| With the speeding up of global warming and the pace of the reform of the ecological civilization system in China,clean energy is now becoming the trend of energy development in the future.As the main force of clean energy,natural gas is facing the greatest opportunities and challenges.However,the natural gas reserves in the conventional oil and gas resources are difficult to meet the growing demand.The natural gas resources in the new block with the characteristics of buried in deep reservoirs,which is difficult to developt.In order to cope with the high temperature and high pressure and strong corrosion environment(corrosive medium such as CO2,H2S)in deep wells and ultra deep wells in the new block,13Cr-L80,super 13Cr tubing and annulus protection fluid are often used.With the advantages of high density,good compatibility and low corrosion,the formate annular protective solution is widely used in high temperature and high pressure gas fields in foreign countries as a high efficiency environmental protection liquid system.Although the super 13Cr stainless steel tubing used in the formate annulus protection liquid in the high temperature and high pressure oil field abroad seldom fails,the failure of the super 13Cr stainless steel tubing is still occurring at the extreme conditions of the ultra deep wells in China.In this paper,under the support of 2017?2019 major project in Tarim Oil field,the failure of super 13Cr oil pipe,which is widely used in deep wells and ultra deep wells,is systematically studied,including the following points.Firstly,this paper introduces the theory of linear elastic fracture mechanics.Based on the core of the material crack and the strain field model of the crack stress field,the stress intensity factor K criterion of material fracture is established,which mainly includes the reason for the crack nucleation and the distribution characteristics of the stress field and strain field after the crack formation,which is extended by the stress field and strain field.The concept and characteristics of stress intensity factor of crack tip are presented.The K criterion for fracture mechanics is eliminated and the stress intensity factor of the material is calculated by considering the plastic zone correction at the crack tip of the actual material.On the basis of the theory of on-line elastic fracture mechanics,the most commonly used tubing in the oil field is analyzed,the maximum internal stress allowed by the material is obtained,and the effect of mechanical factors on the failure of the material is analyzed.Secondly,according to the fracture failure of super 13Cr oil pipe in ultra deep well oilfield,the analysis of the surface crack distribution of material surface was carried out by the magnetic powder detection.Based on the analysis of nondestructive testing,the tensile mechanical properties of the materials,the tensile fracture morphology and and the maximum crack depth of the tensile fracture were analyzed.Then the surface condition of the material was systematically analyzed,including the crack distribution on the surface of the material and the distribution of pitting pit.At the same time,fracture surfaces scanning electron microscope observation and surface corrosion product films XRD phase analysis was carried out.Through the above research,it is found that the downhole tubing produces a large number of longitudinal cracks at the harsh environment of the ultra deep wells.The cracks mainly occur to the bottom of the corrosion pit,extending to the bottom of the pit to the interior,and there are a large number of materials in the well fluid at the top of the crack.The failure mechanism of the super 13Cr stainless steel tubing in the environment of high temperature and high pressure is obtained through the experimental study of the failure tubing,that is,the failure of the harsh environment is the stress corrosion cracking induced by the pitting corrosion.Thirdly,pitting is the main cause of stress corrosion cracking of super 13Cr stainless steel tubing,and pitting corrosion will reduce the safety performance of the material,cause corrosion perforation,and destroy the safety barrier of the well bore.Therefore,the pitting corrosion study was carried out with high temperature autoclave and Kelvin Probe method to test the pitting resistance of materials.It is found that there is an obvious local corrosion on the surface of the material by the high temperature point corrosion test,and it is found that the surface potential changes sharply with the increase in the reaction time,and the local corrosion characteristics have occurred.Through the study of the pitting pit development model,it is found that when the shape parameters of the pitting pit of the oval corrosion pit tend to a stable value of 0.3,the pitting will soon become unstable and develop into a crack.Fourthly,in order to study the reason of stress corrosion cracking of super 13Cr stainless steel tubing in high temperature and high pressure formate annular protection liquid,the stress corrosion cracking performance of the material was studied,including the analysis of the sensitivity of hydrogen embrittlement and the study of the stress corrosion cracking performance.In the analysis of hydrogen embrittlement sensitivity,the theory of hydrogen diffusion was introduced first.The relationship between hydrogen concentration and hydrogen charging current was established.The mechanism of hydrogen induced cracking and the factors affecting hydrogen induced cracking were discussed.The hydrogen production performance of formate solution and the analysis of hydrogen embrittlement sensitivity of super 13Cr stainless steel tubing was carried out.Through the study,it is found that in high temperature and high pressure environment,formate completion fluid will produce a large amount of hydrogen,and the existence of hydrogen atmosphere has a great influence on the mechanical properties of materials and materials.At the same time,the stress corrosion cracking of C ring was introduced to test the cracking resistance of the materials under different environments.The results show that the stress corrosion micro cracks of C ring at high temperature and high pressure appears under different test conditions,and the micro cracks in mud environment are the most serious.According to the experimental test,the mechanism of the stress corrosion cracking of the super 13Cr stainless steel tubing in the polluted formate environment is the anodic stress corrosion cracking.Generally speaking,stress corrosion cracking is caused by oxide film cracking and slip dissolution.In summary,this paper studied the failure of the super 13Cr stainless steel tubing in the high temperature and high pressure environment,pitting performance,hydrogen embitterment sensitivity and stress corrosion cracking.It has an important theoretical guidance and practical application value to solve the failure of deep well in Tarim well and the stress corrosion cracking of stainless steel tubing in high temperature and high pressure formate environment. |
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