Effect of water chemistry on the stability of water-in-crude oil emulsion: Role of aqueous ions and underlying mechanisms

Recent studies have shown that although low salinity waterflooding leads to promising EOR impacts, it is to blame for the formation of unwanted water-in-oil emulsions. Thus, this study aims to explore the effect of water chemistry, namely ionic strength and ion types on the stability of natural water-in-oil (W/O) emulsions at the reservoir temperature of 80 degrees C. To this end, seawater and its dilutions with different concentrations of potential determining ions (PDIs) were used as the aqueous phase. The experimental study was performed by tracking the emulsion droplet size distribution, phase separation study and also IFT measurements. Results showed that the higher the ionic strength of the aqueous phase, the lower the W/O emulsion stability, which could be explained based on the salting-out mechanism. It was found that the IFT parameter did not significantly contribute to the emulsion stability. Regarding the effect of ion types on the stability of emulsions prepared by eight-time diluted seawater with different PDIs, the trend was Mg2+ >= Ca2+ > SO42-. These observations were described in the light of the higher tendency of divalent cations to bond with crude oil polar components. As to the phase separation study, the amount of expelled water in the presence of Mg2+ was 15 vol% of the initial water, compared to 24 vol % for Ca2+ and almost half for SO42- after one-day aging. It was also found that the co-existence of sulfate with divalent cations improved the emulsion stability, while the less stable emulsions were obtained by the sulfaterich brine. This was further supported by the droplet size distribution profile in which a more uniform profile was obtained for emulsions with the co-existence of sulfate and divalent cations as compared to the sulfate-rich samples. Results of this study addressed that the ionic strength of the aqueous phase coupled with its ionic content regulates the emulsion stability, such that there exists a specific range of salinity and ion concentration beyond that the emulsion stability may impair.