Abstract:

The study of the gas hydrate stability zone has great significance in comprehension of the distribution features and the resource assessment of gas hydrates.Many scholars have calculated the thickness of the stability zone by different methods, but there are some big differences among these methods, especially in the selection of parameters. The authors simply evaluate several methods and the selection of parameters for the calculation of the gas hydrate stability zone. The results show that we must consider pore-water pressure, water-column pressure and atmospheric pressure when we select the pressure formula. The second-order polynomial of pressure variation with different latitudes considering atmospheric pressure is best. The obtained equation by experiment is only used to predicting pure methane of pure water or seawater of 33.5‰ salinity. We can calculate the temperature-pressure condition of stability zone with different salinities and gas compositions by the theory of thermodynamics. The phase equilibrium plot is most simple, but its error is great. The calculated thickness of stability zone by the four-order polynomial using the experiment and thermodynamics data is close to practice. The predicted limit of the stability zone by different methods is not always consistent with actual limit of hydrate sediment.

Key words: gas hydrate, stability zone, Gibbs free energy minimization