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With the increased water content in heavy oil gathering pipelines and application of cold transportation methods, understanding adhesion behavior in heavy oil/water/pipeline wall systems has greatly gained in importance. Here, a combined experimental and molecular dynamics (MD) simulation was used to investigate the effect of the heavy oil composition and molecular structures on adhesion. The capacity of heavy oil adhesion and the effect of adding asphaltene molecules to the heavy oil were studied experimentally. MD simulation systems with different asphaltene concentrations were established based on the NPT and NVT ensembles. The results showed that the addition of asphaltene increases the adhesion of heavy oil by as much as 16.60–83.35%. The center of mass, mean square displacement, and radial distribution function of heavy oil droplets were also analyzed, which indicated that the viscosity and thickness of the heavy oil adhesion layer were also affected by the asphaltene content. The functional groups in asphaltene molecules promote the adhesion of oil droplets and inhibit the adhesion of the water phase. Further, asphaltene aggregates are formed by face-to-face stacking, which severely impedes heavy oil flow assurance. The results from this study greatly contribute to understanding the mechanisms of heavy oil adhesion during cold transportation. • Experiments of heavy oil with different asphaltene contents were implemented. • Designed MD simulations to study the heavy oil adhesion under cold transportation. • Effect of asphaltene molecules on the adhesion behaviors was investigated. • The interaction of resin and asphaltene molecules were summarized. • Clarified the adhesion mechanism of heavy oil cold transportation. [ABSTRACT FROM AUTHOR]

After the investigations of experiments and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, the adhesion energy model was proposed to solve the discrimination of the minimum oil-gathering temperature of transportation (adhesion temperature, AT). The results of AT were in good agreement with the experimental results (error values around 0.1 °C–1.2 °C). And a 100-day cooling transportation test was conducted safely, indicating that the model was suitable for engineering application. It was found that the oil droplets were adhered to the initial adhesion layer on the pipeline wall by the dominant contribution of van der Waals (VDW) energy. In contrast, the electric double layer (EDL) energy can resist the coalescence between oil droplets and the initial adhesion layer. And the effects of different temperatures, oil droplet particle sizes, and salinity on the DLVO interaction energy were also analyzed. Furthermore, the effect of DLVO energy on the adhesion of heavy oil was obtained by combining the relationship between adhesion mass and AT. [Display omitted] • An adhesion energy model of heavy oil was proposed based on DLVO theory. • A cold transportation test for 100 days was conducted using adhesion temperature. • VDW interaction can promote adhesion, while EDL interaction can resist adhesion. • Temperature, salinity, and droplet size can affect VDW and EDL interactions. [ABSTRACT FROM AUTHOR]

After surface adhesion apparatus (SAA) experiments, the mass of oil sticking to wall was found to increase slightly in high-temperature region (70-60 °C), while this mass was increased immediately in low-temperature region (35-25 °C). The contact angle and interfacial tension (IFT) of oil droplets were measured, and the work of adhesion and cohesion were cauculated, indicating that the process of oil sticking to pipeline wall includes the adhesion region and cohesion region. However, the work of adhesion does not change with temperature, but the work of cohesion is positively correlated with temperature. Consisting with this, the Results of the work of adhesion and cohesion using molecular simulation (MD) are the same with experiments and the van der Waals (VDW) interaction of polar substances is the main drive force of adhesion. The polar molecules on iron surface are aggregated at 0.5–2.0 nm along the X and Y directions and close to the iron surface at 0.5–1.0 nm along the Z direction. The polar substances in heavy oil are located on the near-iron plate side and the heteroatoms (i.e., N and S) formed adhesive joints in the processes of oil sticking to wall, especially N element. [Display omitted] • A new method for predicting the wall sticking occurrence temperature of heavy oil. • Processes of wall sticking of heavy oil include the adhesion and cohesion region. • The N and S atoms are the bonding joints between heavy oil and pipe wall. [ABSTRACT FROM AUTHOR]