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 separate spreadsheet for three different test scenarios. Further details on the analytical setup, experimental procedures and composition calculation are provided in the following sections.
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.