Experimental Study On Dilatation Damage Of Surrounding Rock Of Salt Cavern Gas Storage Under Different Confining Pressure Unloading Paths

Although salt cavern gas storage is internationally recognized as an ideal place for energy storage,catastrophic events such as cavity failure and oil and gas leakage have occurred in many foreign regions since 1950,which will not only cause major economic losses,but also bring secondary environmental disasters.At present,China is vigorously developing the construction of salt cavern gas storage.Different from the huge thick salt mound salt rock abroad,the salt rock in China is distributed in layers,with relatively thin thickness and more interlayer in each layer.Thin interlayer and salt rock interface are more likely to become the channel of oil and gas leakage,resulting in rapid pressure loss of the cavity.Therefore,higher requirements are put forward for the safe and stable operation of gas storage in China.The change of internal pressure of salt cavern gas storage will cause the change of unbalanced force of surrounding rock of the cavity.During unloading,different initial confining pressure and unloading rate lead to different expansion damage of salt rock.It is particularly important to explain the expansion damage of salt rock under different unloading conditions.At present,there are few experimental studies on the dilatancy damage of salt rock under unloading condition.Therefore,it is of great significance to study the dilatancy damage of salt rock under rapid pressure loss through a series of experiments.In order to reveal the dilatancy damage law of salt rock under rapid pressure loss,firstly,the basic physical and mechanical properties of salt rock are tested;Secondly,the triaxial confining pressure unloading test is carried out,and the effects of initial confining pressure and unloading rate on the unloading dilatancy damage characteristics of salt rock are studied from the perspectives of strain,deformation modulus,wave velocity and energy;Then,in order to simulate the expansion damage of the surrounding rock of the cavity under the condition of rapid pressure loss in the injection production operation stage of the salt cavern gas storage,the triaxial confining pressure cyclic unloading test is carried out,and the effects of the initial confining pressure and unloading rate in the final unloading section after cyclic loading and unloading on the expansion damage characteristics of salt rock are analyzed;Finally,the damage of salt rock under different test conditions is studied by using ultrasonic wave velocity to characterize the damage variable.The main conclusions are as follows:(1)In the triaxial confining pressure unloading test,under the condition of the same unloading rate,the higher the initial confining pressure is,the greater the capacity expansion in the unloading section is.Compared with the initial confining pressure of 6 MPa,the mean volume strain at 12 MPa and 18 MPa increases by 7.1% and 21.4% respectively.The smaller unloading rate of confining pressure can increase the expansion of salt rock.Compared with the unloading rate of 0.2 MPa/s,the mean volume strain at 0.02 MPa/s,0.002 MPa/s and0.0002 MPa/s increases by 60.2%,103.5% and 282.1% respectively.The larger the expansion during unloading,the more serious the internal damage.The deformation modulus of confining pressure unloading section decreases gradually with the unloading of confining pressure,and the larger the initial confining pressure is,the faster the unloading rate is,and the greater the reduction of deformation modulus is.(2)In the triaxial cyclic unloading test,in the cyclic loading and unloading stage(II),the loading curve and unloading curve will form hysteresis loops under certain conditions,and the curves of each cycle are different.The strain produced in the initial cycle process is large,and the strain gradually decreases and tends to be stable with the increase of the number of cycles;When the confining pressure is finally unloaded to 0 MPa(III),large deformation will occur.Under the condition of the same confining pressure unloading rate,the higher the initial confining pressure is,the greater the expansion generated in the unloading section.Compared with the initial confining pressure of 6 MPa,the mean volume strain at 12 MPa and18 MPa increases by 38.7% and 60.9% respectively.The smaller the unloading rate of confining pressure has an increasing effect on the capacity expansion of the specimen.The slower the unloading rate,the greater the capacity expansion in the unloading section.Compared with the unloading rate of 0.2 MPa/s,the mean volume strain at 0.02 MPa/s,0.002 MPa/s and 0.0002MPa/s increases by 18.6%,48.5% and 145.7% respectively.(3)In the triaxial cyclic loading and unloading test,the reduction of deformation modulus is the largest in the first cycle,and the deformation modulus decreases gradually with the increase of cycle times.The deformation modulus of the unloading section is larger than that of the loading section in the same cycle process.the energy density increases with the increase of cycles;the energy absorbed by the first cycle of rock is the largest,and then the energy absorbed by each cycle gradually decreases.With the increase in the number of cycles,hysteresis loops gradually appear.(4)The change trends of energy in final unloading section of confining pressure unloading test and confining pressure cyclic loading and unloading test with initial confining pressure and confining pressure unloading rate are similar;under the same stress conditions,the energy absorbed by salt rock in the unloading section of confining pressure increases linearly with the increase of initial confining pressure,and the energy absorbed by salt rock in the unloading section of confining pressure decreases exponentially with the increase of unloading rate.However,after 10 cycles,the energy of salt rock changes slowly at the initial stage of final unloading stage,and suddenly increases when unloading reaches a certain confining pressure.The more energy absorbed during unloading,the more serious internal damage.(5)Through the characterization of damage by wave velocity,it can be found that the change trends of damage variables before and after confining pressure unloading test and confining pressure cyclic loading and unloading test with initial confining pressure and confining pressure unloading rate at unloading are similar.Under the same stress condition,the damage variable increases linearly with the increase of initial confining pressure,and decreases exponentially with the increase of unloading rate.