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ABSTRACT: Corrosion of tubing steel induced by H2S and CO2 is an important risk factor for oilfield production and development. The trace amounts of H2S can induce local corrosion of X65 carbon steel, but there are few researches on the corrosion of low Cr steel. In this paper, the experimental process is designed to prepare trace H2S at 40°C to study the corrosion effect of N80 carbon steel, 1%wt Cr steel, 3%wt Cr steel and 13%wt Cr steel under CO2 environment that the partial pressure of H2S is 2 × 10−6-6 × 10−6 MPa. The results show that the N80 carbon steel has uniform corrosion behavior, and the Cr steel has different degrees of pitting corrosion characteristics. Finally, after calculating the corrosion rates of different Cr-containing tubing steels, it is found that the pitting corrosion rate of 1%wt Cr steel is more than three times that of 3%wt Cr and 13%wt Cr steels, which further indicates that trace H2S can induce serious pitting corrosion behavior of Cr steel. This discovery can help oilfield provide a theoretical basis for the optimization of the pipe materials used in the upper wellbore with a small amount of H2S, which can bring significant economic benefits. 1. INTRODUCTION The corrosion of tubing and casing seriously restricts the production and development of major oilfields in China. In the traditional CO2 corrosion environment, the presence of H2S will slow down the corrosion process of tubing and casing. However, the CO2 environment containing the trace amounts of H2S will induce local corrosion of the tubing and casing, which has a risk of pitting corrosion and perforation (Navabzadeh et al., 2017; Yan et al., 2018). At present, international scholars have conducted many researches on carbon steel in the CO2 corrosion environment with trace H2S. Those researches show that the corrosion of oil pipe steel is obviously inhibited when the H2S concentration of carbon steel is less than 100 ppm, thereby delaying the corrosion process of the matrix (Bruce et AL., 2004). At the same time, different partial pressure of H2S will lead to pitting corrosion and uniform corrosion of pipeline steel. When H2S pressure is lower than 8 × 10−6 MPa, pitting corrosion occurs on the surface of pipeline steel; when H2S partial is higher than 8×10−6 MPa, there will not be pitting corrosion, when H2S is not contained, no pitting corrosion occurs (Yaakob et al., 2016). |
