Abstract: A classification indicator is proposed and the hazard levels are classified for the carbon dioxide (CO2) dispersion when a well blowout is encountered at a CO2 capture and storage (CCS) site. Taken the topography and climate conditions of Yijinhuoluo Qi in Erdos city, Inner Mongolia, China, as the environmental background, the CALPUFF model is used to simulate a supposed well blowout with a reference import from the Shenhua CCS demonstration project. The dispersion distance of released gases is calculated, then the relationship between the dispersion distance and the blowout speed is chosen to classify the hazard levels of CO2 dispersion, and finally its influence is conducted. According to the maximum dispersion distance of the CO2 concentration in 50 000 ppm, the hazard levels of CO2 dispersion are classified into three levels by using an indicator of blowout speed. A well blowout with a release rate of CO2 between 0 and 2.5 m/s is identified as Level 1 corresponding to the dispersion distance of 0-130 m; a release rate of CO2 between 2.5 and 10 m/s is as Level 2, and the dispersion distance is 130-180 m; a release rate of CO2 between 10 and 20 m/s is as Level 3 with the dispersion distance of 180-420 m. According to the experimental research, it is shown that the persons around the leakage well within 420 m are affected when the hazard level reaches Level 3, which means the workers in the operational site, the residents around and the pedestrians passing on the nearby county road are all affected. Our research on the classification of hazard levels not only contributes to the supervision of relevant government agencies to the CCS projects but also is beneficial to the promotion of coming CCS demonstration projects in China.

Key words: carbon dioxide capture and storage (CCS), CO2 leakage, well blowout, monitoring, risk assessment