Effect of multi-frequency ultrasonic waves on the rheological and physical properties of Iraqi heavy crude oil

Heavy crude oils have unfavorable rheological and thermophysical properties such as high viscosity and low API gravity that hinder transportation and refining operations. In this study, the effect of multi-frequency ultrasonic irradiation (40, 50 and 60 kHz) on the rheological and thermophysical properties of Iraqi heavy crude oil of Qayyarah field was systematically evaluated under controlled conditions. Ultrasonic treatment was performed in a laboratory scale ultrasonic bath (rated ultrasonic power: 480 W) at a constant temperature of 55°C and irradiation times up to 120 min. The initial viscosity of the untreated crude oil was 88 cP. After ultrasonic irradiation at 60 kHz for 120 min, the viscosity was reduced to 33 cP (about 63% reduction). In addition, the crude oil density decreased from 0.9527 to 0.9320 g/cm3, corresponding to an increase in API gravity from 17.01 to 20.32. The pour point was also reduced from 3°C to -3.4°C, which indicates improved low-temperature flowability. To provide compositional information, SARA (Saturates, Aromatics, Resins, and Asphaltenes) fractionation was carried out according to ASTM D6560 / IP-143 for both untreated and treated samples. The results showed a moderate reduction in asphaltenes from 14.7 ± 0.4 to 13.2 ± 0.3 wt% with a decrease in resins and a slight increase in saturates and aromatics after ultrasonic treatment. These variations are attributed mainly to cavitation-induced disruption of resin-asphaltene aggregates and better dispersion of heavy fractions rather than chemical transformation. Overall, the results show that multi-frequency ultrasonic irradiation, especially at 60 kHz, can significantly improve the flowability and handling properties of Iraqi heavy crude oil under mild operating conditions, which supports the potential of this method as an energy-efficient physical upgrading method for improved transportability and pre-refining performance.