Aggregation Behavior of Asphalt on the Natural Gas Hydrate Surface with Different Surfactant Coverages

Abstract

The aggregation behaviors of asphalt on the hydrate surface with different surfactant coverages were studied by molecular simulations. The molecular polarity index of asphalt (10.47 kcal/mol) is higher than that of Span 80 (8.19 kcal/mol). Asphalt is easier to occupy the binding sites between Span 80 and hydrate. Asphalt mainly depends on the π–π interaction to form aggregates. The dispersion interaction up to −114.17 kcal/mol is the main factor to maintain the asphalt aggregate stability. With the increase of surfactant coverage, the polar groups of asphalt are easier to associate with each other, promoting the formation of larger asphalt self-aggregates. The self-aggregation increases the complexity of asphalt stacking, and the proportion of edge-to-face stacking types increases from 40 to 60%. When the Span 80 molecule number reaches 12, the diffusion coefficient of asphalt is only (2.31 ± 0.5) × 10–8 cm2/s. Asphalt with low diffusivity can provide new sites for hydrate nucleation. The results are helpful to understand the aggregation mechanism of asphalt on the hydrate surface. Increasing the concentration of surfactant only is not a good solution to solve blocking-pipeline problems. We need to use surfactant reasonably and give full play to the anti-agglomerant effect of asphalt itself.