Liquid water uptake in unconfined Callovo Oxfordian clay-rock studied with neutron and X-ray imaging

Abstract

The Callovo Oxfordian clay-rock (COx) is studied in France for the disposal of radioactive waste, because of its extremely low permeability. This host rock is governed by a hydromechanical coupling of high complexity. This paper presents an experimental study into the mechanisms of water uptake in small, unconfined, prismatic specimens of COx, motivated by the comprehension of cracking observed during concrete/COx interface sample preparation. Water uptake is monitored using both x-ray tomography and neutron radiography, the combination of these imaging techniques allowing material deformation and water arrival to be quantified respectively. Given the speed of water entry and crack propagation, relatively fast imaging is required: 5 minute x-ray tomographies and ten-second neutron radiographies are used. In this study, pairs of similar COx samples from the same core are tested separately with each imaging technique. Two different orientations with respect to the core are also investigated. Analysis of the resulting images yields with micro-and macro-scale insights into hydro-mechanical mechanisms to be obtained. This allows the cracking to be interpreted as a rapid breakdown in capillary suction (supposed large both to drying and rebound from in-situ stress state) due to water arrival, which in turn causes a loss of effective stress, allowing cracks to propagate with ease, which in turn deliver water further into the material.

Dynamics of Water Absorption in Callovo-Oxfordian Claystone Revealed With Multimodal X-Ray and Neutron Tomography

“…Analysis of Figure 6 immediately reveals that there is indeed swelling occurring on average. The extent of swelling is significantly less than what was expected from the previous study (Stavropoulou et al, 2019)-which is likely explained by a higher initial water content in the sample studied here, together with its different shape, size, and a potential confinement applied by the membrane.…”

“…X-ray and neutron tomographies were acquired simultaneously on NeXT-Grenoble , which is equipped with an almost-parallel cold neutron beam and now a micro-focus x-ray setup with a flat panel detector. The addition of the x-ray setup is a significant improvement from our preliminary study Stavropoulou et al (2019) where twin samples were required for separate scanning with the two different modalities (neutrons and x-rays).…”

“…Combined x-ray and neutron tomographies on shales Chiang et al (2018) indicate that the combination of techniques allows a better identification of the composition of these complex materials. Stavropoulou et al (2019) combined x-ray tomography and neutron radiography to study wate imbibition in COx (albeit on twin samples) linking the cracking and propagation to the water penetration. Even though interesting trends have been revealed, this study is limited by the use of similar but different samples, meaning that the characterisation of a mechanism linking crack opening and water penetration is difficult to quantify locally.…”

“…Even though interesting trends have been revealed, this study is limited by the use of similar but different samples, meaning that the characterisation of a mechanism linking crack opening and water penetration is difficult to quantify locally. This paper presents a significant technical evolution of Stavropoulou et al (2019) with the analysis of a wetting test on Callovo Oxfordian claystone using a simultaneous x-ray and neutron tomography on the same sample.…”

Implementation of Dynamic Neutron Radiography and Integrated X-Ray and Neutron Tomography in Porous Carbonate Reservoir Rocks

“…Neutrons with their peculiar properties (such as ability to penetrate metals) and sensitivity to hydrogen (Schillinger et al, 2000), offer a valuable tool for such studies related to flow problems like capillarity-driven (e.g., Cnudde et al, 2008) or pressure-driven flow (e.g., Yehya et al, 2018). Several studies have used Neutron radiation images to characterize the porosity, moisture, and water absorption in different materials such as concrete (De Beer et al, 2005;Kanematsu et al, 2009;Zhang et al, 2010Zhang et al, , 2011Yehya et al, 2018), steel (Zawisky et al, 2010), building stones (Hameed et al, 2006(Hameed et al, , 2009Cnudde et al, 2008;Zawisky et al, 2010;Dewanckele et al, 2014), porous asphalt (Lal et al, 2014), sandstones (De Beer and Middleton, 2006;Hall, 2013), clay-rock (Stavropoulou et al, 2019), dehydration of molding sand (Schillinger et al, 2011), and cultural heritage artifacts (Fedrigo et al, 2018;Schillinger et al, 2018).…”