The wax deposition experiments under cold and hot flow conditions were conducted in a flow loop apparatus using waxy crude oil. The wax precipitation characteristics and wax molecule carbon number distributions of wax deposit in the radial were investigated emphatically. The wax content and wax appearance temperature (WAT) of the surface deposits were lower than those at bottom. The mass ratio of wax molecules heavier than the critical carbon number (CCN) from high to low was the bottom deposit, surface deposit and crude oil, while in the range lighter than the CCN, the order was opposite. The CCN was C₃₀ and the peak carbon numbers were C₃₅ and C₃₇ for the cold and hot flow experiments, respectively. This work deepens the understanding of the radial distribution characteristics of wax deposit and its intrinsic mechanism, affording a useful reference for the formulation of pigging plan.

Radial properties of wax deposits in crude oil pipeline are of great significance for the selection of wax removal process. Herein, the wax deposits were obtained in a flow loop device and the important characteristics of wax crystal morphology and yield stress were quantitatively analyzed. The polarizing microscopic observations revealed that the amount and size of wax crystals increased in the radial from deposit surface to bottom, while the aspect ratio and boundary fractal dimension varied little, ranging from 1.68–1.80 and 1.61–1.75, respectively. In addition, the yield stress of wax deposit was found to increase with the rise of wax content and deposition duration, ranging from 5310–16,400 Pa in the cold flow experiments and 7550–24,500 Pa in the hot flow experiments. Furthermore, the bottom deposit was stronger in structure and presented higher yield stress than the surface deposit. This work provides a whole understanding on the wax deposit properties in different radial positions and is of great help to the analysis of pig force and pig selection.

Flowing experiments with different water contents in water-in-waxy-crude-oil emulsions were performed in a flow loop apparatus. The real-time variations of rheological properties and microscopic distributions of the emulsions over time during pipe flow were quantitatively analyzed. It was found that the emulsification effect of the emulsion is enhanced due to shear effect, resulting in an increase of gel point and viscosity over time. The average increments in the gel point are approximately 1.5 °C, 1.7 °C and 1.9 °C for the emulsions with water contents of 10%, 20% and 30%, respectively. During the experimental period from 0.5 h to 18 h, the average viscosities measured at the emulsion temperature increase by 31%. Characterization parameters of number-averaged diameter (d_1,0), specific surface area (SA) and polydispersity (PDI) were applied to reflect the microstructure of emulsions from the perspectives of droplet size, dispersity and nonuniformity. The water droplets become smaller and uniformly dispersed as the pipe flow goes on. The Gibbs free energy increases as the emulsion flows through the pipe, which is mainly caused by the shear of the pipe wall. The van der Waals force increases from −0.0678 × 10⁻²¹ J to −1.7657 × 10⁻²¹ J as the emulsion’s water content changes from 10% to 30%. The water droplets can promote crystallization of wax crystals by offering the necessary nucleus surface. Furthermore, the coexistence of water droplets and wax crystals creates a synergistic effect, which can augment the total energy and structural strength of the emulsion system. Graphical Abstract

含蜡原油在油气田开发、运输及储存过程中常因温度变化而导致蜡沉积问题，造成严重的经济损失与安全威胁. 常规的防蜡工艺，如加药、升温、工艺改造、通球清管等，存在成本高、效果不理想、污染大等问题. 为此，通过改变管道材料、对管道表面涂敷特殊涂层及对表面进行改性来进行防蜡的新技术为解决传统蜡沉积问题提供了新选择. 本文系统地阐述了具有低表面能、耐蚀抗磨等特点的非金属及其复合材料在管道防蜡方面的实验机理研究及应用现状. 其次，分析了涂层技术在防蜡深层机理、效果以及应用场景等方面的技术现状. 此外，探讨了材料表面改性技术在防蜡领域的应用，详细阐述了化学防蜡和电化学防蜡两大分支的作用机理及应用现状. 并对上述防蜡技术存在的问题、未来发展趋势和前景进行了深入剖析和展望.