Double-Layer Expansion: Is It A Primary Mechanism of Improved Oil Recovery by Low-Salinity Waterflooding?

Abstract

AbstractSeveral mechanisms which could contribute to improved oil recovery (IOR) by low-salinity waterflooding have been proposed. However, the main mechanism is still debated. Nasralla et al. (2011a) showed that low-salinity water has an impact on the electrical charges at oil/brine and rock/brine interfaces, and it causes expansion of the double layer, which is the main reason behind wettability alteration and high oil recovery. Nevertheless, double-layer expansion by low-salinity water might contribute, but not be the primary mechanism. Therefore, this paper investigates this mechanism and shows how it is involved in the process of IOR.Electrical surface charge is a function of brine's pH, so the injected brine's pH was manipulated to show how oil recovery during low-salinity waterflooding depends on changes in electrical charges. Coreflood experiments were conducted using Berea sandstone cores 20 in. long; different brines (TDS varies from 0.5 to 55 kmg/l) were injected. Furthermore, zeta potential at rock/brine and oil/brine interfaces was measured for the brines used at different pHs. The contact angle technique was used to evaluate the alteration of rock wettability by the same brine at a different pH.Zeta potential results showed that decreasing low-salinity brines’ pH makes the electrical charges at both oil/brine and brine/rock interfaces weakly negative, which reduces the repulsion forces between oil and rock, and as a result the rock becomes more oil-wet, which was confirmed by contact angle measurements. Moreover, coreflood experiments indicated that injecting low-salinity brine at lower pH recovered less amount of oil compared to the original pH because of the elimination of the low-salinity water effect on electrical charges. In conclusion, expansion of the double layer is a dominant mechanism of oil recovery improvement by low-salinity waterflooding.