The Failure Mode And The Safety Analysis Of The Salt Rock Casing For The Gas Storage

Along with the development of the industry and social progress, oil and natural gas has been widely applied, and becomes one of the main source of energy. In recent years many research show that salt rock exhibits pronounced time-dependent deformation or creep under relatively low stress level and has very low permeability and porosity. Salt rock creeps to a very large strain without fracturing and tends to be self-healing. So salt rock is considered as an ideal material for the storage of natural gas, petroleum and wastes, especially nuclear wastes, e.g. It is very important to study site selection and construction of salt rock underground storage cavern for the sake of the safety of nation energy storage. But it becomes prominent that their production, transportation and storage are all limited by the activity life of casing. Therefore, we analyze all the possible factors that account for the affection of the casing stress and strain. So that find out the useful suggest for the designing and repair of the casing.First, by analyzing the casing damage mechanism finds out a certain range of the factors that related to the stress and strain of the casing. It mainly includes that:the mechanical properties of salt, creep, and terrain distribution, cement ring, operation period of the work of the state, casing characteristics of itself. Then, in consideration of the variety in depth direction, establish casing-cement ring-salt of three dimensional combination finite element model instead of the plane model.From the aspect of the casing design, it is critical whether the casing pipe design strength can meet the requirements in particular load conditions. So, during the casing uncertainty analysis mainly discusses rock creep, cement ring settings and salt in different influence. The calculation results show that the influence of the rock creep to the casing stress is very obvious, therefore can not be ignored. Test showed that the cement ring could reduce stress casing, and bonded casing and the salt closely together, effectively enhanced the casing sealing performance.During the operation storage, stochastic characters of geometrical size, load and material of the casing system are analyzed, which are applied in the safety analysis, failure probability calculation and sensitive factors analysis based on response surface method. At last, the calculation result is proved by a three dimensional geological model experiment. According to the calculation, casing and cement sheath are safe during normal operation period. However, failure probabilities of casing are high when the gas pressures of storage are extremely high or low. Extraction and injection rate are also important factors for the safety of casing system. Therefore, operation pressure and also extraction and injection rates should be strictly controlled during operation period. The main conclusion is, casing, cement ring and the elastic modulus of salt are the important factors to influence the casing reliability, but combined with the reality of the technology level, this several factors still exceed our control.