| With the continuous breakthrough and innovation of conventional and unconventional oil and gas exploration and development technology,as well as the transformation of energy structure and the upgrading of environmental protection policies,China’s natural gas industry has entered a period of rapid development.The rapid growth of natural gas consumption demand has brought great pressure on gas transportation and storage infrastructure in China.In the 14 th five year plan,it is clearly pointed out that achieving the goal of doubling the effective energy storage of underground gas storage,this brings great challenges to the construction of underground gas storage.Salt cavern gas storage is considered to be an ideal place for optimization of gas transmission network and seasonal peak shaving of urban gas consumption.Due to the complex geological conditions of many and thick insoluble intercalations and thin single-layer salt beds,the single well cavity construction abroad is lack of adaptability in this kind of strata,which makes the construction of salt cavern gas storage in China difficult,high risk,slow progress and huge pressure to achieve the goal of increasing storage capacity.In recent years,the double well cavity construction technology has been proposed and proved to have the adaptability and advantages of layered salt reservoir construction,but it is still in the exploratory test stage,lack of research and understanding on the reasonable shape,operation parameters and cavity construction process parameters of double well cavity,and the applicability of relevant stability evaluation methods is also unclear,which seriously hinders the construction process of underground salt cavern gas storage.Based on this,in view of the series of problems faced by double well cavity construction in layered salt layer,combined with the geological characteristics of Pingdingshan Salt Mine,this paper explores the mechanical characteristics of layered salt layer,the evaluation method of salt cavity stability,the shape of double well salt cavity,the operation parameters,and the design method of cavity making process parameters.The main work is as follows:1.Experimental study for the mudstone,salt rock and bedded salt rock in Pingdingshan salt mine,the strength test and creep test are performed to obtain the corresponding mechanical parameters of three typical rocks.Comparison and anlysis for the strength characteristics and creep deformation laws of these rocks are carried out.The results show that the layered rock mass composed of three kinds of lithologic rocks still has good properties and has the conditions for constructing underground gas strorage.2.A set of long-term stability evaluation index method for deep salt cavern gas storage is put forward,which involves deformation,yield,damage and sealing.Based on the linear elastic superposition principle,the ellipsoidal inclusion theory in higher elasticity is introduced,and the stress analytical formula of wall rock under triaxial unequal general ellipsoidal salt cavity pressure is derived from the theoretical analysis,and the feasibility of the model is verified by theory and numerical simulation.The influence of internal pressure and cavity shape on the stability of ellipsoidal gas storage is explored by using the derived model combined with Mohr Coulomb plastic yield criterion theory,which enriches the analytical theory of surrounding rock stress of complex salt cavern gas storage.3.The two-well salt cavern models with different top structures and differernt well spacing are established to study the law of displacement,stress,deformation and damage around the cavern wall in connection with static mechanics analysis and rheology analysis.With the decrease of cavern roof height and the increase of well spacing,the indexes of gas storage gradually weaken and the stability gradually decreases.Considering the yield evaluation criterion and the maximum volume principle,the reseanable top shape and well spacing for two-well cavity are proposed.4.The schemes of various internal operation pressure and production rate are designed based on geological condition of Pingdingshan salt mine.The mechanical reponse and deformation characteristics are studied.The results show that the maximum principal stress around the cavity decreases gradually with the increase of the maximum internal pressure,and with the increase of injection production rate,the stability of the cavity gradually weakens,at the same time the operation minimum/maximum pressure and maximum gas recovery rate for two-well cavern are put forward.The model of typical two-well cavern in Pingdingshan salt mine is built and the stability evaluation is carried out under the long-term injection and production condition by elasto-plastic creep analysis.It si concluded that the more irregular the cavity shape is,the worse the stability is,and appropriately increasing the injection production cycle is conducive to the stability of the cavity.5.Based on the principle of single and double well cavity making,combined with the string structure and the expansion characteristics of salt cavity shape,considering the influence of fluid flow state,property,string roughness,salting out scaling on water injection pressure and the loss of solvent resistance caused by the change of well diameter and cavity collapse deformity,the calculation model of water injection pressure for single and double well salt cavity is optimized,and the calculation method of solvent resistance consumption is designed,At the same time,the design software of single and double well cavity making process parameters is developed,which realizes the functions of single and double well string optimization,water injection pump pressure design,oil cushion / air cushion solvent consumption design and cavity making energy consumption cost estimation.The software interface is simple and beautiful,and has strong applicability. |
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