Crack healing in salt: time-resolved 3D microtomography

Rocksalt caverns are considered or already used as storage for nuclear waste, petroleum, hydrogen, CO2, and compressed air energy because of the low permeability and potential of fracture healing of salt. An important concern is the sealing capacity. Undisturbed rocksalt deposits in nature generally have very low permeability. However, as a result of excavation stress, a network of fractures will be induced within the rocksalt formation, increasing the permeability. At low deviatoric stresses and/or at low effective stresses, a fracture network filled with brine is expected to heal, and the connectivity of the brine-filled network, consisting of grain boundaries, pores, and microcracks, is expected to decrease over time. The driving force for such a healing process is the tendency to reduce the interfacial energy by reducing the total interfacial area. In order to assess the rate of pore reconfiguration and permeability evolution in damaged salt and to capture the key process of crack network evolution during healing, we employ time-resolved 3D microtomography to study the long-term evolution of the fracture network of small-scale polycrystalline rocksalt samples. We found that precipitation prefers to occur in open spaces in the early stage of healing, such as new cracks. As a result, flat cracks evolve into zigzag cracks, which create narrow throats, thereby reducing the permeability of the crack network. Our study also offers a way to testify the thermodynamic models quantitatively.