Wax deposition prediction is of great significance for the waxy crude oil pipeline safety. Herein, the wax deposition characteristics of a new crude oil pipeline was experimentally studied in a laboratory flow loop device. It was found that the higher oil temperature and pipe/oil temperature interval could accelerate the wax deposition rate, while the increases of pipe wall temperature, flow velocity and deposition duration reduced the wax deposition rate. The wax molecule concentration gradient and the scouring force played critical roles on the wax deposition process. Furthermore, a wax deposition model was developed considering the mechanism of wax molecule diffusion and shear stripping. This work provides an insight into the wax deposition characteristics and will benefit the pigging maintenance of the new pipeline in the future.
Wax deposition has always been the hotspot in petroleum, which is widely existed in the waxy crude oil pipeline and is affected by many factors (Chi et al., 2017, Fan et al., 2021a). More than 80% of the crude oils in China are waxy crudes with wax content of 2.5%~10%. In the past few decades, some wax deposition mechanisms, such as molecular diffusion, shear dispersion, Brownian motion, gravity sedimentation, aging and shear stripping have been proposed (Singh et al., 2000, Singh et al., 2001, Kannan et al., 2018, Yang et al., 2020). Among them, molecular diffusion and aging are the main internal mechanisms causing wax deposition. Shear stripping, as an external effect, has a certain impact on the deposit, while other mechanisms have little impact on wax deposition and can be ignored (Xiao et al., 2012). The molecular diffusion mechanism is mainly due to the temperature difference between the oil and the pipe wall, resulting in different solubility of wax molecules and the concentration difference of wax molecules, which makes the wax molecules diffuse from the oil flow to the pipe wall. Huang (2011) proposed that molecular diffusion is the main cause of wax deposition. The aging mechanism mainly affects the diffusion process of wax molecules in the porous network structure of the deposits, which is reflected in the distribution of wax molecule carbon number and wax content in the radial direction of the deposits (Fan et al., 2021b, Fan et al., 2022a). The shear stripping mechanism is mainly reflected in the scouring and compaction of deposit by oil flow, which has a significant influence on the wax content and structural strength of deposit (Fan et al., 2021b, Fan et al., 2022b).
