Legacy Wells Integrity Assessment (LWIA) – An Imperative Tool for Assessing the Containment of Carbon Storage Sites

Abstract With the increasing need to reduce carbon emissions in industry as hydrocarbon demand continues to rise, understanding and managing uncertainties in a Carbon Capture and Storage (CCS) project is essential. Storing carbon dioxide (CO2) underground requires a site that can effectively contain the injected CO2 and prevent any leaks. Potential routes through which injected CO2 may leak include: failed caprock, faults, injection wells, or legacy wells in the field. The legacy wells could pose CO2 leakage risks if the integrity of the barriers is not suitable over the CCS project lifecycle. The objective of this work is to understand the role legacy wells will play in project containment. This is accomplished by characterizing offset wells using a Legacy Wells Integrity Assessment (LWIA) workflow that is based on evaluating well construction and abandonment information with respect to the CO2 storage reservoir and understanding complexity of potential well remediation. Identification of abandoned legacy wells in any CO2 storage prospect needs to be completed. This involves collecting available well construction records from public data repositories supplemented with operator records, if available. ExxonMobil's LWIA workflow provides early-stage screening and characterization of wells in the Area of Interest (AoI, potential project area) and then the refined Area of Review (AoR – maximum extent of combined CO2 and pressure plume). The characterization includes evaluations of both barrier certainty and intervention complexity. Post screening and characterization phase, the analysis primarily focuses on identification of potential CO2 leakage paths, probability and leakage rates. From this, consequence levels are defined to understand the unmitigated risk to protect health/safety of people, environment, and assets. Identified challenges can be minimized with a variety of mitigation control techniques including possible intervention and re-establishment of barriers. The characterization of both barrier certainty and intervention complexity guides a fit-for-risk approach that balances the potential leak risk with the risks of remediation actions. ExxonMobil's LWIA workflow is leveraged globally, offering scalability, standardization, and the dissemination of learnings. To date, information for over 10,900 wells has been collated for screening numerous AoIs around the globe and over 1200 legacy wells have been characterized into different groups according to the following factors: (a) interaction of the legacy well with the CO2 injection zone, (b) presence and location of verified and unverified barriers in the wellbore, and (c) wellbore accessibility by conventional means to re-establish the barrier, if required. Additionally, a risk assessment framework has been developed to assess probability and consequence of CO2 leakage risk. This work process facilitates the identification of favorable sites for injection and provides a structured process for investment decision making. A comprehensive legacy wells characterization approach and wells integrity assessment benefits various disciplines to manage risks during initial development phase of a CCS project. Industry-wide standardization of this workflow would greatly enhance appropriate characterization and reutilization of wells assets, enabling more efficient development of CCS.