Thermodynamic properties of hydrate are essential for gas hydrate investigation, especially the phase equilibrium behavior. Gas component can affect the phase equilibrium of hydrate. Through experiments of gas hydrate formation and dissociation in simulated sediments, phase equilibrium of gas hydrate composed of methane, ethane and propane has been studied. It's concluded that the methane hydrate equilibrium needs a lower pressure and a higher temperature with the presence of the ethane and propane. The equilibrium pressure of hydrate decreases with the decrease of methane concentration at the same temperature. The effect of propane concentration on hydrate equilibrium is not significant when the concentration reaches about 15%. The effects of porous medium on hydrate equilibrium have also been investigated.
Clathrate hydrates are ice-like crystalline compounds in which gas molecules are held in cavities formed by water molecules (Sloan,1998). Natural gas hydrate (NGH) is considered to be a potential energy resource, because there is much gas in it, such as methane, ethane and propane. The mineral deposits of hydrate are large in scale with high energy density and shallow burial. NGH is also important for petroleum industry, where the hydrate may form in a pipeline carrying oil/gas and block it. To study how to explore the hydrate from natural deposit, it is necessary to understand the physical characteristics of hydrate such as hydrate phase equilibrium. Thermodynamic investigations of hydrates are also important in the natural gas and petroleum industries, for the prevention of hydrate formation in pipelines and processing facilities (Maekawa, 2008). The former gas component of hydrate affects the phase equilibrium of hydrate. A lot of experiments were carried out to determine equilibrium conditions of gas hydrates containing different hydrocarbon gases. Wang et al. (2006) measured hydrate formation conditions of four natural gases in pure water in a sapphire cell and used a hydrate model to calculate hydrate formation conditions of these four natural gases.