Design And Mechanism Study Of Ultra-Low Interfacial Tension For Oil/Water Interface

Ultra-low oil/water interfacial tension is one of the most important characterization parameter in EOR for surfactant prescription screen and it’s of great importance to deeply investigate its generation regularity. In this thesis, interfacial behavior and capacity of several types of surfactants, such as alkanolamide, petroleum sulfonate and biosurfactant rhamnolipid, all have wide potential in EOR, were studied. A series of ultra-low interfacial tension systems were obtained by adding nonpolar molecules such as alkane and polar molecules such as alcohol into surfactant solution. Molecule Dynamics (MD), Dissipative particle dynamics (DPD) combined with dynamic interfacial tension and interfacial rheology measurements were used to investigate the interaction between molecules arrayed in the oil/water interface layer and the self interfacial assembly regulation of the ultra-low interface tension systems to provide theory foundation for oil displacement process formula design used in EOR.According to the above, this thesis is divided into three parts:Firstly, properties of alkanolamide FFA and FFA with different purification degree were investigated through interfacial tension and interfacial rheology measurement combined with computer simulation. According to the results, the existence of diethanol amine, formic ester, single amine ester and double amine ester can highly enhance interfacial property and oil adaptability. Bulk and interfacial properties of FFA systems solubilizing alkane were also studied. According to the results, FFA molecules aggregated to form vesicle in the solution; a small amount of added alkane made the vesicle transfer to swollen micelle. Interfacial behavior of FFA micelle solubilizing alkane is related to the relative molecule length of the added alkane: When the carbon chain length of the added alkane is shorter than the bulk oil, the added alkane would stay in the interface layer, resulting in less bulk oil inserting into the interface layer thus the influence of oil on the surfactant’s interfacial property is observably lowered and the interfacial property of surfactant is highly increased. On the other hand, when alkane with a longer carbon chain length was added, the added alkane molecules would diffuse into the bulk oil phase and can bring both surfactant and water into the oil phase. The formation of emulsion at the oil side wastes a lot of surfactants which should stay on the interface and stabilize oil/water interface, therefore the interfacial tension increased.Secondly, properties of petroleum sulfonate, petroleum sulfonate/nonionic surfactant and petroleum sulfonate/alkanolamide/alcohol systems were investigated by both experiments and molecular simulation. A series of systems that can rapidly reduce the oil/water interfacial tension to a lower degree were obtained by adding hexanol and octanol to the synergistic systems. The added alcohol molecules were solubilized in micelles and diffused together in the bulk phase. When meets oil/water interface, surfactants and alcohol are released and a new interface layer was formed. Compared with other normal surfactant systems, surfactants here don’t have to insert into the oil phase to form interface layer, therefore, the interfacial tension can be rapidly reduced to a lower degree and the system keeps high interfacial activity for different types of oil. According to the simulation results, the addition of alcohol increased the interfacial activity of surfactants by enhancing surfactant tail extensibility and order degree, thickening the interface layer and lowering the interfacial formation energy.Thirdly, the interfacial property of rhamnolipid with different structure and their synergist effect with different surfactants were investigated using Dissipative Particle Dynamics. The interfacial behavior of RL and the effect of synergistic on the interfacial tension, interfacial density and surfactant interfacial stretching ability were discussed. Interfacial behaviors of different RL structures as well as the interaction mechanisms of the synergistic systems were given in order to direct practical application.On the basis of experiments and simulations, the interfacial property of alkanolamide, petroleum sulfonate and rhamnolipid were investigated. With the addition of alkane and alcohol molecules, the interfacial property and oil adaptability of the system were highly increased. A series of systems that can rapidly reduce the oil/water interfacial tension to a lower degree as well as high oil adaptability were abtained. This will contribute a lot to the formula design used in chemical flooding in EOR in the future.The innovations of the thesis are as follows:1. Combined computer simulations of different scales with experiments to investigate interfacial property of surfactant on molecular scale.2. Purified alkanolamide by different means and studied the interfacial properties. By combing computer simulation with experiments, interfacial effects of weak polar components on enhancing the interfacial property of the system were determined and a new routine in designing systems with high interfacial activity and oil adaptability was put forward.3. Solubilizing alkane to alkanolamide solution, alcohol to petroleum sulfonate solution to obtain a series of systems with high interfacial activity and can rapidly reduce the oil/water interfacial tension to ultra low degree. By comparing the interfacial formation energy of systems, the interfacial position of added molecules and the synergistic mechanism were determined. The interfacial self-assembly regulation of ultra-low interfacial tension systems were put forward, the results of which will contribute a lot to the formula design used in chemical flooding in EOR in the future.4. Investigate the interfacial properties of biosurfactant rhamnolipid and the synergist systems, directing the strains choice and formula design in practical application.