| Hydrate formation poses a great threat to the safe and economic operation of subsea multiphase transportation pipelines.The formation behavior of hydrates in various crude oil systems is obviously different due to the complexity of crude oil components.As the most polar non-hydrocarbon heavy component in crude oil,asphaltene can influence the process of hydrate formation,and complicate the prediction of hydrate formation behavior and the development of flow assurance strategies.However,research on the formation of hydrates in asphaltene-containing emulsions is still in its infancy,and the effect of asphaltenes on hydrate formation and decomposition processes is still unclear.Therefore,the current study systematically investigated the effect of asphaltenes on hydrate nucleation,growth and decomposition processes in asphaltene-containing emulsions,and further explores the synergistic effects of asphaltenes and waxes on hydrate behaviors.The inhibition effect of asphaltenes on hydrate nucleation was clarified by hydrate formation experiments using a high-pressure stirred autoclave.It was found that the inhibition effect increased firstly and then decreased with increasing asphaltene content,and decreased gradually with increasing asphaltene polarity.The 0.15 wt.% content of asphaltenes increased the induction time to 2 to 3 times that of the asphaltene-free scenarios.The critical concentration of asphaltenes for hydrate nucleation was discovered,and the "adsorption-inhibition" mechanism of asphaltenes on hydrate nucleation was revealed by combining the structural composition of asphaltenes,dispersed state,and oil-water interface behavior.An induction time model in asphaltene-containing emulsions was established.The mean relative deviation between the predicted value and the experimental value is 12.12%,providing a forecasting method for quantifying the induction time in the emulsion with asphaltenes.The hydrate growth process in asphaltene-containing emulsions was systematically studied.It was found that the presence of asphaltenes reduced the formation amount of hydrates,and this effect was more significant for the asphaltenes with higher polarity.Molecular dynamics simulations were employed to reveal the inhibition effect of asphaltene aggregates on the diffusion of guest molecules.The influence of water content and other factors on hydrate growth rate was investigated,and the promotion effect of a high stirring rate on hydrate growth was confirmed by a variable rpm experiment.Combining experimental phenomena and molecular simulation results,a kinetic model was proposed to describe hydrate growth rate of water-oil emulsions.The effect of asphaltenes on the decomposition process of hydrates was investigated by experiments under stirring and static conditions,respectively.The promotion of asphaltenes on hydrate decomposition was found in water-oil system and the mechanism was proposed.An evaluation system of hydrate decomposition rate was proposed for the hydrate slurry formed in the emulsion with asphaltenes.The effect mechanism of liquid composition and environmental conditions on the decomposition behavior of hydrates in a water-oil system was elucidated.The interaction of asphaltenes and waxes on the formation and decomposition behaviors of hydrates in water-in-oil emulsions was studied.It was found that the coexistence of waxes and asphaltenes enhanced the inhibition effect of a single component on hydrate formation,prolonged the hydrate induction time and reduced the formed amount of hydrates.The intrinsic relationship between the interaction between the oil components and the hydrate behaviors was revealed,and it was found that the precipitated wax crystals got adsorbed at water-oil interface through the synergistic effect with asphaltenes and inhibited hydrate formation more significantly.For hydrate decomposition process,it was found that the presence of waxes hindered hydrate decomposition,specifically,6 wt.% content of wax reduced the average decomposition rate by about 20%. |
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