Performance of high functional nickel based super alloys in sour oil and gas conditions
The new face of oil and gas discovery will be with deep wells, specifically in deepwater. As compared to shallow wells, deep wells normally need high performance nickel base alloys. Wells are classified as sweet or sour. Sweet wells are just mildly corrosive, whilst sour wells are extremely corrosive. Sour wells contain hydrogen sulfide, carbon dioxide, chlorides and free sulfur. There are different level of corrosive media that are combined by temperatures up to 500oF and pressures about 25000 psi. Deep wells normally have higher temperatures and pressures. Material selection is particularly critical for sour gas wells. The chosen material should be corrosion resistance, economically priced, reliable and have suitable strength for the wells. In the severe conditions, material selection varies from carbon steels for sweet wells to duplex stainless steel to Incoloy 825 for sour well service.
Age hardened nickel base alloys and cold processed solid solution nickel base alloys offer several advantages like nickel base alloys, cold processed solid solution nickel base alloys provide various advantages like high strength, toughness and low magnetic permeability and outstanding corrosion resistance. Properties of Inconel bars are ideal to be chosen for service in the well conditions. These properties include:
Mechanical properties, general corrosion resistance, pitting and crevice corrosion resistance, chloride stress corrosion cracking resistance, sulfide stress corrosion cracking resistance.
The strength values of age hardened materials are increasing in significance, specifically for offshore applications exploiting high pressure deep well reserves where weight factors can influence the economic feasibility of a project. Material choice for down-hole and wellhead components like hangers, safety valves, pumps and packers demand age hardenable alloys to receive essential strength in heavier cross-section that cannot be reinforced by cold processing. Nickel alloys widely used in these applications are:
- Inconel 718
- Inconel X750
- Monel K-500
The age-hardened alloys are used at different strength levels on the basis of applications. Inconel 718 has minimum yield strength level of 827MPa.
Resistance to galvanic corrosion
Galvanic corrosion occurs when different materials are placed in a contact of a conductive fluid. Incoloy and Inconel grades are normally noble and chosen for system design when in contact with less noble materials. Coupling Incoloy 725 to Monel K500 increased corrosion of alloy K500 component.
Resistance to general pitting and crevice corrosion
The minimum required CPT (critical pitting temperature) for an alloy is 40oC for various seawater applications.
Resistance to marine corrosion
Nickel base super alloys with PREN above 40 are found to be extremely resistance to crevice corrosion in natural seawater service. In both stagnant and running water conditions, the weight changes are nominal. Inconel 625 samples received crevice corrosion in a test to a depth of 0.66mm. Concentration of titanium in Inconel 725 seems to have an advantageous effect in enhancing crevice corrosion resistance in seawater.
Resistance to general corrosion in sour conditions
Inconel 625 show good resistance to general corrosion in hydrogen sulfide conditions and significantly higher performance than Inconel 718.