On the Thermo-Hydro-Mechanical Behaviour of a Sheared Callovo-Oxfordian Claystone Sample with Respect to the EDZ Behaviour

Abstract

International audienceTo better understand the impact of temperature elevation on the response of the excavation damaged zone around repository cells and galleries for radioactive waste disposal, the combined effects of shear and temperature elevation were investigated in the laboratory on the Cal-lovo-Oxfordian claystone. To do so, a hollow cylinder thermal triaxial cell with short drainage path specifically developed for low permeability rocks was used. Once properly saturated under stress conditions close to in situ, the specimen was sheared along a constant effective mean stress path mimicking the stress path followed during gallery excavation. The shear stress was afterwards released and an undrained heating test was performed on the sheared specimen. It was observed that the temperature increase under undrained conditions led to a thermal increase in pore water pressure resulting in a decrease in mean effective stress that brought back the sheared specimen to failure, evidencing a thermally induced failure. Steady state radial permeability tests performed at various stages of the test demonstrated that the overall permeability of the sheared specimen was comparable to that before shearing, confirming the excellent self-sealing properties of the Callovo-Oxfordian claystone. This shows that, in spite of being possibly remobilised by temperature elevation, the EDZ will keep an overall permeability constant equal to that of the massive rock, keeping the same isolation properties

High‐stress triaxial direct‐shear fracturing of Utica shale and in situ X‐ray microtomography with permeability measurement

“…Substantially fewer studies have used shale, which can exhibit very low permeability and high ductility behavior due to clay content [e.g., Gutierrez et al ., ]. These prefractured specimen experiments contrast with the more limited studies of permeability through fractures that are formed at under high stress conditions, which have included true‐triaxially confined granite [ Frash et al ., , ] and triaxially confined shale [ Carey et al ., , ; Menaceur et al ., ; Nygard et al ., ]. Laboratory study of permeability through fractures formed in rock at in situ stresses can be expected to better represent fracture properties in the subsurface than similar studies using prefractured or artificial specimens.…”

Drained Triaxial Tests in Low-Permeability Shales: Application to the Callovo-Oxfordian Claystone

“…The satisfactory results obtained with these calculations at a rate of 6.6×10 -8 s -1 are comparable with the values of strain rate adopted in claystones and shales by Wu et al (1997) and Islam and Skalle (2013), equal to 3×10 -8 s -1 and 2×10 -8 s -1 , respectively. They are around two orders of magnitude smaller than those used by Zhang and Rothfuchs (2004) and Masri et al (2014) (6.5×10 -6 s -1 and 1.0×10 -6 s -1 , respectively) and approximately correspond to half the values adopted by Hu et al (2015) and Menaceur et al (2016a) on devices with smaller drainage lengths (1.0×10 -7 s -1 and 0.9×10 -7 s -1 with a 10 mm drainage length) .…”

“…This small drainage length, four times smaller than standard triaxial drainage length (38 mm), reduces the pore pressure diffusion rate by a factor of 16 compared to standard triaxial specimens (Monfared et al, 2011). As shown by Monfared et al (2011), Mohajerani et al (2013), Menaceur et al (2015) and Menaceur et al (2016a) with hollow cylinder specimen and by Hu et al (2014) with small-sized specimen, the 10 mm drainage length made it possible to achieve satisfactory saturation within 3 to 4 days, compared to a period of around one month in standard triaxial tests. Reasonable results of fully saturated and drained isotropic compression tests have also been obtained for the COx claystone with a 10 mm high specimen drained at the bottom, also resulting in a 10 mm drainage length (Mohajerani et al 2012;Belmokhtar et al 2017aBelmokhtar et al , 2017b.…”

Active porosity in swelling shales: insight from the Callovo-Oxfordian claystone

“…The Callovo-Oxfordian claystone is a sedimentary rock deposited 155 million years ago over a layer of Dogger limestone in a 150 m thick layer covered by a layer of Oxfordian 5 limestone. The COx claystone has a very low permeability in the order of 10 -20 m 2 (see Escoffier 2005, Homand et al 2004, Escoffier et al 2005, Mohajerani et al 2011, Menaceur et al 2015a), a high adsorption capacity for radionuclides and a low diffusion coefficient delaying solute transfer. For all these reasons, it has been considered as a potential host rock to store high activity radioactive waste at great depth in France.…”

“…Mohajerani et al 2011) and excellent self-sealing capacity of the COx claystone (e.g. Davy et al 2007;Zhang and Rothfuchs 2008;Menaceur et al 2015a).…”