Imbibition in 3D porous media is shaped by long-range capillary- and shear-driven flow-field dynamics

As a wetting fluid invades a porous material, it displaces a less-wetting fluid through sudden pore-scale events, commonly breaking the latter into isolated ganglia that can be trapped, mobilised and fragmented. Three-dimensional flows inside the ganglia shape the displacement dynamics but are difficult to measure and thus currently poorly characterised. Here, 3D X-ray particle tracking velocimetry reveals how flow inside the nonwetting phase evolves during unsteady imbibition. In the capillary-dominated regime, snap-off and cooperative pore filling disturb the flow over more than ten pore diameters for several seconds, revealing nonlocal dynamic effects in imbibition comparable to those triggered by Haines jumps in drainage. For increased viscous forces, we observe continuous fragmentation of ganglia throughout the pore space, rather than the compact front displacement predicted by phase diagrams derived from 2D micromodels. In this regime, momentum transmitted across fluid menisci drives shear near the interface and recirculation within trapped ganglia. Where previous studies describe rotational flow around a single centre in such cases, our results show that a 3D ganglion can contain a localised recirculation zone that is part of a more complex recirculation pattern. When ganglia are mobilised, this pattern is temporarily replaced by elongational flow, which may hinder the mobilisation of non-Newtonian fluids and enhance internal mixing. The flow patterns inside the nonwetting phase during imbibition thus change with the displacement regime, the ganglion’s dynamic state and with dimensionality. This affects fluid displacement, trapping and mixing in applications such as subsurface CO₂ and hydrogen storage and groundwater remediation.