Capillary phenomena in porous media: Pore and grain scale studies
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Multiphase flow is a common phenomenon in porous media in both natural and industrial processes such as hydrocarbon recovery, geological CO2 storage, remediation of contaminated ground, and flow in unsaturated soils. Capillary phenomena play a critical role in determining flow regimes, residual fluid distribution, displacement efficiency, and all forms of fluid-solid interactions. This study aims to advance the physical understanding of key capillary phenomena in porous media by using pore-scale and bench-scale experiments complemented with physical and mathematical analyses. The non-cylindrical pore geometry has profound effects on multiphase flow. The non-circular transverse pore geometry ensures the connectivity of wetting fluids along pore edges and corners. On the other hand, the converging-diverging longitudinal pore geometry results in non-constant capillary pressures across pore constrictions; interfacial instabilities such as Haines jumps and snap-offs may take place in this case. The evolution of instabilities depends on nearby interacting menisci. Surfactants and particles adsorb to the fluid-fluid interface and change interfacial characteristics such as the interfacial tension and the contact angle. Transient changes in the surfactant surface density in converging-diverging pores alter capillary phenomena. The shell-like mechanical property of particle-coated interfaces results in profoundly different displacement behavior in pores and abnormal fingering instabilities as observed in Hele-Shaw cells. The interfacial tension resists the capillary pressure between immiscible fluids, and it transfers the tension onto the grains. These forces may create openings that facilitate the migration of the non-wetting fluid in deformable porous media such as sediments. Opening-mode discontinuities vary from cavities to fractures depending on the stress state, the mechanical properties of sediments, and the invaded volume.