It has been proved that the flow improver makes the transportation of waxy crude oils in pipeline much more economic and safe, but so far an universal flow improver for various waxy crude oils has not been found because of inadequately understanding the action mechanism of the flow improvers. Therefore it is necessary for the mechanism to be studied further. A series of synthetic waxy oils (SWOs) with or without flow improver GY1, a long chain alkyl acrylate polymer based chemical, are prepared from 25# transformer oil, 50#, 60# (macrocrystalline) and 80# (microcrystalline) wax, single or mixed, and in some cases 60# road asphalt by mixing the ingredients at 100°C for 1 hour. Characteristic temperatures, viscosity-temperature properties and rheological behaviors are studied by using rheological techniques, and microstructures of wax crystals grown from SWOs at 20 °C are analyzed by using a polarization microscopy. Some abnormal viscosity-temperature properties of SWOs are found, which mainly results from wax crystallization and network structure formed by wax crystals. The mechanisms involved in the structure formation and fluidity improved by chemical for SWOs are discussed here. Studies show that the structure formation is followed by the formation of crystal nuclei, growth and interconnection or bridging of the wax crystal particles, which is closely relevant to wax molecular dimension and content, crystalline particle size, shape, concentration and surface characteristics. GY1 added into the SWOs lowers their cloud points by 0–2.0 °C and enhances the amounts of wax precipitated at 30 °C by 10–35wt%, which might not be involved in the mechanisms of the fluidity improving under this study. The extent of pour point depression by GY1 increases with increasing the wax molecular size and decreasing the wax content in the SWOs. As long as the SWO treated by GY1 has a greater yield stress reduction at the temperature closed to its pour point, its viscosity and pour point reduction will be more obvious. The common shortcut of pour point depression and viscosity reduction is to inhibit or desintegrate the formation of paraffin crystal network. The mechanisms involved in fluidity improvement of waxy crude oils by chemicals include modifying surface properties of waxy crystals and promoting crystal particle growth with higher symmetry.

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