Evaluating Seal Quality for Potential Storage Sites in the Norwegian North Sea

Abstract

Caprock quality is crucial when wanting to evaluate the storage potential of a reservoir. The objective of this study is to evaluate the quality of the primary and secondary seal in case of CO2 storage in sandstones form the Mid to Late Jurassic in the Norwegian North Sea. These seals are composed respectively of Lower Cretaceous and Upper Jurassic shales for the primary, and from the three Cenozoic groups (Nordland, Hordaland and Rogaland) for the secondary.The primary seal is in general of sufficient thickness according to the recommendations from the Norwegian Petroleum Directorate (over 50 meters), except in a thin North-South corridor to the West of the platform, in the inner part of the grabens. This seal is brittle in the entire study area due to a deep burial depth. The only area where it could be ductile is to the North of the Utsira High, which is due to a warmer geothermal gradient but also possibly the movement of a basement fault beneath. Since its rheology is brittle, the primary seal could be breached during fault movements. To ensure its efficiency, it is thus important to evaluate the properties of the faults from this area that have a direction less than 30° away from the regional maximum horizontal stress (ENE-WSW)The secondary seal is much shallower and therefore has a ductile rheology which makes it less affected by fault movements. This is due to the nature of ductile deformation which creates less conductive faults that have self- healing properties. The thickness is also at least 500 meters close to the shore, but most commonly between 1000 and 2000 meters. These properties make it a high quality secondary seal, as has been demonstrated by the successful Sleipner storage site in the Utsira sands that are sealed by Nordland shales.