Self-sealing of Pre-Cracked Callovo-Oxfordian Claystone: Implications for Geological Disposal of Nuclear Waste

Self-sealing is a well-known property of mudrocks (clays, claystones and shales). In the context of geological disposal of nuclear waste, understanding the self-sealing mechanism and its efficiency is crucial for the repository safety assessment. This paper presents an experimental study on the pre-cracked Callovo-Oxfordian (COx) claystone to assess the effects of clay swelling and re-compaction on its self-sealing behavior. During experiments, water permeability (Kw) and gas breakthrough pressure (GBP) were measured, respectively, to evaluate the sealing efficiency of water and gas transmissivities. Results demonstrate that, under low lateral stress, Kw significantly decreased due to the clay swelling by water uptake while the GBP values remained low, indicating that residual sealed cracks still impact the water-saturated sample behavior. Apply then confining pressure on the re-sealed samples, led to Kw very close to the intact material one. However, GBP under 14 MPa confinement remains much lower than for intact material. This means that the self-sealing (clay swelling + re-compaction) can significantly reduce the gas and water flow, but is not sufficient to erase the memory of cracks as, despite selfsealing, they still affect the gas transport.