CO2 Storage: Managing the Risk Associated With Well Leakage Over Long Time Scales

Abstract

Summary One of the major challenges associated with the geological storage of carbon dioxide (CO2) is the performance of the confining system over long time scales. In particular, the occurrence of CO2 leakage through existing wells could not only defeat the purpose of storage, but also badly affect human health or the environment. Cement degradation and casing corrosion in injection, production, or abandoned wells can create preferential channels over time, allowing the migration of CO2 from the reservoir to shallower formations (e.g. aquifers), and/or to the surface. In this paper, a risk-based approach is proposed for well-integrity and confinement-performance management. The approach, based on Performance and Risk Management methodology (P&R™), serves as a decision-support tool. The major steps in this methodology are identifying the system and sources of degradation through characterization and system analysis; quantifying their criticality through modeling, in terms of probability and severity; and establishing a risk-mitigation plan. This methodology is based on experience in material aging and risk assessment of complex systems, such as nuclear structures where probabilistic simulations are performed. It accounts for all stakes involved in well-integrity management and enables the full integration of uncertainties as part of risk estimation. The methodology presented here greatly improves common approaches based on "features, events, and processes" because it quantifies risk levels. It provides useful and reliable tools to support decisions for well-integrity-management strategies or emergency plans. To that purpose, mitigation actions such as characterization/inspection, remediation (workover), design improvement, or monitoring are valued on the basis of a cost/benefit ratio. Moreover, updating the risk assessment with incoming data allows for an evolving vision of risk levels to optimize interventions over time. This approach has been applied successfully, leading to recommendations for safer and more-efficient design, maintenance, and monitoring strategies.