Complex Phase Behaviors and Structural Coexistence of Natural Gas Hydrates Containing Large-Molecule Guest Substances

Abstract

The complex phase behaviors and structural coexistence of natural gas hydrates (NGHs) that contain large-molecule guest substances (LMGSs) were examined for their significance in the exploration and exploitation of NGHs as well as natural gas storage and transportation. Methylcyclopentane (MCP) and 2,2-dimethylbutane [neohexane (NH)] were chosen as LMGSs, and the simplest composition of NGHs and natural gas was simulated by a gas mixture of CH4 (90%) + C2H6 (10%). The coexistence of structure II (sII) and structure H (sH) hydrates in the CH4 + C2H6 + LMGS + water mixtures was revealed by 13C nuclear magnetic resonance (NMR) and powder X-ray diffraction (PXRD). CH4 and LMGSs were captured in sH, whereas CH4 and C2H6 were enclathrated in sII. Endothermic heat flow curves, which were obtained by a differential scanning calorimeter (DSC), and pressure (P)–temperature (T) traces exhibited two-step dissociation of formed gas hydrates (sH dissociation followed by sII dissociation). The experimental results for the PXRD patterns, 13C NMR spectra, phase equilibria, DSC heating curves, and P–T traces clearly demonstrated that the CH4 (90%) + C2H6 (10%) + LMGS + water mixtures formed both sII hydrates and sH hydrates and that the final hydrate structure at equilibrium dissociation points was sII.