Effects of wax on the formation of methane hydrate in oil-dominate systems: Experiments and molecular dynamics simulations

Hydrate formation and wax deposition in underwater multiphase pipelines have become two vital problems affecting the flow assurance safety. Experiments and molecular dynamics (MD) simulations were performed to investigate the effect of wax crystals on hydrate nucleation and growth. Our experimental results showed that wax crystals had an inhibitory effect on hydrate growth, but played a dual role in methane hydrate nucleation (promotion and inhibition). The addition of wax could decrease mass transfer coefficient of methane by increasing viscosity of oil phase, which lower methane supply from the oil bulk phase to the oil-water interface. The simulation results revealed that the wax crystals at the interface influenced the mass transfer of methane molecules instead of providing heterogeneous nucleation sites. Small wax crystal adjacent to the oil-water interface accelerated methane supersaturation dissolution and restricted gas diffusion in the aqueous phase because of adsorption to methane, which promoted hydrate nucleation and shifted the nucleation position toward the oil-water interface. While the large wax crystal could effectively block the methane mass transfer from oil phase to aqueous phase by covering the oil-water surface, which reduced the concentration of methane in the aqueous phase and prolonged the induction time of hydrate formation. These results of this work would provide a better understanding of the interaction mechanism between hydrate and wax.