Study On Expansion Law And Stability Evaluation Of Two-well Cavern With Small Spacing In Salt Rock

A sound gas storage system is of great significance to my country's energy strategy.Underground gas storage has obvious advantages over ground storage in terms of storage capacity and safety.Salt cavern gas storage has become an ideal gas storage place because of its low permeability,good creep and self-healing characteristics.Traditional salt rock cavern use the single-well oil cushion method,which has problems such as"slow speed,small cavity,and oil pollution".In response to the strategic needs of national energy development,this paper uses a combination of physical similarity simulation experiments and numerical simulations to study the expansion law and long-term stability of the small-spaced two-well salt cavern gas storage,through the analytic hierarchy process and fuzzy comprehensive evaluation evaluated the long-term stability of the gas storage.The main conclusions are as follows:(1)In the experiment of small-spacing two-well water-dissolving cavity building,the cavity-building efficiency increases with time.Appropriately increasing the wellhead spacing can increase the brine concentration at the early stage of water-soluble cavity building.The influence of wellhead spacing on the cavern shape is concentrated on the top and bottom of the cavern.The greater the wellhead distance,the greater the cavity bottom space,and the higher the upper limit of the cavity volume.At the same time,the greater the roof settlement,the more tolerant it will collapse.There is a large bulge in the middle of the cavity bottom by the naturally dissolution of the two straight wells,which is not conducive to the migration of sediment and reduces the effective volume of the cavern.(2)A numerical model of the salt cavern gas storage was established based on the cavern shape obtained from the cavity-making experiment(the cavern was buried at a depth of 1000m),and the FLAC^3D software was used to simulate and analyze different wellhead distances and circulating internal pressure.When the wellhead spacing is 14m,the volume shrinkage rate of the cavern is the smallest;the roof subsidence increases with the increase of the wellhead spacing,and the maximum settlement is 1.12m;the smaller the wellhead spacing,the more concentrated the plastic zone,the easier it is to cause a large area of communication,leading to the collapse of the wellhead;the lowest circulating internal pressure will have a greater impact on the stability of the cavern,and it is not recommended to use a minimum operating internal pressure lower than 8MPa.(3)Through statistical analysis of the failure risk factors of salt cavern gas storage during the operation period and the process flow of small-spaced two-well cavern creation,the fault tree analysis method was used to establish a small-spaced two-well fault tree analysis model,which determine the basic events that lead to the instability of the gas storage.And through the minimum cut set and structural importance,the main basic events that caused the instability of the two wells with small spacing were determined,which provided a basis for the selection of stability evaluation indicators.(4)According to the main basic events and actual working conditions determined by the fault tree,a small-spacing two-well multi-level stability evaluation system consisting of 17 evaluation indicators is established with four aspects:cavern parameters,casing shoe height,pipe string unit and gas storage pressure as the main body.Through the analytic hierarchy process,the weight of each index is determined,and it is found that the cavern parameters and gas storage pressure have a greater influence on the stability of the small-spaced two-well.At the same time,based on the laws obtained by numerical simulations,the scoring model of the main evaluation indicators such as the wellhead distance and the ratio of the cavern length to the short axis is determined.The fuzzy comprehensive evaluation was used to pre-evaluate the stability of the proposed M well,which provides a basis for the on-site implementation of the small spacing two-well.