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

Overview & Synopsis

Gas monitoring is a prerequisite to understand the exchange, diffusion, and migration processes of natural gases within underground environments, which is 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 a monitoring purpose after CO2 injection. The aim was to develop a real-time chemical monitoring device to follow gas concentrations in water by measurements in water inside the borehole but also at 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 laboratory was carried out. 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, 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 of ± 0.003 mol.kg-1 and ± 0.05 bar, respectively.