Abstract
Impure salt rock strata are extensively distributed in China, giving them great significance in the study of the physical properties of impure salt rock for the construction of underground gas storage in salt mines. To investigate the confining pressure and impurity effects on the mechanical properties and gas permeability characteristics of salt rock, permeability tests under hydrostatic confining pressure and conventional triaxial compression (CTC), on salt rock samples with different impurity contents, were carried out. The results demonstrate that the confining pressure effects cause an increase in triaxial compression strength, but a decrease in permeability. However, impurity enhances the bearing capacity and permeability of the salt rock; both rock strength and permeability increase with an increase in impurity content. Moreover, the broken salt rock specimens were analyzed after the CTC test using Computed Tomography (CT) equipment. To understand the relationships between pore volume and permeability, considering the confining pressure and impurity effects, the cracks were divided into four groups according to different crack diameter ranges: ~0.05 mm, 0.05–1 mm, 1–10 mm and ~10 mm. The CT analysis results show that while the pore volume of smaller cracks shows an “increasing–decreasing” trend by increasing confining pressure, the pore volume of large cracks gradually decreases, indicating that the rock permeability is highly related to macro-cracks. However, impurity has more complicated implications on rock permeability and cracks, and needs further investigation.
Highlights
Due to its excellent self-healing ability, low permeability and good long-term creep behavior, salt rock has been recognized as an ideal medium for underground energy storage and nuclear waste disposal [1,2,3]
Peak strength (MPa) positive and exponential. These results indicate that impurities enhance the strength and stiffness of salt rock, which is important in salt cavern construction
2020, 13, rock specimens with higher impurity content are fragmented after failure, which ofgreatly salt rock, salt increases the connectivity of the gas flow channels
Summary
Due to its excellent self-healing ability, low permeability and good long-term creep behavior, salt rock has been recognized as an ideal medium for underground energy storage and nuclear waste disposal [1,2,3]. While the main composition of pure salt rock is sodium chloride, the composition of impure salt rock is more complicated. Common impurities like anhydrite, glauberite and clay. Salt is deposited as bedded salt, which can transform into domal structures over geological time scales, due to gravity differences between the salt and the overburden. Since various sedimentary cycles usually exist underground, different impure salt layers are formed, ranging from carbonate to high-soluble potash salt. The thickness of the various layers can vary from several meters to hundreds of meters, corresponding to the position in the basin. E.g., in North America, the Gulf of Energies 2020, 13, 1366; doi:10.3390/en13061366 www.mdpi.com/journal/energies
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