Study On The Long-term Safety Analysis And Evaluation Of Horizontal Salt Cavern Gas Storage

Deep salt rock formation is internationally recognized as an ideal medium for natural gas storage.Salt rock has strong creep characteristics,and because the internal gas pressure of the salt cavern gas storage is always lower than the initial formation pressure,the surrounding rocks will produce increasing creep deformation with the extension of the operation time,leading to the continuous loss of the gas storage volume.If the deformation is too large,the roof collapse and plate damage may be induced.In addition,different from the huge thick salt domes in European and American countries,the salt rock strata in China are mostly layered salt rocks containing insoluble interlayer.Because the permeability of the interlayer is usually higher than that of the salt rock layer,the existence of the interlayer increases the risk of natural gas leakage and affects the tightness of the salt cavern gas storage.In order to ensure the long-term operation safety of salt cavern gas storage,this paper studies the stability and tightness of a salt cavern gas storage by combining theoretical analysis and numerical simulation.The main research contents and achievements are as follows:(1)Considering during the long-term operation of salt cavern gas storage the increasing creep deformation of surrounding rocks constantly changes its permeability,based on the gas seepage theory,a seepage theoretical calculation model in the horizontal and vertical direction of salt cavern gas storage is established,and this model is used to analyze the evolution law of gas seepage distance and pore pressure during the operation of the salt cavern gas storage.(2)Using the method of numerical simulation,the long-term operation process of the single-layer horizontal salt cavern gas storage under different minimum operating internal pressures and different initial salt rock permeabilities are calculated.Then,the evolution rules of the stability indexes such as cavity wall displacement,gas storage volume shrinkage rate and the tightness indexes such as seepage distance and pore pressure are analyzed,and the influence of minimum operating internal pressure and salt rock initial permeability on the stability indexes and the tightness indexes are discussed.Finally,the minimum internal pressure of the single-layer horizontal salt cavern gas storage and the initial permeability of salt rock suitable for building the gas storage are determined.(3)Based on the safety analysis results of the single-layer horizontal salt cavern gas storage,the long-term operation process of the double-layer horizontal salt cavern gas storage under different vertical intervals and different injection-production period pressure differences are simulated.Then,the influence of vertical interval and injection-production period pressure difference on the long-term stability and tightness of gas storage are analyzed.Finally,the reasonable vertical interval and injection-production period pressure difference of the double-layer horizontal salt cavern gas storage are determined.(4)The long-term safety factors of the salt cavern gas storage are summarized.Then,using the hierarchical analysis to construct the long-term safety evaluation index system of salt cavern gas storage and calculate the weight of each sub-index and each factor.On this basis,using the fuzzy comprehensive evaluation method establishes the long-term safety subordination matrix of the salt cavern gas storage and determines the fuzzy comprehensive evaluation set,and evaluates the long-term safety of the salt cavern gas storage.The results show that the safety scores of single-layer and double-layer gas storage are respectively 76.42 points and 77.33 points,and the safety levels are general safety,meeting the safety requirements.