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.