Impact of Residual Zwitterionic Surfactants on Topside Water–Oil Separation of Pre-Salt Light Crude Oil Emulsions

Zwitterionic surfactants have been used as foaming agents in Enhanced Oil Recovery due to their foamability, tolerance to reservoir conditions, low toxicity, and low adsorption on reservoir rocks. However, in the case of back-production, the impact of zwitterionic surfactants on water-in-crude oil (W/O) emulsions and produced water remains unclear, representing a knowledge gap in the field. The present study evaluated the effect of three commercial zwitterionic surfactants, at concentrations ranging from 50 to 1000 ppm, on water-oil separation using a Brazilian Pre-Salt crude oil. Emulsion formation, separation kinetics, and water quality were assessed through bottle tests, droplet size microscopy, and oil-in-water content analysis. At the lowest concentration (50 ppm), these surfactants promoted coalescence and sedimentation of W/O emulsions faster than traditional nonionic and anionic surfactants. Separation kinetics increased as surfactant concentration increased up to 200 ppm, but concentrations above 500 ppm induced the formation of multiple emulsions (W/O/W). After 72 h, these emulsions separated, and the aqueous phase still contained residual oil droplets, indicating potential challenges associated with the disposal or reuse of produced water. Surfactant hydrophobicity, reduction in interfacial tension, and critical micelle concentration were critical properties influencing the observed effects. The surfactant with higher hydrophobicity induced a more significant and rapid reduction in interfacial tension, resulting in faster W/O separation at low concentrations but compromising water quality at high concentrations. These results indicate that careful selection and monitoring of surfactants during EOR operations can optimize oil recovery efficiency while minimizing operational and environmental risks upstream.