The effects of aquifer/caprock interface on CO2 storage capacity and security

In simulations of CO2 storage in saline aquifers, it is often assumed that there is a sharp boundary between the aquifer and the caprock. However, this is not always the case. In many cases there is a gradual transition between sand-rich facies in the aquifer and mud-rich facies in the caprock. Moreover, some simulations assume a smooth interface, whereas typically the surface is irregular, due to sedimentological and stratigraphic effects or structural deformation. We have conducted a range of numerical simulations on a variety of heterogeneous aquifer/caprock models to investigate the impact of the different types of aquifer/caprock interfaces. Firstly, the nature of the interface can influence estimates of storage capacity. A transition zone between the aquifer and the caprock leads to uncertainty in the effective aquifer volume. Secondly, the aquifer/caprock interface can affect the security of CO2 storage. A transition zone can increase the security by providing partial baffles to hinder CO2 migration towards the caprock. Rugosity at the aquifer/caprock interface, may assist or hinder structural trapping. Small domes may provide extra storage volume. On the other hand, topographical highs may provide pathways for rapid migration of CO2 from the injector.