Impact of microwave treatment on rheological properties and structural evolution of waxy crude oil

The transportation of waxy crude oil is severely challenged by its high viscosity and tendency to form a gel structure at low temperatures. This study comparatively investigated the effectiveness of microwave treatment versus conventional direct heating in improving the rheological properties of a representative waxy crude oil. The physicochemical alterations were characterized using differential scanning calorimetry (DSC), rotational rheometer, Fourier Transform Infrared Spectroscopy (FTIR), and SARA (Saturates, Aromatics, Resins, Asphaltenes) analysis. Experimental results demonstrated the marked superiority of microwave treatment, which reduced the crude oil's viscosity by approximately 80%, lowered the pour point by 12 °C, and increased the critical wax content for gelation from 3.59% to 6.60%. Furthermore, the yield stress and yield strain of the gelled oil were significantly reduced by 82.45% and 20.05%, respectively. Viscoelastic tests revealed a sustained viscosity reduction effect from microwave treatment, maintaining a 70% reduction rate after 10 days of aging, whereas the viscosity of directly heated samples reverted to their original level within 3 days. FTIR and SARA analyses indicated that the microwave's efficacy stems from its “non-thermal” effect, which facilitates the breakdown of large molecular aggregates, thereby reducing molecular weight and viscosity. In conclusion, microwave treatment fundamentally and persistently enhances the flowability of waxy crude oil, showing great potential for optimizing pipeline transportation.