Cannell: Clearly, if CCS is to achieve its core aim, injected CO₂ has to remain securely contained over the long term, and legacy wells are one of the key areas that must be effectively managed to ensure this happens. Beyond the technicalities of doing so, the issue is also a commercial one, because if wells in a licence area require costly and time-intensive remediation, this can significantly alter the economic outlook of a project at the planning stage.

It’s important to say that this is also a challenge that we can solve with engineering. Not every well will require intervention, as analysis can demonstrate, and for wells that do require intervention, cost-effective approaches to reentry and proper re-abandonment can significantly reduce cost and ensure long-term containment.

There is also an opportunity to convert some legacy wells into assets, fitted with fiber optics, geophones or other equipment to help verify plume behavior and demonstrate storage integrity.

Offshore: With more than 1,400 wells in UK CCS licence areas, how should operators assess and prioritize which wells require intervention?

Cannell: Operators should begin with the CO₂ injection plan and plume migration modeling. The first step is to understand where the CO₂ is expected to move, how quickly it may migrate through the formation and which legacy wells could be reached over time.

Wells should then be assessed against available end-of-well reports, historical abandonment data and barrier status. Wells within or close to the predicted plume path should be prioritized, particularly where there is limited confidence in existing barriers.

Where a well presents a potential containment risk, operators need to consider re-abandonment, relocating injection wells, reducing injection volumes or adding further monitoring.

Offshore: What are the most common integrity risks associated with aging offshore wells, and how could these affect CO₂ containment over time?

Cannell: One of the main risks is uncertainty around historical well data. Older wells may not have the level of documentation required to confidently prove long-term barrier integrity, and some legacy wells may have been correctly abandoned for their original oil and gas purpose but not with future CO₂ storage in mind.

Exploration wells may have been drilled, tested, found to be non-commercial and then abandoned quickly. In some cases, barriers may have been installed to restore caprock integrity above the hydrocarbon target, but not necessarily above a saline aquifer that could later become part of a CO₂ storage site.

Some materials used historically, including certain Portland cements and steel grades, may also be negatively affected by CO₂ exposure over time. If the CO₂ plume reaches a well where barrier integrity is uncertain or insufficient, that well could become a potential migration pathway.

Offshore: What lessons from offshore oil and gas well abandonment can be applied, or need to be adapted, for carbon storage projects?

Cannell: There is a lot that can be carried over from offshore oil and gas abandonment, particularly around well barriers, reentry techniques, abandonment practices and long-term well integrity. However, CCS means that wells are no longer simply being abandoned after hydrocarbon activity and must sit within a storage site that needs to safely contain CO₂ for the long term.

Existing methods such as intersect wells and excavation provide useful lessons, but they can be expensive and may not be the most scalable route for CCS. The industry will need approaches that are technically robust, repeatable and commercially viable, including controlled vertical reentry and the ability to re-abandon legacy wells if required.

Offshore: How are regulators in the UK approaching legacy well risk in CCS permitting, and are current frameworks sufficient?

Cannell: The UK regulatory approach is conservative, which is understandable given that CCS is still an emerging industry. Regulators are focused on public confidence, as even a small leak could undermine confidence before the industry has had time to establish itself.

Operators can expect strong scrutiny around well integrity, legacy well assessment, monitoring plans and risk reduction measures. This framework can also influence project economics, as it could require additional monitoring, lower injection rates or remediation before a project moves forward.

Over time, as more CCS projects are successfully executed and monitored, the regulatory approach may evolve. For now, the priority is likely to remain cautious, with an emphasis on evidence and public reassurance. 

Offshore: What steps does the industry need to take over the next few years to build confidence that CO₂ can be safely stored for the long term?

Cannell: The industry needs to show, through real projects, that CO₂ can be stored safely, monitored effectively and managed over the long term.

Confidence will come from a growing body of operational evidence and a clear track record of identifying and managing risk. It will also depend on regulators, operators and the supply chain learning from early projects and applying those lessons across the wider industry.

Communication matters too. The industry needs to share evidence openly, explain how risks are being managed and show how lessons from early projects are strengthening future developments.