Molecular insights into the effects of surface property and pore size of non-swelling clay on methane hydrate formation

Abstract

Systematic molecular dynamics simulations have been performed to elucidate the effects of basal surface and pore-size on CH4 hydrate formation from the homogeneous CH4 solution in the slit-nanopores of non-swelling clay, i.e. kaolinite. The results indicate that the basal surfaces of kaolinite clay affect hydrate formation mainly by changing the aqueous CH4 concentration via surface adsorption for water and CH4 molecules. Specifically, the highly hydrophilic gibbsite surface strongly adsorbs water molecules and repels CH4 molecules from the interfacial region to the bulk phase, thus, slightly increasing the aqueous CH4 concentration in the bulk phase and is beneficial for hydrate formation. The hydrophobic siloxane surface adsorbs CH4 molecules from the solution and lowers the aqueous CH4 concentration, and then obviously prolongs the induction time and slows down hydrate growth kinetics. Larger pore size is found to be more favorable for hydrate formation, while the extreme narrow nanopores could inhibit hydrate formation. Direct contact of CH4 hydrate solids with the gibbsite surface is almost separated by the strongly adsorbed interfacial water, which is very difficult to be converted into hydrates. In contrast, CH4 hydrate solids can directly interact with the siloxane surface by forming hydrogen bonds or via semi-cages, as the interfacial water is not so strongly adsorbed and can easily form hydrates. These molecular insights into the formation process of CH4 hydrate in the nanopores of kaolinite clay are helpful to understand the formation of natural gas hydrates in marine sediments.