Dynamics of viscosity reduction of a high-waxy crude oil under electric field: Effect of temperature and shear rate

This study explores the use of electric fields to mitigate challenges in transporting crude oil, specifically addressing the issue of increased viscosity due to wax network formation. Using a parallel plate shear rheometer, a significant reduction in viscosity is observed across various shear rates (5-50 s(-1)) when applying a constant electric field (2-3 kV/mm) to waxy crude oil. The reduction in viscosity follows an exponential decay over time, attributed to the breakdown of the wax network. The relationship between the time constant of this exponential reduction with the shear rate and the volume fraction of crystallized wax is also explored. The results reveal a power-law dependence on the shear rate, with an exponent ranging between 0.3 and 0.4, and a linear dependence on the volume fraction (achieved by varying the measurement temperature). The breakdown of the wax network is driven by compressive Maxwell stresses, confirmed by the variation in normal force during rheological measurements. Once the wax network is fully broken, the crude oil exhibits Newtonian behavior, and the final viscosity, after the application of the electric field, depends only on the volume fraction of crystallized wax. This final viscosity is fitted using the Krieger-Dougherty equation using an effective volume fraction much greater than the actual volume of wax. This is due to the significant amount of oil occluded within the broken aggregates. The fraction of occluded oil is found to increase with wax content until it saturates.