Research On Ultimate Strength Of Casing With Corrosion Defects

In the development of the drilling technology,deep well,ultra-deep well,complex formation well and containing corrosive medium well has been widely used in oil and gas exploration,casing has become an essential part of string in the constructions of the oil or gas wells,which lead to a high fault rate day by day.This problem can not only cause significant economic losses but also decrease service life of oil and gas wells.According to the features of mechanical action and chemical effect,the damage of casing can be classified as offset,break,extrusion and corrosion.Among this,the safety problem of corrosion casing has become one of the critical factors to restrict the development of resource.Therefore,the analysis of casing corrosion mechanism and safety evaluation has become an important procedure in exploration of oil and gas reservoirs.Nowadays,some research has been done by domestic and foreign scholars,which mainly focus on prevention and control of casing corrosion.But there were less study on applicability evaluation about it.Moreover,the former can only slow down the corrosion procedure but cannot avoid it,so it is meaningful to make safety evaluation to corrosion casing which are being used.For the complexity of corrosion casing,this research is to analyze strength variation rule of corrosion casing and make a safety assessment criteria based on elastic-plastic mechanics,finite element method and reliability theory,which can provide scientific basis to the efficient utilize of oil and gas resource in our country.The main content includes:(1)This thesis studied the casing damage and collects the casing research background and the related progress at home and abroad.Classifying the types of casing corrosion and analyzing the factors of it such as acid corrosion,salt material corrosion,bacteria corrosion,erosion-corrosion.An influential analysis of corrosion rate has been made which considered concentration of corrosive,velocity,temperature,some kinds of anion and cationic.(2)An investigation has been made on evaluation method of casing residual strength and made corresponding analysis including the rule of ASME/ANSI B31G,API 579,elastic-plastic theory analysis method,finite element method and reliability evaluation method.The finite element method was used to discuss the factors of computational accuracy and efficiency which includes discrete finite element structure,element analysis and integration of total rigidity matrix.(3)Based on the investigation of failure morphology of corrosion casing in practical situation,the assessment-criteria of pressure vessel with defects was applied to regularization process of service casing after corrosion and schematic diagram on different corrosion conditions has been drawn according to GB/T19624-2004.Based on strength calculation theory of API,A three dimensional numerical model of casing with imperfections has been built through simulation contrast verification,which provide a new calculation method of safety-evaluation on corrosion casing.Besides,through the same method,collapsing strength and internal pressure strength of the corrosion casing in different conditions has been get based on the finite element method.And the functions of them were built after numerical calculation and simulation analysis,which can be regarded as theoretical foundation on the practical use of casing.(4)Based on the regularization processing of corrosion casing and the thermo-structural coupling theory,a three dimensional model considering a system containing a casing with ellipsoid shape defects,a cement sheath and formation has been built,the other considered through casing type defect.After conducting experiments with orthogonal test,the Von mises stress of N80,TP90H and P110 were calculated through the finite element method.The influential analysis was made by simulation and range analysis method on residual strength of the two kinds of corrosion casing which considered a single corrosion parameter,axial force,inner pressure,crustal stress,temperature.The main and secondary factors were decided and the result can provide advice to the design and the construction of casing in corrosion situation.(5)With the analysis of the date,the Matlab was used to match the predicted formula of the max mises stress for two different models which considered a casing with ellipsoid shape defects or through casing type defect respectively.The research can make foundation of practical use.