Quantifying the Effect of Salinity on Oilfield Water-in-Oil Emulsion Stability

The effect of salinity on water-in-oil emulsions was systematically studied using a combination of nuclear magnetic resonance (NMR) pulsed field gradient (PFG) measurements of emulsion droplet size distribution complemented by interfacial tension measurements using the pendant drop method. Long-term emulsion stability over periods of up to 5 days was found to increase with salinity; this was shown to be independent of whether a monovalent (NaCl) or divalent (CaCl₂) salt was used. The methodology was applied to water-in-oil emulsions formulated with crude oil, paraffin oil, xylene, crude oil with reduced asphaltene content, and crude oil with reduced organic acid content as the continuous phase, respectively. In all cases, emulsion stability increased consistently with aqueous phase salinity, with no discernible difference between the continuous oil phases with respect to the extent of this stabilization. The enhanced stability could thus not be attributed to differences in density, interfacial tension, or dielectric permittivity. This leaves a potential increased surface accumulation of stabilizing surface-active species driven by increasing salinity as the most plausible explanation for the observations reported here.