Interfacial tension (IFT) reduction, emulsification, and wettability alteration are three main mechanisms for surfactant flooding. In traditional concepts, researchers often connect low IFT with strong emulsifying ability and use microemulsion phase behavior test to choose surfactant systems. However, this is not always necessary, especially for low-permeability reservoir where microemulsion is unwanted due to its potential blocking risk. In this work, we present a new potential direction of low-concentration surfactant flooding.

For low-permeability reservoir, microemulsion that may cause pore blocking is unwanted. Therefore, we try to develop special surfactant formulation with low concentration that can reduce IFT to 10-2 mN/m even ultra-low level and significantly alter wettability, but have a weak emulsifying ability under high temperature and high salinity conditions. IFT, emulsification, and contact angle measurements were performed to determine the surfactant formulation. Furthermore, microfluidic device was employed to investigate the mobilization and transport of residual oil to understand the pore-scale enhanced oil recovery (EOR) mechanism and efficiency of such a surfactant system.

For the special type surfactant, low IFT is not always accompanied by strong emulsifying ability. The anionic-nonionic surfactant was found to have this expected function that can significantly reduce IFT and alter wettability, but have a weak emulsifying ability. This can be accomplished by a low surfactant concentration, especially under high temperature and high salinity. We also found that low IFT always leads to a great alteration of wettabilty, but the reverse is not ture. In porous media, under low IFT condition, the surfactant system with a weak emulsifying ability can effectively displace residual oil, and oil drops could be easily mobilized and no blocking occurred even at narrow throat because of their high deformability. ‘Lateral stripping’ was found to be one of main mechanisms to mobilize oil films and the exterior oil of clusters. The coalescence of stripped oil and its stronger stripping ability to other residual oil plays an important role in EOR mechanism. The surfactant system reduced residual oil saturation from 18.14% (after waterflooding) to 1.28%.

This work presents a new direction of low-concentration surfactant flooding by lowering IFT significantly without the generation of microemulsion, which not only enriches the theory of surfactant flooding, but also provides a great potential application of surfactant flooding in low-permeability reservoir. Simultaneously, this surfactant system can be used at high temperature and high salinity, which also expands the application of surfactant flooding for EOR in harsh reservoir conditions.

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