Imbibition has been proposed as an important means by which to recover oil from low permeability reservoirs. However, the mechanism of imbibition enhanced oil recovery (IEOR) remains controversial. In this study, we attempted to explore these mechanisms using three of the most commonly used chemical systems (surfactant, brine based nano-silica, and surfactant based nano-silica solutions). First, the imbibition efficiency of the three systems was investigated through imbibition tests using Amott cells at 80 degrees C. Then, the interfacial tension (IFT) and contact angle were measured to explore the potential characteristics that may play important roles in IEOR. Finally, the dynamic inverse Bond number was introduced to further analyze the mechanisms of IEOR. The results show that oil recovery can be enhanced by 5.36% when using an ultra-low IFT surfactant solution, which has less ability to alter wettability; up to 4% using brine based nano-silica solutions, which can only alter wettability to more water-wet but cannot reduce the IFT; and 8.78% using surfactant based nano-silica solutions, which can both reduce the IFT and alter wettability effectively. Thus, both low IFT and good wettability alteration are desirable, and the highest recovery was achieved when both characteristics were combined, and maximizing their contributions to IEOR. This work firstly investigated the contribution of IFT reduction and wettability alteration on IEOR separately, and a novel system that produced excellent IEOR efficiency was formulated. This study provides fundamental information to the selection of IEOR chemicals.