Experimental study of wettability alteration and spontaneous imbibition in Chinese shale oil reservoirs using anionic and nonionic surfactants

Wettability alteration exhibits a significant potential to improve the oil recovery of shale reservoirs by means of spontaneous imbibition through shifting the rock wetness from oil-wet to water-wet. The wettability of the oil-wet shale can be modified by adding surfactants in hydraulic fracturing fluids in the field practice. However, the mechanisms of wettability alteration in shale rocks are still unclear and there are reported discrepancies in different types of surfactants solutions. This work attempts to investigate the mechanisms of anionic and nonionic surfactants on wettability alteration of shale rocks of the Sichuan Basin in China, and to analyze their effects of wettability alteration and interfacial-tension (IFT) reduction on the spontaneous imbibition process of the oil-wet shale samples. By measuring the contact-angle, zeta-potential, and IFT, the efficiencies of different surfactants in terms of wettability alteration and IFT reduction are evaluated. The contact-angle experiments show that the water contact angle in rock/oil/water systems decreases when we add more surfactants, which indicates the hydrophilicity of shales is increased by surfactants. Specifically, for the anionic surfactant solutions, the contact angle doesn't immediately change, and it decreases with time, which is different from the sudden change of nonionic surfactants. Moreover, for different surfactants, the IFT decreases in a similar trend with the increase of surfactant concentration. Finally, spontaneous imbibition experiments are conducted and the results show that anionic surfactants are more favorable to recover shale oil than the nonionic surfactants. From the results obtained, it can be concluded that the anion surfactants and nonionic surfactant have different mechanisms for wettability alteration on the shale surface; anionic surfactants have better effect on wettability alteration than nonionic surfactants, however, the mechanism of wettability alteration in anionic surfactants have time delay. The wettability alteration dominates the oil recovery in the way of spontaneous imbibition. More water-wetness tends to yield a higher recovery. In addition, for anionic surfactant solutions, the reduction of IFT increases the final oil recovery and leads to a decrease of the imbibition rate. For nonionic surfactant solutions, the reduction of IFT increases both the imbibition rate and the final recovery, but the enhancement is not significant.