Experimental study on mechanically driven migration of fluids in rock salt

Abstract

Rock salt has been widely used for underground energy storage and waste disposal due to its low permeability, rheological behaviour and self-healing properties. Understanding of rock salt deformation including fluids migration is necessary to ensure the long-term stability of storage and disposal operations. In rock salt, fluids are trapped in fluid inclusions (FIs) formed under various pressure and temperature conditions. The formation of FIs is related to halite crystals growth, dissolution, recrystallisation and deformation. FIs are situated along halite grain boundaries (inter-crystalline) and within halite crystals (intra-crystalline). The FIs influence on mechanical properties of rock salt and deformation mechanisms was studied in natural outcrops and laboratory tests. However, the detailed role of FIs in these processes still requires further research. This study presents experimental laboratory tests performed on pink rock salt from the Kłodawa salt dome (central Poland). Pink rock salt is characterised by FIs occurrence along grain boundaries. FIs are various sizes and shapes. Most of these FIs are multi-phase (gas and liquid phase). Pink rock salt samples (double polished thick sections) were tested under uniaxial compression with simultaneous observations under a polarized microscope. The special device, which fit the size and shape of the microscope stage, was designed and constructed for mechanical tests. The experiment showed that only fluids trapped in FIs located along grain boundaries were active during the whole test. Fluids activity during this experiment were divided into three stages: (1) phase change related to FIs volume change under stress, (2) fluid migration displayed by gas phase movement and (3) fluid release from FIs. For each stage stress values and mechanisms responsible for FIs behaviour were determined. The experimental laboratory tests and their results presented in this paper are unique and provide insight into fluids migration and FIs role in rock salt deformation.