Study On Creep Law And Stability Of Cavern Natural Gas Storage In Thin-bedded Rock Salt

Due to its extremely low permeability,good creep properties and self-recovery properties,rock salt has become an internationally recognized ideal medium for underground energy storage.Compared with foreign giant salt domes,the rock salt in China is with thin single layer,multiple layers and high impurity content.Fundamental differences exist in the basic mechanical properties,creep deformation laws between thin bedded rock salt and complete rock salt,and the cavern shape,creep law and the long-term stability is limited by the geological conditions.Therefore,it’s necessary to carry out new theories and methods through deep research,special design and optimization analysis.This paper firstly summarizes the research results of mechanical properties,strength properties,creep properties and creep constitutive equations of thin-bedded rock salt,and then selects the creep constitutive equations suitable for this study.Secondly,in the aspects of the analysis of sensitive factors,this study mainly focused on the influence of ratio of height to diameter of the cavern,operating pressure(constant pressure and cyclic pressure),interlayer thicknessa and interlayer stiffnes on the creep law and long-term stability of vertical gas storage in thin-bedded rock salt.Thirdly,using the sigle-well vertical gas storage as the control group,this paper studies the influence of the shape and size of horizontal cavern,the working mode,the number,thickness and type of the interlayers on the creep law and long-term stability of the horizontal cavern gas storage in thin-bedded rock salt.And by comparing the numerical simulation results of the control group and the experimental group,a conclusion was drawn that the double-well horizontal gas storage in constant pressure is more economical,applicable and stable under the multi interlayer thin-bedded rock salt geological condituion.Finally,according to the special working mode(when natural gas is injected while the brine is drawn out),suggestions of improving the surface supporting technology and operation management are proposed.Based on the study of the creep law and long-term stability of thin-bedded rock salt cavern gas storage,the following four studies have been carried out:(1)The mechanical properties,strenghth theory,creep propertiers and mechanisms of creep constitutive equations of thin-bedded rock salt have been studied and summarized deeply.And the viscoplastic power law MODEL COPWER is selected to calculate which is suitable for the thin-bedded rock salt cavern gas storage buried underground depth 1000m.(2)Aiming at the traditional single-well vertical cavern gas storage,the effects of the ratio of height to diameter of cavern,operating pressure,the interlayer’s thickness and stiffness on the creep law and long-term operation stability is studied.On the basis of the ellipsoida shape,the optimal ratio of height to diameter is 1.6 through comparing different ratios.While meeting the requirements of hydraulic fracturing criteria to ensure the tightness of the gas storage,it is beneficial to appropriately increase the operating pressure for the creep control and long-term stability.Recommeded operating pressure range of gas storage is 8～18MPa.The increase in interlayer thickness can control the volumetric shrinkage of the caver in a certain extent,so it is recommended to build the gas storage in a thicker interlayer.Due to the difference in elastic modulus,the hard interlayer plays a role in reducing the maximum rheological displacement of cavern surrounding rock and controlling the cavern volumetric shrinkage,and on the contratry,the weak interlayer will redudce deformation resistance of the cavern,and the effect of the equivalent interlayer is between the two.While analyzing each sensitive factor for the vertical cavern gas storage in thin-bedded rock salt,suggestions for cavern construction and long-term stability control messuress are also proposed.(3)Based on the characterstics of rock salt in China are thin-bedded and multiple layers,the feasibility,cavern creep law and long-term operation stability of double-well hrozital cavern gas storage has been studied using the single-well vertical cavern as the control group.The main considerations are the following aspects:cavern shape,cavern size,working mode,the number,thickness and stiffness of interlayers.When the maximum diameter is at least 2 times of its height,the maximum rheological displacement of the ellipsoidal cavern and rotary cavern is almost same,while the rotary cavern control the volume shrinkage better.And it’s larger and more economics.So,it is recommended to bulid the rotary cavern.The effects of different cavern size of the creep law and long-term operation stability are compared and analyzed,and a conclusion is proved that the increase in the volume of the cavern can control the convergence in a certain extent.Rotary cavern with 4.0 ratio height to diameter is recommended in thin-bedded rock salt with single layer is 70m.Comparing and analyzing the two working modes:constant pressure(when natural gas is injected while the brine is drawn out)and circular pressure,it shows that the special constant pressure working mode can control creep law and improve the long-term stability of gas storage since it can keep the internal pressure constant and more flexible in injection.The change of interlayer thickness has little effect on the creep and long-term stability in horizontal cavern gas storage,which indicates that the horizontal cavern has better adaptability to the thin-bedded rock salt stratum conditions.Due to the difference in elastic modulus,the hard interlayer plays a role in reducing the maximum rheological displacement of cavern surrounding rock and controlling the cavern volumetric shrinkage,and on the contratry,the weak interlayer will redudce deformation resistance of the cavern,and the effect of the equivalent interlayer is between the two.All working modes are measured using the singe-well vertical cavern gas storage as control groups.It can be seen that the stability and convergence of the double-well horizontal cavern gas storage is better,and its work volume is lager and more economical,and the injection and gas production is more flexible.So,it is more adaptable to the geological condions of multi-layer thin-bedded rock salt in China.(4)Based on the special operating mode of the double-well horizontal cavern gas storage,corresponding improvement messures are put forward for the ground processing.Under the premise of the old cavernsupporting water injection station and the brine station,the existing equipment should be fully utilized.The design of the cavity water source,brine treatment,and brine tanks proposed effective measures,and proposed operational control measures for the crystallization of the pipeline that may occur during the brine discharge process.