Molecular Dynamics Study of Oil-Water Interfacial Equilibrium in Petroleum Engineering

Interfacial phenomena are ubiquitous in a number of natural and technological processes, and concern areas where chemistry, physics, and engineering intersect. Unlike the bulk material, the interface is non-centrosymmetric. The interfacial phenomena such as surface/interfacial tension, wetting, spreading, mutual diffusion and etc. are governed by interactions between molecules at the interface of their phases, acting usually over a few molecular distances. These length scales are being probed with experimental techniques, such as atomic force microscopy and surface forces measurement, or theoretical tools, such as molecular simulations, to obtain new insights into interface phenomena. Molecular simulations are powerful techniques by which we can obtain thermodynamic properties of the system of given composition at desired temperature and pressure, and it also enables us to observe microscopic phenomena by direct visualization. In this dissertation, Molecular Dynamics (MD, one of molecular simulations) simulations have been performed to investigate the oil–water interfacial equilibrium, related to the primary oil production and the enhanced oil recovery (EOR) techniques in the oil industries. The results are followings: (1) The interfacial equilibrium of a light-oil–water interface system was investigated by using MD method. In advance, the accuracies of each hydrocarbon model were checked by comparing to the experimental values on the thermodynamic properties, such as the bulk density and IFTs at an ambient condition. The light-oil model was modeled as a mixture of eight kinds of hydrocarbons, including n-alkanes, cycloalkanes and aromatics. The results showed the inhomogeneity of the molecular distributions, such as strong accumulation of aromatic molecules at the light-oil–water interface, which were caused by the specific interaction between aromatic and water, called as weak hydrogen bonding. This phenomenon affects a large area of the oil–water interface properties. (2) The interfacial equilibrium of a decane–water–vapor three phase system was investigated by using MD method. The inhomogeneity of the molecular distribution, especially at the vicinity of three phase contact line, and the interfacial tensions of each interface were obtained. In addition, the convenient equation to calculate the line tension of three phases system from local pressure distribution was derived and applied to the decane–water–vapor three phase system. With this method, the line tension of the decane–water–vapor three phase system was calculated successfully. And the size dependence of the contact angle inside of the droplet was estimated from nano to macro scales by using the three calculated interfacial tensions and the line tension with the general form of Neumann triangle relationship. (3) The interfacial equilibrium of a multicomponent-oil–water–vapor three phase system was investigated by using MD method. The multicomponent-oil was modeled as a mixture of heptane, decane and toluene. The line tension, the interfacial tensions and the molecular distributions in the multicomponent-oil–water–vapor three phase system were obtained and discussed. The effects by impurity of the droplet, such as lower interfacial tension and higher line tension, molecular coverage on water surface by toluene molecules, were shown. The line tension value has changed from negative to positive value by adding heptane and toluene molecules into decane droplet. This causes opposite trend in size dependence of the contact angle of droplet.