Raman spectroscopic characterization of CH 4 density over a wide range of temperature and pressure

The Raman spectra of pure N2, CO2, and CH4 were analyzed over the range 10 to 500 bars and from −160°C to 200°C (N2), 22°C to 350°C (CO2), and −100°C to 450°C (CH4). At constant temperature, Raman peak position, including the more intense CO2 peak (ν+), decreases (shifts to lower wave number) with increasing pressure for all three gases over the entire pressure and temperature (PT) range studied. At constant pressure, the peak position for CO2 and CH4 increases (shifts to higher wave number) with increasing temperature over the entire PT range studied. In contrast, N2 first shows an increase in peak position with increasing temperature at constant pressure, followed by a decrease in peak position with increasing temperature. The inflection temperature at which the trend reverses for N2 is located between 0°C and 50°C at pressures above ~50 bars and is pressure dependent. Below ~50 bars, the inflection temperature was observed as low as −120°C. The shifts in Raman peak positions with PT are related to relative density changes, which reflect changes in intermolecular attraction and repulsion. A conceptual model relating the Raman spectral properties of N2, CO2, and CH4 to relative density (volume) changes and attractive and repulsive forces is presented here. Additionally, reduced temperature‐dependent densimeters and barometers are presented for each pure component over the respective PT ranges. The Raman spectral behavior of the pure gases as a function of temperature and pressure is assessed to provide a framework for understanding the behavior of each component in multicomponent N2‐CO2‐CH4 gas systems in a future study.

Section: Resultsmentioning

confidence: 99%

Shift in the Raman symmetric stretching band of N₂, CO₂, and CH₄ as a function of temperature, pressure, and density

Sublett

Sendula

Lamadrid

et al. 2019

“…In the CH 4 ‐bearing fluids, including pure CH 4 , CH 4 ─H 2 O, CH 4 ─N 2 , CH 4 ─C 2 H 6 , and CH 4 ─NaCl─H 2 O systems, the Raman shift (peak position) of the ʋ 1 band of CH 4 is an effective pressure (density) indicator . This is mainly because the peak position of the ʋ 1 band of CH 4 is very sensitive to pressure, and the magnitudes of its pressure‐dependent shifts are sufficiently large enough for pressure determinations.…”

Section: Discussionmentioning

confidence: 99%

Quantitative Raman spectroscopic study of the H₂─CH₄ gaseous system

Fang

Chou

Chen

2018

Self Cite

The Raman spectra of pure H2 and CH4 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 H2─CH4 bearing fluid inclusions. The peak positions of 4 vibrational bands of H2 in pure H2 gas decrease first and then increase after reaching their minima values at about 30 MPa. The wavenumbers of ʋ1 band of CH4 in pure CH4 gas decreases in wavenumber monotonically in the entire investigated pressure range. In H2─CH4 gaseous mixtures, both the peak positions of H2 and CH4 shift to lower wavenumbers with increasing pressure. The peak positions of ʋ1 band of CH4 can be used to determine the pressure of pure CH4 and CH4‐dominated fluids, where the peak position shifts are sufficiently large for precise pressure determination. The peak widths of H2 and CH4 in pure H2 and CH4 gases and H2─CH4 gaseous mixtures broaden as pressure increases. The peak area ratios and the peak height ratios between CH4 and H2 in H2─CH4 binary mixtures are sensitive to composition (i.e., molar ratio between CH4 and H2) but are almost independent of pressure. Thus, the peak area or peak height ratios can be used to determine the composition of H2─CH4 fluids. On the other hand, the peak height ratios of Q1(1) to Q1(3) bands in pure H2 gas or H2‐dominated gaseous mixtures are sensitive to pressure and insensitive to composition. Therefore, in H2‐dominated fluids, the relative peak heights of Q1(1) and Q1(3) of H2 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 H2─CH4‐bearing melt inclusion in quartz from Jiajika granite.

“…Therefore, the derived uncertainties for the calculated pressures tend to be very small. The calibrated Raman shifts for cyclohexane have been applied to evaluate the effect of pressure (or density) and temperature on the Raman shifts of methane ν 1 peak positions …”

Section: Discussionmentioning

confidence: 99%

“…of Lu et al ). Because these differences are not a result due to the small difference in temperatures of these samples, therefore, calibration of each Raman spectroscopic system is needed to minimize the effects of other unknown factors on the measured Raman shifts. Also, it is absolutely necessary to use the same instrument setting and software to determine the position of Raman bands for both the sample and the calibrant.…”

Section: Discussionmentioning

confidence: 99%

“…Raman spectra of these materials were obtained by at least six different labs with both Fourier transform and dispersive Raman spectrometers, and for each of the standards, peak wavenumbers with standard deviations of less than 1 cm −1 were established as an ASTM standard for calibrating the Raman shift axis of Raman spectrometers. However, the quantitative analyses of methane in fluid inclusions (in terms of pressure or density) require accurate CH 4 ν 1 peak wavenumber measurements, with standard deviations much less than 1 cm −1 . In order to establish a reliable Raman shift standard, Raman shifts of cyclohexane were calibrated in this study at room temperature based on the Ne emission lines near the ν 1 peak of CH 4 around 2918 cm −1 .…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Calibration of Raman shifts of cyclohexane for quantitative analyses of methane density in natural and synthetic fluid inclusions

Chou

2015

Self Cite

Raman shifts of cyclohexane were calibrated at room temperature based on two Ne emission lines near the ν1 peak of CH4 around 2918 cm−1. These calibrated Raman shifts of cyclohexane can be used as reference standards for obtaining accurate methane ν1 peak positions and, therefore, the pressures (or densities) of CH4 in natural or synthetic fluid inclusions. Copyright © 2015 John Wiley & Sons, Ltd.