Molecular dynamics simulation to explore the impact of wax crystal on the formation of methane hydrate

Abstract

The combined problem caused by wax and hydrate deposition is the key issue to be considered for the subsea multiphase (crude oil, gas, and water) flow assurance. In our study, molecular dynamics technique was performed to generate the solid state wax crystal and further study the impact of wax crystal on the formation of methane hydrate on the microscopic level. Our simulation results showed that wax crystal played a dual role in methane hydrate formation (i.e. promotion and inhibition effects), depending on its size. Small wax crystal would shorten the hydrate nucleation time from 19 ns to about 12 ns, facilitate the 51262 cage formation and increase the conversion ratio of methane and water molecules to methane hydrate by more than 1.5 times. In this process, the competitive capture of methane in gas–liquid, gas-hydrate and liquid-hydrate phases could be detected. When the size of wax crystal exceeded the critical value, the formation of hydrate would be inhibited and the nucleation time was prolonged beyond 500 ns. In this case, the competition of methane capture between wax crystal and aqueous solution would dominated the whole process. The presence of small wax crystal accelerated the hydrate nucleation and growth by facilitating the molecular migration of H2O and CH4 to a certain extent and promoting the assembly of water and methane molecules into hydrate precursors. However, a further increase in the diffusion velocity of methane and water molecules induced by the stronger gas adsorption performance of more wax would lead to the aggregation of methane molecules into large nano-bubbles, reducing the CH4 concentration in the solution far below the hydrate formation threshold value. Eventually, hydrate cages would not be formed and survival. The results in this work would contribute to a better understanding of the wax-hydrate interaction.