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The Metallurgy Options for Tubing

The metallurgy of tubes is a very important factor while choosing tubes for a particular environment. Usually the tubing is made up of carbon or low alloy steels, martensitic stainless, Duplex stainless or other corrosion repellent alloys like Nickel-base alloy etc.

METALLURGY FOR TUBING:

Carbon steel is an alloy of carbon and flat iron made up of up to 2% carbon and up to at least one 1. 65% manganese and residual quantities of other elements. Steels with a complete alloying component content of less than about 5% but more than specified for carbon material are specified as low alloy metallic. Carbon steel is the most common alloy found in oil industry due to its relatively low cost.

Though corrosion amount of resistance of these steels is limited still they are used in olive oil industry since long satisfactorily. They may be suited to mildly corrosive conditions like low partial pressure of CO2 & low partial pressure of H2S.

A material selected for a specific environment might not remain suitable in the event the environmental conditions change. CO2 can cause extreme weight damage corrosion & localized corrosion, H2S can cause sulphide stress cracking and corrosion. Chlorides at high temperature can cause stress corrosion breaking and pitting of metals, while low pH in general raises corrosion rate.

For example the following material are considered to be resilient to sulphide stress breaking :

  • Low and medium alloy carbon, containing less than 1% nickel.
  • AISI 300 series metal steels (Austenitic) that is fully annealed and free of frigid work.

The following materials have been found to obtain little if any resistance to sulphide stress cracking:

  • AISI Levels 420 and 13% Cr martensitic stainless.
  • All cold completed steels including low and medium alloy steels, many variety of stainless steel.

The constraints of Carbon metallic, 9-Cr-1 Mo, 13-Cr, Duplex stainless are encountered in various environments and downhole procedures.

METALLURGY OPTIONS FOR TUBING:

The various metallurgical options examined for tubes and other downhole equipment are Carbon & Low Alloy Steels, 9 Cr-1Mo metallic, 13% Cr stainless steel, Duplex Stainless steel and nickel centered alloys. A brief of the suitability and constraints of the materials in a variety of environments came across in oil and gas wells:

9Cr-1Mo metallic.

This metallic is immune system to stress corrosion breaking in the presence of chlorides like other nickel free low alloy steels. Corrosion amount of resistance of this steel in the occurrence of H2S is poor. Hence it is not used in tubing metallurgy commonly.

13Cr Stainless steel.

This material can be used upto 100 atms CO2 partial pressure and upto 150 level Celsius temp with chloride upto 50 gms/L. This martensitic level may be susceptible to sulphide stress cracking in sour environment. This material is generally used for great wells where lowest souring is expected.

Duplex Stainless Steel.

Duplex SS has excellent corrosion amount of resistance in CO2 environment. The limitation of their utilization is their susceptibility to stress corrosion breaking at high temperature and limited resistance to sulphide stress cracking, when H2S exists in the produced substance.

Nickel Based mostly Alloys.

Nickel centered alloys are required to be used in extremely corrosive conditions regarding very high incomplete pressure of H2S and CO2 along with presence of free sulphur or oxygen.

SELECTION OF TUBING METALLURGY:

From the many metallurgical options I've analyzed, it can be figured low alloy carbon metal is not suited to the wells where high corrosion risk included, particularly in offshore. If low allow materials were to be used, an intensive corrosion inhibitor treatment program is vital. However, even with the best of programs, the solution to the problem would be trial and error.

Although 9Cr-1 Mo steels are immune to CO2 strike, they shouldn't be considered because of this software since their request in chloride environment is bound up to 10 gms/l (1%). Using the high concentrations of chlorides coupled with the high well bore heat; this materials is not suited to downhole used in these wells.

Duplex stainless steel is susceptible to chloride stress breaking and should not be used with the CaCl2 packer substance. Also, the price for Duplex materials is three to four times the price of 13 Cr SS material, which would make it financially unacceptable.

Hence, in spite of the additional up-front cost for tubing, it is recommended that predicated on the caliper survey results, high corrosion risk wells of field should be re-completed with 13% Cr SS L-80 tubing material.

PROBLEMS OBSERVED:

The incident of metal reduction corrosion in pipeline is triggered by the presence of corrodents in the produced drinking water. Internal corrosion in pipeline can be brought on by the presence of mill range, slag inclusions, improper heat treatment, incorrect welding, too much or too low speed etc. The erosion/corrosion result can be caused by too much fluid velocity. Normal water and sludge build develop with too low liquid velocity that may cause pitting and bacteria infestations. At low liquid velocity, drinking water will have a tendency to segregate to the bottom of the pipeline. After the pipeline is drinking water wetted, the corrosion starts. When corrosion is not handled, period to first failure due to corrosion will be normally from three to twelve years depending on the wall thickness and operating conditions.

Corrosion of all material is inescapable and can rarely be completely eliminated. Nonetheless it can be controlled by carefully selecting material and security methods at the design stage. For instance, as carbon metal is less resistance to corrosion allowance is given in addition to the design width when they are expected to handle reasonably corrosive fluid. In the same way, external surface of the pipeline are protected from corrosive soils by providing protective coatings. Still, there's always unexpected inability which results from one or more of the following reasons :

  • Poor selection of material.
  • Defective fabrication.
  • Improper design.
  • Inadequate security/maintenance.
  • Defective materials.

CONCLUSION:

  • Corrosion anticipated to occurrence of CO2 gas along with unfavorable normal water chemistry is the cause of the piping failures.
  • It is recommended that tubes metallurgy will be of L-80 13 Cr stainless with premium joints.
  • The downhole metallurgy shall be 13 Cr SS.

These elastomeric materials include:

  • Nitrile:

A rubber ingredient with base materials as Butadiene Acrylonitrile.

  • Viton:

A fluoroelastomer created by Dupont.

  • Fluorel:

A fluoroelastomer created by 3M company.

  • Ryton:

A polyphenylene sulfide created by Philips Petroleum Company.

REFERENCES:

  • Effect of microstructure & Cr content in material on CO2 corrsion. By Masakatsu Veda, Aki Ikeda, Japan.
  • Prediction of the risks of CO2 corrosion in coal and oil wells. By Jl Crolet, Mr Bonis.
  • CO2 corrosion in oil wells. By Gunalton, Zadko.
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