Electrical treatment of waxy model oil to improve its cold mobility

Wang, Hao, M.S. in Engineering
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Wax precipitation at low temperatures can pose significant flow assurance challenges during offshore transportation of waxy crude oils. The sharp increase of viscosity due to wax precipitation increases the pressure drop and thus raises the capital and operating cost of transportation. Conventional methods of lowering the viscosity of waxy oils include adding chemicals or externally heating the pipeline, both of which are energy-costly and carbon-intensive. Therefore, there is a need to develop economic and eco-friendly solutions to improve the cold mobility of waxy oils. Previous studies have demonstrated that the electrical treatment of waxy crude oil can significantly lower its viscosity at a relatively low energy cost. Previous research mainly focused on demonstrating the electrorheological behaviors of waxy crude oils, and it has been believed that charged particles in the crude oils, such as resins and asphaltenes, play a critical role in reducing the viscosity of waxy crude oils under electric fields. It remains unknown the performance of electrical treatment on waxy oils in the absence of resins and asphaltenes. To fill in this gap, in this research, the electrorheological responses of waxy model oil systems consisting of mineral oil, decane, and paraffin wax were investigated by a DHR-20 rheometer equipped with an Electro-Rheology accessory. The sample’s viscosity was monitored in three consecutive 10-minute stages: before, during, and after the application of DC electrical fields ranging from 0 to 3.5 kV/mm. Significant viscosity reductions were observed in nearly all model systems, and the maximum viscosity reduction of 82.9% was obtained at 22 °C by applying a 3.5 kV/mm electric field. The magnitude of viscosity reduction depends on several factors, including electric field strength, temperature, and the amount of precipitated wax. Furthermore, the addition of 1wt% n-decane was found to significantly enhance the effectiveness of the electric treatment, increasing the viscosity reduction by nearly 40%. These findings challenge the widely accepted theories in the literature that resins and asphaltene play a critical role in reducing the viscosity of waxy crude oil by electrical treatments and could potentially aid future research in better understanding the underlying mechanisms behind the negative electric effects on the viscosity of waxy crude oils.