Caprock integrity of the Draupne formation, Ling depression, North Sea, Norway

Abstract

The Draupne Formation is the most important source and cap rock for North Sea hydrocarbon plays. It also provides the primary seal for potential CO2 storage on the Norwegian Continental Shelf, but it has been a concern that in situ fracturing may compromise its sealing capacity on both regional and local scales. To evaluate its integrity, mudrocks of the Draupne Formation were cored for mapping and analysis of fracture systems in well 16/8-3S. This well was drilled in the Ling Depression of the central North Sea. The background permeability of the Draupne Formation ranges between 10-6 and 10-3 mD, making it a good caprock for carbon sequestration (CCS). In the 16/8-3S well site, the Draupne Formation is affected by four fracture populations. Fracture population FP1 consists of bedding-parallel fractures reflecting a mechanical strength anisotropy associated with deposition, and probably exaggerated by the effects of diagenetic processes. Fracture population FP2 consists of high-angle pedogenic fractures likely reflecting deformation associated with deposition and burial. Fracture population FP3 consists of inclined (35–600) fractures decorated by different types of slickenlines, whereas Fracture population FP4 is regarded as comprising drilling- and core handling-induced fractures. The bedding-parallel FP1 population is likely to remain closed for fluid conductivity at depth since the stress caused by the overburden likely exceeds the fluid pressure at the top reservoir level. The FP2 is lithostratigraphically restricted and stratabound and therefore cannot contribute significantly to a three-dimensionally connected network. Potential fluid flow is therefore likely to be constrained to the inclined tectonic fracture population (FP3), which is unlikely to connect in three dimensions due to its low fracture frequency unless the FP1 population had become activated by processes such as unloading. Substantial leakage through the Draupne Formation mudstone related to the observed fracture sets is therefore considered unlikely. It is emphasised that injection of CO2 would require fluid pressure control at the reservoir-caprock interface to avoid opening of existing fracture populations, induce (new) or reactivate old fracture systems and thereby corrupting the integrity of the seal.