Abstract
The injection of low-salinity brine enhances oil recovery by altering the mineral wettability in carbonate reservoirs. However, the reported effectiveness of low-salinity water varies significantly in the literature, and the underlying mechanism of wettability alteration is controversial. In this work, we investigate the relationships between characteristics of crude oils and the oils’ response to low-salinity water in a spontaneous imbibition test, aiming (1) to identify suitable indicators of the effectiveness of low-salinity water and (2) to evaluate possible mechanisms of low-salinity–induced wettability alteration, including rock/oil charge repulsion and microdispersion formation. Seven oils are tested by spontaneous imbibition and fully characterized in terms of their acidity, zeta potential, interfacial tension, microdispersion propensity, water-soluble organics content and saturate-aromatic-resin-asphaltene fractionation. For the first time, the effectiveness of low-salinity water is found to positively correlate with the oil interfacial tension in low-salinity water. Oils with higher interfacial activity are found to respond more positively to low-salinity water. Moreover, cryogenic transmission electron microscopy images suggest that microdispersion is essentially macroemulsion, and its formation is an effective indicator – but not the root cause – of wettability alteration. The repulsive zeta potential for the rock and the oil in low-salinity water is found to be an insufficient condition for wettability alteration in carbonate minerals.
Highlights
The injection of low-salinity brine enhances oil recovery by altering the mineral wettability in carbonate reservoirs
The increase in the water content in crude oil after contact with low-salinity water leads to higher desorption of indigenous oil surfactants from the mineral surface leading to wettability alteration and higher incremental oil recovery
The Amott-Harvey wettability index IA-H is measured as −0.5 for Crude A, where the highest amount of additional oil recovery is observed. (Details about IA-H measurement are located in the Supplementary Materials [S3].) The negative wettability index further validates that the cores become oil-wet after oil aging
Summary
The injection of low-salinity brine enhances oil recovery by altering the mineral wettability in carbonate reservoirs. Low-salinity water flooding has drawn increasing attention in the past decade as an emerging low-cost enhanced oil recovery (EOR) technique that improves oil recovery by altering the rock wettability In this technique, the salinity of the injected brine is reduced, and the ionic composition is often modified. Sohrabi et al conducted a series of studies to investigate the critical condition of the crude oils required for low-salinity water to be effective[7,32,33,34,35,36] They claim that the incremental oil recovery in low-salinity water for a specific crude oil is related to the capability of the oil to form water-in-oil “microdispersion” with low-salinity water. The reason microdispersion only forms in some specific oils but not in others has not been clearly explained
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