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A calibration protocol to quantify the compositional information of gas hydrates using Raman spectroscopy is proposed. Structure I pure CH 4 -, CO 2 -and C 2 H 6 -hydrates in their deuterated and hydrogenated forms with known cage occupancies were investigated by Raman spectroscopy. Raman scattering cross sections of CH 4 in the large and small cages were found to be very similar, but not identical. Some C 2 H 6 bands of C 2 H 6 -hydrate were tentatively reassigned or newly reported and assigned. Our results show that the relative cross sections of guest vibrational modes in the deuterated hydrate are in agreement with those in the hydrogenated hydrate, whereas they are considerably different from those in fluid phase. Using our Raman quantification factors, the relative cage occupancies can now be determined more reliably in CH 4 -hydrates. Moreover, with additional assumptions, the absolute cage occupancies, the bulk guest composition and hydration number of pure or mixed gas hydrates become accessible by Raman spectroscopy. m h and m f indicate Raman frequencies of ethane in hydrate lattices and fluid phase, respectively. R is percentage of peak area. a,b and c represent the values given by Helvoort et al. 67 , Fernandez and Montero 71 , and Domingo and Montero 70 , respectively. Dm is the difference between m h and m f .Figure 6. Raman spectra of the deuterated sI CH 4 -CO 2 -C 2 H 6 -hydrate measured at ambient pressure and 113 K.

Reported are the experimental measurements on vapor–liquid equilibria in the H2O + CO2 + CH4 ternary system at temperatures from (324 to 375) K and pressures from (10 to 50) MPa. The results indicate that the CH4 solubility in the ternary mixture is about 10 % to 40 % more than that calculated by interpolation from the Henry’s law constants of the binary system, H2O + CH4, and the solubility of CO2 is 6 % to 20 % more than what is calculated by the interpolation from the Henry’s law constants of the binary mixture, H2O + CO2.

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The exchange process between CO2 and methane hydrate has been observed in numerous laboratory experiments, computer simulations, and recently confirmed in a field test. Yet, to date there is no kinetic model capable of accurately predicting the swapping process at given fluid composition and p-T conditions. Major obstacles on the way to an adequate mathematical description are caused by the insufficient characterization of experimental environments and a nearly complete lack of information on the time-resolved composition of the two-phase fluid at the gas hydrate interface. Here we show that all necessary data can be provided by a combination of cryo-SEM, Raman, and neutron diffraction measurements that deliver accurate space-averaged, time-resolved in situ data on the CH4–CO2 exchange reactions at conditions relevant to sedimentary matrixes of continental margins. Results from diffraction are cross-correlated with ex situ Raman spectroscopy to provide reliable information on the preferential sites for CO2 and CH4 in the (partially) exchanged hydrate. We also show a novel approach based on scattering of neutrons to probe the fluid composition during the in situ replacement in a time-resolved, noninvasive manner. The replacement is seen as a two-step process including (1) a fast surface reaction parallel to a fast enrichment of the surrounding fluid phase with CH4 followed by (2) a much slower permeation-limited gas swapping between the gas hydrate and mixed ambient CH4–CO2 fluid. The main part of the replacement reaction takes place in the second stage. Based on our earlier experimental studies and existing literature we work toward a quantitative gas exchange model which elaborates the hole-in-cage-wall diffusion mechanism to describe the two-component gas replacement.

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Methane-dominated natural gas hydrate deposits have been considered as a potential hydrocarbon resource and as long-term storage reservoirs for the anthropogenic greenhouse gas CO 2 via CH 4 −CO 2 −N 2 replacement in gas hydrates. In this study, N 2 -hydrates of structure type I (sI) were formed, characterized, and quantified in terms of N 2 cage occupancies using synchrotron X-ray diffraction. Pure sI CH 4 -and N 2hydrates with known cage occupancies were used to calibrate the relative Raman quantification factors (F-factors) of N 2 to its H 2 O framework and to CH 4 in sI hydrate phase. The F-factors of CO 2 /CH 4 , CO 2 /H 2 O, and CH 4 /H 2 O in the hydrate cavities were corrected for the presence of ice Ih. Using these empirical ratios of F-factors, the absolute cage occupancies, the bulk guest composition, and hydration number of gas hydrates containing CH 4 , CO 2 , N 2 , and C 2 H 6 molecules can now be determined by Raman spectroscopy without additional thermodynamic assumptions. In this way, one can gain insight into details of the gas composition in mixed hydrates, for example, during the N 2 -assisted CH 4 −CO 2 exchange reaction, as well as into the preference of certain gas species for small or large cages.

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