Dehydration Characteristics Of Crude Oil Emulsion Under Different Electric Field Types

Crude oil dehydration is the important part of crude oil production process, which can not be ignored. With oil field entering the oil exploration stage with high water content, the tertiary oil exploration technology is universal used in the oil field. This technology makes the conductivity and viscosity of crude oil increase, and the phenomenas of large current and unstable electric field appear in the dehydration process, which increase the difficulty of dehydration. Therefore, it is essential to study the mechanism and key technology of electric dehydration for crude oil. In addition, with the decrease of onshore oil reserves, the investment of offshore oil exploration increases gradually. Due to the environmental conditions at sea, marine oil exploration platforms need the character of compact, high efficiency and safety. In order to reduce the costs and improve the efficiency of offshore oil exploration, the efficient and safe electric dehydration technology has become one of the key technologies for offshore oil exploration process to be resolved. In view of the problems appear in the exploration process of onshore and offshore oil, the researchs on dehydration characteristics of crude oil emulsion at different electric field types have carried out to improve the efficiency of existing crude oil electric dehydration and reduce the volume of electric dehydrater.In order to study the dynamic characteristic of water droplet under electric field, the software Comsol Multiphysics is used to simulate based on the method coupled electric field and flow field. The dynamic characteristics of water droplet are simulated and the main influence factors of water droplet deformation, rupture and coalescence are analyzed by establishing the simulation model of water droplet dynamic characteristic in the plate electrode under the power frequency AC electric field. The simulation results show that water droplet deforms along the direction of electric field under the action of electric force and surrounding fluid. The distributions of charge density and electric power on the surface of water droplet gradually increase from the left and right sides to upper and lower ends, and the values reach the maximum at the upper and lower endpoints. The deformation, rupture and coalescence of water droplet are affected by electric field strength, oil phase viscosity, interfacial tension, and water droplet diameter. The higher electric field strength, smaller oil phase viscosity and interfacial tension, larger water droplet diameter could lead to larger deformation degree and faster coalescence rate of water droplet. However, if the electric field strength is too high, water droplet would easily be ruptured. Among these influence factors, the effect of electric field strength and water droplet diameter on deformation and coalescence rate of water droplet are biggest, oil phase viscosity also affects the deformation shape and displacement of water droplet, while interfacial tension affects the rupture shape of water droplet.In order to study the emulsion dehydration effect at different electric field types, electric dehydration experimental system is designed and set up, and emulsion dehydration experiments under AC, DC, AC and DC superposition, and high-frequency pulsed electric field are carried out. Experimental results show that AC electric field is suitable for emulsion with high water content, and it can ensure the stabilization of electric field and improve the dehydration speed. DC electric field is suitable for emulsion with low water content, and it can deep dehydrate the emulsion. The ratio of AC and DC can be adjust to give full play to their advantages when using AC and DC superposition electric field. There are optimum dehydration electric field strength, frequency, and duty ratio in high-frequency pulsed electric field dehydration, the appropriate dehydration electrical parameters can improve the efficiency of dehydration. The coalescence characteristics of water droplet observed by microscopic camera can preferably explain the results of electric dehydration experiments under different electric field types.Through the dehydration experimental results and theoretical analysis, we can get the following conclusions. In the dehydration process, dehydration rate rises with the enhancing external electric field to some extent. Once the external electric field exceeds breakdown electric field strength of water droplet, the dehydration rate will decline. In the high-frequency pulsed electric field, when electric field strength keeps in constant, the vibration amplitude of water droplet will change with frequency. When the external voltage frequency reaches the resonance frequency of water droplet, the vibration amplitude of water droplet is the maximum. The duty ratio affects the chaining of water droplets. When the water content of emulsion is high, the use of lower duty ratio can prevents the chaining of water droplets, which makes the dehydration electric field collapsed. While the water content of emulsion is low, the use of higher duty ratio can prevents water droplets chaining, the final dehydration rate of emulsion can be increased by increasing the duty ratio.The relation between deformation of water droplet and electric field strength can be obtained by establishing the polarization and mechanics model of water droplet. Similarly, the relation between vibration amplitude of water droplet and voltage frequency can be obtained by establishing the vibration dynamical model of water droplet. Meanwhile, the minimum time to dissipate water chains can be obtained by establishing the mathematical model of water droplet recovery time. So that the selection methods of optimum dehydration electric field strength, frequency, and duty ratio can be determine. The experimental verifications show that the theoretical and experimental values have great consistency, and the selection method can be used to select the electrical parameters of dehydration.