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

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.