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Failure analyses of the insoluble interlayers are critical for a salt cavern leaching for natural gas storage. In this study, a series of brine immersion tests was carried out to investigate the softening laws of the mechanical characteristics of an argillaceous anhydrite interlayer. The test results show that brine immersion decreases the mechanical strength of the interlayer. A softening depth model of the interlayer specimens was developed based on the experimental results. This model was used to establish a mathematical model of the softening of an overhanging interlayer in a salt cavern during leaching. The softening model can predict the softening scope and the residual strength of the interlayer in different softening zones at different leaching times. The model was integrated with a numerical simulation to analyze the characteristics of failure of an overhanging interlayer. It was found that the interlayer tends to fail when the brine immersion time reaches a certain value in the simulation. Finally, the influence of environmental and artificial factors on the softening laws of the insoluble interlayer was discussed. It was found that the matrix structure and soluble components of the interlayer are two significant environmental factors and the brine immersion time is the key artificial factor. Therefore, the failure of an interlayer can be controlled by adjusting the brine immersion time during leaching a salt cavern. [ABSTRACT FROM AUTHOR]

Abstract: Apical periodontitis, the inflammation of periapical tissue, commonly requires root canal treatment to achieve apical healing. However, if it is accompanied by pulp canal calcification, the treatment becomes complicated, and locating the root canal can be challenging. This case report describes a novel approach for treating a molar with pulp canal calcification and apical pathology. Due to the risk of perforation during treatment, a digitally printed template was used to assist in accurately locating the root canal. After six months, the patient was asymptomatic and the periradicular radiolucency was gradually reducing in size. [ABSTRACT FROM AUTHOR]

Underground salt cavern storage has become the preferred medium for storing gas energy and strategic substances. Salt caverns are suitable for storing small molecular gases due to the low porosity and permeability of salt rocks. This paper comprehensively analyzes the physical properties of four gases - hydrogen, helium, methane, and carbon dioxide - under subsurface temperature and pressure conditions. It categorizes the flow regimes of these gases in salt rocks under geological pressure conditions based on the Knudsen number. In a typical 1000 m salt cavern, the predominant permeation flow regime of four gases in the surrounding rock is Klinkenberg flow, with helium potentially undergoing transitional flow at low operation pressures. A 3D numerical model is established for an actual salt cavern to compare the permeation and leakage characteristics of these gases within salt rock formations. Results indicate that, under identical operation conditions, the permeation range of the gases decreases in the following order: hydrogen > methane > helium > carbon dioxide. Under the cyclic operation pressures, the cumulative leakage amount of hydrogen, helium, and methane increases over time, while that of carbon dioxide initially rises and then decreases. This behavior is attributed to the fact that the reverse-permeation rate of carbon dioxide at low pressures exceeds its permeation rate at high pressures. Over 30 years cyclic operation, the leakage ratios of the gases are as follows: hydrogen (13.29 %), methane (9.34 %), helium (7.47 %), and carbon dioxide (0.93 %), with hydrogen exhibiting the highest and carbon dioxide the lowest leakage ratios. Larger permeability results in a larger permeation range, while larger porosity leads to a smaller permeation range. When the permeability of salt layer is greater than 1e-20 m2, the permeation range of hydrogen significantly increases. Gas leakage ratios increase with permeability nonlinearly and increase with porosity linearly. The impact of salt layer permeability on leakage ratios is greater than that of porosity. This study provides crucial guidance for the selection of geological formations for storing hydrogen, helium, methane, and carbon dioxide in salt caverns, as well as the investigation of gas permeation characteristics in salt layers. [ABSTRACT FROM AUTHOR]

Deformation stability plays a crucial role in the construction and operation of hydrocarbon storage salt caverns. Monitoring and analyzing wellhead pressure data under closed well conditions provide valuable insights into the evaluation of cavern stability. In this study, an analysis of five factors influencing internal brine pressure was conducted, which revealed that salt creep and brine thermal expansion contribute to an increase in internal brine pressure, while brine penetration, salt dissolution and cavern compressibility lead to a decrease. The atmospheric pressure, surface temperature, and earth tides are the other factors influencing wellhead pressure fluctuations. Through the analysis of wellhead pressure data from brine-filled salt cavern under closed conditions, the creep parameters of salt formations were calculated using the Norton-Hoff creep model. To further understand the behavior of brine-filled salt cavern, numerical simulations were conducted to analyze the brine temperature distribution and deformation stability. The volumetric average temperature of the brine exhibits a nonlinear increase over time, characterized by a rapid rise in the initial stage followed by a slower rate of change. Finally, the brine temperature stabilizes and converges toward the formation temperature. From deformation analysis, it was observed that the closed condition is more favorable for the maintenance of cavern stability in comparison to the open condition. The subsidence of the cavern roof is particularly sensitive and requires careful attention, especially for flat-roof salt caverns. Under closed conditions, it is crucial to control the brine pressure decrease rate and prevent excessively high brine pressure to ensure the mechanical and penetration stability of the brine-filled salt cavern. These findings provide valuable insights into the factors influencing internal brine pressure, the determination of creep parameters, the distribution of brine temperature, and the deformation stability of brine-filled salt caverns. [ABSTRACT FROM AUTHOR]

Based on traditional evaluation indexes, i.e., displacement, volume shrinkage, and equivalent strain, a new safety evaluation system, including the dilatancy safety factor (DSF) of bedded salt rock and failure approach index (FAI), is proposed in this work. Taking the Jianghan gas storage as the engineering background, the feasibility and stability of ultra-large underground gas storages (UGSs) in the Jianghan plain are demonstrated through this evaluation system. A 3D numerical model is implemented based on geological survey results and mechanical parameters of the Jianghan salt district. Subsequently, preferred geometric diameter and operating parameters, including cavern diameter, allowable pressure range, gas production rate, safety pillar width, and operation mode, are determined through a wide range of simulations. Results show that the suitable diameter is 80 m, the allowable pressure range is 16–34 MPa, the reasonable gas production rate is 0.5–0.6 MPa/d; a safe pillar width should be 2–3 times the cavern diameter, and the preferred operating mode should be synchronous injection–production mode. The simulation results, showing a high consistency with the previous research results, indicate the effectiveness and reliability of the proposed evaluation system. This work provides an alternative solution for the safety assessment method and design purposes for the UGSs in bedded salt rock. Highlights: A new safety evaluation system for bedded salt cavern gas storage is proposed. A 3D geomechanical model of Jianghan salt cavern is implemented. The dilatancy boundary curves of bedded salt rock in China are presented. A complete set of operating parameters for Jianghan gas storage are optimized. [ABSTRACT FROM AUTHOR]

Constructing salt cavern gas storage in the ultra-deep strata more than 2000 m in depth is an important strategic technologic development in China. A primary issue is that higher temperature affects the creep behaviors of the surrounding rock. In this paper, triaxial creep tests of rock salt under multi-stage temperatures were carried out to study the influence of temperature on the creep behaviors of rock salt. Synchronous acoustic emission monitoring was performed throughout the creep test to study the microcracks characteristics of rock salt during creep at different temperatures. The results show that the duration of transient creep decreases with the increase of temperature. The steady-state creep rate increases with the increase of temperature. Temperature rise prompts the creep of rock salt to enter the volume dilatation stage. By analyzing the acoustic emission energy/counts, it is found that the microcracks generated during the creep at high temperature are few, but there are more at low temperature. Microcracks are generated intensively during rising temperatures. By analyzing the characteristics of the acoustic emission spectrum, it is found that when the temperature is low, the cracks generated are mainly intergranular cracks with a larger size, and they are mainly intragranular cracks with a smaller size when the temperature is high. The research in this paper is helpful to further understand the creep properties of ultra-deep rock salt and provides an important basis for the design, construction, and operation of gas storage salt caverns. Highlights: The influence of temperature on the creep of rock salt was studied by performing multistage temperature creep tests. The microcracks characteristics of rock salt during creep at different temperatures were studied using acoustic emission technology. The duration of transient creep phase decreases with the increase of temperature. The steady-state creep rate increases exponentially with the increase of temperature. [ABSTRACT FROM AUTHOR]

Salt cavern is one of the best storage for hydrogen, compressed air, and natural gas. However, the current physical/numerical simulation-based construction design cannot yield optimal solutions due to the complex correlation of multiple construction parameters. This paper proposes a novel machine-learning-enabled method for the geometry prediction and design optimization during salt cavern construction. A dataset of 1197 simulations of salt cavern construction is collected using our previously developed program. Construction parameters are included as inputs, and the simulated cavern geometries as outputs. A Gated Recurrent Unit model is selected and well-trained from 600 artificial neural network models. The model achieves a mean absolute error of 1.6m in the test dataset and of 2.83m for the geometric prediction of cavern JT52 in Jintan, meeting the field design requirements. Furthermore, an optimized construction design method is proposed by looping generating construction parameters, shape prediction, and deviation calculation until the deviation meets requirements. It succeed in designing an ideal ellipsoid cavern in approximately 51 min, with a capacity ratio f c 31 % larger than those of the field caverns. This approach demonstrates the potential for machine learning methods to serve as the third generation of construction design methods after physical and numerical modeling. • A model is proposed to predict the geometry of salt caverns during construction. • One prediction costs only 1.14*10−4 s, 2*10−7 times of conventional methods. • The model achieves an error of 2.83 m for predicting cavern JT52 in Jintan, China. • An optimized construction design method is proposed based on the model. • An ideal ellipsoid cavern is successfully designed with a 31 % larger capacity ratio. [ABSTRACT FROM AUTHOR]

Underground energy storage in salt caverns involves operating in high-temperature conditions within deep strata, leading to unloading creep in the surrounding salt rock during cavity construction and natural gas extraction. Therefore, understanding the deformation and damage evolution of salt rock in such conditions is vital for ensuring safe construction and prolonged operation of deep salt cavern gas storage. The present study involves graded unloading creep tests and synchronous acoustic emission tests on the salt rock at 110 °C. The moment tensor inversion theory is employed to analyze focal mechanisms and microscopic crack propagation modes based on the test results. The findings reveal that under constant deviatoric stress, decreasing confining pressure exponentially increases the duration of the deceleration creep stage in salt rock. The creep increment and axial steady-state creep rate exhibited a U-shape trend. Instantaneous strain during stress change mainly involves elastic-plastic strain, where the instantaneous strain decreases first and then increases, exhibiting hardening characteristics in the middle process. The evolution of microcracks in salt rock can be categorized into three stages: compaction stage, stable propagation stage, and accelerated propagation stage. The acoustic emission energy, b-value, and fractal dimension of each stage are different. In unloading creep tests, the peak frequency in acoustic emission signal presents zonal distribution in low-frequency and high-frequency regions. It is found that as the confining pressure decreases, the peak frequency of acoustic emission signal and the proportion of high-frequency signal decrease, and the b-value and fractal dimension exhibit an upward-fluctuation-downward trend. The rupture slip angle of the acoustic emission source can effectively characterize the creep slip behavior in salt rock crystals. The slip gradually shifts from vertical to horizontal, while the strike angle and dip angle of rupture orientation are minimally affected by unloading. The obtained results can provide a reference for the safe construction and the design of the operation pressure for ultra-deep salt cavern gas storage. • The creep behavior of salt rock under high temperature are studied by performing multistage unloading creep tests. • Acoustic emission technology and the moment tensor theory are employed to analyze the microscopic crack propagation modes. • The axial strain increment and axial steady-state creep rate exhibit a U-shape trend as the confining pressure decreases. • The salt rock crystals have a sliding trend from vertical to horizontal with decreasing confining pressure. [ABSTRACT FROM AUTHOR]