Hydrate reservoirs that are partly exposed towards the ocean floor will not be unconditionally stable even though the local temperature and pressure may be sufficient to maintain equilibrium between pure methane, water and hydrate. In this work we study the stability limits with respects to methane concentration in the seawater in contact with the hydrate. Since the average methane concentration in the oceans is very close to zero the hydrate will not be thermodynamically stable when exposed to this seawater. The estimated driving forces for dissociation are limited and the dissociation rates is expected to be correspondingly small. Accurate estimates for the dissociation kinetics in a system with these small driving forces requires accurate values of the interracial tensions as well as other rheological properties involved in the kinetics of dissociation and transport of heat and melted molecules away from the hydrate/seawater interface. Work is in progress on the estimation of these properties using molecular simulations. I have also presented a nucleation theory for the kinetics of hydrate formation and growth based on multi-component diffusive interface theory. Theoretical estimates indicate that methane hydrate initiation from the gas side of the gas/water interface may dominate when compared to initiation and growth from the liquid side of the interface.
One of the problems involved in the modelling of the kinetics of hydrate formation is that many pieces of the mechanisms involved in the total formation process for water and hydrate former to hydrate is still unknown. The nucleation stage is related to that of forming hydrate cores that are thermodynamically stable and as such will grow steadily. The actual content of these two stages, and in particular the nucleation stage, of the formation process in terms of mechanisms and corresponding detailed kinetic contributions remain experimentally unverified today.
