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.
A new in situ experiment is proposed for observing and understanding well integrity evolution, potentially due to changes that could occur during a well lifetime. The focus is put on temperature and pressure stresses. A small section of a well is reproduced at scale 1:1 in the Opalinus Clay formation, representative of a low permeable caprock formation (in Mont Terri Underground Rock Laboratory, Switzerland). The well-system behavior is characterized over time both by performing hydro-tests to quantify the hydraulic properties of the well and their evolution, and sampling the fluids to monitor the chemical composition and its changes. This paper presents the well integrity assessment under different imposed temperature (17-528C) and pressure (10-28 bar) conditions. The results obtained in this study confirm the ability of the chosen design and observation scale to estimate the evolution of the well integrity over time, the characteristics of the flow along the well-system and the reasons of the observed evolution. In particular, the estimated effective well permeability is higher than cement or caprock intrinsic permeability, which suggest preferential flow pathways at interfaces especially at the very beginning of the experiment; the significant variations of the effective well permeability observed after setting pressure and temperature stresses indicate that operations could influence well integrity in similar proportions than the cementing process.
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