Root cause analysis of the Circular hole corrosion failure of N80 tubing

Abstract

During well repair operations in an oil field, a Φ73.02 mm×5.51 mm N80 grade flat oil pipe with a circular hole was found. The depth of the well is between 550 meters to 2100 meters, with a usage cycle of 120 days. In order to figure out the root cause of the circular hole failure, a series of experiments were carried out. The chemical composition, yield strength, tensile strength, and elongation of the oil pipe meet the requirements of API SPEC 5CT-2018 standard. The microstructure of the oil pipe matrix consists of ferrite and pearlite, and there are no obvious cracks. The grain size of the material is all above 8.5 levels, and there are no coarse grains or non-metallic inclusions exceeding the standard. The perforation of the oil pipe is caused by internal corrosion, mainly due to CO2 corrosion and dissolved oxygen corrosion. The coexistence of Ca2+ and Mg2+ ions, iron bacteria, and corrosive bacteria in the wellbore fluid can lead to scale deposition inside the pipe, causing under-deposit corrosion, and the accelerating effect of Cl ions on corrosion ultimately leads to perforation of the pipe.