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2.0 STEELS IN OIL & GAS OPERATIONS.

Steels that are used in the oil and gas operations especially in tubulars for the

production strings fall into four classes:

Low alloy steels:

These steels contain less than 4 percent alloying elements with manganese only

being considered an alloying element if its concentration is greater than 0.8 per cent

by weight of manganese. The higher strength grades for the tubing string are usually

Mn-Cr-Mo-C iron base alloys in a quenched and tempered matrix. The low strength

grades are usually Mn-C iron base alloys annealed and normalized. Low alloy steels

have limited corrosion resistance and depend on coatings or inhibitors for protection

in corrosive wells. The lower strength tubings are more resistant to stress corrosion

cracking in the presence of H2S.

Medium alloy steels

With appropriate alloying like chromium and molybdenum have specific properties. 9

Cr-1 mo, for example, is a heat-resistant material used in the oil field for improved

corrosion resistance. The total Chromium content is lower than that required for total

effective corrosion resistance.

Stainless steels

Contain at least 12 per cent chromium and are of 4 types, namely, martensitic,

ferritic, austenitic and duplex stainless steels whose structures are determined by

their chemical composition. These Stainless steels except the martensitic type are

strengthened by cold working. Martensitic type is strengthened by heat-treating. The

martensitic variety is used fir tubing, the ferritic for down holes accessories. The 12per cent chromium stainless steel is very useful against corrosion in sweet gas wells.

These steels are stable at very low amounts of H2S, less than 1 psi H2S.

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Austenitic stainless steel has excellent corrosion resistance but susceptible to

chloride stress cracking in the presence of oxidizing condition, or H2S and chlorides.

Austenitic stainless steels are susceptible to hydrogen embrittlement. A variety of

alloys are available in this class with increased Cr and Ni contents above the usual

18 Cr/8 Ni compositions.

Duplex stainless steels contain 50 per cent austenite-50 per cent ferrite and can be

cold worked to as much as 50 percent. These steels are susceptible to hydrogen

embrittlement in the presence of hydrogen sulphide. They have poor resistance to

reducing acids like mud acid at a temperature above 650C

Non-ferrous alloys:

(These alloys contain less than 50 percent iron. It is in this class of alloys that

SUPER ALLOYS belong. They contain over 60 percent nickel and are expensive.

These alloys have potentiality to play very important role in the future of the oil and

gas industry. Infact these high nickel alloys, though expensive, might be the best

solution to sour gas environments that also contain high concentrations of chloride

ions.