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Raman spectroscopic characterization of gas mixtures. II. Quantitative composition and pressure determination of the CO2-CH4 system

J. C. Seitz·J. D. Pasteris·I.-M. Chou
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Abstract

Raman spectral parameters were determined for the v 1 band of CH4 and the v 1 and 2 v 2 bands (Fermi diad) of CO 2 in pure CO 2 and CO 2 -CH 4 mixtures at pressures up to 700 bars and room temperature. Peak position, area, height, and width were investigated as functions of pressure and composition. The peak positions of the CH 4 and CO 2 bands shift to lower relative wavenumbers as fluid pressure is increased. The peak position of the lower-wavenumber member of the Fermi diad for CO 2 is sensitive to fluid composition, whereas the peak positions of the CH 4 band and the upper Fermi diad member for CO 2 are relatively insensitive in the CO 2 -CH 4 system. The magnitude of the shifts in each of the three peak positions (as a function of pressure) is sufficient to be useful as a monitor of fluid pressure. The relative molar proportions in a CO 2 -CH 4 mixture may be determined from the peak areas: the ratio of the peak areas of the CH 4 band and the CO 2 upper Fermi diad member is very sensitive to composition, whereas above about 100 bars, it is insensitive to pressure. Likewise, the peak height ratio is very sensitive to composition but also to fluid pressure. The individual peak widths of CO 2 and CH 4 , as well as the ratios of the widths of the CH 4 peak to the CO 2 peaks are a sensitive function of pressure and, to a lesser extent, composition. Thus, upon determination of fluid composition, the peak width ratios may be used as a monitor of fluid pressure. The application of these spectral parameters to a suite of natural CO 2 -CH 4 inclusions has yielded internally-consistent, quantitative determinations of the fluid composition and density.

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J. C. Seitz, J. D. Pasteris, I.-M. Chou. 1996. Raman spectroscopic characterization of gas mixtures. II. Quantitative composition and pressure determination of the CO2-CH4 system. https://doi.org/10.2475/ajs.296.6.577

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