Thermodynamic Characteristics of Phase Transition During Natural Gas Hydrate Decomposition by Depressurization
The exploitation of natural gas hydrate is a major strategic demand of the country,and further improvements in production and efficiency are needed before its commercial develop-ment.How to realize its efficient exploitation depends on the thermodynamic mechanism of the hydrates.Traditional hydrate thermodynamics studies are limited to phase equilibrium character-istics and lack the consideration of non-equilibrium thermodynamics in the hydrate decomposition process.At the same time,the hydrate decomposition process may be accompanied by icing and melting.The thermodynamic characteristics of hydrate exploitation under complex heat and mass transfer conditions need to be clarified.In this study,a 2 L natural gas hydrate depressuriza-tion decomposition experimental system was used to simulate the hydrate reservoirs with different distributions and perform long depressurization(to 1.0 MPa)at a constant exhaust rate of 0.55 L·min-1(normal conditions).The results show that there is a non-equilibrium thermodynamic re-lationship(T[°]=8533.8/{38.98-ln(1000p[MPa])}-275.25)between the temperature and pressure in the hydrate-bearing area during the depressurization process,which is only controlled by the hydrate decomposition and is not affected by the reservoir temperature gradient.According to the Gibbs phase law,the thermodynamic freedom of phase transition process is 1.As a result,the natural gas hydrate decomposition belongs to the phase transition process,and the quantitative relationship between temperature and pressure is consistent with the thermodynamic theory.When the hydrates depressurize to about 2.1～2.3 MPa,instantaneous icing occurs in the reservoir,leading to a sudden increase in temperature and accelerating the hydrate decomposition.Due to the constant exhaust rate,the accumulated gas increases the pressure up to 2.36 MPa.It is found that the temperature and pressure of hydrate-bearing reservoir still satisfy the non-equilibrium thermodynamic phase di-agram of hydrates before and after icing.This study illustrates the thermodynamic mechanism of phase transition in the presence of heat and mass transfer in the natural gas hydrate decomposi-tion process,which can provide a more practical theoretical basis for the process of exploitation site monitoring.
natural gas hydratetemperature gradienthydrate decompositionicing and meltingnon-equilibrium thermodynamics
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