Spectral Analysis on the Raman Peaks of CH4, CO2, and N2 in the Hydrate Phase

Developing a rapid nondestructive measurement of gas-storage density in the hydrate phase has long been a difficulty in hydrate resource assessment and application. In this work, the characteristic peaks of CH₄, N₂, and CO₂ from Raman spectra were correlated with the gas concentrations in the hydrate phase. Water band was chosen as the reference peak to normalize the gas peaks in the hydrate phase. Results showed that the shape of the water band of sI hydrate ranging from 2800 to 3600 cm^–1 was not found to have a noticeable change, which could be viewed as a reference peak. The characteristic peaks of CH₄, N₂, and CO₂ in sI hydrate were then normalized by dividing the integral intensities of the gas peaks and the corresponding water bands. At last, the linear relationship between the normalized gas peaks (Pi) and the mole concentration of gases in the hydrate phase obtained from macroscopic measurements (C_i) was established where P_CH4 = 0.988C_CH4, P_CO2 = 2.759C_CO2, and P_N2 = 9.204C_N2 for CH₄, N₂, and CO₂, respectively. The cage occupancies of CH₄ were also found to agree with calculations from the van der Waals-Platteeuw model. These findings provide a nondestructive path to evaluate the gas-storage density of sI hydrate and also lay a foundation for the quantitative analysis of the in situ Raman measurements on the hydrate reaction kinetics.