Hydrogen-induced-cracking resistant and sulphide-stress-cracking resistant steel alloy

Abstract

The invention relates to a quench-and-temper steel alloy for use in casing for oil and gas wells wherein such casing is exposed to low pH environments. The steel alloy has a carbon range by weight of 0.15% to 0.35%, a manganese range by weight of 0.60% to 1.10%, a molybdenum range by weight of 0.15% to 0.65%, and a sulphur range by weight of less than 0.002%. The steel alloy has a quench-and-temper micro-structure and features precipitated spheroidal molybdenum carbides in manganese- and carbon-rich bands. The steel alloy also has, by weight, a chromium range of less than 0.50%, an aluminum range of less than or equal to 0.08% and a calcium range of less than or equal to 0.0045%.

Description

[0001] The present application is a continuation-in-part application of U.S. application Ser. No. 09 / 036,545 filed on Apr. 9, 2002, which is a continuation application of application Ser. No. 09 / 036,545 filed on Mar. 6, 1998. The '545 application is a continuation-in-part application of U.S. application Ser. No. 08 / 813,374 filed on Mar. 7, 1997. The present application incorporates by reference the entire contents of all of these parent applications.[0002] This invention relates in general to a steel alloy and more particularly to a steel alloy that resists hydrogen-induced cracking and sulfide stress cracking. This alloy is particularly suitable for use in casing for use with sour oil and gas wells. The invention also comprises casing made from such alloy.BACKGROUND TO THE INVENTION[0003] HIC and SSC are two types of hydrogen embrittlement problems of particular concern to steel casing operating in sour gas environments. HIC is a hydrogen sulfide (H.sub.2S) related hydrogen embritt...

AI Technical Summary

Benefits of technology

[0013] In accordance with one aspect of the invention there is provided a quench and temper steel alloy characterized in that the alloy has, by weight, a carbon (C) range of 0.15% to 0.35%, a manganese range of 0.60% to 1.10%, a molybdenum (Mo) range of at least 0.15%, and a sulfur (S) range of less than 0.002%, a chromium (Cr) range of less than or equal to 0.50%, an aluminum (Al) range of less than or equal to 0.080%, a calcium (Ca) range of less than or equal to 0.0045%, a silicon (Si) range of less than or equal to 0.40%, and the substantial balance of the alloy being iron and unavoidable impurities. The steel alloy is further characterized in that the alloy has a quench-and-temper micro-structure and has precipitated spheroidal Mo carbides in Mn and C rich bands. The precipitated Mo carbides result from sustained tempering at high temperatures. The precipitation reduces the carbon content in the matrix of the Mn and C rich segregation bands, and decreases the hardness of the matrix.

[0014] Mo is included in the alloy to harden the alloy, so as to enable boron (B) and titanium (Ti) to be substantially excluded from the alloy and to reduce the Mn content in the steel. This substantially precludes the formation of boron nitride and titanium nitride which may play a role in the formation of HIC and SSC in the steel. As HIC and SSC are found particularly at MnS inclusions in the steel, the relatively low S and Mn content reduce the presence of such inclusions in this steel alloy. Mo has the additional beneficial effect of retarding stress relaxation in steel at elevated temperature; this contributes to the steel alloy being able to withstand stresses for prolonged periods at elevated temperatures. Mo also contributes to slow the rate of corrosion of the steel alloy. As hydrogen is a corrosion product, hydrogen formation in the steel is thus slowed.

[0016] In another preferred alloy chemistry, the carbon range by weight is 0.18% to 0.27%, the manganese range by weight is 0.70% to 0.95%, the molybdenum range by weight is 0.35% to 0.55%, and the sulfur range by weight is less than 0.001%. The steel alloy has a calcium range, by weight, of 0.0020% to 0.0045%, and an aluminum range of 0.030% to 0.050%. This alloy is cheaper and somewhat less resistant to SSC than an alloy of the first chemistry.

Problems solved by technology

Once a certain threshold pressure is exceeded, cracking occurs.

If the stepwise cracking occurs beneath the steel's surface, blistering will occur on the surface.

If the applied load is perpendicular to the elongated MnS inclusion, hydrogen induced cracks nucleated at the inclusion tips will directly lead to final rupture.

Such other structural defects include grain boundaries, carbide-ferrite interfaces, oxides, and dislocation tangles.

Oil and gas well steel casing is commonly exposed to highly acidic conditions.

Many new wells contain significant concentrations of H.sub.2S while older wells become increasingly sour over their production lifetime.

However, the casing is unable to expand due to the physical constraints placed on the casing by the environment.

At the conclusion of the stimulation cycle, the well cools and the resultant thermal contraction of the material results in a tensile load.

This cycling will be repeated several times through the life of the well and places severe fatigue stresses on the casing.

Unfortunately, seamless casing is very expensive to manufacture.

To be effective, boron must be retained in solid solution throughout the processing schedule; however, boron interacts strongly with nitrogen to form boron nitrides which render boron additions ineffective.

However, even alloys that demonstrate good HIC resistance in laboratory trials may perform poorly when stress is applied.

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