Microstructural Study on Dissolution of Natural Methane Hydrate by Multicontrast and Multiscale X-ray Computed Tomography

Understanding the dissolution processes of natural methane (CH4) hydrates is important for the quantification of CH4 released into seawater used to assess unconventional natural gas resources. In this study, the spatial relationships among CH4 hydrates and seawater in frozen natural sediments obtained from the Sea of Okhotsk were successfully visualized using multicontrast and multiscale X-ray computed tomography. It was revealed that dissociation of the CH4 hydrates led to the formation of microbubbles and that the microbubbles were maintained at the boundary in a thin seawater layer with a thickness of 100-200 mu m. These results indicate that CH4 concentrations at the CH4 hydrate-seawater boundary layers were higher than those estimated from CH4 solubility in seawater, suggesting that ocean CH4 hydrates can exist longer even in seawater unsaturated with CH4. Experiments with such high-resolution techniques have enabled the control of dissolution rates, which can be useful for pore-scale CH4 emission and production.