  CO2 capture and storage is an important technology for mitigating climate
change. Design of efficient strategies for safe, long-term storage requires the
capability to efficiently simulate processes taking place on very different
temporal and spatial scales. The physical laws describing CO2 storage are the
same as for hydrocarbon recovery, but the characteristic spatial and temporal
scales are quite different. Petroleum reservoirs seldom extend more than tens
of kilometers and have operational horizons spanning decades. Injected CO2
needs to be safely contained for hundreds or thousands of years, during which
it can migrate hundreds or thousands of kilometers. Because of the vast scales
involved, conventional 3D reservoir simulation quickly becomes computationally
unfeasible. Large density difference between injected CO2 and resident brine
means that vertical segregation will take place relatively quickly, and
depth-integrated models assuming vertical equilibrium (VE) often represents a
better strategy to simulate long-term migration of CO2 in large-scale aquifer
systems. VE models have primarily been formulated for relatively simple rock
formations and have not been coupled to 3D simulation in a uniform way. In
particular, known VE simulations have not been applied to models of realistic
geology in which many flow compartments may exist in-between impermeable
layers. In this paper, we generalize the concept of VE models, formulated in
terms of well-proven reservoir simulation technology, to complex aquifer
systems with multiple layers and regions. We also introduce novel formulations
for multi-layered VE models by use of both direct spill and diffuse leakage
between individual layers. This new layered 3D model is then coupled to a
state-of-the-art, 3D black-oil type model.
