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To meet the requirements of the United Nations Framework Convention on Climate Change, i.e., the Paris Agreement, countries around the world have developed carbon-peaking and carbon–neutral action programs. The use of renewable energy sources is an effective means of meeting this requirement. Compressed air energy storage using salt caverns is an effective way to enhance the efficiency of renewable energy use. To investigate the damage pattern of salt cavern surrounding rocks subjected to cyclic injection and extraction loads during the operation of compressed air energy storage plants, using a MTS815 rock mechanics test system, a series of creep–fatigue mechanics tests on salt rock were carried out, and the effects on the fatigue damage of salt rock were investigated by setting different high stress plateau times and stress levels. (1) The development pattern of the corresponding creep–fatigue stress–strain curve is similar to that of ordinary fatigue, which is also divided into three stages: comb-dense-comb. The cyclic load has a positive effect on creep, but there is a threshold value of plateau time, and the creep deformation is minimal at a plateau time of 15 min. (2) The residual strain before the plateau in A cycle is greater than that after the plateau in B cycle, which is different from the traditional fatigue test, and the difference between the residual strain before and after the plateau is positively correlated with the plateau time. (3) The fatigue life of the salt rock specimens decreases and their creep life increases as the high stress plateau time increases. (4) The effect of the high stress plateau starts to appear under lower stress conditions, and the residual strain difference before and after the plateau increases with increasing stress level. (5) The different responses of the internal structure of the salt rock to fatigue and creep lead to the interaction between creep and fatigue. Highlights: Laboratory creep–fatigue tests of salt rock surrounding a compressed air energy storage plant were conducted under complex stress states. Cyclic loading accelerates the creep deformation during the high stress plateau. The presence of a high stress plateau during cyclic loading affects the plastic fatigue deformation of salt rock. With the increase in the high stress plateau time, a significant negative correlation between fatigue life and creep life arises. [ABSTRACT FROM AUTHOR]

The salt cavern compressed air energy storage (CAES) reservoir undergoes periodic cyclic gas injection and production, which induce coupled fatigue-creep effects on the surrounding rock during operation. In this work, a new constitutive model was developed for salt rocks, and it uses the traditional Norton model to describe the hardening degree with state variables. The fatigue-creep mechanical properties were investigated, the model was validated, and the model parameters were analyzed using triaxial continuous and interval fatigue tests. The findings were as followed: (1) Increasing the time interval will result in a higher residual strain and a shorter fatigue life; the confining pressures increases the compressive strength of salt rock. (2) The model only needs to adjust two parameters to accurately fit the fatigue-creep deformations behavior of salt rock, and it effectively characterizes the relationship between deformation and stress routes. (3) In the model, parameters m and k play a role through state variables. Parameter m affects the magnitude of the overall deformation and the length of the deceleration stage. Parameter k controls the bending of the curve, and has a more sensitive influence on the model. (4) Compared with that of the continuous fatigue model, the floating range of parameters m and k in the interval fatigue model is less than 10%. The model can adapt to the influence of time interval on rock deformation and more accurately predict the deformations of the salt rock surrounding the salt cavern reservoir. [ABSTRACT FROM AUTHOR]

Rock salt, due to its water solubility, low permeability, high plasticity, and damage self-healing ability, is one of the best candidate rock types for underground energy storage. Utilizing salt caves to construct compressed air energy storage (CAES) facilities can effectively enhance the utilization of renewable energy. Due to the need for peak shaving, the surrounding rock of the salt cavern will undergo discontinuous cyclic loading with varying gas injection rates and pressures, namely, alternating creep–fatigue loading. Considering the actual peak-shaving cycle of a CAES plant, long-term creep–fatigue tests of rock salt with different loading cycles and stress levels were conducted. The results indicate that in long-term creep–fatigue tests for rock salt, the lower the loading stress rate is, the greater the deformation of the rock salt. The variation in the stress limit has a greater effect on creep than on fatigue loading and unloading. The deformation rate of rock salt is influenced by alterations in the stress state. Based on the test results, according to the Norton creep model, a new creep–fatigue constitutive model for rock salt was established by defining a state variable that characterizes the level of rock hardening and introducing unloading as well as crack factors. This model can accurately describe the impact of historical loading and unloading processes on the viscoplastic mechanical characteristics of rock salt. The rock salt creep–fatigue test results were used to verify the constitutive model. A comparison of the fitting curve of the different stress loading paths with the test curve reveals good consistency, indicating that the model comprehensively considers the effects of time, load, and state on rock salt creep–fatigue, effectively describing the viscoplastic deformation characteristics of rock salt under different stress paths. These research findings provide important guidance for ensuring the stability of salt caverns used for CAES. • Long-term creep–fatigue tests were conducted on rock salt based on the actual peak operation frequency of a compressed air energy storage (CAES) power plant. • Changes in stress conditions (level and loading rate) significantly impact the deformation rate of rock salt. • A state variable is defined as the hardening factor, which describes the degree of plastic deformation in the rock. • The new creep–fatigue model of rock salt according to the state variable can effectively predict the deformation of rock salt. [ABSTRACT FROM AUTHOR]

Salt rock is characterized by a low porosity, low permeability and self-healing ability, and it is one of the most common materials used for underground energy storage and construction in underground nuclear waste repositories. Studying the healing of macroscopic cracks in salt rock under different conditions is important for ensuring the stability of the rock surrounding a salt cavern. To evaluate the healing ability and effect of cracks in salt rock at the macroscale, two tests were designed to directly assess the healing ability of salt rock by analyzing the tensile strength and permeability recovery of healed crack surface. Furthermore, the microstructure of the healed macroscopic fractures in salt rock was observed using scanning electron microscopy (SEM), and the microscopic mechanisms of salt rock damage healing were analyzed. (1) The results show that the presence of water is an important condition for the healing of cracks in salt rocks. Fully fractured salt rock cracks can heal at a low pressure of 160 kPa, and the uniaxial tensile strength of the healed specimens can reach a maximum of 121.1 kPa. It was found via computed tomography (CT) that a healed crack was still a weak surface in the whole specimen. (2) Increases in temperature, environmental humidity, and normal stress all promote the self-healing of salt rock fractures. Based on the experimental data, a model describing the evolution of salt rock damage healing variables with respect to temperature, humidity, and stress was established. It was also observed that the change in salt rock damage healing variables over time during the healing process can be represented by a first-order exponential decay function. (3) Using SEM, detailed micro healing experiments were conducted on NaCl single crystal and impurity-containing salt rock specimens. The healing characteristics of intracrystalline cracks and intercrystalline cracks were studied. It was observed that salt rock damage healing manifests at the microscopic level as the filling of microcracks and crack segmentation caused by grain growth. Additionally, the presence of a certain amount of impurities was found to promote the growth of internal structures that heal cracks within salt rock. These findings are of great significance for evaluating the stability and tightness of the surrounding rock of salt cavern reservoirs. • Laboratory tests on salt rock crack damage self-healing were conducted at both the macroscopic and microscopic levels. • Elevated temperature, increased environmental humidity, and increased normal stress all promote self-healing in salt rock. • Through CT imaging, tensile strength, and permeability, direct evidence of macroscopic fracture healing was provided. • Dissolution, precipitation and recrystallization, are the primary microscopic mechanisms driving self-healing in salt rock. [ABSTRACT FROM AUTHOR]

Salt rock is a sedimentary rock that is widely distributed in the crust. Since salt rock has a low permeability, rheological behavior, and self-healing characteristics, it has been extensively exploited for subsurface energy storage and waste disposal. The construction of compressed air energy storage (CAES) facilities using salt caverns for solution mining is a practical solution to achieve carbon peaking and carbon neutrality goals worldwide. The stability of the salt cavern surrounding rock is critical to the function of a CAES plant. Considering the triaxial stress condition of the rock surrounding such underground salt caverns, a number of triaxial creep–fatigue mechanics tests were performed on salt rock using an MTS815 mechanical testing device. The creep–fatigue damage to the salt rock was examined under various confining pressures and stress levels. The main research results and conclusions are as follows. First, the triaxial creep–fatigue stress–strain curves show a similar tendency to classical triaxial fatigue results: the strength and ultimate deformation of salt rocks increase as the confining pressure increases. Second, in contrast to typical fatigue test results, the residual strain before the high-stress plateaus is larger than that after the plateaus during a creep–fatigue test. However, as the confining pressure increases, this disparity decreases. Third, fatigue loading increases creep deformation, and this effect weakens when the confining pressure increases. Fourth, the residual strain differential before and after the high-stress plateau increases together with the stress level, and the contribution of creep strain increases correspondingly. Last, as the confining pressure increases, the salt rock behavior transitions from a brittle response to a fully ductile response. A high confining pressure prevents relative sliding between the salt rock particles. Thus, the effect of confining pressure on the creep–fatigue mechanical behavior of the salt rock reaches a threshold. • Creep–fatigue tests of salt rock were conducted under triaxial loading to evaluate the potential of salt cavern for CAES. • The addition of confining pressure influences the impact of high-stress plateaus on creep–fatigue deformation. • Fatigue contributes to creep deformation, but this contribution decreases with increasing confining pressure. • The transition from brittle to ductile behavior is responsible for the different creep–fatigue features of salt rock. [ABSTRACT FROM AUTHOR]

The use of salt caverns to construct the compressed air energy storage (CAES) plants is an important development for the reuse of abandoned mines in response to the Chinese national green mining strategy and is a significant means to accomplish global carbon neutrality. The study of the mechanical behavior of rock salts is the base for ensuring its stability. Power plant operating conditions consist of periodic gas injection and abstraction, resulting in the cyclical mechanical loading of salt cavern walls. To study the mechanical properties of the rock surrounding a salt cavern storage reservoir under representative conditions, uniaxial and triaxial graded creep–fatigue tests on rock salt were designed, and their mechanical and acoustic emission characteristics were analyzed considering the factual operating gas pressure of the CAES plant. The results show that the residual strain produced by the cycle before the high-stress plateau are larger than that produced by the cycle after the high-stress plateau, and this difference may be associated with the hardening effect during creep. The change in acoustic emission parameters can reflect the different fracture patterns of rock salt and can be used to predict the destabilization and failure of the rock salt. Creep and fatigue have different effects on damage. In particular, at higher stress levels, the damage during creep stage predominates. The high-stress plateau drives the propagation of rock fractures at later stages. Therefore, in the design of gas storage reservoirs, the upper gas storage pressure limit must be taken into account. These findings are deemed important for studying and understanding the long-term stability phenomenon in salt caverns of CAES. • Creep–fatigue tests of rock salt under multiple stress levels (multilevel amplitude creep–fatigue loading) were conducted. • The high-stress creep stage affects the fatigue cycle residual strain in rock salts. • The hardening effect during creep increases while the stress level increases, and the presence of confining pressure reduces the increase in this effect. • As the stress level increase, the damage of rock salt during the creep stage gradually dominates. • Acoustic emission parameters can be used as critical indicators to monitor the failure of rock salts under multilevel amplitude creep–fatigue loading. [ABSTRACT FROM AUTHOR]