Coalescence of Crude Oil Droplets in Brine Systems: Effect of Individual Electrolytes

Abstract

To better understand mechanisms of enhanced oil production by smart waterflooding, the coalescence time of crude oil droplets in different brines was measured. Sulfate ions were found to hinder the coalescence of crude oil droplets, whereas magnesium ions significantly enhanced coalescence of the crude oil droplets. To shed light on the observed effect of an individual types of water-soluble inorganic ions in brine on coalescence of crude oil droplets, interfacial shear rheology of crude oil–brine interfaces was measured at both ambient and elevated temperatures. ζ potential of crude oil in brine emulsions was measured at ambient temperature. The major objective of these measurements is to investigate interactions of individual water-soluble inorganic ions with the crude oil–water interface and study their impact on the coalescence of crude oil droplets. Interfacial shear rheology results showed significantly higher viscoelastic moduli for brines comprising of sulfate ions, while the brines with sodium, calcium or magnesium ions showed comparable interfacial shear rheology. The transition time, for the interface to become elastic-dominant from a viscous-dominant regime, was found to be the shortest for sulfate brine, followed by sodium brine and calcium brine, with the highest being magnesium brine. The most negative ζ potentials, indicating the strongest electrostatic repulsion between two oil droplets, were observed in the sodium brine and sulfate brine. The magnesium brine and calcium brine showed the lowest negative ζ potentials. The results on coalescence time showed a good agreement with the combined effect of film interfacial rheology and ζ potential. This study establishes the impact of individual water-soluble inorganic ions, for the first time, on coalescence between oil droplets in specific relation to waterflooding and enhanced oil recovery.

An overview of the oil-brine interfacial behavior and a new surface complexation model

“…The discrepancy between the model and the experimental evidence could be a consequence of the higher ionic strength of the brines in these experiments; note that the optimization of the model parameters was performed using zeta potentials obtained at lower ionic strength. Moreover, in this work a high oil/brine volume ratio is used in the electrophoretic measurements (20%), whereas much lower values (<1%) were reported in other works 18,25,26,36 . The modeling of these data would probably benefit from more information on the experimental conditions (i.e., oil drop size, closed or open system, etc.).…”

“…Ayirala et al . 18 performed zeta potential measurements on a crude oil with very low AN and high BN (Table 1) in four different brines. Theoretically, a positive surface charge would be expected throughout the whole pH interval, since the fraction of acids undergoing dissociation at higher pH is expected to be very low.…”

“…The authors mention that the negative surface charge of the oil can be explained by the brine pH of around 6. However, knowing the actual pH of the oil-brine system would facilitate the comparison between the results from the model and the experimental data.

Figure 8Zeta potential of crude oil in four different brine compositions 18 . All the measurements were performed at a pH around 6.

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“…84 Crude oil A0.181.650.60.6Crude oil B0.110.620.30.55Ayirala et al . 18 Crude oil0.050.70.50.35Takeya et al . 85 Crude oil0.391.860.750.75Lu et al .…”

“…On the other hand, Ayirala et al . 18 reported that, even though the connectivity of the oil phase increases with increasing viscoelasticity of the interfacial film, very high viscoelasticity does not necessarily imply a higher oil recovery. Instead, they considered the coalescence time to contribute more to oil connectivity, since this factor indicates the time that takes for the snapped-off droplets to reunite.…”