Three small-scale mock-up tests were carried out on artificially fractured Opalinus Clay samples from the lower sandy facies of Mont Terri site: the first with the synthetic water of neutral pH and at an osmotic suction of 1 MPa, the second with a highly NaNO 3 saline solution with a neutral pH and at osmotic suction of 34 MPa, and the third with a hyperalkaline solution at pH = 13.5 and at osmotic suction of 1.16 MPa. The radial swelling pressure was monitored using a total pressure sensor along with the fracture opening using a displacement transducer. The tests ran for over 290 days. After that, the three tests were dismantled. A detailed petrophysical characterization of 45 subsamples was conducted, and the pore size distribution was determined at different positions. X-ray diffractometry measurements were also conducted to identify any mineralogical changes. Results show that, in the case of the synthetic water and the alkaline solution, the friction forces exerted by the lid and the base on the sample were significant and probably hampered the self-sealing of the fracture. Saline solutions affect the self-sealing process of claystones. Indeed, even though increasing the salinity caused the contraction of the diffuse double layer and generated additional fractures within the sample, which increased the permeability and thus accelerated the hydration process, a complete self-sealing of the fracture opening was not observed.