Experimental study on the imbibition mechanism of the Winsor type Ⅰ surfactant system with ultra-low IFT in oil-wet shale oil reservoirs by NMR

Surfactants enhanced spontaneous imbibition is an effective method to improve the oil recovery of shale reservoirs. The popular perspective believes that the wettability alteration and the capillary force are the primary mechanisms. Thus a water-wet condition and a relatively high interfacial tension (IFT) are required. However, in this work, we found that the Winsor type I surfactant solution with ultra-low IFT performed better than other relatively higher-IFT solutions with the capillary effect in oil-wet shale reservoirs. The contact angle, spontaneous imbibition recovery and phase behavior of the Winsor type I surfactant solutions were measured and compared with the solutions with relatively high IFT, including water, nonionic, and anionic surfactant solutions. The results demonstrated that anionic surfactants and Winsor type I surfactant solutions could alter the wettability to a water-wet condition, while the nonionic surfactants could only change to an intermediate-wet state. Both the anionic surfactants and Winsor type I surfactant solutions could displace the oil from micropores and mesopores. Still, the Winsor type I surfactant solutions with ultra-low IFT showed a better performance than the anionic surfactants with relatively high IFT. The interfacial micrographs of the Winsor type I surfactant solutions in contact with the oil phase manifested that the Winsor type I surfactant solutions changed the recovery patterns from two-phase flow driven by capillary force to the migration of microemulsions and emulsions driven by diffusion and gravity. The over-equilibrium Winsor type Ⅲ microemulsions and emulsions are rapidly formed near the oil-water interface. After that, the over-equilibrium Winsor type Ⅲ microemulsions are gradually diluted to Winsor type Ⅰ microemulsions and dissolved in a fresh surfactant solution. At the same time, the emulsions diffuse firstly, then coalesce and migrate out of the pore mediums driven by buoyancy. The water-wet condition and capillary force are not significant for this flow pattern, thus the Winsor type I surfactant solutions with ultra-low IFT are especially effective in recovering the oil of oil-wet micropores and mesopores. This work provides a new way of exploiting the oil in organic matter pores that can barely change to water-wet conditions.