Experimental evidence for self-sealing of fractures in claystone

► Fracture aperture decreases exponentially with increasing normal confining stress. ► The resulting decrease in permeability is related to the aperture by the cubic law. ► Water flow leads to a drastic decrease inpermeability compared with gas flow. ► Thermal loading has no remarkable impact on the permeability of fractured clay. ► Fractures in clay under the repository conditions can be fully re-sealed. Various laboratory sealing experiments have been carried out on strongly cracked samples of different sizes from the Callovo-Oxfordian argillite and the Opalinus clay under relevant repository conditions. The fractured samples were compacted and flowed through with gas or synthetic pore water under confining stresses up to 15 MPa and elevated temperatures from 20 °C to 90 °C. The sealing behavior of the fractures was detected by measurements of deformation, permeability, and wave velocity. Under the applied thermo-hydro-mechanical conditions, a significant sealing process of the fractures was observed with fracture closure and permeability decrease to the very low levels of 10 −19–10 −21 m 2 within the time period of months to years. All test results provide strong evidence for the high self-sealing capacity of the studied claystones.