Multicomponent Mass Transfer in Dissolved Gas Analysis: The Impacts of Headspace Pressurization on Reliable Measurement

Measuring dissolved gas concentrations such as methane (CH₄), carbon dioxide (CO₂), and hydrogen (H₂) (e.g., groundwater or surface water samples) is important for ensuring safe and effective subsurface energy development and storage. A common method is to collect water samples in fixed-volume sealed vessels and then use static headspace equilibrium techniques to quantify the dissolved gas concentrations by gas chromatography. Previously, the presence of multiple gas components was not considered during the analysis of water samples but is necessary. A mass balance approach considering multicomponent mass transfer was developed and validated, and the impact on dissolved gas measurements was quantified. It was found that mass transfer occurring in fixed-volume vessels leads to pressurization of sample headspace during analysis, causing error. Higher solubility gases (e.g., CO₂) exhibit higher headspace pressurization and larger errors than lower solubility gases (e.g., CH₄). In addition, it was found that the volume of the headspace induced and the co-occurrence of multiple dissolved gas species in a sample can exacerbate headspace pressurization and error. Overall, caution must be taken when using static headspace equilibrium techniques; if multicomponent mass transfer is not considered, error and potential under reporting of dissolved concentrations is possible.