The Safety Analysis Of Casing String In The Deep Stratum Cavern Underground Gas Storage

The article is aimed to solve the safety issues of the construction of the salt cavern gas storage.The research areas include tubing material selection research in the corrosion environment,in-situ stress back analysis of the injection and production wells and stress distribution research of wellbore,casing string vibration of UGS and gas friction resistance measurement research,casing strength calculation and remaining life evaluation.The main conclusions are listed below:By analyzing OCTG material selection standard of six steel companies in four countries and considering the different factors which influence the safety of the tubing,the sphere of application of different material selection standard in various environments is determined.The OCTG material selection software in the acid environment is developed based on the different material selection standard.The software includes six material selection standards,which are Sumitomo metal material,NKK material,DMV material,Kawasaki material,LoneStar material and Cabval material selection criterions.The erosion resistance and safety issues are compared and analyzed based on the certain acid environment.Multiple choices are provided to users as the safety issue is satisfied.Users can make the final selection based on the economic issues and other design criterions.Based on the multiple objective optimized back analysis method,back analysis of in-situ stress is carried out on the Turkey salt cavern.The calculation results of the objective wells are similar with the measured values,the error is below 8%.By the analysis of the in-situ stress field of the research area,the in-situ stress magnitude and orientation distribution are obtained.According to the numerical values of in-situ stress field,the stress distribution of wellbore in the injection and production well are calculated.By the analysis of radial stress,circumferential stress and shear stress in different distances of the wellbore,it is concluded that the wellbore can be maintained safety.The stress and strain distribution of salt cavern is calculated and the results show that the strain and vertical displacement decrease as the distance with the cavity increases.The reserverd height of casing shoe decreases when the height-diameter ratio of cavity and inner pressure of salt cavern increases.The Von-Mises stress and strain decrease rapidly when the distance with the cavity increases.The vibration and gas friction resistance measurement unit of casing string in the UGS are designed by the similarity principle.The gas friction resistance coefficient and its variation tendency of the string of different diameter are measured.The measurement and theoretical results are compared and are proved consistent.The measurement unit and the variation tendency of gas friction resistance provide the basic criterions to the vibration and dynamic analysis of the string.The flow field analysis model of casing string in the UGS is established by the finite difference method.The flow field and pressure distribution are studied in different injection-production pressures and well deviation angles.The results show that the gas velocity in the casing string changes from the annular distribution to the helical distribution with the increase of vorticity.The velocity in the inner string increases and produces the pressure vacuum,which makes other pressure larger and inner pressure smaller.As the vorticity decreases in the casing string,the flow velocity of fluid in the string tends to be stable and the pressure difference between the outer and inner string decreases.Therefore the bending section of string is prone to vibrate and fracture.The casing collapse strength and internal pressure strength in different diameter to thickness ratio are compared in the API standard and ISO standard.The strength analysis and remaining life evaluation are carried out on the casing string of Turkey salt cavern.According to the Norsok corrosion model,the corrison velocity is calculated in different well depth of well UGS-2 of the Turkey salt cavern.The influence of the corrosion protection film on the corrosion velocity is studied and it is proved that the corrosion protection film is efficient when the well depth is above 500 m.According to the remaining well thickness of string obtained by the experiment,the corrosion velocity of the well is studied and it is consistent with the theoretical results.The relationship between the remaining life of casing string and well depth is obtained and the minimum life of casing string in all well intervals is above 30 years.