Some Factors That Affect the Sour-Service Performance of Carbon-Steel Oil-Country Tubulars
- Stephen W. Ciaraldi (Amoco Corp.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- June 1986
- Document Type
- Journal Paper
- 233 - 241
- 1986. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 4.1.5 Processing Equipment, 1.6 Drilling Operations, 4.2 Pipelines, Flowlines and Risers, 3.2.4 Acidising, 4.2.3 Materials and Corrosion
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Carbon-steel oil-country tubular goods, such as API 5AC Grade L-80, combine high strengths for deep-well applications with restricted hardness for resistance to sulfide stress-cracking in sour wells. Despite these general characteristics, field experience with such tubulars has not been completely trouble-free. Problems have been encountered with hardness control and testing. Metallurgical factors involved with heat treatment and upsetting have affected tubular performance. Problems associated with threaded connections and tool damage have occurred. Special considerations are required for electrical-resistance-welded (ERW) tubulars, field welding, and completion operations.
Carbon steels generally contain only incidental amounts of alloying elements other than carbon, manganese, and silicon. Such tubulars have overwhelmingly dominated oil-industry needs for downhole casing and production tubing. With increased interest in deep hydrocarbon reserves, higher-strength tubulars have been required. Higher strengths also provide increased collapse resistance while maximizing wellbore diameter. However, a major limitation to the use of high-strength carbon-steel tubulars in sour wells (i.e., wells that contain more than 0.05-psia [0.35-kPa] hydrogen sulfide) is sulfide stress-cracking (SSC). This phenomenon is the cracking failure of steels under tensile loading in the presence of aqueous hydrogen sulfide. Although many factors influence the occurrence of SSC, increased steel strength and hardness generally result in decreased SSC resistance.
Historically, SSC has been minimized by restricting carbon-steel maximum hardness to 22 Rockwell C (HRC). This practice has formed the basis for carbon-steel restrictions detailed in NACE Standard MR-01-75. The hardness restriction effectively limits the specified minimum yield strength (SMYS) of carbon steels to a maximum of about 85 ksi [586 MPa]. API tubulars within this SMYS limitation include Grades C-75 and L-80. Inclusion of hardness restrictions in API Specification 5AC on Grade L-80 have resulted in widespread use of this tubular grade in sour service because of easier hardness testing compared with tensile testing. Grade L-80 is further required to have quench-and-temper heat treatment, which results in increased SSC resistance. Proprietary versions of Grade L-80, referred to as Modified L-80, have been developed by manufacturers. These steels often offer enhanced collapse resistance or tighter hardness/ chemistry restrictions but generally maintain the carbon-steel classification and applicability to sour production. production. L-80-type (API 5AC L-80 and Modified L-80) tubulars offer high strength, good SSC resistance, and relative economy. Higher-strength tubulars (C-85, C-90, and C-100) can be obtained by alloying with chromium and molybdenum. However, heat treatment of these materials is critical and SSC resistance generally must be ensured by lengthy laboratory tests. Practically, Grade L-80 is probably at the upper strength limit for acceptable SSC resistance in carbon steels. Because of this, problems are encountered when such tubulars are used in sour service. This paper describes some of these problems, which generally pertain to SSC resistance. Internal corrosion, most often controlled by batch or continuous inhibitor treatments, is not substantially addressed.
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