Workflow for characterization of the Nini West storage site seal, Danish North Sea

The Greensand project aims to develop Paleogene sands in the Siri Canyon offshore Denmark for safe long-term CO2 storage with the initial focus on the Nini West depleted oil field. The seal consists of a primary caprock succession of marine shales from the Horda to mid Lark formations and a secondary caprock succession from the mid to upper Lark formation, totalling approximately 900 meters in thickness including sandy siltstone beds in the secondary seal. The applied work stream extensively examines the geological seal parameters using a multidisciplinary framework. This includes determining elemental composition, mineralogy, grain size distribution, porosity, permeability, specific surface area, pore throat size distribution, and capillary entry pressure data from core and cuttings samples in the Nini area. Digital rock analysis was conducted on cuttings representing sandy siltstone beds within the secondary seal. A petrophysical workflow is described, incorporating algorithms for calculating grain size fractions, total porosity, grain density, specific surface area, and permeability utilizing wireline logs based on analytical data. Highest seal capacity was found in the smectite rich primary seal compared to the illite-kaolinite dominated secondary seal. The study documents that the seal exhibits a high capacity for withholding CO2 in the subsurface.