Characterization of carbon dioxide leakage process along faults in the laboratory

It is important to understand the process of multiphase carbon dioxide (CO2) leakage in faults for the risk assessment of carbon capture and storage (CCS). To quantitatively characterize the CO2 leakage process in the fault, pressure sensors, fiber Bragg grating (FBG) temperature and strain sensors were simultaneously used to monitor CO2 leakage in the fault. Ten experiments were carried out, including five groups of gaseous CO2 leakage tests with initial pressures of 1–5 MPa and five groups of liquid CO2 leakage tests with initial pressures of 6–10 MPa. The results indicate that when liquid CO2 leaked with an initial pressure of 7–10 MPa, the pressure and temperature of CO2 dropped rapidly in the first few seconds and then remained unchanged. The behavior that CO2 continues to leak while maintaining temperature and pressure unchanged is defined as “temporary pseudo-sealing (TPS)” behavior, which continues for the first 1/3 of the leakage period. However, this TPS behavior did not occur in gaseous CO2 leakage. If only the pressure and temperature data were used to evaluate whether CO2 leakage occurred, we would misjudge the risk of leakage in CCS projects during the TPS period. The causes and conditions of TPS behavior were further studied experimentally. The results show that: (1) TPS behavior is caused by the phase transition energy generated when liquid CO2 leaks. (2) The condition for TPS behavior is a small leak aperture (0.2 mm). Only a small leakage rate can make the phase transition energy and pressure change from a dynamic equilibrium, and (3) The compression zone caused by the Bernoulli effect and fault “barrier” could reduce the CO2 leakage rate and further promote the occurrence of TPS behavior. This study provides technical and theoretical support for the quantitative characterization of the CO2 leakage process in faults of CCS projects.