Foam-like Drainage Dynamics Follow Rapid Passage of Fluid Displacement Fronts in Porous Media

Abstract:

Liquid drainage in porous media is characterized by complex interfacial processes in which pore space geometry and drainage boundary conditions jointly determine entrapped liquid distribution behind an invading fluid front. For water drainage at the pore scale, the rapid emptying of ‘full’ pores at their air entry value leaves liquid in corners retained by capillary forces that subsequently slowly drain by film thinning in a process similar to foam drainage. The invasion patterns of drainage fronts reflect an interplay of capillary and viscous forces where higher drainage rates favor preferential invasion through more conductive pore spaces and result in a liquid distribution behind the front that deviates from hydrostatic conditions. The study describes drainage dynamics through the residual corner-film network using similarities with foam drainage. Experimentally we quantified pore scale geometry and resulting liquid distribution at different flow rates by rapid X-ray tomography (3D scan per second) of water drainage from coarse sand and glass beads. We define the fraction of pore liquid that drains with the front and the fraction left in corners based on pore shape parameters and estimate rates for gravity drainage of the corner film network. From imaged liquid distribution we distinguish conditions for entrapment of pore clusters that subsequently drain at secondary times scales defined by the foam drainage equation.