Gas Hydrate Formation and Dissociation with Thermodynamic and Kinetic Inhibitors

Abstract

Abstract This work presents new data on the kinetics of gas hydrate formation and dissociation for methane with water and seawater, with thermodynamic and kinetic inhibitors in static and in dynamic conditions, and several results of the morphology study of gas hydrate crystals. Effective porosity of methane hydrate ranges between 0 - 80%. Hydrate formation and dissociation can proceed simultaneously with transit of hydrate formers from one crystal to the other, both in bulk water and in gas phase with dissolving or sublimation of some crystals and formation of others. At the same time there can be recrystallization of hydrates with change of structure, porosity, and density of crystals. Metastable state of the system in static conditions may last many hours even at large supercooling with no hydrates. Induction time of hydrate formation depends on structure of water, water and gas composition, pressure and supercooling, cooling rate, dynamic or static conditions. Presence of thermodynamic or kinetic inhibitors may decrease induction time, decrease supercooling and increase rate of hydrate formation. We have tested four different kinetic inhibitors with fresh water and seawater with concentration of kinetic inhibitors from 0.5 mass % up to 5 mass %. Fresh water and seawater formed hydrates in presence of all kinetic inhibitors, which we tested. Content of natural gas dissolved in water in presence of hydrate at the interface is significantly lower than in water, which is in direct contact with gas. As a consequence, the formation of gas hydrate deposits in nature may proceed more rapidly than the formation of free gas deposits.