Abstract

As regulatory requirements and integrity management expectations for Underground Gas Storage (UGS) facilities continue to evolve, operators are increasingly motivated to move beyond qualitative, index-based risk ranking approaches toward fully quantitative risk assessment (QRA) methodologies. This shift is driven by the need for greater transparency, defensibility, and consistency in risk-informed decision-making related to well integrity, public safety, and asset management. This paper presents the development of a structured and defensible QRA methodology designed to estimate the risk of loss of containment events for UGS wells, with a primary focus on gas releases to the atmosphere via the wellhead and wellbore.

The proposed methodology employs a hybrid approach for estimating Probability of Failure (POF). Detailed leak path characterizations are used to construct fault trees that explicitly represent credible failure mechanisms and barrier interactions. Key well barriers considered include the production casing, cement sheaths, annulus, and surface wellhead components. To ensure practical applicability and alignment with existing operator datasets, the model incorporates well-specific adjustment factors applied to industry derived historical baseline failure frequencies. These adjustment factors can account for variations in well design, construction practices, operating conditions, age, inspection results, and integrity management activities, allowing for differentiation of risk at the individual well level. Consequence estimation is performed using mechanistic modeling approaches to quantify the potential impacts of atmospheric gas releases. These models consider release rates, duration, and dispersion behavior to support consistent estimation of safety, environmental, and operational consequences. When combined with the POF results, the methodology produces quantified risk estimates that can be directly compared across wells, facilities, and mitigation scenarios.

Overall, this methodology represents a step forward in bridging the gap between relative risk indices and absolute risk quantification for UGS wells. By providing a transparent and repeatable framework, it enables operators to make risk informed decisions regarding inspection intervals, integrity testing, well interventions, and capital allocation, while supporting compliance with emerging regulatory and industry best practices.

Author:

Worth, Darren

Publisher:

ASME The American Society of Mechanical Engineers
2026 International Pipeline Conference

Year Published:

2026

Purchase Link:

Coming Soon.