Low-salinity water flooding of formation water in rock cores is, potentially, a promising technique for enhanced oil recovery (EOR), but details of the underlying mechanisms remain unclear. The salinity effect on the interface between water and oil was investigated here using the molecular dynamics (MD) simulation method. n-Decane was selected as a representative oil component, and SPC/E water and all-atom optimized potentials for liquid simulations (OPLS-AA) force fields were used to describe the water/oil/ionic interactions for saltwater and n-decane molecules. Equilibrium MD simulations were first conducted to study the n-decane/vapor and saltwater/vapor interface systems at six different NaCl concentrations (0, 0.05, 0.10, 0.20, 0.50, and 1.00 M). The water/oil interface was then investigated by calculating bulk density distribution, radial distribution function, interface thickness, and water/oil interfacial tension (IFT). Sufficiently long MD simulations of water/n-decane/vapor were performed, followed by an analysis of the effect of salinity on the water/oil/vapor interface. The IFT values for the water/vacuum interface, n-decane/vacuum interface, and water/n-decane interface were obtained from the pressure tensor distribution after system equilibration, with values of 71.4, 20.5, and 65.3 mN/m, respectively, which agree well with experimental and numerical results reported in the literature. An optimal salinity of ∼0.20 M was identified corresponding to a maximum interfacial thickness between water and the oil phase, which results in a minimum water/oil IFT value and a maximum value for the oil/water contact angle, a condition beneficial for EOR.

中文翻译：

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盐度对正癸烷/水/蒸气界面平衡的影响的分子动力学模拟

岩芯中地层水的低盐度注水可能是提高石油采收率（EOR）的有前途的技术，但其潜在机理的细节仍不清楚。本文使用分子动力学（MD）模拟方法研究了盐度对水和油之间界面的影响。选择正癸烷作为代表性的油组分，并使用SPC / E水和用于液体模拟的全原子优化势能（OPLS-AA）力场来描述盐水和正癸烷分子的水/油/离子相互作用。首先进行平衡MD模拟以研究n-在六种不同的NaCl浓度（0、0.05、0.10、0.20、0.50和1.00 M）下的癸烷/蒸气和盐水/蒸气界面系统。然后通过计算堆积密度分布，径向分布函数，界面厚度和水/油界面张力（IFT）来研究水/油界面。进行了足够长的水/正癸烷/蒸气的MD模拟，然后分析了盐度对水/油/蒸气界面的影响。水/真空界面，正癸烷/真空界面和水/ n的IFT值癸烷界面由系统平衡后的压力张量分布获得，其值分别为71.4、20.5和65.3 mN / m，与文献报道的实验和数值结果吻合良好。确定了约0.20 M的最佳盐度，对应于水和油相之间的最大界面厚度，这导致最小的水/油IFT值和最大的油/水接触角值，这是有利于EOR的条件。