Impact of salinity on spontaneous emulsification near water/oil interfaces

Abstract

Natural surfactants that are present in complex crude oil may induce spontaneous emulsification in the oil and brine phases that co-exist in rock pores. This process is known to be affected by the salinity of brine. However, the role of salinity in water-oil micro-emulsification is not fully understood. In this paper, we report on our experimental studies of the effect of salinity on spontaneous emulsification in a “mixture” of dodecane and brine. The dodecane contains SPAN 80 surfactant and brine with different salinity values, varying from 0.2 % to 20 % (by weight). For our observations, we use dynamic light scattering (DLS) technique to capture nano-scale emulsion formation and pendant drop method to observe micro-scale emulsion dynamics. The DLS experiments show that small (2.2 nm) and medium-sized emulsions (100 nm) are formed at low salinities, while at higher salinities only smaller droplets are formed and emulsification is reduced. In pendant drop experiments, dodecane and heptane systems were tested over 13 h. Heptane exhibited faster emulsification at water-oil interfaces in the cases with pure water and low salinity brine (0.2 %), where the changes at interfacial area occurring within two hours and significant droplet shrinkage by 13 h. Lower salinity enhances micelle activity and emulsification, while higher salinities (2 %, 5 %, and 20 %) stabilize the oil-water interface and suppress emulsion formation. Dodecane exhibits a similar trend in emulsification but forms more stable emulsions and maintains a more stable water-oil interface compared to heptane. Additionally, we present the theory of reverse micelle exclusion through a theoretical derivation, providing a deeper understanding of the emulsification mechanism. Four distinct scenarios are schematically presented to explain the influence of salinity on spontaneous emulsification, illustrating how varying salinity levels affect micelle formation and emulsion behaviour. This study provides valuable insights into optimizing salinity levels in enhanced oil recovery.