Permeability changes in fractured Tamusu mudstone in the context of radioactive waste disposal

Abstract

Tamusu mudstone, located in Inner Mongolia, is being recognized to be the potential host formation in deep HLW disposal across China. This paper describes diverse laboratory experiments performed using the fractured Tamusu mudstone, with concern to the study of the combined mechanical and thermal effects on its hydraulic properties, as well as its self-sealing effect. A single rough fracture is generated down the sample diameter direction before the sample’s installation. Considering a heating-cooling cycle, the evolution of the samples’ hydraulic properties under different stress conditions has been monitored. The hydraulic conductivity is proved to be not only related to the temperature-induced water viscosity and density variation but also the change of stress state. At the same thermal conditions, the increase of confining pressure (or deviatoric stress) may weaken the flow capacity of the fractures and thus reduce the permeability. The non-coincidence for the curve of permeability versus temperature at the heating and cooling stage indicates that the permeability alterations in the fractures are irreversible. Fracture zone changes were observed by X-ray computer tomography (CT) before and after the tests. The self-sealing behavior, which may be beneficial from high confining pressure, was detected for fractured Tamusu mudstone. Morphologies of fracture surfaces were detected by the 3D scanning technique, which shows that the fracture surface roughness, due to the alteration of asperity contacts and void spaces under high stress, may take an important part of the responsibility to the change of permeability.