Impact of model complexity on CO₂ plume modeling at sleipner

The goal of geologic carbon sequestration (GCS) is to store carbon dioxide (CO₂) in the subsurface for time periods on the order of thousands of years. Mathematical modeling is an important tool to predict the migration of both CO₂ and brine to ensure safe and permanent storage. Many modeling approaches with different levels of complexity have been applied to the problem of GCS ranging from simple analytic solutions to fully-coupled three-dimensional reservoir simulators. The choice of modeling approach is often a function of the spatial and temporal scales of the problem, reservoir properties, data availability, available computational resources, and the familiarity of the modeler with a specific modeling approach. In this study we apply a series of models with different levels of model complexity to the 9th layer of the Utsira Formation. The list of modeling approaches includes (from least complex to most complex): numerical vertical-equilibrium model with sharp-interface, numerical vertical-equilibrium model with capillary transition zone, vertically-integrated model with dynamic vertical pressure and saturation reconstruction, and fullycoupled three-dimensional model. The model domain consists of a 3 x 6 km section of the 9th layer, as described in the IEAGHG benchmark dataset. The layer thickness varies in space, ranging from 5 to 30 m, while porosity and permeability are close to constant at 0.36 and 1.8 Darcy, respectively. The models are all based on the same input data, and initial and boundary conditions are chosen in a way that ensures the different models are comparable. In addition, a simple box model is used for preliminary simulations. The models are compared based on the predicted CO₂ plume footprints and saturation cross-sections. The predicted CO₂ plumes are also compared to the actual CO₂ plume footprint from seismic surveys to determine the ability of the different models to predict the actual CO₂ plume footprint. The results show that vertical-equilibrium models are sufficient to model CO₂ migration in the 9th layer of Sleipner, due to the formation's higher permeability and relatively thin capillary transition zone. None of the models used in this study was able to accurately predict the actual plume footprint; this suggests the modeling approaches used here are missing essential physics or that some parameters in the site model (e.g., topography of the caprock) are inaccurate.