The metallurgy of tube is a really of import factor while taking tubing for a peculiar environment. By and large the tube is made up of C or low metal steels, martensitic chromium steel steel, Duplex chromium steel steel or other corrosion immune alloys like Nickel-base metal etc.
METALLURGY FOR Tube:
Carbon steel is an metal of C and Fe incorporating up to 2 % C and up to 1.65 % manganese and residuary measures of other elements.Steels with a entire alloying component content of less than approximately 5 % but more than specified for C steel are designated as low metal steel.Carbon steel is the most common metal used in oil industry because of its comparatively low cost.
Though corrosion opposition of these steels is limited still they have been used in oil industry since long satisfactorily. They are suited for mildly caustic environments like low partial force per unit area of CO2 & A ; low partial force per unit area of H2S.
A stuff selected for a peculiar environment may non stay suited in the instance the environmental conditions change.CO2 can do utmost weight loss corrosion & A ; localized corrosion, H2S can do sulphide emphasis snap and corrosion. Chlorides at high temperature can do stress corrosion snap and roughness of metals, while low pH in general additions corrosion rate.
For illustration the undermentioned stuff are considered to be immune to sulphide emphasis snap:
- Low and average metal C, incorporating less than 1 % Ni.
- AISI 300 series chromium steel steels ( Austenitic ) that is to the full annealed and free of cold work.
The undermentioned stuffs have been found to hold small or no opposition to sulphide emphasis snap:
- AISI Grades 420 and 13 % Cr martensitic chromium steel steel.
- All cold finished steels including low and average metal steels, many assortment of unstained steel.
The restrictions of Carbon steel, 9-Cr-1 Mo, 13-Cr, Duplex chromium steel steel are encountered in assorted environments and downhole operations.
METALLURGY OPTIONS FOR Tube:
The assorted metallurgical options examined for tube and other downhole equipment are Carbon & A ; Low Alloy Steels, 9 Cr-1Mo steel, 13 % Cr chromium steel steel, Duplex Stainless steel and Ni based alloys. A brief of the suitableness and restrictions of these stuffs in assorted environments encountered in oil and gas Wellss:
9Cr-1Mo steel.
This steel is immune to emphasize corrosion checking in the presence of chlorides like other nickel free low metal steels. Corrosion opposition of this steel in the presence of H2S is hapless. Hence it is non used in tubing metallurgy normally.
13Cr Stainless steel.
This steel can be used upto 100 atms CO2 partial force per unit area and upto 150 degree Celsius temperature with chloride upto 50 gms/L. This martensitic class is known to be susceptible to sulphide emphasis checking in rancid environment.This stuff is by and large used for Sweet Wellss where lower limit souring is expected.
Duplex Stainless Steel.
Duplex SS has first-class corrosion opposition in CO2 environment. The restriction of their use is their susceptibleness to emphasize corrosion checking at high temperature and limited opposition to sulphide emphasis snap, when H2S is present in the produced fluid.
Nickel Based Alloys.
Nickel based metals are required to be used in highly caustic conditions affecting really high partial force per unit area of H2S and CO2 along with presence of free sulfur or O.
Choice OF TUBING METALLURGY:
From the assorted metallurgical options I have analyzed, it can be concluded that low metal C steel is non suited for the Wellss where high corrosion hazard involved, peculiarly in offshore. If low allow stuffs were to be used, an intensive corrosion inhibitor intervention plan is indispensable. However, even with the best of plans, the solution to the job would be test and mistake.
Although 9Cr-1 Mo steels are immune to CO2 onslaught, they should non 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 good bore temperature ; this stuff is non suited for downhole usage in these Wellss.
Duplex chromium steel steel is susceptible to chloride emphasis snap and should non be used with the CaCl2 bagger fluid. Besides, the monetary value for Duplex stuff is three to four times the cost of 13 Cr SS stuff, which would do it economically unacceptable.
Hence, in malice of the extra up-front cost for tubing, it is recommended that based on the calliper study consequences, high corrosion hazard Wellss of field should be re-completed with 13 % Cr SS L-80 tubing stuff.
PROBLEMS OBSERVED:
The happening of metal loss corrosion in grapevine is caused by the presence of corrodents in the produced H2O. Internal corrosion in grapevine can be caused by the presence of factory graduated table, slag inclusions, improper heat intervention, improper welding, excessively high or excessively low speed etc. The erosion/corrosion consequence can be caused by excessively high fluid speed. Water and sludge construct develop with excessively low fluid speed that may do roughness and bacterium infestations. At low fluid speed, H2O will be given to segregate to the underside of the grapevine. Once the grapevine is H2O wetted, the corrosion begins. When corrosion is non controlled, clip to first failure due to corrosion will be usually from three to twelve old ages depending on the wall thickness and operating conditions.
Corrosion of most stuff is inevitable and can rarely be wholly eliminated. But it can be controlled by carefully choosing stuff and protection methods at the design phase. For illustration, as C steel is less opposition to corrosion allowance is given in add-on to the design thickness when they are expected to manage reasonably caustic fluid. Similarly, external surface of the grapevine are protected from caustic dirts by supplying protective coatings. Still, there is ever unexpected failure which consequences from one or more of the undermentioned grounds:
- Poor pick of stuff.
- Defective fiction.
- Improper design.
- Inadequate protection/maintenance.
- Defective stuff.
Decision:
- Corrosion due to presence of CO2 gas along with unfavourable H2O chemical science is the cause of the piping failures.
- It is recommended that tubing metallurgy shall be of L-80 13 Cr chromium steel steel with premium articulations.
- The downhole metallurgy shall be 13 Cr SS.
These elastomeric stuffs include:
- Nitrile:
- Viton:
- Fluorel:
- Ryton:
A gum elastic compound with basal stuff as Butadiene Acrylonitrile.
A fluoroelastomer manufactured by Dupont.
A fluoroelastomer manufactured by 3M company.
A polyphenylene sulphide manufactured by Philips Petroleum Company.
Mentions:
- Consequence of microstructure & A ; Cr content in steel on CO2 corrsion. By Masakatsu Veda, Aki Ikeda, Japan.
- Prediction of the hazards of CO2 corrosion in oil and gas Wellss. By Jl Crolet, Mr Bonis.
- CO2 corrosion in oil Wellss. By Gunalton, Zadko.
