Effect of Pressure and Surfactants with Different IFT and Wettability Alteration Abilities on Imbibition Oil Recovery in Tight Sandstone Reservoir Under High Pressure

The water huff-n-puff imbibition oil recovery technique has been recognized as an important approach to supplementing formation energy and recovering the remaining oil, attracting increasing attention. To further improve imbibition efficiency, a surfactant-aided huff-n-puff imbibition technique under high pressure was proposed. However, the imbibition mechanisms under high pressure, particularly under variable pressurization modes, remain insufficiently understood. In this study, the effects of different pressurization methods (constant vs. variable pressure) and surfactant types on imbibition behavior were systematically investigated. The results show that, compared with spontaneous imbibition, high-pressure imbibition increases oil recovery by 7–10% and the imbibition rate by 1–2 times, with the variable pressurization mode demonstrating a more pronounced enhancement. Surfactant selection should not pursue ultra-low interfacial tension (IFT) alone; instead, the wettability alteration ability is more critical. An optimal IFT–wettability synergy window is identified, through which the best imbibition performance is achieved when the IFT ranges from 10−2 to 10−1 mN/m and the contact angle ranges from 30° to 60°. Furthermore, the slug injection mode provides a synergistic effect with high-pressure variable pressurization and surfactant action. Compared with high-pressure formation water imbibition, surfactant-aided imbibition increases oil recovery by 10.44% and the imbibition rate by three times. These findings provide a deeper understanding of the key factors governing imbibition behavior and support the application of surfactant-aided huff-n-puff imbibition under high pressure in tight sandstone reservoirs.