Geochemical experiments in boreholes in the Andra’s Meuse/Haute-Marne Underground Research Laboratory (URL) made it possible to extract pore-water over years from the Callovian-Oxfordian argillaceous rock and to characterize its chemical and isotopic composition.
This pore-water contains dissolved gases. Obtaining representative data about their natural content in the rock is not straightforward because sampling operations are particularly disturbing for gases in low permeability clay-rich rocks like the Callovian-Oxfordian one. Some of the gases (CO2, CH4, noble gases...) were already analyzed in deep boreholes core samples (Bigler et al. 2005, Girard et al. 2005, Prinzhofer et al. 2009, Lerouge et al. 2015). In the URL, dedicated borehole equipment allowed the circulation of gas, initially pure argon, in contact with the rock in a closed circuit, at a pressure of 1 to 1.5 bar. Online infrared spectrometers (Cailteau et al. 2011) and gas sample analyses helped monitoring the circulating gas composition over months to years. We observed N2, CO2, CH4 and other light alkane content evolution in the circulating gas.
One dimension, radial diffusion modelling gave insights on the natural content of these gases in the rock pore-water and on their transfer properties (diffusion and retention) in the rock. Furthermore, isotopic measurements helped discussing the gas origins.
Results showed that N2 is the most abundant dissolved gas (≈ 3 mmol/L) in the Callovian-Oxfordian pore-water in the URL. All together, estimated values for the dissolved gases content lead to a total gas pressure value at equilibrium with pristine Callovian-Oxfordian pore-water larger than 5 bar at the location of the experiments. A consequence of this result is that a gas phase will form in the water-saturated porosity of the rock surrounding the drifts and other openings, wherever the pore pressure becomes smaller than this value.
Reference
Bigler, T., Ihly, B., Lehmann, B.E., Waber, H.N. (2005) Helium production and transport in the low-permeability Callovo-Oxfordian shale at the Site Meuse/Haute Marne, France. Nagra Arbeitsbericht NAB 05-07, Nagra, Wettingen, Switzerland.
Cailteau, C., Pironon, J., de Donato, P., Vinsot, A., Fierz, T., Garnier, C., Barres, O. (2011) FT-IR metrology aspects for on-line monitoring of CO2 and CH4 in underground laboratory conditions. Analytical Methods 3, 877-887.
Girard, J.-P., Fléhoc, C., Gaucher, E. (2005) Stable isotope composition of CO2 outgassed from cores of argillites: a simple method to constrain δ18O of porewater and δ13C of dissolved carbon in mudrocks. Appl. Geochem. 20, 713–725.
Lerouge, C., Blessing, M., Flehoc, C., Gaucher, E.C., Henry, B., Lassin, A., Marty, N., Matray, J.M., Proust, E., Rufer, D., Tremosa, J. and Vinsot, A. (2015) Dissolved CO2 and Alkane Gas in Clay Formations. Procedia Earth and Planetary Science 13, 88-91.
Prinzhofer, A., Girard, J.P., Buschaert, S., Huiban, Y., Noirez, S. (2009) Chemical and isotopic characterization of hydrocarbon gas traces in porewater of very low permeability rocks: The example of the Callovo-Oxfordian argillites of the eastern part of the Paris Basin. Chemical Geology 260, 269-277.