Optimization Design Of Downhole String For Multiple-fluids In Thermal Recovery

The string for multiple-fluids in thermal recovery of offshore heavy oil is an important class of downhole string and its specificity is determined by the complex wellbore environment and multiple load cases.On the one hand the string has features of slender and flexible and it deforms greatly and has nonlinear contact with the wellbore when placed in a three-dimensional horizontal wellbore.On the other hand,the string is affected by a variety of loadings during working.A series of problems occur when the string is working.In the process of well completion or well workover,contact force and friction force exist between string and casing during the lowering and raising of the string.Friction between string and casing affects the lowering and raising of the string as well as the service life of the string.Meanwhile,the string withstands other loadings such as gravity,pressure and thermal stress during the steam injection,which brings a great test for the string strength.Therefore,the structure design and strength analysis of the string are necessary.The work in this paper is carried out from the following aspects.1.The parameters of well track are calculated by using the cylinder helix method.The three processes of lowering,raising and steam injection of the string are studied.Nonlinear finite element mechanical model is established.The matrix is assembled in the node coordinate system using three-dimensional beam element.The large linear algebraic equation is solved by improved root-squaring method and the nonlinear contact problem is solved by spring iterative method.The finite element analysis results of three processes of lowering,raising and steam injection of the string are obtained.2.The positions and number of centralizers are designed according to the distribution of contact force and friction force in the processes of lowering,raising of the string and the well track.The stroke of the thermal compensator is designed according to the bottom displacement in the steam injection process of the string.The lowering possibilities of the string are studied according to the analysis of the hook load in the processes of lowering,raising of the string.The strength of the string is checked by the fourth strength theory in the processes of lowering,raising and steam injection of the string.3.The optimization design system of string for multiple-fluids in thermal recovery is developed by using the Visual Basic 2010.The optimization design system of string for multiple-fluids in thermal recovery has 12 modules including basic data,well track description,string's load analysis,string's finite element analysis,matching tools' layout design,string's structure combination,string's lowering analysis,string's structure safety evaluation,string's structure optimization,report,industry standards and help.It's used for the structure design and strength analysis of the string to guide the engineering practice.