FT-IR metrology aspects for on-line monitoring of CO2 and CH4 in underground laboratory conditions

Abstract

The French National Radioactive Waste Management Agency (ANDRA) has recently developed a new experimental set-up which allows sampling water from marl rock formations, together with an in situ characterisation of the composition and migration mechanisms of the gases dissolved in the marl porewater. Gases and liquids are collected from vertical borehole drillings in underground laboratories. The analytical design, Fourier transformed infrared spectroscopy based, allows powerful and long term on-line monitoring of gases released by low-permeability media. The IR system is designed to cope with the unfavourable measurement conditions occurring in an experimental underground laboratory (moisture, dust, etc.). Because the working conditions in such an underground laboratory make complete purging of the IR spectrometer difficult, the IR spectra of geological gases are often perturbed by contributions from atmospheric CO 2 and water vapour. The metrology aspect is based on an IR low resolution sensor equipped with two measurement compartments. In the internal compartment linked to the borehole layout, gases are monitored on-line through a cell with a variable optical path, whereas in the external compartment, atmospheric CO 2 is measured through a short open path configuration. The experimental method and data processing procedure used to determine the real partial pressure of CO 2 arising from the marl rock formation are described in this paper. Results of the on-line gas (CO 2 and CH 4 ) monitoring conducted in the Mont Terri underground laboratory are presented and compared with punctual gas chromatography analyses.

In situ diffusion test of hydrogen gas in the Opalinus Clay

“…All of these gases were originally dissolved in the rock porewater. Indeed, their occurrence in Opalinus Clay porewater has been described (Pearson et al 2003;Vinsot et al 2008a;Cailteau et al 2011). The surface area of the rock supplying these gases in the test interval was c. 1.2 m 2 .…”

Fourier Transform Infrared Absorption Spectroscopy for Quantitative Analysis of Gas Mixtures for Homeland Security Applications

“…Therefore, delivery of analyte concentrations at AEGL + 5 % was targeted. Under these experimental constraints, there is approximately 95 % or greater Quantitative FT-IR Spectroscopy of TIC Test Gases [7] confidence that the true test concentration will be between the relevant AEGL value and a concentration 10 % greater than that AEGL value. Of those values that do not fall within the 95 % confidence interval, the probability that the true test concentration will be below the interval rather than above is 50 %, provided that all uncertainties are normally distributed [13].…”

“…This criterion is met in the present referee method, which makes an absorption measurement of the gas using a commercial Fourier transform infrared (FT-IR) absorption spectrometer. FT-IR spectroscopy has been utilized previously for quantitative analysis of gasphase components, using a variety of approaches (see, for example, [3][4][5][6][7]). The method that we demonstrate relies upon the use of primary reference spectra from available spectral libraries, thus providing quantitative concentration measurements with defined levels of uncertainty.…”

Three decades of underground research laboratories: what have we learned?

“…In the Pore-water Chemistry (PC and PC-C) experiments at Mont Terri (Vinsot et al 2008a;Wersin et al 2011) and the Prélèvement et Analyse Chimique (Sampling and Chemical Analysis, PAC) experiments at CMHM (Appelo et al 2008;Vinsot et al 2008b) the boreholes were designed following the recommendations of the Geochemical Modelling Group (Pearson et al 2003). Moreover, naturally dissolved gases were characterized by circulating initially pure argon in some of the boreholes and monitoring the change in the gas by infrared spectrometry (Cailteau et al 2011) and by sampling.…”