Influence Of Surfactant Molecular Structure On Oil/Water Interface Aggregation Behavior And Oil Displacement Efficiency

Surfactants generally consist of polar hydrophilic head groups and non-polar hydrophobic tail groups,contributing to their amphiphilicity and stable adsorption at the oil/water interface thereby reducing interfacial tension.Nowadays,surfactant flooding has been widely applied in enhanced oil recovery processes.The selection of suitable surfactant for different reservoir environments is crucial to surfactant flooding,since the interfacial adsorption properties and oil flooding performance are significantly influenced by the surfactant structure.However,the researches on the effect of surfactant hydrophobic tail structure on oil displacement effect are still insufficient and mainly focus on experimental studies at macroscopic level due to the complex reservoir environment and the difficulty of obtaining micro-level information experimentally.In this paper,the influence of surfactant structures on oil/water interface aggregation behavior and oil displacement effect were investigated through a combination of molecular dynamics simulations and experiments.The structural properties of the surfactant monolayer and the interfacial adsorption mechanism at the microscopic level were explored to elucidate the effect of the surfactant structure on molecular properties.The clarification of the intrinsic relationship between macroscopic oil flooding efficiency and microscopic mechanism can facilitate the guidance of theory to experiment.The main research work was divided into the following parts:(Ⅰ)Effect of PO and EO groups on the aggregation behavior of surfactant at oil/water interfaceThree kinds of alkyl benzene sulfate acid anionic surfactants were designed to study the aggregation behavior at oil/water interface by molecular dynamics simulation.The difference in the molecular structure of surfactants lies in the number of PO and EO groups.We investigated the aggregation behavior in detail,such as the oil/water interfacial thickness,interfacial emulsification efficiency,and molecular adsorption conformation.The simulation results show that 9C18Ф2P3ES and 9C18Ф4PES,which containing PO and EO groups,increase the interfacial thickness from 2.24 nm to 3.26 nm and 3.18 nm after adsorption at the oil/water interface.They formed a monolayer structure that is more orderly and uniform,resulting in the interface emulsification efficiency increased from 0.150 to 0.184 and 0.189.The benzene groups of 9C18Ф2P3ESs are in a weaker oil atomsphere resulting in a weaker π-π stacking interaction.(Ⅱ)Effect of the hydrophobic tail branching on aggregation behavior at oil/water interface and surfactant aqueous solution propertiesThree SDBSs with different branching degrees were synthesized in this work.The surfactant adsorption models at the oil/water interface was constructed to investigate the effect of the hydrophobic tail branching on aggregation behavior.The experimental results show that the most branched 6C12ΦS exhibits the lowest interfacial tension and the strongest salt tolerance.The molecular dynamics(MD)simulations reveal that the surfactant oil/water interface aggregation behavior was significantly affected by hydrophobic tail branching.The higher the degree of branching,the more inclined the surfactant molecules are to be vertically arranged at the oil/water interface.For the three kinds of surfactant molecules C12ΦS,4C12ΦS and 6C12ΦS,the angle between the molecular axis and the oil-water interface normal decreased from 35.4° to 32.7° and 30.4°,and the molecular arrangement was more orderly,which was conducive to reducing the oil/water interfacial tension and improving the salt tolerance.(Ⅲ)Simulation of surfactant flooding process in silica poreNon-equilibrium molecular dynamics simulations of mixed surfactant/polymer in hightemperature and salt-rich silica nanopores were constructed to investigate the microscopic mechanisms of dynamic oil flooding processes.Compared with C12ΦS,the oil displacement efficiency of 4C12ΦS and 6C12ΦS with a branching structure increased from 0 to 63 % and71 %.It is shown that the branched hydrophobic tail and the benzene substituents are both favorable for enhancing oil recovery.The anionic-nonionic surfactant DBES with long alkoxy groups interact more strongly with the aqueous phase and exhibits an increasing thickness of the surfactant monolayer,which can effectively enhance oil recovery.Analysis of structural features indicates that the polymers are arranged parallel to the oil/water interface and possess a large interfacial occupation area,forcing the surfactant molecules to be vertically distributed at the oil/water interface,which can effectively utilize the bridging effect of surfactant to effectively enhance oil recovery.(Ⅳ)Practical application of theoretical simulation results about hydrophobic tail branching of alkyl benzene sulfonate surfactantsGuided by the simulation results,the branching degree and symmetry of the existing alkyl benzene sulfonate surfactants were improved to optimize the composition and structure of the industrial heavy alkyl benzene sulfonate surfactants used in oil flooding.A new ASP combinational flooding system was prepared,which significantly improved the ability of reducing oil/water interfacial tension and resisting the adsorption by oil sands.The oil recovery efficiency of the new ASP system in the laboratory Beret core flooding experiment has increased by more than 4 percentage points compared with that before optimization.The SN development zone of DQ oilfield was selected for on-site application evaluation of the application effect of the new ternary composite system.The proportion of effective wells in the entire area was 92.5 %,with a stage increase in oil recovery rate of 17.88 percentage points and a recovery rate of 20.46 %.