Micro FT-IR spectroscopic measurements of CH4: Experimental calibration with high-pressure and high temperature optical cell (HPOC)

Yahao Huang; Chongyang Wu; Xiaowan Tao; Kuan Lu; Wenjing Wang; Xiuyan Liu

doi:10.1016/j.saa.2023.123703

Micro FT-IR spectroscopic measurements of CH₄: Experimental calibration with high-pressure and high temperature optical cell (HPOC)

Spectrochim Acta A Mol Biomol Spectrosc. 2024 Mar 5:308:123703. doi: 10.1016/j.saa.2023.123703. Epub 2023 Nov 30.

Authors

Yahao Huang¹, Chongyang Wu², Xiaowan Tao³, Kuan Lu¹, Wenjing Wang⁴, Xiuyan Liu⁴

Affiliations

¹ Key Lab of Exploration Technologies for Oil and Gas Resources of Ministry of Education, Yangtze University, Wuhan, 430100, China.
² Research Institute of Petroleum Exploration and Development, SINOPEC, Beijing 102206, China.
³ Research Institute of Petroleum Exploration & Development, PetroChina, Beijing 100083, China. Electronic address: taoxiaowan@petrochina.com.cn.
⁴ Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geoscience (Wuhan), Wuhan, Hubei 430074, China.

PMID: 38061105
DOI: 10.1016/j.saa.2023.123703

Abstract

Fourier transform infrared spectroscopy (FTIR) is a prevalent nondestructive in situ analytical technique widely employed for the qualitative characterization of natural fluid inclusions. Presently, the quantitative determination of the temperature, pressure, and composition of natural inclusions stands as a pivotal challenge in the geological application of laser spectroscopy. Through the integration of the capillary high-pressure optical cell (HPOC) technique and Fourier transform infrared spectroscopy, infrared quantitative models for the C-H symmetric stretching band (v₃) in the vapor phase of CH₄ under varying temperature (40-200 °C) and pressure (20-500 bar) conditions are established for the first time. This accomplishment is achieved through fitting and calculation of infrared spectra. A linear quantitative relationship and the dynamic evolution of infrared spectral parameters (peak area, full width at half maximum, peak height ratio, and peak shifts) concerning temperature, pressure, and density are meticulously investigated. The findings reveal that the peak area and full width at half maximum exhibit an upward trend with increasing pressure and density, and are inversely proportional to temperature. The height ratio (P band/Q band, R band/Q band) experiences an increase as density escalates. Moreover, the CH₄v₃ band position shows a higher wavenumber shift with increasing temperature, while its position correlates negatively with pressure and density. Thus, this method demonstrates its applicability in deducing the PVT-x properties of CH₄-bearing systems, including CH₄-rich fluid inclusions, CH₄ content within hydrocarbon inclusions, and experimental fluid investigations. In contrast to the Raman quantitative approach, the infrared quantitative formula can be universally adopted in any infrared laboratory owing to the quantitative theory (Beer-Lambert law) governing infrared absorption spectra. By collecting Raman spectra and infrared spectra of pure methane fluid inclusions from the central region of the Alps and subsequently calculating the results using two distinct quantitative methods for fluid inclusions, we have demonstrated the feasibility of applying infrared spectroscopy in natural fluid inclusion studies.

Keywords: Fluid inclusion; Fourier infrared spectroscopy; PVT-x; Quantitation.