CO2 data on gas and pore water sampled in situ in the Opalinus Clay at the Mont Terri rock laboratory

Since 1996, experimental and modeling approaches allowed understanding of the main mechanisms controlling the Opalinus Clay porewater composition. By 2003, one of the main remaining uncertainties concerned the carbonate system. To reduce this uncertainty, an innovative device for porewater sampling was implemented in 2004 in the Mont Terri Rock Laboratory. An ascending borehole was drilled, using nitrogen and aseptic tools to avoid rock oxidation and microbiological perturbation. The borehole equipment allowed the circulation of gas, initially pure argon, in contact with the rock in a closed circuit and the sampling of formation water produced in the test interval. This experiment allowed to monitor the composition of the circulating gas over several months and to collect water over more than 3 years. The sampled water composition remained stable over time and coherent with the previous understanding. CO2 partial pressures calculated from water analyses are between 1E-2.4 and 1E-1.7 bar. Modeling of the evolution of CO2 concentration in the circulating gas involved advection, diffusion, calcite precipitation and proton buffering surface processes. CO2 partial pressure deduced from water analyses is a few mbars higher than that measured on the gas. Calcium carbonate precipitation and capillary effects could explain this discrepancy.