The effect of surfactant partition coefficient and interfacial tension on oil displacement in low-tension polymer flooding

Abstract

This paper presents an experimental study of low-tension polymer (LTP) flooding with a short-hydrophobe surfactant as a sole additive. Such a simple surfactant makes low-tension displacement fronts in polymer flooding (e.g., 10−2 mN/m) without involving micro-emulsions of ultra-low interfacial tension (IFT). The envisioned application of LTP flooding is to enhance the displacement of a continuous oil phase with such a moderate reduction in IFT as an effective improvement of polymer flooding.In our previous research, 2-ethylhexanol-7PO-15EO (2-EH-7PO-15EO) was selected as an optimal short-hydrophobe surfactant that resulted in the lowest IFT between polymer solution and oil, and achieved the greatest final oil recovery. However, this paper presents the effect of surfactant partition coefficients on the LTP flooding as an additional important factor for surfactant optimization.A series of LTP floods showed that the IFT primarily affected the final oil recovery when the sandpack was swept sufficiently by LTP. Comparison of two cases with similar IFT values (2-EH-4PO-15EO and 2-EH-7PO-25EO) showed that the surfactant partition coefficient affected the oil recovery through its impact on the surfactant in-situ propagation. The case with 2-EH-7PO-25EO resulted in a greater oil recovery because the surfactant propagated more efficiently with a smaller partition coefficient than 2-EH-4PO-15EO.Results collectively showed that optimization of 2-EH-xPO-yEO for LTP flooding involves two competing factors. One is to minimize the water/oil IFT for increasing the local oil displacement efficiency, and the other is to minimize the partition coefficient for more efficient in-situ propagation of the surfactant. It is critical to take a balance between these two factors for the surfactant used for LTP flooding. The importance of the surfactant partition coefficient became more obvious when a limited amount of surfactant was injected.