High performance nickel alloys solve problems in sour oil and gasMost of the easy to pick fruits of the industry have already been taken with shallow field development. Compared to shallow wells, deep wells generally require equipment built with more high performance nickel base alloys. Wells are categorized as either being sweet or sour:
- Sweet wells are only mildly corrosive
- Sour wells are highly corrosive and contain hydrogen sulfide, carbon dioxide, chlorides and free sulphur
In addition, different levels of corrosive conditions are compounded by temperatures up to 260°C and pressures up to 25,000psi and deep wells generally have higher temperatures and pressures.
Material selection is especially critical for sour gas wells. The materials of choice must be corrosion resistant, cost effective, reliable and have the required strength for the well conditions. As these conditions become more severe, material selection changes from carbon steels for sweet wells to duplex (austenitic-ferritic) stainless steel, to nickel based alloys such as INCOLOY® alloys 825 and 925, INCONEL® alloys 725HS and 725. The limiting chemical compositions of the nickel based alloys are displayed in Table 1.
Materials selection properties
The strength levels of age hardened materials are increasing in importance, particularly for offshore applications exploiting high pressure deep well reserves, where weight considerations can affect the economic viability of a project. Material selection for down hole and wellhead equipment such as hangers, sub surface safety valves, pumps and packers call for age hardenable alloys in order to obtain the necessary strength in heavier cross sections, which cannot be strengthened by cold work. Nickel alloys commonly used for these applications include Incoloy® 925, Monel® K500 and Inconel® 718, X750, 725 and 725HS. Typical mechanical properties of high performance nickel alloys used in oil country applications are shown in Table 2.
The age hardened alloys are used at different strength levels depending on the application. Generally Incoloy® 925 is used at 758 Mpa (110 Ksi) min yield strength level. The min yield strength level for Inconel® 718 and 725 is 827 Mps (120 Ksi). Inconel® 725HS is used at a 965 Mpa (140 Ksi) min yield strength level. The enhanced strength properties of Inconel® 725HS have been achieved through optimized thermal and mechanical processing.
