Effect of EVA wax inhibitor on the W/O emulsion stabilized by asphaltenes and paraffin wax

Ethylene-vinyl acetate copolymer (EVA) is a typical wax inhibitor. It is plausible the EVA could influence the stability of waxy crude oil emulsion because it was influencing both the dispersion state of asphaltenes and the wax precipitation properties. In this paper, the number after the EVA represents the VA content, and the more VA content means the stronger polarity of EVA. Based on the physical characteristics of model crude oil, the emulsion stability, and interfacial properties, the influence of EVA polarity (from EVA 10 to EVA 40) on the stability of model waxy crude oil emulsions (the oil/water volume ratio is fixed at 7:3) containing both asphaltenes and paraffin wax was systematically studied at two different temperatures (30 ℃ and 15 ℃). The results demonstrated that the increment of EVA polarity can not only greatly improve the dispersion state of asphaltenes, but also significantly improve the flowability of model crude oil by changing the morphology of wax crystals. The competitive adsorption of EVA to the interfacial film can be facilitated by the increased polarity of EVA, thus decreasing the asphaltene mass ratio in the interfacial film and then causing the decrease of the interfacial tension and the dilational modulus. The test temperature impacted the stability of the W/O emulsion. At the temperature of 30 ℃ which was higher than the wax appearance temperature (WAT), the interfacial modulus was the main factor affecting the emulsion stability and then increasing EVA polarity caused the emulsion stability to decrease. At the temperature of 15 ℃ below the WAT, due to the formation of large amount of wax crystal particles, the low-temperature flowability of the oil sample was greatly aggravated, which can improve the emulsion stability—increasing the EVA polarity greatly improved the oil rheology, thus leading to a decrease in the emulsion stability. In addition, the size of wax crystals formed in the emulsion was much smaller than those formed in the oil phase, but the amount became larger. We speculated that the emulsified water droplets could serve as wax nucleators, promoting the formation of wax crystal layers around the droplets, which will also favor the emulsion stability at 15 ℃.