Alkali/Surfactant Improved Low-Salinity Waterflooding

Abstract

Extensive laboratory studies have shown that oil recovery from water flooding is dependent on the salinity and composition of the injected water. The potential of low-salinity waterflooding (LSWF) has been observed in field trials, with relatively good agreement with the measured laboratory data. However, the incremental recovery from LSWF is relatively modest (2–10 % OOIP) compared to other water-based EOR methods such as chemical methods, particularly when applied in tertiary mode. In this paper, we investigate low-salinity flooding combined with alkali to improve the incremental recovery. The recoveries are also compared with low-salinity brine combined with surfactant. This is studied in a system, which is first shown to be responsive to low salinity. The low-salinity recovery result is used as a baseline for comparison. A clay-rich core from a sandstone reservoir and crude oil were used. The flooding experiments were performed by successive injection of high-salinity formation brine and low-salinity water or low-salinity water combined with a surfactant or alkali (SDS/NaOH). Based on the results, without adding alkali or surfactant, low-salinity flooding recovered \(\sim \)4 % additional oil over the recovery from high-salinity injection. However, when combined with 1 wt% alkali/surfactant, the oil recovery increased to 7–17 % OOIP. Minor formation damage was observed in all experiments. Interfacial tension (IFT) reduction (capillary desaturation) in each combined method is envisaged to be the driving mechanism for the enhancement of oil recovery. Interfacial tension reduction decreases capillary pressure, thereby decreasing trapping or re-trapping of the mobilized oil by low-salinity flooding. Comparison of the recovery from surfactant-improved low salinity and alkali-improved low salinity indicates that higher oil recovery can be achieved with surfactant than with alkali. Higher efficiency with surfactant can be attributed to the lower attainable IFT with surfactant than alkali (higher capillary numbers). Nevertheless, due to lower costs, alkali is more cost effective than surfactant and is advantageous because it reduces adsorption of in-situ generated petroleum surfactant. The results of the study emphasize the benefits of hybrid methods for the improvement of oil recovery. Particularly where a reservoir is responsive to low salinity, recovery can be enhanced by the addition of a small amount of alkali or surfactant.