Migration of buoyant non-wetting fluids in heterogeneous porous media

Abstract:

The buoyant migration of a non-wetting fluid in porous media occurs in several natural contexts, such as during CO₂ sequestration, methane in cold seeps, DNAPLs infiltration in groundwater, oil recovery and magma chambers. In this study, we use numerical modeling and laboratory experiments to investigate the migration of buoyant non-wetting fluids in time-dependent (reactive) or spatially heterogeneous porous media. We find that the stress balance at the pore scale greatly influences the migration dynamics and regime of viscous energy dissipation of the flow (low Re) and therefore impacts the transport of non-wetting fluid even at the field scale. We consider two complementary pore-scale studies. In the first case, the migration of the non-wetting fluid is impacted by the concurrent dissolution of the porous medium because of the reactivity of the buoyant invading fluid. In the second scenario a chemically inert buoyant non-wetting fluid migrates across an heterogeneous medium, from a low porosity (and permeability) layer into a high porosity (high permeability) layer. We find that these two cases lead to a similar and counter-intuitive outcome: the migration of the buoyant non-wetting fluid is reduced at high porosity/permeability. These counter-intuitive results stem from the effect of confinement of the non-wetting fluid (volume available for invasion) and viscosity ratio between the immiscible fluids on the fluid migration at the pore scale. An important solid confinement (low porosity) stabilizes fingering pathways and promotes efficient transport, while high porosity promotes the formation of an emulsion with discrete bubbles or slugs of non-wetting fluids.