Pore-scale investigation of solute dispersion behavior in porous media under a two-phase co-flow condition

Phase distributions and hydrodynamic flow field at the pore scale can greatly affect the tracer mass transfer process in a two-phase co-flow system, yet the underlining mechanism is not well understood. In this work, dispersion of non-reactive tracers in the aqueous-phase flow field in porous structures (reconstructed from high-resolution Micro-CT images) is numerically investigated under an immiscible two-phase co-flow condition. The spatial-temporal evolution of tracer fronts is observed over a range of Sw from 0.45 to 0.95, and Péclet numbers from 130 to 650. The topology of the two-phase flow field is evaluated in terms of the fluid-phase morphology, rescaled Eulerian velocity distributions, and the WP fractional flow rate (fw)-saturation (Sw) curve, etc. Meanwhile, solute dispersion and mixing are comprehensively studied through tracer breakthrough curves, residence time distribution, dispersion coefficient, mean concentration gradient, and dilution index. A well-defined Fickian transport regime was achieved for Sw ≥ 0.75, where uniform invading fronts with finer fingers were discovered, strengthening the dispersion process through merging of fingertips and transverse mixing. In contrast, multipeak and strong tails were discovered for lower saturations of Sw = 0.45 and Sw = 0.55, where ramified dispersive finger structures were observed, regulating solute transport into oriented channels while weakening its transport into diffusive barriers of disconnected/isolated clusters. In addition, transverse mixing contributes to 25 %-50 % of the overall mixing, which increases with Sw due to finger merging. The majority of mixing takes place in the well-connected branches, while ganglia and singlet may act as sources and sinks that weakens tracer transport especially for low Sw. The dilution index is 2 or 3 orders larger in the branches than that in the isolated clusters of ganglia or singlet. This research provides insight into the relationship between solute transport properties and two-phase hydrodynamics at the pore scale.