Abstract
Due to the gas injection and production of underground salt caves during the operational phase, rock salt is often subjected to a combined stress of cyclic pressure and constant pressure. In order to investigate the damage evolution of rock salt under different combined stresses, the uniaxial cyclic loading test and cyclic creep test were carried out. The stress‐strain curves, energy characteristics, energy dissipation, and damage of rock salt in the two experiments were analyzed and compared. The test results show that the stress‐strain curves of the two tests presented three stages of “sparse”‐“dense”‐“sparse.” As the maximum stress increases, the stage of “dense” will decrease and the rock salt cycle life will decrease. The relationship between cycle life and Δσ (difference between maximum and minimum stress in the tests) is an exponential function under cyclic loading and a linear relationship under cyclic creep. Based on the experimental data, the energy dissipation of rock salt is analyzed. The damage variables were defined from the perspective of energy dissipation, and the damage evolution of rock salt under two tests was obtained. There are three corresponding stages of energy dissipation and damage: initial, constant speed, and acceleration. The damage model is obtained by inverse functioning the s function, and then the correction coefficient is added to the model to obtain the modified damage model. The modified damage model is compared with the experimental data. The results show that the model can accurately describe the three stages of rock salt damage. The significance of parameters in the modifying damage model is also discussed.
Highlights
Rock salt is considered as an ideal medium for underground storage of energy such as oil and natural gas due to its excellent characteristics of low porosity, low permeability, highplastic deformation capacity, and self-repair capacity [1, 2]
In order to simulate the gas injection and gas production processes in the operation of the salt cavern, the two types of load paths are simulated by the cyclic loading and cyclic creep tests, as shown in Figure 3. e test adopts the load control method, and the loading and unloading speed is 2 kN/s. e maximum and minimum stresses are maintained for 1 hour in the cyclic creep test. e maximum and minimum stress values in the test are calculated by using the uniaxial peak strength, and the uniaxial peak strength σc 45.66 MPa of rock salt is obtained by the uniaxial compression test with a loading speed of 0.5 MPa/s
E relation curve of damage variables of rock salt and the relative number of cycles under different maximum stress values is obtained by using data of cyclic loading tests and cyclic creep tests and damage variables calculated by Equation (2), as shown in Figure 8. e evolution process of rock salt damage shows three stages corresponding to energy dissipation
Summary
Rock salt is considered as an ideal medium for underground storage of energy such as oil and natural gas due to its excellent characteristics of low porosity, low permeability, highplastic deformation capacity, and self-repair capacity [1, 2]. Erefore, it is of great significance to study the mechanical properties, energy characteristics, and damage evolution process of rock salt under cyclic loading for the long-term stability evaluation of salt cavern gas storage. A lot of scholars have carried out in-depth research studies on deformation characteristics, fatigue properties, acoustic emission rules, and energy features of various rocks under cyclic loading [4,5,6,7,8,9,10]. Some scholars [16, 17] have carried out cyclic creep loading tests of rock salt. Is paper analyzes the deformation characteristics and energy features of rock salt through the cyclic loading test and cyclic creep test under different maximum stress values. E damage variable is defined from the aspect of energy dissipation, the evolution law of rock salt damage in the two kinds of the cyclic test is found, and the modified damage model is established on the basis of inverse S function. e model can serve as a reference in the engineering design in the operating stage of rock salt gas storage
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