Geological modeling and dynamic flow analysis as initial site investigation for large-scale CO₂ injection at the Vedsted structure, NW Denmark

The paper will describe a site investigation study that has been performed on the Vedsted structure in NW Denmark, where upper Triassic and Lower Jurassic sandy formations are planned target reservoirs for large-scale CO₂ injection. The study is carried out in the screening stage where only existing data are used for the characterization, for geo-modeling and for the dynamic modeling of storage capacity and injectivity. In this early stage of site investigation, the regional geological model and sequence stratigraphic interpretation are important contributors to the construction of a realistic geo-model. The reservoir rocks are mainly constituted by estuarine and shoreface sandy facies, interfingering with offshore muddy intercalations. This facies assemblage gives rise to internal seals or baffles, which potentially have marked influence on the lateral distribution of the injected CO₂. The sealing properties of the interfingering shaly layers are therefore very important during the future petrophysical characterization studies. The two reservoir layers of interest are at depths of 1300 and 1900 m respectively, and therefore the CO₂ is injected at supercritical conditions. The flow simulation study is carried out with a commercial black oil simulator, and the use of capillary pressure and relative permeability functions for the simulation causes concern about the upscaling effects, and especially how to handle the effects from small-scale heterogeneities at the sub-gridcell scale. Especially the additional storage effect from capillary trapped CO₂ during the long-term equilibration period with associated water imbibition should be investigated as dependent on the small-scale heterogeneities. Potentially, the different heterogeneities at small- to medium-scale can add an element of increased capacity and added containment safety that has not yet been fully recognized and quantified.