Imbibition in a porous medium plays a central role in diverse energy, environmental, and
industrial processes. In many cases, the medium has multiple parallel strata of different
permeabilities; however, how this stratification impacts imbibition is poorly understood. We
address this gap in knowledge by directly visualizing forced imbibition in three-dimensional
porous media with two parallel strata. We find that imbibition is spatially heterogeneous: for
a small capillary number Ca, the wetting fluid preferentially invades the fine stratum, while …

Imbibition, the displacement of a nonwetting fluid by a wetting fluid, plays a central role in
diverse energy, environmental, and industrial processes. While such flows are typically
studied in homogeneous porous media with uniform permeabilities, in many cases, the
media have multiple parallel strata of different permeabilities. How such stratification impacts
the fluid dynamics of imbibition, as well as the fluid saturation after the wetting fluid breaks
through to the end of a given medium, is poorly understood. We address this gap in …