Experimental Study On Mechanical Properties Of Static Fracture And Creep Fracture Behaviors Of Gypsum Interlayers In Corrosive Environmentn Of Brine

Rock salt is recognized as an ideal host for hydrogen and hydrocarbon energy storage because of its low porosity and permeability,simple hydrogeology,self-healing properties and wide geographical distribution.Compared with foreign thick salt dome,the bedded salt formation in china is characterized by thin monolayer and deep burial.Besides,the salt formation contains impurities like mudstone,gypsum and glauberite.The gypsum interlayers are widely distributed in Yunying and Ping Dingshan.Note that the interlayers will have a negative effect on the constructing and operating of salt cavern,which causes a serious of potential risks.In order to evaluate the effect of gypsum interlayers on the safety of salt cavern,in this study,the microstructure evolution,weakening mechanism,static fracture and creep fracture behaviours of gypsum are studied.Moreover,based on the experimental results,the periodical collapse pace and creep damage evolution process are investigated by using theoretical research method and numerical simulation.The main contents are as follows:To investigate the microstructure evolution and weakening mechanism of gypsum under corrosive environment,a serious of lab tests,including SEM scanning,MCT scanning,indentation test and factorial design.The weakening of gypsum soaked in water and brine is attributed to the combined effects of water,temperature,and chlorine ions.Notably,the water-temperature effect contributes to a severe weakening of gypsum interlayers,but the chlorine ions have little effect.Then,based on the experimental results,the damage model is proposed.Also,to determine the parameters,the model is imbedded into PFC.By analyzing the fracture results of gypsum soaked in brine,it can be obtained that the fracture toughness value increases with the increase of brine.However,the results remain essentially unchanged with increasing brine concentration.Then,for comparison purpose,the experimental fracture envelopes of gypsum under varying conditions are determined to compare with the conventional criterions.Further,to evaluate the initiation angle and fracture trajectory of gypsum,a unified approach combining the interaction integral method and the finite element method was provided to calculate the dimensionless parameters of stress intensity factor and T-stress for the CSTBD specimens by using MATLAB.Based on the calculation results,the equations of MMTS criterion are input into MATLAB to invert the parameter by using a back-propagation neural network.It is obtained that the FPZ value increases with increasing brine temperature.The predicting results of MMTS criterion show a great consistent with the experimental envelopes of gypsum soaked in different brine.By analyzing the fracture results of gypsum under varying confining pressure,it can be obtained that the fracture toughness value increases with the increase of confining pressure.The results also show that the confining pressure significantly affects the final fracture mode.The macroscopic behaviour of gypsum ranges from brittle to ductile depending on the confining pressure.With increasing confining pressure,the residual energy gradually decreased,while the brittleness index gradually increased.For confining pressures of 1 MPa and 3 MPa,cleavage fracture characterized by cleavage steps,river lines and tearing ridges was shown on the surface,indicating that typical brittle failure occurred in the specimens.However,under confining pressures of 5 MPa and 7 MPa,intergranular microcracks and dimples were more frequently observed,indicating that ductile failure occurred in these specimens.Further,based on the calculation results,the equations of MMTS criterion were input into MATLAB to invert the parameter using a back-propagation neural network.It is obtained that the FPZ value increases with increasing confining pressure.The predicting results of MMTS criterion show a great consistent with the experimental envelopes of gypsum under varying confining pressure.Based on Charles stress corrosion model,a micromechanical creep fracture model is proposed by considering both of the tensile stress and shear stress.Besides,the model also takes into account the effect of tensile stress and shear stress on creep fracture and crack propagation rate.Then,the real-time tests of creep fracture of gypsum under different loads were carried out.Then,the initiation time,steady crack propagation rate and accelerating crack propagation rate are determined to calibrate the model parameters.Further,the model is imbedded into PFC to simulate the creep fracture process and fracture time of gypsum under different loads.Also,the simulation results are compared with the theoretical results and experimental results.Based on the annular plate model,a computing formulas of periodical collapse pace of gypsum interlayers is proposed.Then,the collapse pace is studied by using the method of ABAQUS simulation.The results reveal that the collapse pace is closely related to the brine temperature and soaking time.During the operation of salt cavern,the creep damage evolution processes of gypsum interlayers are simulated using PFC.The relationship between cracks propagation law and internal pressure is determined.