New insights into the deposition of natural gas hydrate on pipeline surfaces: A molecular dynamics simulation study

Natural 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 water on Fe surface, the deposited hydrate cannot convert the adsorbed water into hydrate, thus, a water film exists. As water affinities decrease (Fe > Fe₂O₃ > FeO > Fe₃O₄), adsorbed water 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.