The effect of drainage on the capillary retention of CO2 in a layered permeable rock

Buoyant plumes of CO2 spreading through water-saturated permeable rock, bounded by layers of lower permeability, tend to spread laterally. As they advance, they may gradually leak through fractures or discontinuities in the lower permeability boundary, leading to a gradual waning of the original plume and dispersal of CO2 higher in the formation. With a finite release of CO2, the trailing edge of the plume recedes with time, and capillary forces tend to trap a fraction of this CO2 within the pore space. This also leads to a gradual waning of the plume with time and limits the mass of CO2 which can leak through the boundary and rise higher into the formation. We explore the balance between these two effects and calculate some of the controls on the fraction of the CO2 plume which becomes trapped within the original layer of rock.