  Imbibition 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 (3D) porous media with two parallel
strata. We find that imbibition is spatially heterogeneous: for small capillary
number Ca, the wetting fluid preferentially invades the fine stratum, while for
Ca above a threshold value, the fluid instead preferentially invades the coarse
stratum. This threshold value depends on the medium geometry, the fluid
properties, and the presence of residual wetting films in the pore space. These
findings are well described by a linear stability analysis that incorporates
crossflow between the strata. Thus, our work provides quantitative guidelines
for predicting and controlling flow in stratified porous media.
