Concentration loss and diffusive fractionation of methane during storage: Implications for gas sampling and isotopic analysis

Concentration loss and diffusive fractionation of methane during storage: Implications for gas sampling and isotopic analysis

Methane

Diffusion

Coal deposits

Gases

Bottles

Isotopes

Coal bed methane

Genes

Carbon isotope composition (13C) has been used as a standard geochemical tool to explore the genetic and post-genetic information of natural gas systems. However, little attention has been paid to the influence of the diffusion loss on the real 13C value of gas samples. In this study, diffusion experiments were conducted to explore the variations in concentration and isotopic composition during methane storage. The results showed that the gas concentration reduced by 82.7%, and 13C of methane increased by 2.0 after one month of storage in 240 mL bottles, far from the main viewpoint that diffusion loss during storage is negligible. Comparative experiments revealed that the diffusive fractionation during storage is mainly controlled by the collection volume, cross-sectional area, and sealing performance. One month of storage requires 10 mL of methane to be collected for isotope testing. A glass bottle with a wide-body, narrow-neck, and a good sealing cap should always be preferred when selecting a gas container. Furthermore, a quantitative model was established based on Fick's second law of diffusion for characterizing the diffusive fractionation during storage. The diffusion coefficient calculated by the presented model ranged from 6.5 109 to 6.0 1011 m2/s, with an average value of 6.0 1010 m2/s. The diffusion coefficient ratio of 13CH4 to 12CH4 ranged from 0.990 to 1 and decreased with increasing storage time. This study may provide a behavioral basis for gas sampling and storage in fieldwork and laboratory and a reasonable correction to reduce additional diffusive fractionation on isotopic analysis. 2022 Elsevier B.V.

101

Journal of Natural Gas Science and Engineering

2022

Li, Wenbiao

Lu, Shuangfang

Li, Junqian

Feng, Wenjun

Zhang, Pengfei

Wang, Jun

Wang, Ziyi

Li, Xiao

journalArticle

18755100

10.1016/j.jngse.2022.104562