Numerical Aspects of Using Vertical Equilibrium Models for Simulating CO₂ Sequestration

Storage of CO₂ in deep saline aquifers is considered an important means to reduce anthropogenic CO₂ in the atmosphere. Assessing the risk of storage operations requires accurate modeling of migration of injected CO₂. However, since potential injection sites typically are very large and time-scales long, flow simulation with traditional methods from the petroleum industry is often not feasible. Also, CO₂ is very mobile and the flow is usually confined to thin layers, which put severe requirements on the vertical grid resolution. Using a vertical equilibrium assumption, the flow of a layer of CO₂ can be approximated in terms of its thickness to obtain a 2D simulation model. Although this approach reduces the dimension of the model, important information of the heterogeneities in the underlying 3D medium is preserved. In this paper, we consider the Johansen formation, a candidate for CO₂ sequestration, to compare the use of 3D simulations to simulations with a vertical equilibrium 2D model. We discuss numerical aspects of using the different methods, and demonstrate that the vertical equilibrium model provides more accurate results when the vertical grid resolution is low. Moreover, we investigate how averaging of parameters influences the accuracy of the vertically equilibrium solution.