Nanoparticle Stabilized Oil in Water Emulsions: A Critical Review
Arab, D., Kantzas, A., Bryant, S.
Journal of Petroleum Science and Engineering, 163, April 2018, Pages 217-242.
Thermal recovery techniques and solvent-based EOR methods are not applicable in many of thin post cold heavy oil production with sands (CHOPS) heavy oil reservoirs in Western Canada. Therefore, alkaline-surfactant flood has been suggested as an alternative to develop these reservoirs. The main mechanism behind these processes has been attributed to the effect of synthetic surfactant and natural soap, created due to the reaction of alkaline with acidic crude, to form an oil in water emulsion. The created emulsions can plug the very high permeable wormholes in post CHOPS heavy oil reservoirs to improve sweep efficiency. In these processes the efficiency of the process is improved with increasing the stability of the emulsions. Therefore, this review was aimed to investigate the potential application of nanoparticles, with the main focus on silica nanoparticles as the most cost-effective ones, along with synthetic or natural surfactants created by alkaline-surfactant flooding to boost the stability of oil in water emulsions.
It was found that very hydrophilic silica nanoparticles (NP) tend not to adsorb at the oil-water interface and so they need to be surface activated to be able to work as emulsifiers. Different techniques including in-situ (i.e. surfactant-assisted) versus ex-situ (i.e. either surface-coating the nanoparticles or physically-enforced attachment of NP to the interface) techniques have been used to reduce nanoparticles hydrophilicity among which in-situ methods seem to be more practical in oil field applications. All types of surfactants and their effects on the hydrophilicity of silica NP were discussed in detail. All the literature discusses NPs effects to emulsify simple oils like decane, dodecane, 1-octadecene, n-hexane, kerosene, toluene, mineral oil, vegetable oil, paraffin, etc. in water. There is very little research done with crude oil, which is not comparable to the corresponding ideal systems work.
It is concluded that although there are some clues in the literature addressing the emulsion stability due to mixing of surfactant with NPs of similar charge, the most stable oil in water emulsions can be obtained when the system contains oppositely-charged surfactants and NPs i.e., anionic NPs along with cationic surfactants or cationic NPs along with anionic surfactants. Therefore, it seems that the most influential factor is having two emulsifiers with different charges and the system chemistry is of less importance. In these cases, at specific surfactant concentrations the electrostatic interactions between NPs and surfactant molecules result in the least stable and most flocculated NPs which is reported as the most suitable condition (corresponding to the most favorable state for NPs hydrophobicity) to stabilize oil in water emulsions. Furthermore, adding a large amount of energy into the system by ultrasonication decreases the size of oil droplets which in turn, increases their surface area on which NPs have a higher chance to adsorb.