Evidence for gas-induced pathways in clay using a nanoparticle injection technique

Abstract

Corrosion, water radiolysis and microbial degradation will result in the generation of gas within repositories designed for the geological disposal of high-level radioactive waste. It is therefore crucial in the design of such facilities that the relevant mechanisms allowing gas migration through repository materials, both engineered barriers and clay-based candidate host rocks, are correctly identified. In Belgium, the Boom Clay represents a candidate host material for which the advective gas breakthrough characteristics and transport properties have been extensively tested and are well defined by numerous studies. The Boom Clay displays a significant capacity for self-sealing and both laboratory and field tests indicate that advective gas transport occurs not by visco-capillary flow, but instead through the formation of pressure-induced dilatant pathways. In this study, we present results from a gas injection test designed to demonstrate the presence of these features by injecting nanoparticulate tracers with helium gas into a sample of Boom Clay. The results provide conclusive evidence for the formation of transient, dilatant gas pathways within a candidate clay-based host rock. This technique provides a novel diagnostic tool for the identification of processes governing multi-phase flow, supporting robust long-term assessments of repository performance.

Impact of the electron donor on in situ microbial nitrate reduction in Opalinus Clay: results from the Mont Terri rock laboratory (Switzerland)

“…Continuous denitrification could lead to the formation of a separate gas phase when the concentration of produced N gases would exceed the solubility limit of the gases. This might cause fissuring of the host rock and might thus result in the formation of preferential pathways for radionuclide migration (Mallants et al 2007;Harrington et al 2012).…”

Gas transport properties through intact and fractured Callovo-Oxfordian mudstones

“…Similar observations were reported by Harrington et al (2009Harrington et al ( , 2012aHarrington et al ( , 2014, who measured changes in the volume of the confining system during gas flow. Additional evidence for dilatant gas is provided by Harrington et al (2012b), in which gold and titanium oxide nanoparticles were injected into an unlithified clay sample. Post-test scanning electron microscopy analysis found clay draped around aggregates of gold particles, the latter only able to enter the clay if dilation of the fabric had occurred during gas flow.…”

Gas Transfer Through Clay Barriers

“…Similarly, images showing the traces of previous 'dilatant' features within a sample of Boom clay were presented by Harrington et al (2012b) in which a mixture of gold and titanium oxide nano powders were injected (within a carrier gas) through the material. Post-test scanning electron microscopy imaging showed layers of clay material draped around aggregates of nano powder as well as a line of aggregates left along the trace of a sealed fracture.…”