INTRODUCTION
In selecting materials for corrosive sour oil field environments, both age-hardened and coldworked solid solution nickel-base alloys offer many advantages such as high-strength, toughness and excellent corrosion resistance. Alloy 028 (UNS N08028), alloy 825 (UNS N08825), alloy G-3 (UNS N06985), alloy 050 (UNS N06950) and alloy C-276 (UNS N10276) are among the primary solid solution high nickel CRA's currently used in the cold worked condition for OCTG's mechanical tubing in sour gas wells. A comparison of corrosion resistance of these nickel-base alloys for applications in severe sour service is presented. Many commercially significant alloys are ranked as to corrosion resistance up to 230°C (450ºF) depending on the chloride concentration and H2S content.
Natural gas still remains one of the world?s most abundant sources of energy. In the past, drilling and production tubulars were steel only, while stainless steels and nickel alloys were used primarily for valves or instrumentation. Today high strength, Corrosion Resistant Alloy (CRA) tubulars made from nickel alloys are needed because of:
1. Deeper wells involving higher temperatures, up to 230ºC (450ºF), and higher pressures.
2. Enhanced recovery methods such as steam injection, carbon dioxide injection and fire flooding.
3. Increased weight reduction considerations, especially for offshore.
4. The need for greater corrosion resistance in sour wells containing hydrogen sulfide, carbon dioxide, elemental sulfur, and chlorides.
DISCUSSION
Material selection is especially critical for sour gas wells, those containing an H2S partial pressure of at least 0.0003MPa (0.05 psi). The materials of choice must be corrosion resistant, costeffective, reliable, and have the required strength for the well conditions. The need for higher corrosion resistance and increased strength increases with well depth as temperature, pressure, acidity, CO2, chloride, and H2S levels also increase. As these conditions become more severe, tubular material selection goes from Fe-Cr or C-Mn steels used for shallow, sweet wells (<0.05 psi H2S), to duplex (austenitic-ferritic) stainless steel, INCOLOY alloys 028, 825 or 925, to INCONEL alloys G-3 or alloy 050, for sour well service. Under the most severe conditions, INCONEL alloys C-276 or 725 have been used. The limiting chemical composition is displayed in Table 1. In general, resistance to stress corrosion cracking (SCC), hydrogen embrittlement (sulfide stress cracking, SSC), increases with increasing alloy nickel, chromium, molybdenum, tungsten and niobium content. These materials are cold worked or age-hardened to specified levels in order to obtain the strength needed to support the weight of several thousand meters of tubing and withstand the intense pressure.
