Evaluation of the impact of low-salinity water on wettability alteration and oil recovery in Berea sandstones

Abstract

Low-salinity water (LSW) flooding is an emerging technique to enhance oil recovery from carbonate and sandstone reservoirs. Despite growing interest in the field, significant disparities remain regarding the underlying mechanisms. While most researchers agree that wettability alterations may be the dominant mechanism, their quantitative impact on oil recovery remains elusive. This study quantitatively evaluated the effects of LSW flooding on rock wettability alteration and oil recovery enhancement using a non-polar crude through a series of U.S. Bureau of Mines (USBM) tests and contact angle measurements on synthetic clay-free Berea samples with similar textural properties and pore structures. Experimenting on clay-free samples and non-polar crude evaded the effects of (i) mineral dissolution, (ii) interfacial tension reduction due to in-situ saponification, and (iii) wettability alteration due to multi-ion exchange between clays and the crude-oil polar compounds. When water salinity varied from 85,000 to 4,000 ppm, the USBM index increased from approximately 0.26 to 0.38, thereby confirming a minor change in wettability alteration toward a more water-wet condition. These changes were consistent with a decrease in the contact angle from approximately 55.6° to 35.3°. As water salinity decreased to approximately 5 % of its original value, centrifuge data indicated a drop in the residual oil saturation from 0.247 to 0.202, corresponding to a minor increase (of approximately 4 %) in the oil recovery factor at the end of the forced imbibition cycle. This slight oil recovery enhancement is consistent with the minor wettability alterations observed in this study. However, the relatively large recovery factor values reported in the literature were not reproduced in this experiment. The reported findings of this study concerning the evaluation of wettability alteration and recovery factor improvement caused by LSW flooding are unique because they (i) are based on reproducible measurements obtained using state-of-the-art ultrahigh-speed rock centrifuge and core test system that simultaneously measures capillary pressure, evaluates rock wettability, and generates relative permeability data, and (ii) are focused on clean sandstones.