Effects of Salinity on Formation Behavior of Methane Hydrate in Montmorillonite

Abstract

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.

Effects of the NaCl Concentration and Montmorillonite Content on Formation Kinetics of Methane Hydrate

“…Montmorillonite absorbs more water into its interlayer with the increase of the montmorillonite content. Most of the absorbed water exists in the bound water, which links with montmorillonite by hydrogen bonds, making it more difficult to form hydrate on the surface of montmorillonite [29,39]. In other words, montmorillonite also has a certain thermodynamic inhibition on hydrate formation at a high content [25].…”

“…However, the silica sands were mainly used in previous studies, which cannot reflect the actual reservoir's characteristics. Montmorillonite is an essential part of the marine sediments in the Shenhu area of the South China Sea [29]. As a result of the unique surface property and layered structure, montmorillonite adsorbs exchangeable cations (Na + , Ca 2+, Mg 2+ ) and water molecules to achieve a charge balance between the layers [30], changing the pore volume and salinity during hydrate formation, and further affects hydrate formation kinetics.…”

Recent Advances in Methods of Gas Recovery from Hydrate-Bearing Sediments: A Review

“…However, these inhibitors only work well at higher concentrations (usually >10 wt %), and the application of too low a concentration of inhibitors may abnormally facilitate the NGH growth. Consequently, high injection rates and concentration of inhibitors are often required on site to prevent the anomalous behavior from happening, which is why we often call the THIs “high-dosage NGH inhibitors”. , In the low-temperature environment caused by the endothermic reaction of NGH dissociation, the amount and concentration of inhibitors similarly need to be increased to maintain the sustained inhibitory effect. When the temperature of HBS is too low, the THIs will not work .…”

Enhancing the Properties of Water-Soluble Copolymer Nanocomposites by Controlling the Layer Silicate Load and Exfoliated Nanolayers Adsorbed on Polymer Chains

“…Montmorillonite (MMt) is a common inorganic phase in the synthesis of water-soluble polymer nanocomposites, and its nano-effects have received considerable research attention [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. MMt is a natural two-dimensional layered silicate material whose crystal structure is composed of two sheets of silicon–oxygen tetrahedra sandwiched between sheets of aluminum–oxygen octahedra [ 18 , 19 , 20 ]. The surface of a MMt layer has ample hydroxyl groups and negative charges, and abundant exchangeable ions exist between layers [ 21 , 22 ].…”