Development of Asphaltenes-Triggered Two-Layer Waxy Oil Gel Deposit under Laminar Flow: An Experimental Study

Abstract

In this work, we investigated the development of asphaltenes-triggered two-layer wax deposit with deposition time using an in-house wax deposition device, DSC, HTGC, SARA, and microscopic observation. For the pure waxy oil (Waxy Oil A), the formed wax deposit is a homogeneous and loose waxy oil gel. The wax deposit mass is relatively high and increases with increasing deposition time, while the wax appearance temperature (WAT), wax content, and ΔW of the wax deposit are relatively low and increase slowly with increasing deposition time. The critical carbon number (CCN) of the wax deposit is C24. The precipitated wax crystals (needlelike) in the wax deposit are liable to form a continuous network structure, which is adverse for the diffusion of wax molecules. The presence of asphaltenes in the pure waxy oil (Waxy Oils B–D) causes the formation of a two-layer wax deposit, which can be clearly characterized by the significant difference of structural strength. Different from the outer-layer wax deposit, the inner-layer wax deposit has smaller mass but higher WAT, wax content, CCN (C26), and asphaltene content. The WAT and wax content of the inner-layer/outer-layer wax deposits increase with increasing deposition time, while the asphaltene content of the inner-layer wax deposit stays almost the same at deposition time ≥ 1 h. The presence of asphaltenes greatly modifies the precipitated wax crystals’ morphology (spherical-like flocs) and then favors the diffusion of asphaltenes and wax molecules. Both molecular diffusion of waxes and Brownian diffusion of asphaltenes result in the formation of the two-layer wax deposit, which is formed by a four-step process. The increase of the original asphaltene content decreases the mass but increases the WAT and wax and asphaltene contents of the inner-layer wax deposit.

Molecular Dynamics Simulation of Waxy Crude Oil Multiphase System Depositing and Sticking on Pipeline Inner Walls and the Micro Influence Mechanism of Surface Physical–Chemical Characteristics

“…And the relationship between the thickness of deposition layer and time was explored. Chuanxian et al , studied the influence mechanism of asphaltene on the gelation and deposition behavior using the Couette deposition experimental device. It was found that the associated particles of asphaltene could accumulate in the inner wax deposition layer through Brownian motion and participate in the process of wax crystallization and precipitation, which would improve the waxy crystals’ structure and result in the higher structural strength of the inner deposition layer than that of the outer layer.…”

A review of heat‐transfer mechanism for solid deposition from “waxy” or paraffinic mixtures

“…[18,72] 4.4 | A model for deposit aging: shear deformation of a cubical cage A number of studies have reported that, with time and under shear stress, the incipient gel-like structure undergoes compositional changes through a process called deposit aging. [33,50,54,[75][76][77][78]93,[129][130][131][132][133] During deposit aging, the concentration of lighter paraffins in the deposit decreases with time and that of the heavier paraffins increases. Experimental results have shown that a harder deposit, with a lower fraction of the entrapped oil, is formed under higher shear rates.…”

Multifunctional anti-wax coatings for paraffin control in oil pipelines

“…However, the inner deposit was very compact and hard and required sharp instruments to scrape it away. Yang et al also investigated the effect of asphaltenes on the stratification in wax deposits (Yang et al 2016). The results showed that there was interaction between asphaltenes and wax molecules, and the asphaltenes were enriched in the wax deposit layer by Brownian diffusion.…”