Study On Mechanism Of Stress Corrosion Cracking Of Oil Casing In Common Completion Fluid

In the completion process, the oil casing is long-term in the corrosive completion fluid,the service condition is bad and the coiled tubing is also subjected to alternating stress such as bending stress, normal stress and torque. Under the combined action of corrosion and alternating stress, Tubing prone to stress corrosion cracking, resulting in a sudden breakage of the tubing, oil production caused huge losses, and even cause casualties. Therefore, it is very important to study the mechanism of corrosion and stress corrosion of oil casing in completion fluid, and to study the corrosion mechanism and the fracture mechanism, fracture critical stress and crack initiation and propagation of oil casing.In this paper, two kinds of commonly used oil casing casing materials P110, 13Cr in the completion of the static electrochemical analysis, taking into account the impact of temperature. Electrochemical polarization curves show passivation properties and exhibit good corrosion resistance. With the increase of temperature, the corrosion resistance of the material decreases, and at 70℃, the corrosion rate decreases and the corrosion rate tends to be stable. Corrosion to local corrosion-based, but did not appear comprehensive corrosion,compared to the two materials show that 13Cr has good corrosion resistance. Under the condition of stress, the corrosion regularity of 13Cr steel in formate and chlorate completion fluid is similar. With the increase of stress, the passivation current density of 13Cr steel increases gradually, the increase of high stress is more obvious, the corrosion resistance decreases The In the formate completion fluid, with the increase in stress. P110 steel corrosion resistance decreased. Unlike the formate completion fluid, the corrosion potential of P110 steel decreases with the increase of stress in the chlorate completion fluid, which indicates that the corrosion tendency is reduced. Compared with no stress conditions, once applied stress, P110 steel passivation current density increased dramatically.The corrosion resistance of 13Cr was higher than that of P110 by the weight loss method. The corrosion rate of the two kinds of oil casing was measured by pressure,temperature and completion fluid. With the increase of temperature and pressure, 13Cr and P110 steel in the chlorate completion fluid passivation stability decreased, 6MPa when the most serious corrosion in the formate completion fluid there is a critical value, 3MPa, 90℃corrosion The most serious. 13Cr and P110 steel pitting in the chlorate completion fluid pitting, in the formate completion fluid in the form of uniform corrosion occurred. The fracture morphology of 13Cr and P110 steel was observed by scanning electron microscopy(SEM), which was characterized by typical brittle fracture.The finite element method is used to model the oil casing. The analysis shows that the triangular defects in the oil casing are more dangerous than the conical defects. The analysis shows that the fracture critical stress decreases gradually with the depth of the gap and is the smallest when the wall thickness is half. With the increase of the arc length of the crack, the critical stress value is the smallest when the crack angle is 30°. The transverse crack of P110 and 13Cr is more sensitive to tension than longitudinal crack. Under the action of internal pressure or external pressure, the longitudinal crack is more sensitive than the transverse crack to the internal pressure load. At the same time, the critical tensile stress of crack propagation is decreased under the action of internal and external pressure and tensile stress.The scanning electron microscopy (SEM) shows that the port has no necking phenomenon at the time of final fracture, the port is transiently fractured, the port is flat and the crack propagation direction is perpendicular to the direction of tensile normal stress. The micro -fracture morphology is cleavage fracture.