Material selection is very important in the oil and gas industry, especially when it comes to deep sour wells. Unlike ‘sweet’ wells, which are only mildly corrosive, sour wells are highly corrosive. Sour gas is natural gas that contains high levels of hydrogen sulphide, carbon dioxide, chlorides and free sulphur. Hydrogen sulphide is toxic – even at relatively low concentrations – flammable and corrosive.
These deep sour wells present many problems for engineers. The gas itself is difficult to work with and the environmental conditions are harsh, with both high temperatures and pressure. There are large natural gas resources around the world that are ‘sour’. Since the advent of high-performance, corrosion-resistance materials, alongside advanced manufacturing processes, it has been possible to tap into these resources.
The oil and gas industry is increasingly exploring and extracting from deep wells, particularly in underwater. While it’s possible to use some lower-cost materials for sweet wells, sour wells require the very best nickel alloys. Materials used in these extreme environments need to have outstanding corrosion resistance as a priority. They also need to exhibit other key properties like high strength and toughness to handle the intense pressure.
When selecting materials for use in a sour gas situation, the engineer will have to consider a wide range of factors. This includes the mechanical properties of the alloy and its resistance to pitting and crevice corrosion, chloride stress corrosion cracking, sulphide stress corrosion cracking and general corrosion.
Alloys are often age-hardened, especially for offshore applications. This is because of the extra strength that the age-hardening process imparts on a material and the high pressure of a deep well. Many down-hole and well-head components rely on age-hardenable alloys, including valves, pumps, packers and hangers.
Corrotherm’s range of nickel alloys includes some good options for sour gas environments. INCONEL alloy C-276, for example, has outstanding corrosion resistance to both acid and alkali media. It’s a good choice for severely corrosive environments. It has a low carbon content, which helps to minimise carbide precipitation during heat treatment. This helps it to retain its excellent pitting and crevice corrosion. As hydrogen sulphide is even more corrosive in water – causing sulphide stress cracking – INCONEL alloy C-276’s resistance to seawater corrosion is useful for deep-sea wells. However, it can’t be hardened by heat treatment, only cold working, which means it isn’t suitable for all applications.
INCONEL alloy 625 is widely used across a range of industrial applications, mainly due to its exceptional corrosion resistance. It is a standard material of construction in many areas, and is in wide use for oil extraction, including where sour gas is present. Its combination of nickel, chromium, molybdenum and niobium give the alloy its superior corrosion resistance. They also impart particularly high strength, so that it can perform under high-temperature and high-pressure conditions.
If you require an even higher-strength nickel alloy, INCONEL alloy 725 can be age-hardenable for extremely high strength. It also has broadly the same corrosion resistance as INCONEL alloy 625.
To find out more about Corrotherm’s range of corrosion-resistant nickel alloys, please Contact Us or visit our Grades page.