Capillary trapping of CO2 in a heterogeneous horizontal aquifer

We consider the capillary trapping of carbon dioxide in a horizontal aquifer. Motivated by the heterogeneous nature of reservoir rocks, we allow the permeability to vary vertically across the aquifer. The CO 2 spreads under buoyancy following the end of the injection period. We derive a parabolic governing equation for the motion, which accounts for the trapping of CO 2 at the trailing edge. The flow behaves in a self-similar fashion at early times when it is confined and at late times when it is effectively unconfined. We determine how these similarity solutions are influenced by vertical heterogeneity. We quantify the late-time position of the leading edge of the CO 2 and show that it is highly sensitive to the permeability at the top of the aquifer but rather insensitive to the permeability structure elsewhere. Our results have important implications for the volume of CO 2 that may be stored at a particular geological site.