The phenomenon of spontaneous emulsification occurring when crude oil is gently contacted with water was investigated and is presented in this paper. Various crude oils and a model system composed of asphaltenes in toluene have been studied. The latter contributed to simplify the observations and allowed to have a better insight into the underlying mechanism. It was established that the water micro-droplets appear in the oil phase due to an osmotic phenomenon: the molecular water diffuses from the water reservoir and the water micro-droplets swell; at the same time, “osmogenic” species (hydro-soluble asphaltenes in the case of the model system) penetrate in the water micro-droplets increasing the osmotic pressure. The saltier the contacted brine, the more this process is slowed down.
A higher oil recovery has been measured at a microfluidic scale in presence of water micro-droplets whatever the wettability. In a hydrophilic system, the oil recovery is even higher than in the hydrophobic system. Besides, we have noticed that these water micro-droplets do not have a significant impact on the dilatational visco-elasticity; hence, the visco-elasticity and interfacial tension cannot explain the higher recovery. The particular organization of the water micro-droplets suggest an explanation for these observations. We have observed that some water micro-droplets are stuck at the oil/water interface, others nucleate at the oil/solid interface and others sediment on the solid surface. The accumulation of water micro-droplets observed at the oil/water/solid interface suggest a change of contact angle. The sedimentation and nucleation of the water micro-droplets lead to a more hydrophilic surface.