Durability of the magnetorheological effect of waxy crude oil during dynamic cooling

Magnetic treatment is used to improve the cold flowability of waxy crude oils by leveraging the magnetorheological effect. It is typically most effective when treatment is applied at a temperature close to the wax appearance temperature (WAT) of crude oil. Previous studies have primarily focused on the performance of magnetic treatment under static cooling, with limited attention paid to the behaviors under dynamic cooling scenarios—specifically, cooling accompanied by shear—a condition frequently encountered in industrial applications. This study investigates four typical waxy crude oils to elucidate how shear imposed during cooling alters the viscosity reduction caused by treatment of stationary magnetic fields and the underlying mechanisms. The results show that sustained low shear (5–100 s−1) during cooling either reduces or even eliminates the viscosity reduction induced by magnetic treatment. For high shear (1200 s−1), influence depends on the temperature at which the shear is applied: if sheared above the WAT, viscosity reduction keeps unchanged, but high shear applied near the pour point temperature fully eliminates the viscosity reduction effect. Combining the results of microscopic observation, impedance analysis, and x-ray diffraction characterization, we demonstrate that shear during cooling interrupts wax particle growth, thereby improving cold flowability. However, shear during cooling causes reaggregation of the disaggregated resins and asphaltenes induced by the magnetic field, thus impeding their adsorption on the surface of wax particles, thereby weakening or even eliminating the magnetorheological effect. The predomination of this reaggregation effect results in the reduced or even eliminated viscosity reduction under dynamic cooling.