DOI: 10.5880/gfz.3.1.2022.008
title: Raman spectroscopic data from dissociation behavior of sI CH4 hydrates, sII CH4-C3H8 hydrates and multicomponent mixed gas hydrates in terms of thermal stimulation

description: Natural gas hydrates are non-stoichiometric crystalline compounds containing water and guest molecules such as CH4, C2H6, C3H8, CO2, etc. They are considered as a promising energy resource, a potential geohazard and a contributor to global climate warming. An accurate knowledge of the dissociation behavior of gas hydrates is a necessity for the recovery of natural gas hydrates and the assessment of potential risks of CH4 release from destabilized deposits. To explore the dissociation behavior of gas hydrates, Raman spectroscopy is regarded as a non-destructive and powerful tool. This technique enables to distinguish between guest molecules in the free gas or liquid phase, encased into a clathrate cavity or dissolved in an aqueous phase, therefore providing time-resolved information about the conditions of the guest molecules during the hydrate dissociation process. Experiments were carried out at the Micro-Raman Spectroscopy Laboratory, GFZ. Since the dissociation kinetics of sI hydrates may vary from that of sII hydrates, sI CH4 hydrates, sII binary hydrates and sII multicomponent mixed hydrates were investigated during the experiments. For the in situ Raman measurements, hydrates were synthesized in a high-pressure cell from pure water and the specific continuous gas flow which was the CH4-C3H8 gas mixture for binary hydrates and CH4-C2H6-C3H8-CO2 gas mixture for mixed hydrate system. The p-T condition of the experiment was initially set at 274 K and 7.0 MPa for the sI hydrates whereas 278 K and 3.0 MPa for sII hydrate systems. After the stabilization of the hydrates in the reactor, the temperature of the system was increased one step at a time to mimic global warming and initiate hydrate dissociation. In situ Raman spectroscopic measurements and microscopic observations were applied to record changes in hydrate compositions over the whole dissociation period until the hydrate phase was completely decomposed. Apart from this, hydrates were formed from ice powders and the specific gas/gas mixtures in batch pressure vessels for several weeks. Gas hydrates were recovered and placed into a Linkam cooling stage for further ex situ Raman spectroscopic measurements. Again, the temperature of the stage gradually increased from 168 K onwards to study the dissociation process. In all three hydrate systems, one in situ Raman measurements and at least two repetitions of ex situ Raman measurements (3 repetitions for the CH4 hydrate system) were carried out, therefore resulting in 10 separate experimental tests. This dataset encompasses raw Raman spectra of the 10 experimental tests (4 tests for CH4 hydrates, 3 tests for CH4-C3H8 hydrates and 3 for mixed gas hydrates) which contained Raman shifts and the respective measured intensities. Each Raman spectrum was fitted to Gauss/Lorentz function after an appropriate background correction to estimate the band areas and positions (Raman shift). The Raman band areas were then corrected with wavelength-independent cross-sections factors for each specific component. The concentration of each guest molecule in the hydrate phase was given as mol% in separate spreadsheets for three different hydrate systems as. Further details on the analytical setup, experimental procedures and composition calculation are provided in the following sections.
description: In this study, the dissociation process of sI CH4 hydrates, sII binary and multi-component gas hydrates were investigated via experimental simulations (in situ and ex situ Raman spectroscopic measurements). sI CH4 gas hydrates were synthesized in a custom-made pressure cell in the laboratory from water and a certified CH4 gas. Binary sII gas hydrates were synthesized from deionized water and a binary gas mixture containing CH4 and C3H8, whereas the multicomponent gas hydrates were formed from a gas mixture comprised of CH4, C2H6, C3H8 and CO2. Initially, the sample cell was filled with 150 μl deionized and degassed water, carefully sealed and pressurized with the respective gas mixture at 7.0 MPa for CH4 hydrates and 3.0 MPa for both sII hydrates. When the pressure reached the set point and the flowrate was constant, the cell was cooled down to 253 K to induce the spontaneous crystallization of hydrate and ice. After the formation of hydrates and ice, the cell was slowly warmed up to allow the dissociation of ice and most hydrate crystals until only a few hydrate crystals were left. Subsequently, the cell was cooled down again to a temperature within the stability field of the hydrate phase, but above the melting temperature of the ice. Under these conditions set, euhedral gas hydrate crystals were allowed to grow. This “melting-cooling” process was carried out three times before the p-T condition was fixed at 7.0 MPa and 274 K for the formation of CH4 gas hydrates and at 3.0 MPa and 278 K for the sII binary and multicomponent gas hydrates for around 5 days. In addition, all CH4 hydrates, CH4-C3H8 hydrates and multi-component gas hydrates were prepared from ice powder and the gas mixtures mentioned above in medium-volume batch pressure vessels. Fine-grained ice was initially prepared by spraying deionized water into liquid nitrogen bath and milled in a 6750 Freezer Miller (SPEX CertiPrep). The vessels loaded with ice powder (35 ml) were sealed, pressurized with the respective gas mixture at 7.0 MPa for CH4 hydrates and 3.5 MPa for sII hydrates. The vessels were subsequently stored in a cooling box with the temperature fluctuating between 263 K and 268 K for several weeks while monitoring the pressure drop. Gas hydrate samples were recovered and quenched into liquid nitrogen once there were no further changes in the pressure of the vessels. The recovered hydrate samples were quickly placed into a Linkam cooling stage at ambient pressure and 168 K for ex situ Raman spectroscopic measurements.