The dual-functional roles of N-2 gas for the exploitation of natural gas hydrates: An inhibitor for dissociation and an external guest for replacement

Abstract

In this study, the dissociation behavior and the time-dependent guest distributions of methane (CH4) hydrate after the injection of gaseous N-2 were closely investigated using gas chromatography, in situ Raman spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy for two different N-2 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 N-2 injecting pressure. Time-dependent Raman spectra confirmed that the N-2 molecules began to be captured in the hydrate cages immediately after the N-2 gas injection at 268.8 K, and the extent of N-2 incorporation in the hydrate phase increased at a higher N-2 gas injecting pressure. The C-13 NMR spectra revealed that an increase in the N-2 composition in the hydrate phase induced a structural transition of the CH4 + N-2 hydrates from sI to sII and that N-2 molecules were preferentially captured in the small (512) cages of the CH4 + N-2 hydrates. The overall results provide deep insight into the exact role of N-2 mol-ecules in the inhibitor injection method and CH4 - CO2 + N-2 replacement methods for the exploitation of natural gas hydrates. (C) 2021 Elsevier Ltd. All rights reserved.