Objective Unlike crude oil and natural gas pipelines, supercritical CO₂ pipelines encounter phase characteristic changes during the transient shutdown process. To ensure post-shutdown safety of a supercritical CO₂ pipeline, it is crucial to establish the safe operational process (transport pressure and temperature) boundaries prior to shutdown.

Methods A hydrothermodynamic calculation model was developed using OLGA to accurately depict the transient process of a post-shutdown pipeline, with the supercritical CO₂ pipeline demonstration project of Xinjiang Oilfield Branch serving as a case study. At the same time, the accuracy of the commercial software model was validated through Matlab programming calculations utilizing the equation of flow continuity, equation of motion, energy equation, PR equation of state, and thermodynamic relations. Based on the observed fluctuation law from the coordinated variations of temperature, pressure, density, and phase state in the pipeline during shutdown, it was proposed that the safe shutdown time for the pipeline should be determined by the step change of CO₂ density under the synergic action of pressure and temperature. This approach reframes the concern of safe shutdown time to preventing the transition of supercritical CO₂ into the gas phase in the transportation system.

Results Based on the operating pressure and temperature parameters in this demonstration project, eight typical operational process boundaries were identified for summer and winter scenarios, including high-pressure & low-temperature, high-pressure & high-temperature, low-pressure & low-temperature, and low-pressure & high-temperature process boundaries. Furthermore, an analysis was conducted to compare and study the fluctuation characteristics of parameters in the pipeline, the coordinated variation between temperature and pressure, as well as the phase transition path and behavior during shutdown processes under various seasonal and boundary conditions.

Conclusion The findings revealed that the high-pressure & low-temperature boundary is the safest, while the low-pressure & high-temperature boundary poses the highest risk. Additionally, the safe shutdown time for the pipeline in winter was significantly reduced compared to summer. To guide engineering practices, process boundary ranges and functional expressions for the safe shutdown of the demonstration project were provided for summer and winter scenarios. The research results can offer theoretical support and technical assurance for the safe operation of supercritical CO₂ pipelines.