(425i) Forced Imbibition in Stratified Porous Media

Imbibition, the process by a wetting fluid displaces a non-wetting fluid from a porous medium, plays a central role in enhanced oil recovery and groundwater remediation. It is therefore critically important to understand the factors that control the fluid displacement. Studies typically consider homogeneous media with random and uncorrelated pore sizes; however, in reality, porous media have parallel strata of different permeabilities. Here, we use direct visualization to investigate the dynamics of flow rate-controlled imbibition in 3D porous media with two parallel strata. We find dramatically different flow dynamics depending on the capillary number Ca. For low Ca, the wetting fluid invades the fine stratum preferentially, while above a threshold Ca, the wetting fluid invades the coarse stratum instead. Moreover, the threshold value of Ca depends on the presence of residual wetting films in the pore space. Our experimental results can be rationalized by a linear stability analysis that considers the balance of capillary and viscous forces, along with crossflow between the strata. Together, these results provide quantitative guidelines for predicting and controlling flow in heterogeneous porous media, such as those in oil reservoirs and groundwater aquifers.