Characterization of Legacy Wells for Integrity Assessment in a Depleted Offshore CO2 Storage Prospect
Authors: Patil P. A. , Mishra P. , Shandilya P. , Aylor A. W. , Montgomery J. K. , Moffett T. J. 
Published: February 2025

Understanding and managing uncertainties is critical for Carbon Capture and Storage (CCS) projects. Storing carbon dioxide (CO2) underground in an identified depleted offshore field requires effective containment preventing any leaks. CO2 may leak through caprock, faults and traps, or legacy wells in the field. These 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 containment challenges in a depleted offshore storage site by characterizing legacy 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 all the 24 legacy wells in the Area of Interest (AoI, potential project area) was carried out. This involved collecting available well construction records from the repositories. ExxonMobil's LWIA workflow provides early-stage screening and characterization of wells in the AoI 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. After screening and characterization phase, the analysis primarily concentrates on identifying potential CO2 leakage pathways, assessing the probability of leaks, and estimating leakage rates. Rates for CO2 leakage and reservoir fluids are determined for a worst-case scenario using a steady-state model. 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.

For this study, 24 wells in the AOI were analyzed for tubular and cementing records, wellbore barrier information, formation tops, drilling hazards, etc., therefore to be characterized into different groups according to following factors: (a) interception of the legacy well with zone of CO2 injection zone, (b) presence and location of verified and unverified barriers in the wellbore, and (c) wellbore accessibility means to re-establish the barrier, if required. This work process facilitates qualification of 13 out of 21 development wells for possible asset reutilization and 3 abandoned wells for further monitoring during the initial development phase of a CCS project. The workflow fulfills structured decision-making process of risk assessment providing mitigative approach for any residual CO2 leakage risk along with some consequences for any data acquisition activity. These legacy wells characterization approach and wells integrity assessment benefits various disciplines to manage risks during initial development phase of a CCS project.