Oxygen corrosion of N80 steel under laboratory conditions simulating high pressure air injection: Analysis of corrosion products

High pressure air injection (HPAI) is an effective and economic improved oil recovery process, however, the corrosion of tubing and casing in the injection well is one of the serious safety concerns which have limited its widely application. Until now the structure and composition of corrosion products formed in HPAI condition are still not fully clear because of the difficulty in simulating such a harsh environment in laboratory. In this work, the corrosion rates of N80 steel under laboratory conditions with different oxygen content (21-5% vol%), total pressure (50-20 MPa) and temperature (120-70 °C) simulating HPAI in Dagang oil field in China were investigated using weight loss measurement. The corrosion products on N80 steel surface were analyzed using scanning electron microscopy coupled with energy dispersive spectroscopy (SEM + EDS), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the corrosion rate decreased with the decreasing oxygen partial pressure and temperature. Furthermore, for all the conditions, the corrosion rate was incredibly high in the simulating HPAI environment, consequently, large amount of corrosion products were formed. The corrosion products showed a two-layer structure: outer layer and inner layer. The outer layer was porous, brittle and easily flaked off, while the inner layer was more compact than outer layer. However, both layers could not effectively lower the corrosion rate of N80 steel in HPAI environment. The corrosion products were composed of hematite (α-Fe2O3), goethite (α-FeOOH), magnetite (Fe3O4) and trace amounts of Ca(OH)2 and CaO. In addition, the structure and composition of corrosion products did not change with the temperature and oxygen partial pressure in the simulating HPAI conditions.