Research On The Corrosion Mechanism Of Casing Failure In Jiangsu Oilfield

The actuality of casing failure of inland oilfield and the affecting factor of casing failure were reviewed in this paper, and the circumstantialities of casing failure of Jiangsu oilfield were also introduced. The electrochemical factors of N80 steel casing corrosion and the stress corrosion cracking (SCC) of N80 steel were studied by polarization curve and slow strain rate tension (SSRT) separately, and the fracture surface morphology was examined by scanning electron microscopy (SEM), and the mechanism of SCC process was discussed. Moreover, corrosion fatigue properties of N80 steel was studied by corrosion fatigue experiment. Furthermore, the inhibition performance of corrosion inhibitor which was used in oilfield-water, was researched by X-ray photoemission spectroscopy (XPS). The results showed:(1) The sequence of different gas accelerating corrosion rate of N80 steel in oilfield-water is:H₂S>CO₂>O₂>N₂(2) The temperature and rotating rate have effect on corrosion of N80 steel when the oilfield-water is saturated by different gas, and the corrosion rate of N80 steel arrives at the top at about 75 degree centigrade and 1500 rpm.(3) The oilfield-water of Jiangsu oilfield has distinct effect on mechanics property of N80 steel. N80 steel displays the fracture features of the dimple in air, which appears excellent ductile fracture. However, the ductility, yield stress and maximum bear stress decrease of N80 steel decreases in oilfield-water, which expressed partial intergranular fracture.(4) The mechanical property of N80 is affected by strain rate and temperature.(5) N80 steel has definite SCC sensibility in oilfield-water compared in air, but lower strain rate administer to relieving the ductility loss and the SCC sensibility of N80 steel.(6) The mechanical property of N80 steel has increased obviously after adding corrosion inhibitor, where the ductility of N80 steel increased, and crack fissure of fracture surface decrease.(7) Corrosion fatigue test shows that the state fatigue limit S_0.1 of N80 steel is 326N/mm² in oilfield-water.(8) XPS analysis shows the presence of Zn,P,Ca etc. on the surface of N80 steel after corrosion inhibitor is added, but the corrosion products are mainly ironic oxidation without corrosion inhibitor.