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Equilibrium conditions of CH 4 , CO 2 , and C 3 H 8 hydrates confined in small pores of porous glass were determined. The dissociation temperature of each hydrate at a given pressure shifted lower than that for bulk hydrate; the largest shift for CH 4 hydrate was -12.3 K ( 0.2 K for 4-nm-diameter pores and the shift decreased to only -0.5 K for 100-nm pores. CH 4 hydrate experiments at temperatures lower than the quadruple point of 270.6 K in 30-nm porous glass showed no shift of the equilibrium line. All temperature shifts were fitted by the Gibbs-Thomson equation; the best fits for CH 4 , CO 2 , and C 3 H 8 hydrates predicted hydrate-water interfacial energies of 1.7(3) × 10 -2 J/m 2 , 1.4(3) × 10 -2 J/m 2 , and 2.5(1) × 10 -2 J/m 2 , respectively. Both type-I hydrates of CH 4 and CO 2 had interfacial energies within 20% of each other but significantly smaller than the type-II hydrate of C 3 H 8 . Ice formation in the same porous glass fit the Gibbs-Thomson relation with an interfacial energy of 2.9(6) × 10 -2 J/m 2 , which is in good agreement with established values. The estimated interfacial tensions between gas hydrates and water were found to be only weakly affected by the kinds of gas. This indicated that the pore effect on the phase equilibrium was mainly due to the water activity change. The wide range of experiments on pore size, temperature, and the kind of gas allowed us to evaluate the validity of previous model predictions for pore effects on gas hydrate stability.

The dissociation conditions of methane hydrates in confined small pores were measured by the gradual temperature increase method. Significant downward shifts of the dissociation temperature were observed in porous glasses, which had small pores ranging from 100 to 500 Å in diameter, compared with that of the bulk hydrate at a given pressure. Systematic measurements revealed that the temperature offset was in inverse proportion to the pore diameter. The Arrhenius plot of the dissociation conditions suggests that the heat of methane-hydrate dissociation tended to be small compared to that of bulk hydrates in pores smaller than 300 Å in diameter. Applying the Gibbs−Thomson effect to the quantitative analysis of the phenomenon indicated that the dissociation condition of methane hydrates in small pores shifted because of changes in the water activity. The apparent interfacial free energy between methane hydrates and water in the confined condition was estimated to be approximately 3.9 × 10-2 J m-2, which is comparable to that between ice and water in the similar condition.

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In situ observations of CH4 hydrate dissociation using X-ray diffraction were carried out at atmospheric pressure and at both 168 and 189 K. Dissociation rates of the hydrate and the rate of transformation into hexagonal ice were measured using time-resolved energy-dispersive X-ray diffraction. The dissociation of CH4 hydrate had an initially fast regime followed by slower dissociation. Thus, the data support a previously suggested two-step process. In addition, we observed dynamic behavior of the X-ray diffraction intensities of ice Ih, which implies a transient crystal structure at the beginning of the dissociation. Our analyses indicates that the first step, which lasted several tens of minutes, was the formation of an ice Ih layer around the CH4 hydrate, and the second step was relatively slow because the CH4 had to diffuse through the thickening ice layer. This second step determined the hydrate lifetime. The resulting diffusion coefficients were estimated at 2.2 × 10-11 m2/s at 189 K and 9.6 × 10-12 m2/s at 168 K.

We measured the times to nucleate CO 2 hydrates from CO 2 dissolved water under pressure and 8.6 K supercooling using different methods to prepare the water. These times ranged from 50 min to more than 7200 min, depending on the preparation method. The nucleation rates were calculated by fitting the observed nucleation probability distributions to a nucleation rate equation. The nucleation rates significantly increased when the water had previously frozen as ice and melted (freezing-memory effect), except when the meltwater was heated to 298 K before nucleation. The nucleation rates also increased with O 2 -saturated meltwater, but decreased with degassed water.