Effects of Surfactant Partition Coefficient and Interfacial Tension on Oil Displacement in Low-Tension Polymer Flooding

Abstract

Abstract Complex surfactant formulations have been applied to induce ultra-low interfacial tension (IFT) (e.g., 10−3 mN/m) between the displacing water phase and the displaced oil phase in chemical enhanced oil recovery (CEOR), where the residual oil after waterflooding can be largely recovered as an oil bank. This paper is concerned with a simpler, lower-cost CEOR, in which a sole additive of surface active solvent (SAS) makes low-tension displacement fronts in polymer flooding (e.g., 10−2 mN/m) without involving ultra-low IFT microemulsion phase behavior. The envisioned application of SAS in this research is to displace a continuous oil phase with low-IFT displacement fronts with mobility control by polymer. Previous research found that 2-ethylhexanol-7PO-15EO (2-EH-7PO-15EO, or "7-15") as SAS was able to reduce the IFT between polymer solution and the reservoir oil from 15.8 mN/m to 0.025 mN/m. In this research, the effect of SAS partition coefficient on the low-tension polymer (LTP) flooding was studied as an additional factor for SAS optimization. In particular, the comparison between two SAS species, 2-EH-4PO-15EO (4-15) and 2-EH-7PO-25EO (7-25), was important, because they had similar IFT values, but markedly different partition coefficients. The IFT was 0.20 mN/m with 4-15 and 0.18 mN/m with 7-25; and the partition coefficients were 1.61 with 4-15 and 0.68 with 7-25 at the experimental temperature, 61°C. These two SAS species were compared in the secondary-mode LTP flooding with a slug of 0.5 wt% SAS for 0.5 pore-volumes injected (PVI). Although these two SAS species gave comparable IFT values, their oil-displacement characteristics were quite different. The oil recovery factor at 1.0 PVI was 65% with 4-15 and 67% with 7-25. At 5.0 PVI, it was 74% with 4-15 and 84% with 7-25. The SAS recovery at the effluent was 67% for the 4-15 SAS and 86% for the 7-25 SAS. With a similar IFT reduction, the SAS with a smaller partition coefficient (i.e., 7-25) resulted in less SAS retention and more oil production for a given amount of injection. In comparison to the previous research with 7-15, the oil recovery factor with 7-25 was smaller at 5.0 PVI, but similar at early times up to 1.5 PVI. When the SAS concentration was reduced to 0.1 wt% SAS for 0.5 PVI, the oil recovery factors with 7-25 and 7-15 were close to each other at both early and final stages. In these smaller slug sizes, the SAS recovery was detected only for the 7-25 case, which means 7-25 went through the sandpack whereas 7-15 did not. That is, the optimization of SAS likely requires taking a balance between lowering the partition coefficient and lowering the IFT.