Fundamental Investigation of Auto-Emulsification of Water in Crude Oil: An Interfacial Phenomenon and its Pertinence for Low Salinity EOR

Abstract The phenomenon of auto-emulsification occurring when crude oil is gently contacted with water was investigated using various techniques. This spontaneous emulsification which creates a micro-droplet layer at the oil/brine interface is believed to be linked to the improved oil recovery during low salinity Enhanced Oil Recovery (EOR). Crude oils and a model system (asphaltenes solubilized in toluene) have been studied. Observations were facilitated when using the model system, this allowed to have a better insight into the underlying mechanism of micro-droplet formation. It was established that the water micro-droplets appear in the oil phase due to an osmotic phenomenon: molecular water diffuses from the bulk water which provokes the water micro-droplets swelling. The kinetics of the micro-droplet formation is directly linked to the brine salinity in contact with the crude oil: salt addition slows down the emulsification process. This was further confirmed by the evaluation of the water chemical activity in the oil phase from calorimetry measurements. Micromodel experiments showed a higher oil recovery when water micro-droplets are present in the system, irrespective of the initial wettability imposed to the micromodel material. Dilatational rheology measurement did not show significant visco-elasticity arising from the water micro-droplet presence; hence, the visco-elasticity difference cannot completely explain the higher recovery. Manipulation of crude oil droplet during dilatational rheology experiments highlighted the impact of micro-droplets on the shape of the macroscopic oil droplet. The nucleation of micro-droplets at oil/brine or solid/oil interface suggests an explanation for the EOR effect. We have observed that micro-droplets organize at the oil/water interface, while others nucleate at the oil/solid interface or sediment on the solid surface. The interaction of asphaltenes with water molecules dissolved in the oil phase may promote wettability alteration. The micro-droplet formation indicates the magnitude of this interaction for a given asphaltenes/brine system.