Material selection is determined by the abrasion and corrosion properties of the fluid, the pressures and the mechanical and hydraulic loadings on the completion string under operating conditions, e.g. stimulation, injection and production conditions.
Tubing stress calculations will allow specification of threaded coupling type, and material as well as identifying the need for a dynamic seal assembly to accommodate tubing movement.
There are six distinct material groups which are used in well applications:
- Low alloy steels
- Medium-alloy steels
- Stainless steels
- Non-ferrous alloys
- Plastic coated tubing
- Fiberglass Reinforced Plastic (FRP)
4.1 Low alloy steels
Low-alloy steels will contain less than 4% alloying elements with manganese being considered an alloying element if its concentration is greater than 0.8% by weight. The higher strength tubular grades (L-80 to V-150) are usually Mn-Cr-Mo-C. The lower strength tubulars are lower alloyed, usually with manganese and carbon. Low-alloy steels possess limited corrosion resistance and must rely on natural and artificial coatings or inhibitors for protection in corrosive wells. The lower strength tubings are more resistant to stress corrosion cracking in the presence of H2S.
4.2 Medium-alloy steels
Medium-alloy steels such as the 9 Cr-1 Mo are usually special grade steels with specific properties. This particular alloy is a heat resistant material used in the oil field for its improved corrosion resistance. The chromium content is lower than that required for total effective corrosion resistance.
4.3 Stainless steels
Stainless steels normally are found to contain at least 12% chromium and are one of four types. These include martensitic, ferritic, austenitic, and duplex stainless steels whose structures are determined by their chemical composition. They are all strengthened by cold working except the martensitic tubing which is strengthened by heat treating.
The 12% chromium steels have very useful resistance to corrosion in sweet gas wells. The martensitic variety is used for tubing; the ferritic for downhole accessories. Only low amounts of H2S can be allowed with these steels (<1 psi H2S).
Austenitic steel has excellent corrosion resistance. This material is susceptible to chloride stress cracking in the pressure of oxidising conditions, or H2S, and chlorides. It has low susceptibility to hydrocarbon embrittlement. A variety of alloys is available in this class with increased Cr and Ni contents above the usual 18 Cr/8 Ni composition.
Duplex stainless steels contain 50% austenite-50% ferrite and can be cold worked to as much as 50%. These steels appear to be susceptible to hydrogen embrittlement when hydrogen sulphide is present, and they have poor resistance to reducing acids, e.g. mud acid, at temperatures above 65°C.
4.4 Non-ferrous alloys
Non-ferrous alloys are identified as containing less than 50% iron in their composition. The high nickel alloys might be the best solution to sour gas environments which also contain high concentrations of chloride ions. Since some of these metals contain over 60% nickel, they are very expensive.
Corrosion resistant alloy (CRAs) tubulars have been found to improve flow efficiency. Typical carbon steel production tubing is limited according to an API 14E empirical formula to a certain flow rate in order to avoid erosion, which is caused by increased flow velocity. Corrosion scales do not build up in CRA tubing, so operators can obtain a higher flow rate (up to 100 ft/sec) while decreasing tubing size.
4.5 Plastic coated tubing
Plastic coated tubing, to prevent corrosion or wax deposition has been tried, its effectiveness very much related to the integrity of the plastic coating. Any damage to the coating by wireline operations results in increased localised corrosion and wax deposition. Problems have also been experienced at tubing connections and in cases where pup joints have been installed which were not factory coated.
4.6 Fiberglass Reinforced Plastic (FRP)
Fiberglass Reinforced Plastic (FRP) tubulars are effective against corrosive fluids, however, their use as well conduits is still being developed. Special care should be given with regards material "creep", buckling and cyclic loading when applying FRP tubulars.