Electric-field-induced instantaneous and reversible viscosity reduction of waxy crude oil above the wax appearance temperature

Electrorheological (ER) responses in waxy crude oil remain poorly understood. As the first step in a systematic investigation of the ER responses of waxy crude oil from colloid to suspension, this work focuses on the viscosity response of colloidal waxy crude oil above the wax appearance temperature (WAT) to an electric field. An instantaneous reduction in viscosity was observed upon electric field application, followed by an immediate recovery after removal of the electric field, while the viscosity remained unchanged under a continuous action of the electric field, in clear contrast to the time-dependent and persistent viscosity reduction reported for waxy oils below WAT. To clarify the origin of this behavior, comparative experiments were conducted on one waxy model oil without polar components and one wax-free heavy oil with polar components, such as resins and asphaltenes. The results indicate that the instantaneous viscosity reduction observed in the waxy model oil above WAT is consistent with an electric-field-induced bulk-phase hydrodynamic response, whereas the gradual viscosity reduction in the heavy oil is consistent with field-induced redistribution of polar colloidal particles. Taken together, these results suggest that the instantaneous and reversible viscosity reduction of waxy crude oil above WAT is associated with coupled bulk-phase and colloidal responses under the electric field. In addition, a possible contribution from field-modulated transient molecular organization cannot be excluded. This study provides a new insight into the origin of electric-field-induced viscosity reduction in colloidal waxy crude oil.