If hydrates were dissolved under a constant volume condition, decomposition would occur along with a H-Lw-V three phases equilibrium line. Thus the H-Lw-V three phase equilibrium line can be considered to be equivalent with the decomposition condition. So, to determine the H-Lw-V three-phase equilibrium line accurately is very important task in constructing simulation programs of the methane production rate from a hydrate reservoir. There are several methods to improve the prediction accuracy. In three-phase coexistence state, not only a hydrates phase but also a liquid phase and a vapor phase may play important roles. Consequently, by using the correct model of the liquid phase and the vapor phase we can estimate the H-Lw-V threephase equilibrium condition more accurately.
Differing from the forming condition, the dissolving condition of hydrates is recognized to coincide with the equilibrium condition if the state of a system changes along with quasi-static processes. So the accurate estimation of the equilibrium condition plays an important role in constructing simulation programs that can predict the methane production rate from a hydrate reservoir. Usually phase equilibrium calculation is performed based upon classical statistical models. To use these models, data describing molecular properties are necessary, for instance UNIQUAC model and UNIFAC model demand the interaction potential parameters between molecules in addition to the van der Waals area and the van der Waals volume of each molecule, and the van deer Waals & Platteeuw model needs Kihara potential parameters that specify the potential between a water molecule and a guest gas molecule. Of course, It is a difficult task to directly measure these values, so the values of these parameters are determined in the way such that the values of some measurable physical quantities (pressure, mole fraction, etc) coincide with the predicted values derived from these models.
