Quantitative Raman spectroscopic study of the H₂─CH₄ gaseous system

Abstract

Abstract The Raman spectra of pure H 2 and CH 4 gases and their 5 gaseous mixtures were collected at pressures from 1 to 40 MPa at ambient temperature. They were systematically analyzed in order to establish methodology for quantitative determination of composition and pressure of H 2 ─CH 4 bearing fluid inclusions. The peak positions of 4 vibrational bands of H 2 in pure H 2 gas decrease first and then increase after reaching their minima values at about 30 MPa. The wavenumbers of ʋ 1 band of CH 4 in pure CH 4 gas decreases in wavenumber monotonically in the entire investigated pressure range. In H 2 ─CH 4 gaseous mixtures, both the peak positions of H 2 and CH 4 shift to lower wavenumbers with increasing pressure. The peak positions of ʋ 1 band of CH 4 can be used to determine the pressure of pure CH 4 and CH 4 ‐dominated fluids, where the peak position shifts are sufficiently large for precise pressure determination. The peak widths of H 2 and CH 4 in pure H 2 and CH 4 gases and H 2 ─CH 4 gaseous mixtures broaden as pressure increases. The peak area ratios and the peak height ratios between CH 4 and H 2 in H 2 ─CH 4 binary mixtures are sensitive to composition (i.e., molar ratio between CH 4 and H 2 ) but are almost independent of pressure. Thus, the peak area or peak height ratios can be used to determine the composition of H 2 ─CH 4 fluids. On the other hand, the peak height ratios of Q 1 (1) to Q 1 (3) bands in pure H 2 gas or H 2 ‐dominated gaseous mixtures are sensitive to pressure and insensitive to composition. Therefore, in H 2 ‐dominated fluids, the relative peak heights of Q 1 (1) and Q 1 (3) of H 2 is a better alternative for pressure determination. Based on our established Raman quantitative analysis method, we determined the composition and internal pressure of a natural H 2 ─CH 4 ‐bearing melt inclusion in quartz from Jiajika granite.