It is a well-known EOR mechanism that residual oil in swept zone after water flooding (including oil drops, films and clusters) can be mobilized by low interfacial tension (IFT) during surfactant flooding. The aim of this work is to investigate if trapped oil in low-permeability zone (unswept by water flooding) can be mobilized or not by ultra-low IFT in dilute surfactant flooding without any additional conformance control treatment using polymer, gel, or foam, etc. Anionic–nonionic surfactant was used to attain ultra-low oil-water IFT of 10-4 mN/m at a low surfactant concentration of 0.2wt% under high salinity without adding any additional solubilizer, alkali, alcohols, etc. Using this surfactant solution, low field NMR core flooding experiment was conducted using heterogenous core. This core was drilled from artificial rectangular-sandstone core with two layers (gas permeability in low- and high-permeability layer is 90mD and 1200mD, respectively). Then, two-parallel core flooding experiment was conducted, where two core holders were parallelly installed (one for low-permeability core, one for high-permeability core) to simulate the heterogenous permeability condition.

NMR core flooding results showed that the reduction percentage of oil saturation in large pores (mostly distributed in high-permeability layer) was much higher than that in small pores (mostly distributed in low-permeability layer) during water flooding. After a high water cut of 98%, surfactant was injected. It was found that oil saturation in both high- and low-permeability layer was decreased, and the reduction percentage in small pores was much higher than that in large pores, which partly indicates that ultra-low IFT cannot only mobilize residual oil in swept zone in high-permeability layer, but also effectively mobilize trapped oil in unswept zone in low-permeability layer. This EOR mechanism was further verified by two-parallel core flooding experiment. Under the permeability max-min ratio of 705.1 to 48.96 mD (water permeability), there was no oil production from low-permeability core during water flooding even when water cut was 98% in high-permeability core. This means that it is difficult for injected water to penetrate into low-permeability core under such a heterogeneity. However, when changed to surfactant flooding, low-permeability core started to produce oil. A surfactant injection of 0.3 PV can yield an oil recovery of about 5% and 16% from low- and high-permeability core, respectively, which proves that ultra-low IFT has ability to mobilize trapped oil in low-permeability zone unswept by water flooding under heterogenous permeability without any additional conformance control treatment.

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