Risk evaluation method for converting previous water injection wells to CCUS injection wells

The evaluation and analysis of well integrity were performed, considering the high risks of well sealing failure during the conversion of previous water injection wells to CO₂ injection wells in the carbon capture, utilization and storage (CCUS) testing area of the Aonan oilfield, Daqing. Based on the analysis of conditions of existing water injection wells, the technical system of the well integrity evaluation was built, and the weights of indexes were determined via the analytical hierarchy process. Given that in the previous well integrity evaluation, the assessment of each index is solely based on the expert survey, an innovative numerical simulation approach was developed, and the determination principles for the single-factor risk evaluation index, such as casing and cement sheath, were clarified. Furthermore, the quantitative risk ranking method was proposed for the conversion of water injection wells to CO₂ injection wells. This research showed that among 361 injection wells of the testing area, wells with medium risks and above account for 41%, requiring immediate treatment or abandonment; for wells with low risks, the casing should be inspected and monitoring should be strengthened. It is suggested that the risk evaluation method and management strategy should be promoted across major CCUS testing areas in China to ensure safe operations of existing water injection wells.