Well integrity risk assessment is of great importance to the safe production in CO2 flooding. In this study, the well integrity of injection well in CO2 flooding is analyzed firstly, and the assessment criteria of injection well integrity are put forward. Secondly, the injection well integrity failure risks are analyzed, and the Bow-tie model of well integrity failure is established. By transforming the Bow-tie model into a hierarchical structure, the weights of well integrity influencing factors are obtained with the use of analytic hierarchy process (AHP). Next, the probability and consequences of each single well integrity risk are assessed by using risk matrix method. Finally, the well integrity risk assessment model of injection wells in CO2 flooding is established, and the well integrity risk value and risk level are calculated.

The injection well integrity assessment of CO2 flooding can be divided into three assessment units: wellhead, pipe string and wellbore. The well integrity influencing factors and assessment criteria involve the following fifteen aspects: christmas tree, wellhead seal, tubing, safety valve, packer, casing, cementing, etc. The probability of well integrity risk and the consequence caused by the risk are both considered in the well integrity risk assessment. Based on the risk assessment results, the injection well integrity risk can be divided into four levels: critical risk, high risk, medium risk and low risk. With the use of the above method, the well integrity of certain CO2 injection well in Jilin oilfield of China has been assessed, and the main risk influencing factors and the risk level of well integrity have been determined.

The well integrity analysis and risk assessment of injection wells in this study provide a reliable basis for safety risk decision making and management, and are helpful to reduce the risk of safety accidents in CO2 flooding.

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