EDZ permeability monitoring through barometric pumping : two case studies, in the Opalinus Clay (Mont Terri URL, Swiss Jura) and in gneissic rock (Roselend URL, French Alps)
Résumé
Free oscillations of the atmosphere induce variations of the barometric pressure (Lamb, 1911). The barometric signal includes i) short period variations – diurnal and semi-diurnal variations – due to daily temperature variations of the atmosphere and tides, ii) medium period due to weather such as cyclonic and anticyclonic phenomenon, iii) and also long period related to seasonal cooling or heating of the atmosphere. Atmospheric or barometric pumping is a piston-like process, cyclic and nearly reversible, responsible of gas motion between subsurface permeable rock mass and the atmosphere (e.g. Nilson et al., 1991). A decrease of atmospheric pressure induces gas carrying out of the permeable rock; an increase of atmospheric pressure pushes the air into the rock subsurface. Barometric pumping is classically described as a low-cost and passive method used in the extraction of contaminated soil gas in the vadose zone (e.g., Olson et al., 2001 ; Rossabi and Falta, 2002). Barometric signal propagation into the subsurface rocks constitutes also a simple technique to characterize gas permeability of subsurface medium (Nilson et al., 1991 ; Alhers et al., 1999 ; Wassermann et al., 2011 ; Guillon et al., 2013). Indeed the gas diffusivity (a function of the ratio of air-permeability and air-filled porosity) describes the attenuation and time-lag that characterized the propagated barometric signal into the subsurface. A long-term measurement of barometric signals in obturated boreholes and at the surface could constitute, through the deduced time-lag and attenuation, a monitoring of potential gas permeability variations. In the context of nuclear waste geological disposal, the permeability of the Excavation Damaged Zone (EDZ) is one of the most important parameter that will influence the safety of the confinement of the radio-nuclides by the host rocks. In order to test the reliability of the method in monitoring EDZ permeability variations, we look at pressure monitoring performed during several months in boreholes into two different damaged rocks. Firstly, into an EDZ surrounding a tunnel in a crystalline rock mass (Roselend Underground Rock Laboratory, Roselend URL). Secondly, into the EDZ surrounding gallery excavated in Opalinus Clay of the Mont Terri URL (Swiss Jura). The Roselend URL occupies a subhorizontal dead-end tunnel drilled by blasting within the Méraillet crystalline massif (French Alps). The Roselend URL tunnel is located near the west bank of the Roselend artificial lake. This nearly straigth excavation about 128 m long crosses fractured micaschists and gneisses successively. The natural fracture pattern is related to the Alpine orogeny and presents an heterogeneous metric fracture density along the tunnel. Water content in the massif and flow rates of percolating waters depend on seasonal precipitation (drought, rain and snow alternating periods) characterizing the mountainous Roselend environment. The Mont Terri URL is a 300 m underground laboratory consisting of 600 m of excavated galleries along the security gallery of the Mont Terri motorway tunnel within a clay rich formation, the Opalinus Clay. Which is a proposed host rock for future deep geological disposal of radioactive waste. Cyclic seasonal variations of fractures and cracks openings were observed by Möri et al. (2010) linked to environmental conditions (temperature and humidity) in the URL galleries. Within few year, self-sealing and healing processes have been observed when the Opalinus clay is in contact with wet air (e.g. Bossart, 2004) ; inducing EDZ permeability decrease. The pressure monitoring system was designed to follow the response of the different EDZ to barometric pumping. It consists in a borehole obturated by a simple mechanical packer and absolute barometric transducers that measured the pressure into the borehole and in the tunnel. The temperature was also measured into the obturated borehole and in the tunnel with high resolution autonomous probes. Hygrometry was also monitored both into boreholes and gallery. In Roselend URL, this pressure monitoring system was placed into four boreholes during several months in 2009. In Mont Terri URL site, the pressures has been measured since winter 2011. The estimations of potential time-lag and attenuation variations from pressure signals – related to gas permeability variations – are performed through signal analysis using Lissajou curves analysis, and simulated annealing algorithm. Modeling of these time-lag and attenuation are also performed with different external sollicitation conditions such as groundwater table level fluctuation in the case of Roselend URL, or moisture content.