Dissipative Particle Dynamics Simulation Of Crude Oil Emulsion And Its Demulsification Process

Currently, oil exploitation in China has entered the second and tertiary oil recovery stage. The produced "crude oil" is usually commingled with quantities of water and surfactants, making it necessary to carry out the dehydration and demulsification process before the transportation and refining of crude oil.Chemical demulsification, in which chemical demulsifiers were added into the emulsions, is the most frequently used demulsificationmethod.Various experimental approaches have been developed to investigate the demulsification process and the demulsification mechanism. However, although they can evaluate the demulsification efficiency and interpret the demulsification mechanism at macro level, they usually fail to interpret the demulsification mechanism at micro level and impossible to design effective demulsifiers at the molecular level. In space, the crude oil emulsions are generally in the form of water-in-oil emulsions, with the diameter of the water droplets in the range of 0.1μm-50μm, and the coalescence of the droplets is supposed to occur at the nanoscale. In time, the coalescence rate of the droplets is extremely fast, only high-speed cameras are able to record the overall coalescence process under milliseconds (ms) scale. Thus, it is very difficult for experimental approaches to investigate the microscopic demulsification mechanism.Computer simulation method, which is emerging in recent years, can not only make up for the shortage of experimental researches, but can also provide better interpretation of the experimental phenomenon. Moreover, it can be employed as a tool to predict experimental results, thereby providing novel research ideas for experimental researches. In this paper, two kinds of mesoscopic scale were adopted based on dissipative particle dynamics to investigate the stability mechanism, the demulsification process and the demulsification mechanism of emulsions. This method can not only investigate the stability mechanism of asphaltene, resin, and polyacrylamide emulsions at the molecular level and observe the static structure of the system visually, but can also track and record the dynamic evolution process of the demulsification in real-time just like the high-speed camera, thus acquiring information about the interaction mechanism between asphaltene and demulsifiers in depth.Through comparison of the simulation and experimental results, the stability mechanism of the emulsions was investigated. It was revealed that there were varieties of interfacially active components existing in the crude oil emulsions. Among which, asphaltenes were discovered to self-assemble at the interface in the form of "lying" (with the thickness of the interfacial film was in the range of 8-20 nm). The produced interfacial film was characterized by high viscoelasticity as well as strong rigidity, which explained the excellent emulsification ability of asphaltenes. For resins, they usually absorbed at the interface in the form of "slant" (with the thickness of the interfacial film was in the range of 5-11 nm). In the case of polyacrylamides, they tend to absorb at the interface in the form of "reticular", and the interfacial film was featured by strong closeness, ultrahigh viscosity as well as rigidity. The viscoelasticity, rigidity and emulsion ability of interface films are following as:asphaltenes > polyacrylamides> resins. Furthermore, the synergetic effect between asphaltenes and resins can readily enhance the emulsification ability. The viscoelasticity, rigidity and emulsion ability of mixing interface films are following as:asphaltenes and polyacrylamides> resins and polyacrylamides> asphaltenes and resins.The demulsification process and the mechanism can be described as follows:(1) flocculation stage, when droplets aggregate together and grow rapidly; (2) coalescence stage, in which the number of the flocculating constituents remain unchanged while the size change dramatically. Thereinto, polyether demulsifiers will form micelles at the interface, which might drag the droplets together in the way of "bridging", leading to the coalescence of the droplets. Different polyether demulsifiers were determined to have different flocculation and coalescence constants, between which the coalescence constant is thought to be the key to influence the demulsification efficiency. During the demulsification process, the root mean square of the demulsifiers usually undergo significant changes with time, implying that the demulsifiers can stretch at the interface freely, thereby displacing the initially absorbed asphaltene molecules at the interface. Once the rigid asphaltene film is displaced, a "hole" might be formed in the interface, which favors the coalescence process of droplets.