Leakage Dispersion Characteristics and High-Concentration Risk Zoning of Liquid CO2 Storage Tanks in Oilfield CCUS System Based on a PHAST-KFX Two-Stage Simulation Framework

The leakage and dispersion behavior of CO2 in the ground injection stage of oilfield CCUS systems is highly complex under high-pressure, low-temperature, and continuous-operation conditions, while quantitative criteria for emergency control distances remain insufficient. To address this issue, a liquid CO2 storage tank in an oilfield CO2 cyclic injection station was selected as the research object. A 1:1 two-dimensional and three-dimensional computational model was established. A two-stage simulation framework was developed in this study, where PHAST was first employed for rapid multi-scenario consequence screening, followed by three-dimensional CFD validation using KFX under representative high-risk scenarios. The proposed framework was used to systematically investigate CO2 leakage and dispersion characteristics under various leakage aperture sizes, ambient temperatures, and wind speed conditions. Risk zoning and risk quantification were further conducted based on high-concentration CO2 toxicity thresholds of 10%, 15%, 20%, and 30%. The results show that the leak aperture is the dominant factor controlling dispersion consequences. As the aperture increased from 5 mm to 50 mm, the maximum dispersion distance increased from 8.8–9.4 m to 132–146 m, and the maximum cloud footprint increased from 4.8–5.4 m2 to 2204.1–2599.6 m2. Higher temperature enhanced CO2 dispersion capacity, whereas wind speed exerted a dual effect involving near-field dilution and downwind transport. The KFX three-dimensional simulation indicated that buildings and equipment arrangements induced near-ground flow separation and local accumulation. Under a 25 mm leak and an average wind speed of 4.6 m·s−1, the cloud approached full coverage of the injection station within approximately 300 s and tended to spread beyond the site boundary. Liquid CO2 leakage was accompanied by flashing, dry-ice formation, and sublimation, leading to a leakage-rate pattern characterized by an initial decrease followed by an increase and gradual stabilization. This indicates that emergency response should account for the ice-plugging and de-plugging process. The risk zoning results show that the downwind region can be divided into fatal, severe-injury, minor-injury, adverse-reaction, and relatively safe zones. A risk level on the order of 10−6 still existed at 100 m from the leakage source. It is recommended that CO2 concentration monitoring and alarm systems be deployed within 100 m of the station, warning signs be placed near the maximum impact boundary of 146 m, and 300 s be used as a critical time window for coordinated evacuation between the plant area and surrounding residential areas. The findings provide a technical basis for safety-distance verification, monitoring layout design, and emergency-plan development for ground injection systems in oilfield CCUS projects.