Potential secondary seals within overburden: Observation from the CO₂ storage sites Aurora and Smeaheia, northern North Sea

This study evaluates potential secondary seals within the overburden in two CO2 storage sites, Aurora and Smeaheia within the Horda Platform area, of the northern North Sea. Secondary sealing intervals provide for safe and reliable CO2 storage as they prevent injected fluids from migrating into the atmosphere in the case of primary seal failure. This study analyzes the possible fate of the CO2 plume in the case of primary seal failure for the Aurora and Smeaheia CO2 storage sites by evaluating the secondary caprock and sealing potential of polygonal fault systems within the overburden intervals using structural reliability methods. Our findings suggest that a ductile secondary caprock is present in the overburden intervals of the storage sites, which could act as a secondary seal in the case of primary seal failure. The trapped hydrocarbon leaked from the Troll field within this ductile unit indicating a good secondary sealing potential. The study also reveals the presence of polygonal fault systems in the entire study area, but their frequency and orientation vary between layers. However, polygonal fault systems are not structurally stable, posing a risk of fault-parallel flow within the overburden interval. Nonetheless, the ductile clay-rich secondary caprock provides a seal for the polygonal fault systems within the interval. Due to either erosion or non-deposition, the ductile rock interval is not present in the east of the study area. As such, the critical risk for the Alpha structure in the Smeaheia injection site is the Vette fault and the efficiency of the primary Draupne caprock. Moreover, the crucial factor in the western Aurora site is the connectivity between the depleted Troll reservoirs and the CO2-injected pressurized reservoirs underneath. Based on our qualitative assessment, it is concluded that the overburden interval in the Horda Platform has adequate secondary sealing potential to hold and accumulate CO2 in the case of primary seal failure. Still, further numerical simulations are recommended to quantify our findings for both the Aurora and Smeaheia sites.