Catastrophic Mechanism Induced By Damaged Surrounding Rock And Mitigation Principle During Bedded Salt Cavern Construction Period

At present, China is turning into a large-scale construction period of salt cavernenergy storage. Owing to there existing a large number of undissolved or insolubleinterlayer in domestic salt mine which makes it more difficult and more possibility ofcatastrophe during storage construction period, so the keys to guaranteed salt cavernsafe and successful building were fully grasping the induced mechanism of catastrophebuilding in salt cavern with interlayer and putting forward the reasonable scientifictheory and scheme of reducing disaster.The most important induced factor during storage construction period was thedamage and failure of surrounding rock which caused a series of disasters. For instance,casing shoe damage aroused by large deformation of surrounding rock, casing smashedbending or breaking up by softening interlayer collapse, surrounding rock damage dueto the excessive stress concentration, etc. Therefore, the key points of this paper weremastering the mechanism of induced disaster by surrounding rock damage in storagebuilding in salt cavern with interlayer and proposing the reasonable scientific scheme ofreducing disaster based on corresponding disaster mechanism.(1) Using fluorescence and mesoscopic analysis method to carry out experimentalresearch on expansion characteristics of surface crack on salt rock and interlayer, notonly it was mastered but also a criterion of causing crack aimed at tension fracture hadbeen built. The research found that the grain size, mineral composition of interlayer, theinterface shape between salt rock and interlayer in layered salt rock both had effects onthe evolution and extension of crack. In generally, the strength and deformation abilityof coarse grain salt rock were less than fine grain salt rock with high purity.(2) From the analysis of influence of cavity surrounding rock caused by brine,temperature, stress of loading and unloading and other factors, it acquired the damageweakening rule of cavity construction period. The experiment results show that brinemainly showed dissolution effect on salt while soak weakening effect was hardlyexisting. The main factor leading interlayer to weakening was brine soak also it can bepromoted with increasing temperature.(3) Through the confining pressure reduction tests, it revealed the deformationmechanism of salt rock in cavity construction period and setted up a constitutiveequation of damage considering unloading effect and creep. In the process of constant axial pressure and confining pressure decreasing gradually, the salt rock would notappear volume shrinkage but expansion deformation. It would appear acceleratedunloading flash when the confining pressure decreases to a certain value which marksthe internal crack began to develop. After unloading was completed, the deformation ofthe specimen turned into creep whose rate was controlled by partial stress and confiningpressure.(4) Through the analysis of self-healing characteristics of damaged salt rock indifferent temperature, humidity and stress, the self-healing mechanism in salt cavityconstruction period had been mastered and a relation between damage and healing hadbeen established. The research showed that with more internal opening crack inspecimen more disadvantageous to self-healing, but if the cracks were consolidated bycertain confining pressure it would have some help. Higher temperature would promotethe intracrystalline grain recrystallization under water supply, but only undertemperature effect, with moisture continuously losing it would restrain self-healing.(5) A similar model experimental platform of flow field in construction period ofsalt caverns was established based on the similar theory of "selected laws of physics ". Itacquired the migration law of flow field and indicated that it could be divided into sixrole zones: buoyant plume zone, convection-diffusion zone, buffer diffusion zone,saturated precipitation zone, border dissolution zone and waterfall flow zone. It foundthat the water flow, casing spacing, corresponding space position, occurrence state andquantity of interlayer and cycle mode of water injection affected the migration of flowfield. In addition, the non-uniform flow of brine also could produce different degree ofdisturbance water pressure to interlayer based on the calculation, but this hydrodynamicpressure was very smaller than its own gravity that was not enough to lead interlayer tocollapse and destroy.(6) A corresponding scheme of reducing disaster had been proposed based on themechanism of the damage and weakening of surrounding rock in construction period ofsalt caverns. A model to predict the weakening and collapse of interlayer also had beenestablished according to the characteristics of cavity construction with interlayer and thewater softening mechanism of interlayer. Aimed at the salt cavern shape with interlayerwas not regular and it was disadvantageous to control cavern shape, a cavernconstruction software of Salt Cavern Builder V1.0(SCB1.0) had been developed whichcould simulate cavity construction with interlayer and better solve disasters caused byirregular shape of the cavity. (7) Through simulating the process of cavity shape expansion in solution miningfor salt cavern with many interlayer by the cavern construction software SCB1.0,selecting several key cavern construction stages and considering the characteristics ofdamage and weakening of surrounding rock in salt cavern construction, a regularity ofdistribution of damage zone of surrounding rock was found.