Microfabrics impact on rock salt geomechanical diversity – an experimental investigation

Abstract

Geomechanical studies of rock salt serve as the foundation for designing and ensuring the long-term stability of storage caverns. Research conducted in recent years has shown that rock salt exhibit highly variable mechanical parameters. Even macroscopically similar rock salt samples from the same formation have very different mechanical parameters in standard strength tests. In this paper, the effect of microfabrics on the mechanical behaviour of rock salt was studied using a uniaxial compression device with simultaneous observation under a microscope. The tests were carried out on rock salt samples, previously tested on standard uniaxial compression (UCS). Tested samples showed different strength values despite macroscopic similarity and similar anhydrite content. The tests revealed that the microfabrics, such as fluid inclusions and anhydrite crystals distributed along the halite grain boundaries, play a significant role in the strength of a rock salt sample. Four types of grain boundaries were recognized in both group of samples: (1) dominated by anhydrite crystals with a small number of fluid inclusions (FIs), (2) with FIs prevailing over anhydrite crystals, (3) only with FIs, and (4) without anhydrite crystals or FIs. The experimental uniaxial compression tests in microscale (mUCS) showed that the damage process is initiated by FIs activity at the halite grain boundaries and consists of three main stages. The samples dominated by anhydrite crystals at halite grain boundaries were characterised by the highest mUCS. Conversely, if FIs prevail at the grain boundaries the lowest mUCS values were registered. Therefore, anhydrite crystals located along halite grain boundaries influenced the mechanical behaviour and deformation process. On the contrary, anhydrite crystals dispersed within halite grains had no impact on failure and mechanical parameters. The experiment showed that apart from impurity content, the distribution of these impurities within halite grains should be examined during geomechanical evaluation of rock salt samples. Thus, the overall strength of rock salt samples determined in standard test results from the local variations of petrological features in micro- and macro scale.