-diverging microfracture with CO₂ flow, choked flow and a shock wave were generated as outlet pressure was decreased to less than 6.8 MPa. The H₂O flow did not generate choked flow at any outlet pressures. For CO₂-H₂O mixtures, choked flow was generated when the CO₂ void fraction was greater than 0.7 with an outlet pressure of 6.5 MPa, indicating that presence of H₂O inhibited occurrence of choked flow. Choked flow and shock waves can occur in various geologic environments including geologic CO₂ sequestration, geothermal energy development, geysers, and volcano eruptions.