Study On Triaxial Creep Mechanics Test And Damage Evolution Law Of Salt Rock

Salt rock is recognized as the ideal medium for energy storage in the world for its good performance in creep and damage recovery.The excessive creep is the main failure mode of oil and gas storage in the cave of underground salt.Deterioration of mechanical properties caused by salt rock damage accumulation is the main reason of salt rock creep failure.Therefore,studying the creep properties and damage evolution laws of salt rocks is of great significance to guide the design,construction and operation of salt rock gas storage.Based on the method of laboratory tests and numerical simulations,this paper studied the creep characteristics and damage evolution laws of salt rocks from both macroscopic and mesoscopic views.(1)Triaxial creep test and acoustic emission test of salt rock are designed to conduct simultaneously.The sample was loaded by step and the rock creep curve obtained in the test was analyzed deeply.The study found that with the increase of the deviatoric stress,the instantaneous strain,creep strain and steady-state creep rate of the salt rock all increase.The steady-state creep rate shows exponential growth with the increases of deviatoric stress.Constitutive model and parameters were inverted based on creep curve data,and Burgers constitutive model can accurately match the creep characteristics of salt rock.(2)The evolution process of rock creep damage was studied based on the acoustic emission events distribution and acoustic emission localization.The study found that the active level of acoustic emission gradually decreased from decay creep to steady-state creep.Acoustic emission locations were concentrated in the middle and end of the sample.(3)Based on the PFC2 D software,the macroscopic parameters of salt rock obtained from laboratory tests were used to calibrate the mesoscopic parameters of the numerical model and the creep simulation tests were performed to analyze the creep characteristics and damage evolution of the salt rock from mesoscopic angle.The study found that when the deviatoric stress was small,the sample showed a tendency of contracting to the middle,and when the deviatoric stress was large,the sample showed a tendency to expand outward from the middle.As the stress level increased,the percentage of damage generated during the decay creep stage increased,and the percentage of damage generated during the steady-state creep stage decreased.During the creep damage process,the spatial evolution of cracks is randomly dispersing-aggregating into cores-strip shape expanding-connecting,and the whole of the crack is mostly at an angle of approximately 40-50° to the principal stress direction.The conclusions will provide theoretical basis for guiding the design,construction and safe operation of salt rock storage.