Methane hydrate formation and dissociation behaviors in montmorillonite

A Review of the Effect of Porous Media on Gas Hydrate Formation

Most gas hydrates on the earth are in sediments and permafrost areas, and porous media are often used industrially as additives to improve gas hydrate formation. For further understanding its exploration and exploitation under natural conditions and its application in industry, it is necessary to study the effect of porous media on hydrate formation. The results show that the stacked porous media affects the phase equilibrium of hydrate formation depending on the competition water activity and large specific surface areas, while integrated porous media, such as metal foam, can transfer the hydration heat rapidly and moderate the hydrate phase equilibrium. A supersaturated metal-organic framework is able to significantly improve gas storage performance and can be used as a new material to promote hydrate formation. However, the critical particle size should be studied further for approaching the best promotion effect. In addition, together with the kinetic accelerators, porous media has a synergistic effect on gas hydrate formation. The carboxyl and hydroxyl groups on the surface of porous media can stabilize hydrate crystals through hydrogen bonding. However, the hydroxyl radicals on the silica surface inhibit the combination of CH₄ and free water, making the phase equilibrium conditions more demanding.

Molecular Insights into the Effect of Nitrogen Bubbles on the Formation of Tetrahydrofuran Hydrates

In this work, a molecular dynamics simulation was conducted to study the microscopic mechanism of how nitrogen bubbles affect the formation of THF hydrates at the molecular level. The results obtained reveal that the nitrogen bubble can promote the formation of THF hydrates. In the system with a nitrogen bubble, more THF-filled cages were generated, and the crystal structure was more orderly. The promotion of nitrogen bubbles on hydrate crystallization comes from the dissolution of nitrogen molecules. Some of dissolved nitrogen molecules can be enclosed in small hydrate cages near the nitrogen bubble, which can serve as stable sites for hydrate crystal growth, resulting in the fact that THF-filled cages connected with N₂-filled cages are much more stable and have a long lifetime. The results in this work can help to understand the promotion effect of micro- and nano-air bubbles on the crystallization of THF hydrates.

Effects of Salinity on Formation Behavior of Methane Hydrate in Montmorillonite

In marine sediments, seawater influences the phase behavior of natural gas hydrate. As a porous medium, the water distribution and physical properties of montmorillonite are influenced by the salt ions in seawater. In this work, the bound-water content in, and crystal structure of, montmorillonite is measured to investigate the effect of salt ions on the water distribution in montmorillonite. It can be determined from the results that the bound-water content in montmorillonite decreases as the salt-ion concentration increases. Salt ions affect the intercalation of water molecules in montmorillonite, and they then inhibit the expansion effect of montmorillonite. Next, the phase behaviors of methane hydrate in montmorillonite with NaCl solution are investigated using high-pressure micro-differential scanning calorimetry. The phase behavior of hydrate in montmorillonite with NaCl solution is discussed. In montmorillonite with NaCl solution, the phase equilibrium temperatures and the conversion rate of methane hydrate both decrease with increasing NaCl concentration. The results show that methane hydrate in montmorillonite is influenced not only by the phase-equilibrium effect of salt ions, but also by the formation effect of the salt ions on the bound-water content in montmorillonite.