Impact of capillary imbibition on development of unconventional oilfields: A five-year review

The imbibition process, which is characterized by the displacement of non-wetting phase by wetting phase in porous media, plays a crucial role in enhancing oil recovery of unconventional reservoirs. However, due to the complex nature of reservoir characteristics and fluid properties, existing research on this phenomenon presents contradictory conclusions and interpretations. This paper aims to fill this knowledge gap by providing a comprehensive review of the fundamental concepts, primary mechanical factors, and critical influences in oil-water imbibition. Primary mechanisms, including capillary, viscous, gravitational, hydrostatic, inertial, capillary back, dead end, and osmotic forces, are analyzed in terms of their key roles under varying boundary conditions and scales. To further clarify the conflicting research findings, we systematically analyze the factors influencing water imbibition, encompassing rock and fluid properties, rock-fluid interactions, and reservoir conditions. Based on this analysis, an integrated framework is established to describe the underlying coupled mechanisms. Furthermore, this review details the imbibition mechanisms and fluid distribution characteristics across the entire development cycle of unconventional oil reservoirs, from injection and shut-in to flowback and production. It subsequently offers practical recommendations for optimizing development strategies. Finally, we identify persistent knowledge gaps and propose critical directions for future research. By synthesizing these insights, this work provides valuable insights for advancing the understanding of oil-water imbibition and improving recovery processes in unconventional oil reservoirs.