Summary

The corrosion behavior of 3% Cr steel is tested by a high-temperature/high-pressure autoclave. The corrosion environment is categorized into CO₂–alone and CO₂/H₂S conditions. At 90°C, with the addition of H₂S to the CO₂, the surface corrosion condition improved greatly, and the corrosion rates declined compared with the CO₂–alone condition. Under CO₂/H₂S condition, with an increasing CO₂/H₂S partial-pressure ratio, the corrosion rate reached a peak value at pCO₂/pH₂S = 100, and then declined. Through the analysis of the corrosion products of the samples in different conditions by scanning electron microscope (SEM), EDS, and X-ray-diffraction (XRD) methods, it was found that the inner film is finer and denser than the outer scale. The partial-pressure ratio of H₂S corrosion regime should be between 10 and 100 and is not the previous 200.

Introduction

Corrosion has wide-ranging implications for the integrity of materials used in the petroleum industry.  The implication of CO₂ corrosion can be viewed in terms of its effect of capital and operational expenditures and health, safety, and the environment (Kermani and Harrop 1996), which brings huge losses and poses security threats to the development of oil and gas (Minxu et al. 2002; Kinsella at al. 1998). Medium Cr-containing steels showed improved corrosion resistance compared with carbon steel and lower cost than 13% Cr steel (Ueda and Takabe 2002; Muraki et al 2002; Nose et al. 2001). The effect of medium chromium-additions was attributed to the formation of chromium enriched corrosion products (e.g., observed on 3% Cr-steel). Steels alloyed with 3% CR or more were found to show improved corrosion performance over CR-free steel by a factor of two or more (Nose et al. 2001). The corrosion becomes complicated when H₂S is added into the CO₂ corrosion system. The current research should focus on the formation mechanism of the corrosion product film, properties of corrosion product of different structures and protection to the matrix. The corrosion experiment that has been carried out with 3% Cr tubular steel in pure CO₂ and CO₂/H₂S environments. The results would be to supplement the CO₂/H₂S corrosion theory of 3% CR steel and provide theoretical basis for the proper selection of the tubing and casing in oil and gas fields.
 

Read or download the full SPE paper 157302-PA.

Wei Yan and Jingen Deng, China University of Petroleum; Xingliang Dong and Chunyang Zhang, China National Offshore Oil Company; and Wenliang Li, China University of Petroleum