Competition between viscous and capillary forces triggers diversity of fluid distribution and imbibition modes

Spontaneous imbibition is the process in which the wetting phase displaces a non-wetting phase under the action of capillary forces. However, variations in interfacial properties, imbibition directions, and fractures result in different imbibition modes, posing challenges to a comprehensive understanding of the process. In this study, microfluidic chips representing matrix and fracture–matrix systems were designed. Imbibition agents with varied interfacial properties were selected to conduct experiments under counter-current and co-current conditions. A flow factor (γ), related to fracture geometry and imbibition direction, was defined and used together with the microscopic capillary number (Ca_micro) to characterize the imbibition process. Three distinct imbibition modes were observed for different Ca_micro and γ, clearly separated by γ–Ca_micro boundaries. During co-current imbibition, an unusual capillary-driven displacement process was observed, leading to fingering in the fracture–matrix model and leaving a large area of macroscale remaining oil. Smaller Ca_micro and fracture development will facilitate this process. In addition, various forms of microscale remaining oil, caused by bypass flow snap-off and Saffman–Taylor instability/Rayleigh–Taylor instability, were also observed across different imbibition processes. This study elucidates the imbibition mechanisms under the combined influence of capillary and viscous forces, providing deeper insights into the imbibition process in porous media.