Review of effects and mechanisms of forced imbibition on enhanced oil recovery: Huff-n-puff and improved wettability

Imbibition plays a key role in the performance of injected fluids in enhanced oil recovery from low-permeability, tight reservoirs. Huff-n-puff with well-soaking and improving reservoir wettability can promote imbibition, resulting in forced imbibition. However, our understanding of its concept, influencing factors and the underlying mechanisms remains limited. Therefore, first, this work systematically reviews the relevant research and clarifies the concept of forced imbibition. Next, the mechanisms and enhanced oil recovery contributions of huff-n-puff and wettability improvement during the forced imbibition process are highlighted and summarized. Huff-n-puff and low-salinity water flooding are two key forced imbibition methods. Regarding huff-n-puff development, this work compares and analyzes the key controlling mechanisms and enhanced oil recovery effects of imbibition enhancement using three fluids: Water, gas and activated water. Gas is currently the most widely used huff-n-puff medium in oilfields because of its stronger mass transfer and diffusion capabilities, making it can enter smaller pore throats. Besides, water is also an irreplaceable huff-n-puff medium because of its fast energy replenishment, low cost, and environmentally friendly. Active water with the addition of surfactants, nanofluids and displacement systems can enhance the effect of water huff-n-puff, achieving a broader application potential. Subsequently, the enhanced oil recovery contribution rate of forced imbibition during the huff-n-puff process is discussed. Regarding low-salinity water flooding, this paper focuses on the mechanism of improving wettability and its effect on enhancing CO₂ imbibition. It not only offers a comprehensive understanding of the concepts mechanisms and enhanced oil recovery effects of the forced imbibition process but also provides valuable insights for theoretical research and field applications of forced imbibition-based enhanced oil recovery technologies. Document Type: Invited review Cited as: Chen, X., Tang, K., Zhao, W., Yue, X., Zhao, X., Wang, W. Review of effects and mechanisms of forced imbibition on enhanced oil recovery: Huff-n-puff and improved wettability. Capillarity, 2026, 18(2): 50-67. https://doi.org/10.46690/capi.2026.02.02