Behavior of high functional Nickel alloys in oil and gas wells

In the recent years the new face of oil and gas exploration will be in deep wells, and specifically those in deepwater. As compare to shallow wells, deep wells normally need equipment made with high performance nickel base alloys. Wells are classified as sweet or sour. Sweet wells are mildly corrosive whilst sour wells are extremely corrosive and contain hydrogen sulfide, carbon dioxide, chlorides and free sulfur. Besides to different levels of corrosive media compounded by temperatures up to 260oCand pressures to 25000 psi and normally deep wells have higher temperatures and pressures.

Material choice is particularly critical for sour gas wells. The materials of choice should be corrosion resistant, economical, reliable and have the desired strength for the well conditions. These conditions become more vigorous material selection varies from carbon steels for sweet wells to duplex stainless steel to nickel base alloys like Incoloy 825 and Inconel 725.

Materials required to meet criteria for corrosion resistance and mechanical properties in application media need increased reliability throughout the life of well. Age hardened nickel based alloys and cold worked solid worked nickel base alloys offering various benefits like high strength, hardness, low magnetic permeability and outstanding corrosion resistance. The choice of material for specific set of well environments depends on wide selection factors such as:

• ·         Mechanical properties
• ·         General corrosion resistance
• ·         Pitting and crevice corrosion resistance
• ·         Chloride stress cracking resistance
• ·         Sulfide stress corrosion cracking resistance

The strength levels of age hardened materials are increasing in significance, specifically for offshore applications showing high pressure deep well reserves where weight factors can influence the economic feasibility of a project. Material choice for down hole and wellhead equipment including hangers, sub surface safety valves, pump and packers need age hardenable alloys to receive the resuired strength  in heavier cross section that cannot be reinforced by cold processing.

Nickel alloys used in oil well applications are Monel 400wire, Incoenl 718, Inconel X750 and 725. The age hardened alloys are used at different strength levels considering the applications. Incoloy 925 is used at 758 MPa minimum yield strength level. The minimum yield strength level for Inconel 718 is 827 Mps. Inconel 725 is used at 965 Mpa minimum yield strength level. The improved strength properties of Inconel 725 HS have been received by suitable thermal and mechanical processing.

Galvanic Compatibility

Galvanic corrosion occurs when different materials are placed in contact in a conductive fluid. The Inconel grades are usually noble materials. In galvanic compatibility, tests conducted in ambient temperature seawater for three months for corrosion mechanism. Inconel 725 and alloy 625 were found to be galvanically compatible. Coupling a large area of Inconel 725 to Monel K500, increased corrosion of the component of Monel K500.

General Pitting and Crevice Corrosion

Traditionally corrosion resistant alloys are graded first by their PREN and then by the equivalent cracking data produced in sour brine condition.

The critical pitting temperature for an alloy is found by subjecting samples in acidified 6% ferric chloride solutions. New unexposed test samples and fresh ferric chloride solution are used at every test temperature. The minimum critical pitting temperature for alloy is 40oC for various offshore applications.