4D microtomography of brine-assisted healing processes in deformation-damaged rocksalt: A first look

Abstract

Rock salt formations represent key options for storage of natural gas, hydrogen, and compressed air energy, and for storage or disposal of radioactive waste. At depths beyond a few tens or hundreds of meters, undisturbed halite-dominated (>90%) rock salt deposits are usually impermeable and have very low porosity. However, as a result of excavation, nearfield microcracking and associated dilatancy occur in rock salt, increasing porosity and permeability. The connectivity of a brine- or water-vapour-filled microcrack network in deformation-damaged salt, is expected to decrease over time, partly due to dissolutionprecipitation healing. Here, we employ 4D (i.e., time-resolved 3D) microtomography to study the long-term evolution of dilated grain boundary and microcrack networks developed in deformation-damaged natural salt by such brine-assisted processes. We found substantial microstructural modification or “healing” over periods of days to a few months. Cracks and dilated grain boundaries became crystallographically faceted, necked, discontinuous, and disconnected, and often migrated to “recrystallize” the material, producing an increase in tortuosity and a decrease in connectivity of the crack network. The magnitude and rate of associated permeability reduction and its evolution with time remain to be determined in future studies.