PHASE-EQUILIBRIUM OF GAS HYDRATE - IMPLICATIONS FOR THE FORMATION OF HYDRATE IN THE DEEP-SEA FLOOR

We calculate the solubility of methane gas over a range of pressure an d temperature. The gas is dissolved in liquid water, which coexists wi th free gas at high temperature or solid hydrate at low temperature an d high pressure. We show that solubility is significantly altered by t he presence or absence of the hydrate phase. When hydrate is absent at high temperatures, our calculations reproduce experimentally observed increases in solubility with decreasing temperature. When hydrate is present, however, we find that the gas solubility decreases sharply wi th decreasing temperature. Such an abrupt decrease in solubility permi ts hydrate to crystallize directly from the aqueous solution, without the need of any free gas. This result has important implications for t he formation of gas hydrate in marine environments, where the gas supp ly may not be sufficient to provide free gas. We apply our calculation s at typical pressure and temperature conditions in marine sediments t o establish the gas concentration needed to stabilize hydrate. Estimat es of the vertical distribution of hydrate in marine sediments and the rate of accumulation are obtained using simple models of hydrate form ation.