This paper aims to develop a mathematical model to predict the wax deposition rate of waxy crude emulsions, combining heat and mass transfer mechanisms. According to the flow loop experimental results, the wax deposition rate increases with the decreasing average temperature of oil/wall in a manner of linear regularity, and shows a downtrend with the increase of water cut due to diffusion resistance. An applicable model is developed regarding emulsion properties, radial temperature gradient, shear stress, and wax diffusion coefficient. In model validation, the prediction results are in good agreement with experimental data with the relative errors within 28.87%. GRAPHICAL ABSTRACT

Wax deposition problem occurred during crude oil production and transportation process is a hot one and has brought huge economy losses around the world. This paper focused on the mass and heat transfer process during wax deposition with cold-finger experiments by method of differential scanning calorimetry (DSC) and gas chromatography (GC). Two sets of experiments with 15 °C and 20 °C temperature difference were conducted using the model oil with wax content of 10 wt. % for 120 h. During each experiment, the wax-solvent mixtures at two radial positions of two horizontal planes and deposits are sampled for DSC and GC analysis. The dissolved wax content gradient of the wax-solvent mixture at the same height is from the tank wall to the cold-finger, which is just opposite of its temperature gradient. As the diffusion proceeds, the total proportion of wax with carbon numbers of nC21~nC39 gradually decreases near the tank wall, but increases near the cold-finger. Furthermore, compared with the initial wax-solvent sample, the absolute mass ratios of wax molecules heavier than nC31 become greater in the deposits under 15 °C and 20 °C temperature difference, while the mass ratios of those lighter than nC32 decrease. This variation trend indicates that under this cold-finger temperature, heavy wax molecules continually diffuse into the deposition layer and meanwhile light wax molecules are displaced out of the deposition layer. The nC32 is the critical carbon number (here we defined the carbon number from which can deposite on the cold-finger as the critical carbon number) heavier than which the wax molecules can deposit in this condition. This paper intensely analyzed the wax molecule distribution conditions from the view of heat transfer and mass transfer theories. It will undoubtedly promote the investigation of wax deposition mechanisms of waxy crude oil.

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.