In situ gas monitoring in clay rocks: mathematical developments for CO2 and CH4 partial pressure determination under non-controlled pressure conditions using FT-IRspectrometry

Abstract

The quantification of dissolved gases in water at equilibrium with rock assemblages is very important for modelling the long-term behaviour of radioactive wastes and other materials in a deep geological disposal. This paper presents the mathematical development allowing to quantify the partial pressures of evolved gases from the rock that were collected from a borehole and transferred to a gas cell coupled to a low resolution FT-IR spectrometer. The areas of the different ν₃ bands of CO₂ between 2400 and 2220 cm⁻¹ and the CH₄ Q-branch between 3022 and 3002 cm⁻¹ have been calculated using a peak integration method. It has been shown that the equation between the integrated area and the partial pressure for CH₄ and CO₂ for a spectral resolution of 1 cm⁻¹ does not follow Beer–Lambert law and is dependent on the bulk pressure. A non-linear continuous calibration model has been developed for a range of bulk pressures from 0.9 to 1.4 bar, giving, for each gas, a relation between partial pressure and both integrated area and bulk pressure. Explored partial pressure ranges are between 0.3 and 4 mbar for CO₂, and 0.3 and 12 mbar for CH₄ with an equivalent optical path of 1 m. The relative root mean squared error of the prediction of the models is included between 1 and 3% in the explored pressure ranges for each gas.