Salt-tolerant surfactant for dilute surfactant flooding in high-salinity reservoirs: Residual oil stripping and displacement mechanism and efficiency by ultra-low interfacial tension

© 2020, Society of Petroleum Engineers Dilute surfactant flooding is a promising EOR method. It is more economical and easier to be controlled in reservoirs compared with microemulsion flooding where high surfactant concentration is needed. In this work, we present a salt-tolerance surfactant that can attain ultra-low interfacial tension (IFT) by a low surfactant concentration (0.2wt%) under high salinity condition. And its oil stripping and displacement mechanism and efficiency under ultra-low IFT condition were investigated. A novel anionic-nonionic surfactant with a low concentration of 0.2 wt% was used to prepare surfactant solutions at different salinity conditions. Ultra-low IFT (10-3 mN/m) was obtained at high salinity condition. These surfactant solutions were used for oil film stripping experiments in visual oil-wet quartz cell and filtration experiments in transparent sand pack model as well as two-parallel core flooding model under heterogeneous permeability conditions to study oil stripping and displacement mechanism and efficiency by ultra-low IFT. Oil film can be effectively stripped only under ultra-low IFT condition at high salinity condition with the occurrence of wettability reversal (from oil-wet to water-wet). Oil film is stripped in the form of oil drops by automatically shrinking. The stripping mechanism by ultra-low IFT can be attributed to the reduction of water-oil IFT, water-solid surface IFT, and wearability reversal, which thus reduces the work of adhesion. Filtration experiments showed that a high displacement efficiency (additional oil recovery is about 16-23% depending on the heterogeneity) was yielded under ultra-low IFT condition at high salinity condition in dilute surfactant flooding process. In addition, a new EOR mechanism was found that ultra-low IFT can re-mobilize the trapped oil in low-permeability layer that cannot be displaced by water flooding under heterogeneous permeability conditions without any additional conformance treatment. This anionic-nonionic surfactant can obtain ultra-low IFT at high salinity condition at a low surfactant concentration (0.2 wt%), which can effectively strip residual oil film and re-mobilize trapped oil in low-permeability zone that cannot be mobilized by water flooding under heterogeneous permeability. The development of this salt-tolerance surfactant and finding of its EOR mechanism will improve the application of dilute surfactant flooding in those reservoirs with high salinity