The stability and integrity of salt caverns are critical for ensuring their safety and long-term viability, especially when used for storing hydrogen on large scales. This study is aimed at understanding the failure mechanisms of rock salt under coupled thermo-mechanical stress regimes based on the study of rock salt dilatancy-compression boundary stress levels. Several tests were conducted using a high-temperature true triaxial testing facility on 50 mm high-purity rock salt cubes. Different temperatures of 30 °C and 100 °C were considered, along with a variety of stress values of 0, 5, 10, and 20 MPa for confining pressures. The results showed that the applied deviatoric stress and temperature both have significant coupling impacts on the peak strength and dilatancy boundary of rock salt.
This Hydrogen is a promising solution, in response to the increasing demand for sustainable and low-emission energy sources in Australia and worldwide. However, due to its highly flammable nature, safe and effective storage methods with large capacities are necessary. One potential option is to store hydrogen underground in salt caverns, which has been proposed as one of the most viable options for large-scale storage. Rock salt caverns are ideal for the storage of hydrogen, oil, natural gas, petroleum products and other hydrocarbons, compressed air, carbon dioxide, and chemical and hazardous waste disposal due to their impermeability, and structural stability (Djizanne et al., 2014). They are usually constructed deep underground, typically between 1,500 and 2,500 meters, and can reach high temperatures of up to 90 °C due to geothermal gradients. Because of the associated high geological stresses and high temperatures involved, constructing and operating these caverns is challenging, making accurate design crucial.
The complete stress-strain curve of rock salt consists of several stages, including compaction, elasticity, plasticity, and failure as shown in Figure 1a. Under loading, there is the stress level at which cracks cause permanent damage called dilatancy boundary. It signifies the end of stable crack growth and the beginning of a transition from compaction to dilatancy, leading to an increase in pore volume and significantly higher permeability. Dilatancy in rock salt is equivalent to the crack damage threshold of brittle materials. The volumetric strain graph, illustrated in Figure 1, is a reliable indicator of dilatancy (Stormont, 1997; Schulze et al., 2001; DeVries et al., 2002; Serati et al., 2016, Serati et al., 2018).
