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A series of phase equilibrium tests were performed to explore the influence of various experimental conditions, including initial water content, dry density, and salt content, on the phase equilibrium behavior of methane hydrate in fine- and coarse-grained sediments. The results indicate that the phase equilibrium condition of pore hydrate is significantly influenced by the adopted experimental conditions and the sediment types. It is shown that in equilibrium, a unique relationship (the soil hydration characteristic curve (SHCC)) exists among the temperature shift, the unhydrated water, and the amount of dissolved salt. Under salt-free conditions, the SHCC is independent of dry density for coarse-grained specimens, whereas it is slightly influenced by the dry density for fine-grained specimens. A recently developed phase equilibrium model of pore hydrate, which can account for osmotic, capillary, and adsorptive effects, is introduced to interpret the experimental results. It is shown that the SHCC of the coarse-grained specimens can be very well described by the model; for fine-grained specimens, however, the effect of salt content on matric suction has to be taken into account in the model prediction.

The low-field NMR method is introduced to explore the effect of various experimental conditions on hydration number during hydrate dissociation in methane hydrate-bearing sediments. It is shown that the NMR signal peak value of water is independent of the dry density of sediment, gas pressure, and salt concentration and significantly influenced by temperature. A relationship among NMR signal peak value, substance content, pressure, and temperature is developed to estimate the hydration number of pore hydrates during hydrate dissociation. It is revealed that the hydration number (N h) of synthesized methane hydrates remains practically unchanged during hydrate dissociation in fine-grained sediments and is trivially influenced by experimental conditions, including initial pressure, density, initial water saturation, and salt content. Our experimental results indicated that the hydration number of methane hydrates is about 6.5, which is slightly higher than that of natural hydrates.