Mechanism Of Electrical Treatment To Improve The Cold Flowability Of Waxy Oils:A Study On The Role Of The Charged Colloidal Particles

The waxy crude oils behave poor flowability at ambient temperature,posing many flow assurance problems to the long-distance pipeline transportation.Improving the cold flowability of crude oils is the fundamental way to resolve the flow assurance issues.In recent years,enhancing flowability of waxy oils with high-voltage electric field has become an international research hotspot,but there are many doubts about the mechanism of electrical treatment.Based on the impedance spectrum(IS)of waxy oils before and after the electrical treatment,the role of charged colloidal particles(asphaltenes and resins)is recognized.It is proposed that interfacial polarization of wax crystals is the reason of flowability improving.It is found that in addition to wax crystals,the presence of charged colloidal particles in model oils is an essential condition for the electric field to function.After being subjected to an electric field,the viscosity of the waxy model oils without charged colloidal particles,as well as the model oils containing charged colloidal particles but no wax,is almost unchanged.When the electric field applied temperature is below wax appearance temperature(WAT),the waxy oil containing charged colloidal particles exhibit viscosity reduction effect.A novel apparatus is assembled independently to obtain the IS of waxy oils.It is found that the impedance of the waxy oil increased and electrical conductivity decreased after high-voltage electric field treatment,indicating that the charged colloidal particles in the waxy oil that can be migrated directionally are reduced.For waxy oils,the above results show that some charged colloidal particles are bound to the surface of wax crystals.In addition,although the combined treatment with pour point depressants(PPDs)and electric field can lead to better flowability improvement than single treatment,the addition of PPDs actually weakens the effect of electrical treatment.The reason is that PPDs charge wax crystals,which hinders the accumulation of charged colloidal particles on the surface of the wax crystals under electric field.The results of IS showed that the interfacial polarization of wax crystals is the key of flowability improvement,i.e.,charged colloidal particles accumulate on the surface of wax crystals under electric field,whereas ionic polarization,electronic polarization,and orientation polarization are not the main electrical response.The interfacial polarization of wax crystals increases the electric double layer repulsion between wax crystals,and weakens the van der Waals attraction,resulting in the improvement of cold flowability of crude oil.After the electric field being removed,the charged colloidal particles bound to the surface of wax crystals gradually diffuse back to the bulk oil under the action of thermal motion.Therefore,the viscosity of the crude oil gradually returns to the state before electric field treatment.Experiment results show that the larger increasing rate of impedance(?R)before and after electrical treatment,the greater viscosity reduction rate induced by electric field(??)becomes.Quantitatively,there is a good positive correlation between??and?R:??=0.00682?R²+0.56205?R.In addition,the influence of seven kinds of SiO₂ nanoparticles on the electric field effect is studied.Since SiO₂ nanoparticles do not change the interfacial polarization of wax crystals,they have no effect on the viscosity reduction induced by electrical treatment.