| The tubing and cementation structures constitute the major parts of Underground Gas Storage(UGS)wellbore,where the cementation structures include the casing,cement sheath as well as the surrounding strata.These structures are referred to in the literature as“UGS wellbore structures”and their failure mechanisms have been the subjects of many studies in recent years.Due to the ultrashort period of alternate injecting and producing operation under the conditions of high pressure and high gas flow velocity,severe challenges are posed for the integrity of the UGS wellbore.However,previous studies on failure evaluation of UGS tubing are mainly based on static analysis,which says,the dynamics of tubing in injection-production process especially during open-shut operation,rare studies are conducted.On the other hand,up to now,most failure evaluation processes of cementation structures have been developed from the ideal simplified models.The mechanical states and failure mechanism of cementation structures under the effects of temperature difference and non-uniform geo-stresses need more focus.In this work,under the engineering background of a UGS(so called UGS-T4)in a gas field locating in southwest of China,failure mechanisms for both tubing and cementation structures are comprehensively and richly studied through theoretical methods,numerical calculations and experimental verifications etc.Furtherly,a method for evaluating the wellbore integrity of UGS is proposed,verified and implemented.The achievements of this dissertation are expected to provide guidance for the design and safety evaluation of UGS.The detailed content is summarized and organized as follows:(1)Fluid-structure interactions(FSI)between gas and tubing under dynamic pressures are investigated.The FSI model used herein is established through the Euler equations describing the transient mechanical state of the cross-section of the gas-tubing system.To implement Glimm's method,analytical solutions for Euler hyperbolic equations of initial value problem are presented based on the transformation of Euler equations and Riemann problem.By recombining the FSI model and Glimm's solution,the expressions of tubing's velocity,stress along with bending moment in local coordinate system from timet(28)t_n to momentt(28)t_n(10)(35)t are derived.Furtherly,by FLUENT UDF(user-defined functions)programming on the Riemann-Glimm(R-G)method,the influences of dynamic pressure during the well-opening process on the acceleration,velocity and displacement of the gas-tubing system in the cross-section are studied.The effects of parameters like tubing internal diameter and pressure difference on are also explored.(2)Experimental verification of the R-G method is conducted and the near-wall pressure of tubing in the production process is analyzed.Taking the UGS-T4 as the prototype,an experimental device of simulative gas-tubing system is built.With Tijsseling's work referred,experimental procedures are designed to verify the accuracy of the aforementioned FSI model and R-G method.From the measured data of near-wall pressure,acceleration and strain of tubing under certain impact loads,the prediction of R-G method agrees well with the experimental results.On experimental basis,a detailed analysis on the optimal mesh quantity and the reasonable range of CFL(Courant-Friedrichs-Lewy)coefficient is carried out.Simultaneously,a quasi-similarity experiment is developed according to the geometric similarity conditions and Reynolds self-similar conditions and by which,the injection and production process of UGS-T4 is simulated.Combing with the numerical computation,the values and distributions of near-wall pressures of the tubing with the variations of internal diameter,dogleg severity and joint dimension of the tubing as well as the well pressure difference are discussed.(3)Mechanics and failure mechanism of UGS tubing in production process is investigated.A set of complete iterative procedures,based on CEM(cellular element method)method,are derived to solve the mechanical problems of tubing with dynamic internal pressure.Comparing with FEM,CEM is verified both in accuracy and efficiency using the software of ANSYS.The stresses and deformations of tubing in both stable production process and well-opening process are explored by FSI and CEM.A conclusion is drawn that the deformation of tubing and its neutral point are mainly determined by the axial force.For convenience,two indexes of“tubing failure index(TFI)”and“tubing failure factor(TFF)”are defined according to the failure modes of the tubing.Quantitative formulas for evaluating the extent of tubing failure are thus derived.The case study indicates that the deformation failure caused by buckling is the main mode of tubing failure forms in both stable production process and well-opening process,and the TFF rises with the increasing of the tubing inner diameter and pressure difference.Initially,the deformation failure caused by buckling is prone to occur with a relatively small pressure difference,while with the pressure difference rise,strength failure will gradually become the leading failure form of the tubing in turn.(4)Calculations of geo-stresses around UGS wellbore based on layering model are conducted.A method to obtain the dynamic and static lithology parameters of the target block using well logging data and Gristensen's formulas is proposed and is verified by triaxial tests of rock cores.Then,based on the multi-objective constrained optimization theory,numerical inversion is implemented to invert the geo-stresses of the target block where UGS-T4 is located.Afterwards,the geo-stress field around the wellbore of UGS is obtained through the formulas that derived from the layering geo-stress model and hole-plate model,considering the tectonic geo-stresses and additional geo-stresses.Furtherly,the distributions and variations of geo-stresses around the UGS wellbore with horizontal azimuth and distance from well-wall are discussed.(5)Mechanics and failure mechanism of UGS cementation structure are studied.Taking displacement continuity as the boundary condition,an improved mechanical model for cementation structure is established considering the effects of both non-uniformed geo-stress and temperature difference.Formulas to calculate the stresses in cementation structure are also derived based on the aforementioned studies of geo-stresses around the wellbore.In combination with the calculation results of geo-stresses,the distributions of stresses in the cementation structure and interfaces are presented.Moreover,the influences of temperature difference on the stresses of the cementation structure are given.Similar to what has been done to the tubing,two indexes of“cementation structure failure index(CFI)”and“cementation structure failure factor(CFF)”are defined,and the quantitative formulas to evaluate the degree of cementation structure failure are derived.Further investigations are focused on the effects induced by the changes of non-uniform coefficient,annulus pressure,stratum temperature and material parameters,which are rather influential to the cementation structure but instead,always overlooked.(6)A quantitative evaluation method for UGS is proposed and applied in engineering.Before the systematic study on the evaluation method of integrity for UGS,an analysis model of contributors of wellbore integrity failure is deduced and implemented based on the statistical theory and reliability calculation.On this basis,with the principle of minimum loss of information,classification and importance ranking of“wellbore integrity failure contributors(WIFC)”is carried out.Taking UGS-T4 as an example,the number of categories of WIFC is reduced from 23 to 7,and the event of"equivalent stress failure of the casing"is ranked the first.For further study,formulas of reliability of wellbore integrity are derived through central-point method and comparatively verified by Monte-Carlo method.Built upon these formulas,relationship between“wellbore integrity reliability probability(WIRP)”and“wellbore integrity reliability index(WIRI)”is established and expatiated.On this basis,the applicability and accuracy of the proposed method is verified by case of UGS-T4.The results show that the WIRP of UGS-T4 is 0.8966 and the integrity grade is“Minor reliable”using the proposed method in this dissertation,while the WIRP of UGS-T4 is 0.9106 and the integrity grade is“Reliable”using Monte-Carlo method.It is proved that the evaluation method proposed herein is accurate and relatively conservative which is more suitable for practical application. |
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