Complex Coupled Effects of Seawater Ions and Clay Surfaces on CH₄ Hydrate Formation in Kaolinite Janus-Nanopores and Bulk Solution

Abstract

Molecular insights into the kinetic effect of seawater ions and its coupling with the clay surface effect on CH4 hydrate formation help to understand the complex formation process of natural gas hydrate resources in marine sediments. Molecular dynamics simulations are performed to explore CH4 hydrate formation from a homogeneous salty solution containing seawater ions (Na+, K+, and Ca2+, in the form of salts NaCl, KCl, and CaCl2) in the kaolinite Janus double-nanopore and outside bulk phase, by analyzing water order parameter, aqueous CH4 concentration, flows of CH4 and seawater ions, and formation of hydrate-cage clusters. The kaolinite double-nanopore consists of a hydrophilic nanopore with gibbsite surfaces and a hydrophobic nanopore with siloxane surfaces. Simulation results show that in fresh water, the gibbsite surfaces of hydrophilic nanopore do not inhibit hydrate formation, while the siloxane surfaces of hydrophobic nanopore are adverse to hydrate formation due to formation of a large surface nanobubble. By contrast, hydrate formation in saline water is affected by the complex coupled effects of seawater ions and kaolinite surfaces. In the hydrophilic nanopore, most seawater ions quickly adsorb to the gibbsite surfaces and exert very weak effect on hydrate formation, whereas in the hydrophobic nanopore, seawater ions show a certain inhibitory effect on hydrate formation, as few ions adsorb to the siloxane surfaces. Interestingly, more hydrate solids form in saline water than in fresh water, as hydrate solids in the outside bulk solution grow into the throat of the hydrophobic nanopore and decompose the surface nanobubble. Additionally, transport of CH4 molecules and seawater ions between the kaolinite double-nanopore and the outside bulk solution further affects hydrate formation in the systems.

Effect of Glucose on CH4 Hydrate Formation in Clay Nanopores and Bulk Solution: Insights from Microsecond Molecular Dynamics Simulations

“…Park and coworkers observed that CH 4 hydrate solids can stably intercalate within montmorillonite interlayers under conditions of lower pressure and higher temperature, suggesting that a portion of such clay mineral surface actively involved in promoting CH 4 hydrate formation . Recently, Ren et al reported that montmorillonite shortens the induction time of gas hydrate formation by providing additional nucleation sites but retards the growth kinetics of gas hydrate due to hindered mass transfer. , Several experimental studies have found that the phase equilibrium of CH 4 hydrate is inhibited in porous media composed of solid particles. − Our previous study revealed that clay surfaces affect gas hydrate formation by changing the concentration of guest molecules and ions via the adsorption of water molecules, ions, and guest molecules. , On the other hand, the abundant organic matter in hydrate reservoirs will also affect the formation behavior of gas hydrates. , Liu and coworkers found that organic matter can kinetically promote gas hydrate formation by enhancing gas–water contact, and this effect was further enhanced by sulfur-containing acid-dissolvable organic matters. , Some studies revealed that organic matter molecules inhibit the formation of gas hydrates by association with water molecules. , It is worth noting that recent studies have observed that organic matter and clay minerals have a synergistic effect on the formation of gas hydrates, which significantly shortens the induction time for gas hydrate formation. − Molecular dynamics (MD) simulations have been often employed to investigate the microscopic behavior of gas hydrates from the molecular scale. − Based on MD simulation, some studies had reported that lignin, protein, humic acid, fatty acid, amino acid, and other organic matters , exhibit different effects on the formation and growth of gas hydrate. Furthermore, some MD simulation results found that clay mineral surfaces can repel organic matter or form organo-mineral complexes, significantly influencing the wettability of the clay surface. , The interaction between organic matter and clay surfaces complicates the formation processes of gas hydrates.…”