In situ sampling and characterization of Callovo-Oxfordian pore water

摘要

The geochemical composition of pore water is one of the elements required for characterizing the confining properties of the Callovo-Oxfordian argillaceous formation. For this purpose, an innovative device for sampling of the pore water has been implemented in the Andra URL. A vertical ascending borehole was drydrilled, using N2 gas only and aseptic tools to avoid rock oxidation and microbiological perturbation. Equipment with a 5 meter long test interval at its far end was installed, and an Ar gas circulation was established. After a few weeks, due to the hydraulic gradient between the test interval and the rock formation, the water flows freely at a rate of 0.5 to 1.3 liters per month in the borehole and it is sampled. Collected water samples show a very low level of chemical perturbation. The chemical composition of this water is close to a theoretical composition deduced from core analyses and thermodynamic modelling.