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. Key Points: Numerical simulations were performed on CO2, H2O, and CO2‐H2O mixtures to characterize choked flow in microfracturesThe sound speed of the CO2‐H2O mixture is significantly smaller than those of pure CO2 or H2OWhen CO2 is injected to subsurface, choked flow was harder to generate in the CO2‐H2O mixture flow than in CO2 flow [ABSTRACT FROM AUTHOR]