Formation Mechanism and Phase State of Hydrates in Fractured Layers of Permafrost

Natural gas hydrate is widely distributed in the fractured layers of permafrost. In order to explore gas hydrate in the fractured layers of permafrost, it is necessary to study its formation, distribution and phase state. In this paper, the formation and phase equilibrium experiments of gas hydrate were conducted on the artificial cores which were prepared in reference to the characteristics of fractured layers in the area of Qilian Mountains. The phase equilibrium of hydrate in NaCl and KCl solutions were predicted according to the gas hydrate thermodynamic model created by A.H.Mohammadi, and the phrase equilibrium migration of hydrate in drilling fluids under the conditio of low temperature and salt bearing was discussed. It is shown by experimental results that gas hydrate is mainly distributed at the fracture surface and end surface of samples. When the pressure is 4 MPa, the phase equilibrium temperature of hydrate in NaCl solutions that have mass fraction of 5% and 1% declined about by 2.2 K and 4.6 K, respectively, and that in KCl solution with mass fraction of 10% decreased by 3.4 K. Research results indicated that the gas hydrate in fractured layers of permafrost is mostly in the massive form and slightly filled in the pores, and its phase equilibrium condition is consistent with that in macroscopic water phase. And under the same salinity, the effect of NaCl on the phase equilibrium of gas hydrate is greater than that affected by KCl.