ABSTRACT
In selecting materials for corrosive sour oil field environments, the materials of choice must be reliable and cost-effective. Materials have to meet criteria for both corrosion resistance and mechanical properties in sour environments for the required service life. This paper shows the results of the stress corrosion cracking evaluation of two different austenitic corrosion resistant alloys (CRAs), N08028 and N08825 under different conditions of temperature and pressure of H2S and CO2. Those testing conditions are demanded by several end users with oil and natural gas wells operating in different zones onshore and offshore. Depending on the well location the severe environment varies. Corrosion data are presented showing the performance of each grade using the slow strain rate according to NACE TM0198, the NACE TM0177 method A and the NACE TM0316.1-3 Both cold worked materials are typically specified as type 4c and are approved by the NACE MR0175 up to a hardness level of 40 HRC and a maximum yield strength of 150 ksi.
INTRODUCTION
As a result of the increasingly harsh environments in high pressure and high temperature (HPHT) wells, corrosion resistant alloys (CRAs) are often used for downhole tubulars and in other production equipment. Materials selection is especially critical for sour gas wells containing H2S, CO2, chlorides and elemental sulphur. Depending on the place where the gas well is located, the sour conditions are different and consequently the material selection is critical to avoid any problem in service.
Cold worked solid solution nickel base alloys as N08028 and N08825 combine high strength and toughness with an excellent general corrosion resistance in sour service conditions. Regarding the mechanical properties, oil country tubular goods (OCTG) products are usually delivered as cold hardened to fulfill a minimum of strength but at the same time, with a limit in the maximum hardness. With regards to the stress corrosion cracking (SCC) resistance, there are different papers published by Hibner et al. and Wenle et al. for both grades N08028 and N08825 in different sour conditions.4-6 In this paper, more data are provided by taking into account the in-service conditions where the tubing and coupling stock produced with both grades have been installed.
