Containment Risk Assessment of the Northern Lights Aurora CO2 Storage Site

Abstract

Assessment of containment (storage site integrity) is a key activity in the evaluation, and a requirement for approval, of potential CO2 storage sites. Here we present the workflow used for, and key results of, the containment risk assessment (CRA) in the Northern Lights project, which is the transport and storage part of the Norwegian “Longship” full-scale CCS project. In the Northern Lights project, CO2 storage is planned to take place at the Aurora site (Exploitation license EL001), southwest of the Troll Field. Injection is planned into the sandstone-rich Johansen Formation (Lower Jurassic Dunlin Group). The argillaceous upper member of the Dunlin Group, the Drake Formation, provides the main seal for the storage reservoir. Several further barrier and baffle units and intervening sandstones are present between the Drake Fm. and the sand-rich units of the Viking Group, which contain the giant Troll field with a high level of depletion to date. The injection site is located downdip of – and stratigraphically deeper than – the Troll West gas province (TWGP). Injected CO2 is expected to move slowly over time buoyancy-driven up-dip (northwards), with a small proportion reaching eventually the structural trap underneath the TWGP. A key method utilized in the Aurora CRA is the bowtie method. Bowties were prepared for each identified “leakage” pathway (according to the definition in the EU CO2 Storage Directive, thus not implying any emission to the water column) instead of per “leakage” mechanism, as has been the case in previous CO2 storage CRAs. The bowties were designed to allow for updates, which took place as new data or interpretations became available. A major update occurred after drilling of a confirmation well at the planned injection site. Key results of the CRA are that the probability of any emissions into the water column due to “leakage” of CO2 out of the storage complex is very low to negligible. Important arguments for this conclusion are (i) the hydraulic separation of the Johansen Formation from the shallower, depleted Viking Group, as attested by pressure data in the confirmation well, and (ii) the proven effectiveness of the seal to the Viking Group sandstones, which have accumulated and stored hydrocarbons over geological time. Further, any CO2 reaching and blending into the hydrocarbon accumulations nearby (such as the Troll Field) is regarded to also have very low probability. Storage Complex Monitoring (SCM) is an integral element of the CRA to enable mitigating actions in case of indications for unpredicted behaviour or “leakage” of the injected CO2.