The dual-functional roles of N2 gas for the exploitation of natural gas hydrates: An inhibitor for dissociation and an external guest for replacement

In this study, the dissociation behavior and the time-dependent guest distributions of methane (CH4) hydrate after the injection of gaseous N2 were closely investigated using gas chromatography, in situ Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy for two different N2 gas injecting pressures (4.0 and 8.0 MPa) at three different temperatures (268.8, 274.2, and 278.2 K). The dissociation kinetics of the CH4 hydrate were accelerated at a higher temperature and a lower N2 injecting pressure. Time-dependent Raman spectra confirmed that the N2 molecules began to be captured in the hydrate cages immediately after the N2 gas injection at 268.8 K, and the extent of N2 incorporation in the hydrate phase increased at a higher N2 gas injecting pressure. The 13C NMR spectra revealed that an increase in the N2 composition in the hydrate phase induced a structural transition of the CH4 + N2 hydrates from sI to sII and that N2 molecules were preferentially captured in the small (512) cages of the CH4 + N2 hydrates. The overall results provide deep insight into the exact role of N2 molecules in the inhibitor injection method and CH4 – CO2 + N2 replacement methods for the exploitation of natural gas hydrates.