Study On Failure Mechanism And Stability Of Gallery Storage Cavern In Bedded Salt Rock

Rock salt deposit is considered as an ideal medium for natural gas and oil storage because of its low permeability and strong creep characteristics. The commonsalt formations in China in which there exist hundreds of gallery salt caverns are shallower and thinner than salt domes, and are layered and interspersed with non-salt sedimentary materials such as anhydrite, shale, dolomite, and limestone. Compared with the huge salt domes with deep embedding depths in other countries, the presence of thin interlayers in the salt beds adds additional risks for safety of the storage cavern, and cavern development and operations in China present additional engineering challenges. With the completion of “west to east gas transmission pipeline” project in China, it is badly in need of utilizing these caverns effectively for the storage of natural to ensure adequate supplies of energy in China.For providing operators with improved geotechnical design and operating guidelines for gallery salt caverns in bedded salt formations, fundamental analysis on the damage of interbed and building theory on the stability of the gallery storage cavern are needed.Based on building the gallery storage cavern in the typical bedded salt rock deposit, the uniaxial and triaxial compression tests have been carried out on specimens of rock salt, interlayers, and composite rock salt(containing interlayers); this paper combines the testing results with the theory analysis to study the influence of interlayer on mechanical propertise and the damage characteristics of the composite rock salt; based on energy method, a new failure criterion is developed for the rock salt and interlayer. The failure rule and damage range of interbed are determined in this paper and the influence of interbed on the stability of salt cavern and pillar are also discussed.The following are the main research contents:1. The uniaxial and triaxial compression tests have been carried out on specimens of rock salt, interlayers, and composite rock salt(containing interlayers); at the condition of uniaxial compression, the effects of the varations of interlayer thickness, interlayer number, interlayer inclined angle on the mechanical properties of the composite rock salt are revealed; at the condition of triaxial compression, the difference between rock salt and composite rock salt are also investigated.2. According the features of salt damage, a non-linear triple shear energy yield criterion for salt rock is developed based on the shear strain theory; Comparative study between the nonlinear criterion predictions and experimental results of salt rock shows that the non-linear triple shear energy yield criterion fits quite well with both conventional triaxial testdata and the true triaxial test data; compared with the results of other criteria, the predictive capability of the non-linear triple shear energy yield criterion is clearly better; The availability of the non-linear triple shear energy yield criterion can also be confirmed by comparative analysis between theoretical values and experimental values for non-salt rocks.3. Focus on the interface between salt rock and thin interlayer in roof or wall, a rule to limit slippage of the interface is developed by theoretical analysis on the gallery storage cavern with a cross-section of a circle or ellipse;the range of damage on the interface is also determined including the influence of several main factors,such as cavern pressure,cavern shape,cavern depth,dip angle of interbed, et al.4. The non-linear triple shear energy yield criterion is compiled to the program for numerical simulation; and aiming at the planned gallery storage cavern, numerical simulation analysis is conducted for the damage of the interbed, including several main factors, such as interbed thickness, interbed site, interbed number and the mechanical properties of interbed.5. Based on the geological conditions in Jintan area of Jiangsu province,numerical model of planned gallery storage cavern is constructed in the program for numerical simulation, and the stability of the planned gallery storagecavern affected by the roof interbed is discussed; The determination of the allowable width for pillars between two adjacent caverns is presented; comparison study of the influence on allowable width for pillar between synchronous injection-production mode and asynchronous injection-production mode is also performed; finally, the parameters for the design and operation of horizontal salt cavern are determined.