Regular Article

Quantitative monitoring of dissolved gases in a flooded borehole: calibration of the analytical tools

¹ Université de Lorraine, CNRS, GeoRessources laboratory, F-54506 Vandœuvre-lès-Nancy, France
² University Grenoble Alpes, CEA, LETI, Grenoble, France
³ Solexperts France, 10 allée de la Forêt de la Reine, 54500 Vandœuvre-lès-Nancy, France

^* Corresponding author: marie-camille.caumon@univ-lorraine.fr

Received: 16 December 2022
Accepted: 11 July 2023

Abstract

Gas monitoring is a prerequisite to understanding the exchange, diffusion, and migration processes of natural gases within underground environments, which are involved in several applications such as geological sequestration of CO₂. In this study, three different techniques (micro-GC, infrared, and Raman spectroscopies) were deployed on an experimental flooded borehole for monitoring purposes after CO₂ injection. The aim was to develop a real-time chemical monitoring device to follow dissolved gas concentrations by measurements in water inside the borehole but also at the surface through a gas collection system in equilibrium with the borehole water. However, all three techniques must be calibrated to provide the most accurate quantitative data. For this, a first step of calibration in the laboratory was carried out. A new calibrations were required to determine partial pressure and/or concentrations of gases in water or in the gas collection system. For gas phase analysis, micro-GC, FTIR spectroscopy, and Raman spectroscopy were compared. New calibration of the micro-GC was done for CO₂, CH₄, and N₂ with uncertainty from ±100 ppm to 1.5 mol% depending on the bulk concentration and the type of gas. The FTIR and Raman spectrometers were previously calibrated for CO₂, and CO₂, N₂, O₂, CH₄, and H₂O, respectively with an accuracy of 1–6% depending on concentration scale, gas and spectrometer. Dissolved CO₂ in water was measured using a Raman spectrometer equipped with an immersion probe. The uncertainty on the predicted dissolved CO₂ concentration and partial pressure was ±0.003 mol·kg⁻¹ and ±0.05 bar, respectively.