Self-sealing tests with water injection were performed on artificially fractured core samples of the Callovo-Oxfordian claystone using an X-ray transparent triaxial cell. 3D X-ray scans and permeability measurements were performed continuously to assess the evolution of fracture volume and permeability. The impact of sample orientation (parallel and perpendicular to the bedding plane), calcite content and gas injection was analyzed. It resulted that the higher the calcite content, the less effective the self-sealing process. No significant influence of the sample orientation on the self-sealing process was identified. Self-sealing is generally fast at the beginning of the test and then stabilizes after one month. The initial permeability of the healthy claystone is partially restored, which is a promising result concerning the restoration of the initial sealing properties of the host rock. Injection of an inert gas has a delay effect on the self-sealing process due to the crack desaturation.
Excavation of underground galleries can result in the formation of an Excavation Damaged Zone (EDZ), which is a network of cracks in the surrounding rock. These flaws desaturate the host rock and can cause a deterioration of its mechanical and hydraulic (i.e., low permeability) properties. In the case of Underground Research Laboratory (URL) of the National Agency of Radioactive waste Management (ANDRA) located in Meuse/Haute-Marne (Bure, France) and excavated in the Callovo-Oxfordian (COx) claystone, the structures will be resaturated and during this stage the cracks in the EDZ can self-seal, resulting in a decrease in water permeability and a partial restoration of the rock's mechanical properties. In the context of radioactive waste disposal, knowing this long-term self-sealing phenomenon is of great importance.
Many scholars have investigated the occurrence of self-sealing fractures in claystone caused by water percolation. Self-sealing is induced by a rearrangement of minerals and pores inside the fracture region, according to Auvray et al. (2015). Hence, these structural changes are expected to ameliorate mechanical and transfer properties of the fractured zone. Bastiaens et al. (2007) defined self-sealing as the lowering of permeability in the EDZ by any hydro-mechanical-bio-chemical mechanism. Furthermore, Bastiaens et al. (2007) and Van Geet et al. (2008) investigated in-situ studies on Opalinus clay and Boom clay presenting their capabilities to self-seal rather than self-restore. De La Vaissière et al. (2015) have shown the partial restoration of the permeability of the COx claystone during in-situ resaturation experiments. Over a year, the hydraulic conductivity in the boreholes reduced by up to four orders of magnitude, nearing the value of healthy claystone, demonstrating the COx claystone's potential to self-seal.
