Towards Evaluating and Managing Risks Associated with Legacy Wells and Offshore Gas Storage in Scotland

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Carbon capture and storage (CCS) will play a vital role in mitigating greenhouse gas emissions and combating climate change during global efforts to reach NetZero. Offshore storage of CO2 presents a promising solution for long-term storage; however, it is crucial to assess, manage and mitigate the risks associated with the loss of containment at offshore CO2 storage facilities. The UK Continental Shelf is estimated to have a storage capacity of 80-90GT of CO2 (ETI UKSAP, 2011). Crown Estates Scotland (CES) are responsible for leasing assets for gas storage between 12 to 200 miles from the Scottish coast, an area of seabed that has a history of extensive oil and gas exploration and production, whereby more than 7,000 wells have been drilled since the late 1960s. The seabed available for potential gas storage managed by CES is approximately 7500 km2 in area and spans the Scottish coastline. When selecting optimal sites to lease for gas storage in any region, it is crucial to consider risk factors that may lead to loss of containment and evaluate the severity of consequences were containment to be compromised. Such risk screening exercises are critical to identify optimal areas for gas storage, to screen out areas with greater risk and to be used in the process of risk-based decision-making. Factors that determine the overall risk profile for an offshore storage area include geologic integrity and architecture, legacy infrastructure and integrity, seabed stability and the presence and nature of any environmental receptors that may be impacted by loss of containment. Of particular concern are legacy wells and their integrity for which many thousands exist within the Scottish offshore environment. Legacy wells are a major risk to containment during offshore geological storage, where integrity may become compromised allowing fugitive gas migration towards the seabed and into the marine environment. Currently, no legacy well risk screening tools, methods, or strategies specific to the Scottish offshore environment exist that can help manage, optimize and prioritize leasing of offshore gas storage in Scotland. Meanwhile, various data held by regulatory authorities (e.g., the NSTA National Data Repository) relate to legacy wells, their integrity and associated geological attributes. Such data could aid in assessment of risk posed to gas storage containment by legacy wells across the Scottish seabed, yet to date has not been utilized in such context. This PhD research will involve evaluating existing legacy well risk screening methods and strategies, appraising data availability related to the offshore Scottish environment with focus on legacy wells, collating said data and developing a custom database that can be used to aid in risk screening. Subsequently, the student will perform data analyses to better understand key risk factors associated with loss of containment and legacy wells, develop a risk screening approach, and apply it to legacy wells present within CES lease areas to aid in decision-making.