Both polymeric pour point depressants (PPDs) and asphaltenes can improve the flowability of waxy oils. However, the effect of polymeric PPDs together with asphaltenes on the flowability of waxy oils is not clear. In this paper, the synergistic effect of ethylene–vinyl acetate (EVA) PPD (100 ppm) and resin-stabilized asphaltenes (0.75 wt %) on the flow behavior of model waxy oils (10–20 wt % wax content) was investigated through rheological tests, DSC analysis, microscopic observation, and asphaltenes precipitation tests. The results showed that the asphaltenes disperse well in the xylene/mineral oil solvent as small aggregates (around 550 nm) with the aid of resins. The EVA or asphaltenes alone moderately improve the flow behavior of waxy oils by changing the wax crystals’ morphology from long and needlelike to a large, radial pattern or fine particles, respectively. The wax precipitation temperatures (WPTs) of waxy oils are also slightly decreased by adding EVA or asphaltenes, meaning that the cocrystallization effect between the additives and waxes is dominant. The addition of EVA together with asphaltenes cannot further decrease the WPT, but it can dramatically decrease the pour point, gelation point, G′, G″, and apparent viscosity of waxy oils, indicating that a synergistic effect exists between EVA and asphaltenes. The synergistic effect deteriorates upon increasing the wax content of waxy oils. The EVA molecules can adsorb on the surface of asphaltene aggregates, thus inhibiting the asphaltenes precipitation and forming the EVA/asphaltenes composite particles. The formed composite particles can act as wax-crystallizing templates and then greatly change the wax crystals’ morphology into large, compact, and spherelike wax crystal flocs, thus dramatically improving the waxy oil flow behavior. This work enriches the theory of micro/nano composite PPDs, which is helpful for developing new PPDs with high efficiency.

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In the last two published papers, the influences of wax and asphaltene content on the synergistic performance of ethylene-vinyl acetate (EVA) copolymer together with resin-stabilized asphaltenes on the flow behavior improving of model waxy oil were systematically investigated, and a relevant mechanism has been proposed. Here, the effects of vinyl acetate (VA) content (12–40 wt %) on the synergistic performance between EVA and asphaltenes is continuously studied to develop and complete the synergistic theories. Results show that different VA contents slightly influence the rheological properties of model waxy oils doped with neat EVA but play a significant role in the flow behavior improvement of the oil doped with EVA and asphaltenes. EVA with moderate VA content (28 wt %) possesses the best flow-improving efficiency among the neat EVA PPDs, but associated with asphaltenes, EVA with a higher VA content (33 wt %) does the best. According to the DSC tests, when the VA content is low or moderate (12–33 wt %), the wax precipitation temperature (WPT) of the waxy oil is found to be decreased after adding neat EVA, while the phenomenon for the EVA with too high VA content (40 wt %) is the opposite. The WPT of oil would not be further suppressed by EVA/asphaltenes, but EVA/asphaltenes can facilitate the crystallization of paraffin waxes and accelerate the precipitation process of wax crystals below WPT. Increasing the polarity of EVA (12–33 wt %) can strengthen the polar interaction between EVA and asphaltenes in the oil phase, promoting EVA molecules to adsorb onto the asphaltene aggregates to form the EVA/asphaltenes composite particles. The composite particles favor the formation of large, compact, and spherical wax flocs to release more of the liquid oil phase, reduce the solid–liquid interfacial areas, and weaken the interactions between wax crystals, therefore facilitating the outstanding rheological improvement of waxy oils. As VA content increases to a much higher level (40 wt %), however, the rigidity of EVA molecules is high, which is adverse for the good oil-dispersing ability of EVA and the corresponding interactions with wax molecules. Meanwhile, the high polar EVA disperses the wax crystals into smaller sizes. Both of these two sides enlarge the solid–liquid interfacial area and strengthen the interactions between wax crystals, making them more able to build up a continuous wax crystal’s network structure and leading to the performance deterioration of the EVA together with asphaltenes. This conclusion that the modest increase of PPD’s polarity facilitates the improving efficiency between PPDs and asphaltenes gives another powerful proof to the correctness of the EVA/asphaltenes composite particles mechanism.

At low ambient temperatures in offshore environments, the water-in-oil emulsions of waxy crude oil develop a combined structure of wax crystals and water droplets, resulting in gelling and other complicated flow problems which may severely challenge flow assurance of the multiphase production and transportation system. In this study, the viscoelastic and yield behaviors of waxy crude emulsion gels were investigated, and analyses were then made by investigating the roles of wax particles and water droplets. Small amplitude oscillatory measurements were first carried out to study the effects of dispersed water on the structure and its evolution with time elapsing. Then, creep and recovery tests were conducted within the linear viscoelastic region to further investigate the viscoelastic behaviors of the emulsion gels. Further, the influence of dispersed water on the yield behaviors was studied by stress sweep measurements, and the effects of temperature, i.e., the precipitated wax, on the yield stress and yield strain were investigated by shear-rate-controlled loading measurements. The emulsion was found to become more elastic with the increase of the water cut, exhibiting phenomena such as the loss angle decreasing, storage modulus growing-up, the emulsion gelling at higher temperature, and strain recoverability increasing. The creep and recovery behavior may well be described by a mechanical analogy model with one Maxwell model and two Kelvin–Voigt models associated in series. Compared to the brittle structure of the gelled waxy crude oil as was reported in previous studies, the emulsion gels become more ductile with the increase of the water cut. The yield stresses of both the crude oil and the emulsion gels increase monotonically with the increase of the precipitated wax, and the yield strain of the emulsions with few precipitated wax particles increases with decreasing temperature, which is contrary to the waxy crude oil and the emulsions with low water cut, and interestingly the yield strain of emulsions may show both of these opposite trends, first increasing and then decreasing with the continuous decrease of temperature. All structural behavior differences between the emulsions and the waxy crude oil may be attributed to the roles that the dispersed water droplets may play and the interactions of the wax particles and the water droplets.

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