A simple model to represent damage and fracturing in rock salt caused by heating and subsequent cooling: Understanding brine inflow data from the Brine Availability Test in Salt (BATS)

Abstract

Salt deposits are being considered in several countries as potential host rocks for deep geological disposal facilities (GDFs) for radioactive waste. Although the very low porosity and lack of water content are favorable properties of salt that make it well-suited for this purpose, brine is present in amounts that are not insignificant when considering potential corrosion of waste canisters and pathways for radionuclide migration. It is therefore important to understand the processes and mechanisms that can cause brine to flow into the GDF. In this paper a relatively simple model is described that represents damage and fracturing in rock salt caused by heating and subsequent cooling. The model is used to simulate brine inflow to a heated borehole, using data from the Brine Availability Test in Salt (BATS) which was carried out at the Waste Isolation Pilot Plant (WIPP) in New Mexico, USA. Key characteristics of BATS and other, similar, experiments are increased brine production during periods of elevated temperature and a very large but short-lived increase shortly after the heat source is removed. The latter is not directly relevant to the disposal of heat-generating radioactive waste, where the heat source will gradually diminish over time, but the ability to simulate this phenomenon demonstrates an understanding of the coupled thermo-hydro-mechanical (THM) processes that operate in salt-based systems. The modelling demonstrates that a certain level of complexity is needed (such as the inclusion of viscoplastic effects and damage) to capture the key behaviors, but complexity can be tailored to the purposes of the study. In this case, a 1D model and a simple representation of the damage directly linked to permeability change was sufficient to gain a detailed understanding of the system. This includes the importance of the excavation damage zone (EDZ) in controlling the availability of brine.