Study On Influencing Factors Of S135 Drill Pipe Steel Oxygen Corrosion And Corrosion Fatigue

In the petroleum well drilling project, drill pipe corrosion and corrosion fatigue are main factors to cause drill pipe failure. In recent years, with the well drilling to the development of highly difficult wells, such as deep well, large displacement well, horizontal well, etc. the problem of drill pipe corrosion and corrosion fatigue is becoming more serious, demanding prompt solution. The S135 drill pipe steel was selected as the research object in the paper;the oxygen corrosion and corrosion fatigue behavior of drill pipe steel was studied.On the basis of the theory analysis systematically and the X-ray diffraction analysis of corrosion products, the oxygen corrosion mechanism in 3%NaCl solution of drill pipe steel was studied. The corrosion of drill pipe steel in the neutral or weak alkaline solution was the function of oxygen depolarization;at the same time oxygen corrosion was control by oxygen in 3%NaCl drill fluid diffusion course of the liquid. Though the dynamic simulated test of the indoor autoclave, the influence law that oxygen content, drill fluid temperature, drill fluid velocity of flow and salt density of the drill fluid had studied separately. The result indicates that the increase of the oxygen content of the drill fluid and velocity of flow will promote oxygen corrosion rate of the drill pipe steel, and it have double affection that drill fluid temperature and salt content on the oxygen corrosion rate of the drill pipe steel.A three-point bending test of corrosion fatigue in 3.5%NaCl solution for drill pipe steel was carried out in the paper, combine with the theory and analysis, the behavior of corrosion fatigue of near threshold area and Paris area of drill pipe steel of BNK and NKK drill pipe steel has studied deeply and systematically. Origin software was adopted to fit to the drill pipe steel linearly while analyzing on the corrosion fatigue crack of Paris area, two kinds of drill pipe steel expands the relation between the corrosion fatigue crack rate and amplitude of factor of intensity of stress had established, the theoretical foundation in advance for the surplus fatigue life with defect drill pipe steel had estimated and offered. Detecting instruments, such as the metallographic electron microscope, picture analysis instrument, etc. used to announce that the relation between the corrosion fatigue crack and microstructure ofthe drill pipe steel, the result indicates that the impact toughness and material segregation of the material has obvious influence to propagate the corrosion fatigue crack. The morphology of the drill pipe steel was studied by using equipments, such as digital camera and scanning electron microscope, etc. were adopted. Finally the corrosion fatigue mechanism in 3.5%NaCl solution of drill pipe steel systematically had overall announced, the corrosion of the drill pipe steel relies mainly on oxygen depolarization in near threshold area, but with the increase of the amplitude of factor of intensity of stress, the anode dissolves and acts on the hydrogen brittleness together was the mechanism of corrosion fatigue of the drill pipe steel.