Migration, Leakage, and the EU Directive on Geological Storage of Carbon Dioxide

An escape of injected carbon dioxide (CO2) from a subsurface storage site is not a leak according to the Council and Parliament of the European Union (EU), who suggest it is migration of CO2 into the surrounding storage complex. This definition is given in the EU Directive 2009/31/EC for geological storage of CO2, which has been adopted and adapted by the countries that border and operate in the North Sea. However, when assessing the potential capacity of a deep saline aquifer of large areal extent, how do stakeholders delineate the storage complex? Can the monitoring grid size change over time? And how might this impact the monitoring of the injected CO2 volume? From data available in the public domain, and from discussions with a Norwegian Operator and the Regulator of Norway, the authors explore the assumptions and perceptions of the storage potential and containment risks of identified sites in the North Sea. Examples are given of a pre-FEED project and an ongoing storage project in the region and how the risk of lateral containment of the injected CO2 is perceived and how it may be mitigated. With only a limited database of examples, it is a challenge to assign values to the likelihood of CO2 leakages occurring. This paper offers some suggestions on how the directive on the geo-sequestration of CO2 might be improved to define a storage site vs. a storage complex, leveraging lessons learnt from case studies which now exist.