Buoyant dispersal of CO₂ during geological storage

[1] Carbon capture and storage is currently the only technology that may allow significant reductions in CO₂ emissions from large point sources. Seismic images of geological CO₂ storage show the rise of CO₂ is influenced by horizontal shales. The buoyant CO₂ spreads beneath impermeable barriers until a gap allows its upward migration. The large number and small scale of these barriers makes the prediction of the CO₂ migration path and hence the magnitude of CO₂ trapping very challenging. We show that steady buoyancy dominated flows in complex geometries can be modeled as a cascade of flux partitioning events. This approach allows the analysis of two-dimensional plume dispersal from a horizontal injection well. We show that the plume spreads laterally with height y above the source according to (y/h)^1/2L, where L is the width of the shales and h is their vertical separation. The fluid volume below successive shale layers, and therefore the magnitude of trapped CO₂, increase as (y/h)^5/4 above the source, so that every additional layer of barriers traps more CO₂ than the one below. Upscaling small scale flow barriers by reducing the vertical permeability, common in numerical simulations of CO₂ storage, does not capture the dispersion and trapping of the CO₂ plume by the flow barriers.