10.5880/GFZ.3.1.2021.003 Pan, Mengdi Mengdi Pan 0000-0003-0632-1799 GFZ German Research Centre for Geosciences, Potsdam, Germany Schicks, Judith M. Judith M. Schicks 0000-0003-1106-0693 GFZ German Research Centre for Geosciences, Potsdam, Germany GFZ Data Services 2021 mixed gas hydrates in situ Raman spectroscopy Earth Remote Sensing Instruments > Active Remote Sensing > Spectrometers/Radiometers > Lidar/Laser Spectrometers EARTH SCIENCE > SOLID EARTH > ROCKS/MINERALS/CRYSTALS > GAS HYDRATES > GAS HYDRATES FORMATION EARTH SCIENCE > SOLID EARTH > ROCKS/MINERALS/CRYSTALS > GAS HYDRATES > GAS HYDRATES PHYSICAL/OPTICAL PROPERTIES > STABILITY resource > energy resource Pan, Mengdi Mengdi Pan 0000-0003-0632-1799 GFZ German Research Centre for Geosciences, Potsdam, Germany Pan, Mengdi Mengdi Pan 0000-0003-0632-1799 GFZ German Research Centre for Geosciences, Potsdam, Germany Schicks, Judith M. Judith M. Schicks 0000-0003-1106-0693 GFZ German Research Centre for Geosciences, Potsdam, Germany Schicks, Judith M. Judith M. Schicks 0000-0003-1106-0693 GFZ German Research Centre for Geosciences, Potsdam, Germany Micro-Raman Spectroscopy Laboratory (GFZ German Research Centre for Geosciences, Germany) GFZ German Research Centre for Geosciences, Potsdam, Germany Schicks, Judith M. GFZ German Research Centre for Geosciences, Potsdam, Germany Dataset 10.3390/molecules26103039 10.1016/S0024-4937(00)00043-8 10.1002/9783527615438 10.1007/978-3-642-81279-8_4 CC BY 4.0 Natural gas hydrates encase predominantly methane, but also higher hydrocarbons as well as CO2 and H2S. The formation of gas hydrates from a changing gas mixture, either due to the preferred incorporation of certain components into the hydrate phase or an inadequate gas supply, may lead to significant changes in the composition of the resulting hydrate phase. To determine the overall composition of a hydrate phase during the hydrate formation process, 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 guest molecules during the hydrate formation process.
Experiments were carried out at the Micro-Raman Spectroscopy Laboratory, GFZ. Mixed gas hydrates were synthesized in a high-pressure cell from pure water and a specific gas flow containing CH4, C2H6, C3H8, iso-C4H10 and n-C4H10 at 274 K and 2.20 MPa. Three potential different gas supply conditions were selected for the formation of mixed gas hydrates, namely an open system (test scenario 1) with a continuous gas supply, a closed system (test scenario 2) with no gas supply after initial pressurization with the gas mixture, and a semi-closed system (test scenario 3) with only an incoming gas but a disrupted outlet. In situ Raman spectroscopic measurements and microscopic observations were applied to record changes in both gas and hydrate compositions over the whole formation period until it reached a steady state. In all three test scenarios, 12 hydrate crystals were selected and continuously characterized for 5 days with single point Raman measurements to record the formation process of mixed gas hydrates. Each test scenario was repeated for 3 times, therefore resulting in 9 separate experimental tests.
This dataset encompasses raw Raman spectra of the 9 experimental tests (.txt files) 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 / gas phase was given as mol% in a separate spreadsheet for three different test scenarios. Further details on the analytical setup, experimental procedures and composition calculation are provided in the following sections.
Mixed gas hydrates were synthesized in a custom-made pressure cell in the laboratory from water and a certified gas mixture containing CH4, C2H6, C3H8, iso-C4H10, and n-C4H10. Initially, the sample cell was filled with 150 μl deionized and degassed water, carefully sealed and pressurized with the respective gas mixture. When the pressure reached 2.20 MPa 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 2.20 MPa and 274 K for the formation of mixed gas hydrates.
To investigate the hydrate formation process, three different test scenarios were carried out with different gas flows but under identical p-T conditions. The inlet and outlet valves located outside the pressure cell were set to the desired position once the mixed gas hydrates started to form. In test scenario 1 (open system), the inlet and outlet valves were kept open throughout the whole experiment. Test scenario 2 (closed system) was carried out with the inlet and outlet valves being closed right after initial pressurization to mimic a system with a limited gas supply. The outlet valve was closed in test scenario 3 (semi-closed system) while the inlet valve was open. These changes on the gas flow were maintained throughout the whole formation process. Each test scenario was repeated for 3 times during the experiments.