In situ X-ray diffraction measurements of the self-preservation effect of CH₄ hydrate

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.