Osmond, J. L.¹*, Mulrooney, M. J.¹, Holden, N.¹, Skurtveit, E.²𝄒¹, Faleide, J. I.¹, and A. Braathen¹
¹University of Oslo. ²Norwegian Geotechnical Institute, *Corresponding author
Capture of industrially sourced CO₂ and transport to the Aurora subsurface storage site in the northern North Sea are approved to commence in 2024 under the direction of the Longship and Northern Lights projects. Results from well 31/5-7 drilled in early 2020 within exploitation license EL001 confirmed suitable parameters at Aurora (e.g., porosity, injectivity, etc.). While the geology of the site proves promising for CCS, it remains imperative to mature additional locations in order to meet current climate mitigation targets and establish the Horda Platform as a European storage hub.
Planned injection and containment at Aurora will be hosted by the Lower Jurassic Dunlin Gp stratigraphic storage complex (storage aquifer and seals), however, the Upper Jurassic Viking Gp represents an additional storage complex. Moreover, Aurora is located in the western-most of three large, basement-rooted fault blocks, each showing storage potential. Hundreds of thick- and thin-skinned faults create two- and three-way structural traps for both storage complexes in all three fault blocks. Some Viking Gp traps contain hydrocarbons (e.g., Troll field), providing direct analogs, but should be avoided for CO₂ storage until the end of their production life around 2050. Nevertheless, the remaining structural traps currently make the most attractive storage prospects, as they can focus injected CO₂ in a predicable fashion, particularly during the early stages of the sequestration process before other trapping mechanisms take over (e.g., residual trapping).
As both top and lateral seals must completely envelop the storage aquifer, understanding the distribution and nature of the seals is critical for predicting subsurface CO₂ containment. In order to provide insight towards additional CCS potential in the Horda Platform, we present a summation of top and lateral seal mapping, modeling, and observations for the Dunlin and Viking Gp storage complexes in the three major fault blocks.
For the Dunlin Gp storage complex, interpretation of its top seal distribution from 3D seismic and wellbore data confirm seal presence in all three fault blocks, including that of the Aurora site. The majority of small thin-skinned faults at the Jurassic stratigraphic level and create aquifer juxtapositions against the top seal, while larger thick-skinned faults must provide membrane seals along the largest closures. In these latter cases, the Dunlin Gp sandstone aquifer is up-thrown and juxtaposed against the overlying Viking Gp sandstone aquifer, but shale gouge ratio analysis and regional aquifer pressures suggest favorable membrane fault seal potential. Top seal formations above the Viking Gp aquifer are determined to be present throughout the Horda Platform, but only the eastern-most fault block is currently prospective for CO₂ storage, given the high risk of contaminating producing fields in adjacent fault blocks. Fault seals in this case appear to be juxtaposition-controlled, even for thick-skinned faults, which are analogous to Troll East. Considering the availability of structural traps for expanding storage activities in the Horda Platform, our work infers that the presence top and lateral seals is probable for both the Dunlin and Viking Gp storage complexes.