Molecular Dynamics Simulation of Methane Hydrate Formation and Dissociation in the Clay Pores with Fatty Acids

摘要

Natural gas hydrate is a promising energy resource, but it is challenging to recover methane from the clay pores rich with sediment organic matter due to the inadequate information on hydrate evolution in the organoclay complex. Molecular dynamics simulations were conducted to investigate the methane hydrate formation and dissociation in the sodium montmorillonite interlayer (Na-MMT) with fatty acids by characterizing the four-body structural order parameter, radial distribution functions, and cage types. Results demonstrated the slight inhibition of fatty acid types (butyric and isovaleric acid) at molar concentrations (0.6% and 1.5%) on the methane hydrate formation in the context of this study. This finding was a result of several main processes including the formation of "quasihydrate" water structures around the hydrophobic carbon chains of fatty acids, the disruption of hydrogen bond networks by the hydrophilic carboxyl of fatty acids, and the restriction on the thermal motion of the sodium ions by coordination with the carboxyl of fatty acids. Results also demonstrated the promotion of fatty acids on the methane hydrate dissociation in the Na-MMT, which highlighted the role of fatty acid accumulation in the accelerated breakdown of hydrates at relatively low decomposing temperature. Overall results provided theoretical support for better understanding the formation mechanisms and decomposition strategies of methane hydrates in the heterogeneous sediment environments since fatty acids are microbial intermediates of special importance in the hydrate-bearing sediment.