Self-Sealing of Boom Clay After Gas Transport

In the geological disposal of high-level radioactive waste in argillaceous rocks, studying the barrier integrity after gas transport and the pathway closure thanks to self-sealing capacity is a crucial aspect for the safety assessment. This paper presents experimental research in Boom Clay (a potential host rock in Belgium) to evaluate the effectiveness of self-sealing and possible fissure reactivation during a second gas invasion event. Initial water permeability under oedometer conditions was first measured on samples at two bedding orientations, being higher the sample with bedding planes parallel to flow, highlighting marked anisotropy. Then, gas injection tests at a constant volume rate were performed. Results indicated that Boom Clay underwent expansion and degradation during gas injection due to the development of fissures that were quantified using microstructural techniques. The computed effective gas permeability was not significantly dependent on bedding orientation and was slightly larger than the initial intrinsic water permeability. The re-saturation of the samples led to a recovery of the initial water permeability for both orientations, replicating the original anisotropy. The microstructural analyses confirmed the gas pathways’ closure, indicating good self-sealing and the regaining of the hydraulic barrier function. However, a small volume of large unconnected pores was detected on undrained unloading before the microstructural study. An additional gas injection after the self-sealing resulted in a higher effective gas permeability and a larger increase in pore volume, suggesting the reopening of fissures generated during the first injection. Finally, the experimental data were compiled within a multi-scale phenomenological model to relate the microstructural information to macroscopic flow transport properties capturing the intrinsic permeability increase on gas invasion and its recovery during self-sealing.