When CO2 is injected into a saline aquifer for storage purposes, it rises through the brine to form a thin plume underneath a caprock (or under an intermediate shale layer). This is challenging to model, because a fine grid is necessary to resolve the plume and, since aquifers are extensive, many grid cells are required. On the other hand, simulations may be speeded up by using vertical equilibrium (VE) which assumes instantaneous separation of fluids due to gravity.
The focus of this work is to compare VE models with conventional black oil (BOS) and compositional simulations (CS) to determine conditions under which the VE approximation is valid. A simple 2D homogeneous pilot model was tested first. The base case model had 81×1×81 cells, and the refined models had 243×1×243 and 729×1×729 cells. Subsequently, a realistic 3D model representing the Bunter formation in the southern North Sea was used. This type of formation is expected to be used for CO2 storage in the future.
The results from the pilot model showed that the VE model could adequately represent the plume with a coarser grid than the BOS or CS models. For a homogeneous model, the VE approximation is therefore very useful. However, when applying VE to a heterogeneous 3D model, the results must be treated with caution. For the CS model, the areal distribution of the CO2 at the top of the aquifer showed that CO2 had migrated only through the high-permeability cells while the VE model showed evidence of CO2 migration through the high and moderate permeability cells.
The VE model saved processor time and gave reasonable results at low grid resolution. Even though the results may be less accurate for heterogeneous aquifers, the use of VE is important for the extended reservoir simulations needed to generate multiple realizations to capture the uncertainties and their effects on the aquifer-description parameters.