A Comparison of Screening Methods for Assessing Legacy Wells Within CO2 Storage Licences: Application to Offshore Norway

Abstract

Legacy wells represent a critical consideration for offshore CO₂ storage projects, as they may provide potential pathways for fluid or gas migration if poorly constructed, undocumented or inadequately abandoned. Systematically assessing large numbers of legacy wells remains challenging due to heterogeneous data availability and the diversity of existing evaluation approaches. This study presents an open-source, QGIS-based framework that integrates well construction data, geological context and multiple established screening methodologies to enable transparent and reproducible legacy-well screening at the licence scale. The framework is applied to two offshore CO₂ storage licence areas on the Norwegian Continental Shelf, covering legacy wells drilled across several decades and reflecting a range of construction and abandonment practices. The results show that different screening methods produce systematically different classifications when applied to the same wells, highlighting the method-dependent nature of legacy-well screening. Screening outcomes vary substantially within and between licence areas and cannot be explained by individual well attributes such as drilling year or well depth alone, underscoring the need for multi-criteria screening approaches rather than reliance on single proxies. A small subset of wells with previously reported seabed gas occurrences was analysed for contextual comparison. These wells display heterogeneous screening outcomes, and the limited availability of quantified methane flux data precludes any general inference regarding relationships between screening results and leakage behaviour. The presented framework does not replace detailed well integrity analysis or risk assessment. Instead, it provides a practical, transparent and spatially enabled approach for early-stage screening and prioritisation of legacy wells at the licence level, supporting site evaluation and regulatory review in offshore CO₂ storage projects.