New insights into the deposition of natural gas hydrate on pipeline surfaces: A molecular dynamics simulation study
Jun Zhang, Hai-Qiang Fu, Muzhi Guo, Zhao Wang, Liwen Li, Yin Qi, Youguo Yan, Wei Wei, Weifeng Han, Jie Zhong
Abstract
Nature gas hydrate (NGH) can cause pipeline blockages during the transportation of oil and gas under high pressures and low temperatures. Reducing hydrate adhesion on pipelines is viewed as an efficient way to prevent NGH blockages. Previous studies suggested the water film can greatly increase hydrate adhesion in gas-dominant system. Herein, by performing the molecular dynamics simulations, we find in water-dominant system, the water film plays different roles in hydrate deposition on Fe and its corrosion surfaces. Specifically, due to the strong affinity of waters on Fe surface, the deposited hydrate cannot convert the adsorbed waters into hydrate, thus, a water film exists. As water affinities decrease (Fe > Fe2O3 > FeO > Fe3O4), adsorbed waters would convert to amorphous hydrate on Fe2O3 and form the ordered hydrate on FeO and Fe3O4 after hydrate deposition. While absorbed water film converts to amorphous or to hydrate, the adhesion strength of hydrate continuously increases (Fe < Fe2O3 < FeO < Fe3O4). This is because the detachment of deposited hydrate prefers to occur at soft region of liquid layer, the process of which becomes harder as liquid layer vanishes. As a result, contrary to gas-dominant system, the water film plays the weakening roles on hydrate adhesion in water-dominant system. Overall, our results can help to better understand the hydrate deposition mechanisms on Fe and its corrosion surfaces and suggest hydrate deposition can be adjusted by changing water affinities on pipeline surfaces.