Synergistic Improvement of Rheological Properties in Changqing Waxy Crude Oil by Ethylene Vinyl Acetate and Surfactant Combinations

Despite the widespread use of surfactants as wax and asphaltene inhibitors, studies investigating the synergistic effect between polymer PPDs and surfactants are limited. This study explored the synergistic effects between different surfactants (Span, Tween, and PAE series) and ethylene vinyl acetate (EVA) PPD on improving the rheological properties of Changqing waxy crude oil through the rheological measurement, differential scanning calorimetry (DSC) test, microscopic observation, and asphaltene stability analysis. The results indicated that all 11 surfactants and EVA could disperse and stabilize the asphaltenes, and the sequence of stabilizing ability is Tween > PAE > Span > EVA. The surfactants cannot improve the crude oil rheology alone in the dosage range 20–200 ppm, but adding 20 ppm of EVA together with 20 ppm of surfactant can synergistically improve the crude oil rheology. The Tween or PAE series has a stronger synergistic effect with EVA PPD, and increasing the alky chain length of surfactants also enhances the synergistic effect. With the increase of PAOE dosage from 20 to 200 ppm, the synergistic effect between PAOE and EVA first increases and then reduces; the best synergistic effect is found at the dosage of 60 ppm. We speculate that the adsorption of both EVA molecules and surfactants brings more long alkyl chains on the asphaltene particles, which facilitates the interactions between the EVA/surfactant/asphaltene composite particles and the paraffin wax and makes the precipitated wax particles more compact, thus improving the crude oil rheology further. A higher dosage of surfactants (≥100 ppm) may greatly depress the EVA adsorption on asphaltene particles due to the competitive adsorption, thus weakening the synergistic effect.