The Metallurgy Options for Tubing

The Metallurgy Options for TubingThe metallurgy of tubing is a very important factor while choosing tubing for a particular purlieu. Generally the tubing is made up of carbon or pitiable alloy brand names, martensitic stainless steel, semidetached house stainless steel or other corrosion resistant alloys like Nickel-base alloy etc.METALLURGY FOR TUBINGCarbon steel is an alloy of carbon and iron containing up to 2% carbon and up to 1.65% manganese and residual quantities of other elements.Steels with a tot alloying element content of less than about 5% but more than specified for carbon steel atomic number 18 jut outated as low alloy steel.Carbon steel is the close to common alloy used in oil industry be father of its relatively low cost.Though corrosion resistance of these steels is limited still they be possessed of been used in oil industry since long satisfactorily. They are suitable for mildly corrosive environments like low partial pressure of carbon dioxide low part ial pressure of H2S.A material selected for a particular environment may non remain suitable in the case the environmental conditions change.CO2 smoke cause extreme weight loss corrosion localized corrosion, H2S can cause sulphide direction cracking and corrosion. Chlorides at noble temperature can cause song corrosion cracking and pitting of metals, while low pH in general increases corrosion rate.For example the following material are considered to be resistant to sulphide stress cracking Low and medium alloy carbon, containing less than 1% nickel.AISI 300 series stainless steels (Austenitic) that is fully annealed and free of dust-covered work.The following materials have been found to have little or no resistance to sulphide stress crackingAISI Grades 420 and 13% Cr martensitic stainless steel. either cold finished steels including low and medium alloy steels, many variety of stainless steel.The limitations of Carbon steel, 9-Cr-1 Mo, 13-Cr, Duplex stainless steel are enc ountered in variant environments and downhole operations.METALLURGY OPTIONS FOR TUBINGThe various metallurgical options examined for tubing and other downhole equipment are Carbon Low Alloy Steels, 9 Cr-1Mo steel, 13% Cr stainless steel, Duplex blameless steel and nickel based alloys. A brief of the suitability and limitations of these materials in various environments encountered in oil and gas wells9Cr-1Mo steel.This steel is immune to stress corrosion cracking in the heraldic bearing of chlorides like other nickel free low alloy steels. Corrosion resistance of this steel in the presence of H2S is poor. Hence it is not used in tubing metallurgy commonly.13Cr Stainless steel.This steel can be used upto 100 atms CO2 partial pressure and upto cl degree Celsius temperature with chloride upto 50 gms/L. This martensitic grade is known to be susceptible to sulphide stress cracking in sour environment.This material is generally used for tasty wells where minimum souring is expected.D uplex Stainless Steel.Duplex SS has excellent corrosion resistance in CO2 environment. The limitation of their usage is their susceptibility to stress corrosion cracking at high temperature and limited resistance to sulphide stress cracking, when H2S is present in the produced fluid.Nickel Based Alloys.Nickel based alloys are required to be used in extremely corrosive conditions involving very high partial pressure of H2S and CO2 along with presence of free sec or oxygen.SELECTION OF TUBING METALLURGYFrom the various metallurgical options I have analyzed, it can be concluded that low alloy carbon steel is not suitable for the wells where high corrosion risk involved, particularly in offshore. If low allow materials were to be used, an intensive corrosion inhibitor treatment program is essential. However, heretofore with the best of programs, the solution to the problem would be trial and error.Although 9Cr-1 Mo steels are resistant to CO2 attack, they should not be considered for this application since their application in chloride environment is limited up to 10 gms/l (1%).With the high concentrations of chlorides coupled with the high well bore temperature this material is not suitable for downhole use in these wells.Duplex stainless steel is susceptible to chloride stress cracking and should not be used with the CaCl2 packer fluid. Also, the price for Duplex material is three to four generation the cost of 13 Cr SS material, which would make it economically unacceptable.Hence, in spite of the additional up-front cost for tubing , it is recommended that based on the caliper survey results , high corrosion risk wells of field should be re-completed with 13% Cr SS L-80 tubing material.PROBLEMS OBSERVEDThe occurrence of metal loss corrosion in air is caused by the presence of corrodents in the produced water. Internal corrosion in pipeline can be caused by the presence of mill scale, slag inclusions, improper combust treatment, improper welding, besides h igh or too low velocity etc. The erosion/corrosion effect can be caused by too high fluid velocity. Water and sludge build develop with too low fluid velocity that may cause pitting and bacteria infestations. At low fluid velocity, water will tend to segregate to the bottom of the pipeline. Once the pipeline is water wetted, the corrosion begins. When corrosion is not controlled, time to first gear failure due to corrosion will be normally from three to twelve years depending on the wall thickness and operating conditions.Corrosion of most material is inevitable and can seldom be completely eliminated. But it can be controlled by carefully selecting material and protection methods at the design stage. For example, as carbon steel is less resistance to corrosion allowance is given in addition to the design thickness when they are expected to mete out moderately corrosive fluid. Similarly, external surface of the pipeline are protected from corrosive soils by providing protective co atings. Still, there is always unexpected failure which results from integrity or more of the following reasons Poor choice of material.Defective fabrication.Improper design.Inadequate protection/maintenance.Defective material.CONCLUSIONCorrosion due to presence of CO2 gas along with unfavorable water chemistry is the cause of the piping failures.It is recommended that tubing metallurgy shall be of L-80 13 Cr stainless steel with premium joints.The downhole metallurgy shall be 13 Cr SS.These elastomeric materials includeNitrileA rubber compound with base material as Butadiene Acrylonitrile.VitonA fluoroelastomer fabricate by Dupont.FluorelA fluoroelastomer manufactured by 3M company.RytonA polyphenylene sulfide manufactured by Philips Petroleum Company.REFERENCESEffect of microstructure Cr content in steel on CO2 corrsion. By Masakatsu Veda, Aki Ikeda, Japan. prediction of the risks of CO2 corrosion in oil and gas wells. By Jl Crolet, Mr Bonis.CO2 corrosion in oil wells. By Gunal ton , Zadko.