Thursday, July 23, 2009

Sour Oil and Gas

High Performance Age-
Hardenable Nickel Alloys Solve Problems in Sour Oil and Gas Service
S. A. McCoy.
Special Metals Wiggin Ltd
Holmer Road
Hereford, UK

B. C. Puckett & E. L. Hibner
Special Metals Corporation
Huntington, WV


The new frontier of oil and gas exploration will be with deep wells, particularly in deepwater. Most
of the “easy-to-pick” fruit have been taken with shallow field development. Compared to shallow
wells, deep wells generally require more high-performance, nickel-base alloys. Wells are
categorized as being either “sweet” or “sour.” Sweet wells are only mildly corrosive, while sour
wells are very corrosive. Sour wells can contain hydrogen sulfide, carbon dioxide, chlorides, and
free sulfur. There are different levels of corrosive conditions that are compounded by temperatures
up to 500F (260C) and pressures up to 25,000 psi (172 MPa). Deep wells generally have higher
temperatures and pressures. Material selection is especially critical for sour gas wells. The materials
of choice must be corrosion-resistant, cost-effective, reliable, and have the required strength for the
well conditions. As these conditions become more severe, material selection changes from carbon
steels for “sweet” wells, to duplex (austenitic-ferritic) stainless steel, to INCOLOY alloys 825 or
925™, to INCONEL alloys 725HS and 725™ for sour well service.

Materials have to meet criteria for corrosion resistance and mechanical properties in service
environments needing increased reliability for the lifetime of the exploited asset. Age-hardened
nickel-base alloys and cold-worked solid solution nickel-base alloys offer many advantages such as
high-strength, toughness, low magnetic permeability and excellent corrosion resistance. The choice
of material for a particular set of well conditions is based on a number of selection criteria
*Mechanical properties
*General corrosion resistance
*Pitting & crevice corrosion resistance
*Chloride stress corrosion cracking resistance
*Sulfide stress corrosion cracking resistance
Mechanical Properties
The strength levels of age-hardened materials are increasing in importance, particularly for offshore
applications exploiting high-pressure deep well reserves, where weight considerations can affect the
economic viability of a project. Material selection for down-hole and wellhead equipment such as
hangers, sub-surface safety valves, pumps and packers require age-hardenable alloys to obtain the
necessary strength in heavier cross-sections which cannot be strengthened by cold work. Nickel
alloys commonly used for these applications include INCOLOY alloy 925, MONEL alloy K-500,
and INCONEL alloys 718, X-750, 725, and 725HS. Typical mechanical properties of highperformance
nickel alloys used in oil country applications.

The age-hardened alloys are used at different strength levels depending on the application.
Generally INCOLOY alloy 925 is used at a 758 MPa (110 ksi) minimum yield strength level. The
minimum yield strength level for INCONEL alloys 718 and 725 is 827 MPa (120 ksi). INCONEL
alloy 725HS is used at a 965 MPa (140 ksi) minimum yield strength level. The enhanced strength
properties of INCONEL alloy 725HS have been achieved through optimized thermal and
mechanical processing.

Galvanic Compatibility

Galvanic corrosion can be a concern when dissimilar materials are in contact in a conductive fluid.
The INCOLOY and INCONEL alloys are generally noble and consideration is given towards the
system design when in contact with less noble materials. In galvanic compatibility tests performed
in ambient temperature seawater for 92 days at LaQue Center for Corrosion Technology, INCONEL
alloys 725 and 625 were determined to be galvanically compatible. Coupling a large surface of
INCONEL alloy 725 to MONEL alloy K-500 promoted corrosion of the alloy K-500 component.

General Pitting and Crevice Corrosion Resistance
Traditionally, corrosion-resistant alloys are screened first by their pitting resistance equivalent
number (PREN), and then by the equivalent cracking data generated in sour brine environments1.
Equation 1 shows a typical formula used to compare the pitting resistance of stainless steels and
nickel-base alloys.

PREN = %Cr + 1.5( %Mo + %W + %Nb) + (30 x %N) Equation 1

The critical pitting temperature (CPT) for an alloy is determined by exposing samples in acidified
6% ferric chloride solutions, according to ASTM Standard Test Method G48, Method C 2, and
raising the temperature by incremental amounts until the onset of pitting. New unexposed test
specimens and fresh ferric chloride solution are used at each test temperature. The tests are only
valid up to 85C because at higher temperatures the test solution becomes unstable. The minimum
accepted CPT for an alloy is 40C for many offshore applications (e.g., the North Sea).
Determining the critical crevice temperature (CCT) of an alloy involves exposing samples to the
same aggressive test solution but with a multiple crevice device (TFE-fluorocarbon washer)
attached to the surface of the test specimen. The temperatures shown in Table 3 indicate the onset of
crevice corrosion.

Resistance of Age Hardened Nickel-Base Alloys to Corrosion by Seawater.

Nickel alloys with a PREN greater than 40 are very resistant to crevice corrosion in natural seawater
service. Table 4 compares the crevice corrosion resistance of corrosion-resistant alloys in seawater.
Under both stagnant and flowing conditions, the weight losses are extremely low.

Resistance to General Corrosion in Sour Environments

In mineral acids3, INCONEL alloy 725 in the age-hardened condition has comparable corrosion resistance to INCONEL alloy 625. Good general corrosion resistance can be important in resisting the various chemicals injected as inhibitors and dispersants.
Environmental Cracking
Wellhead and downhole components must resist stress corrosion cracking (SCC). The potential for
SCC becomes greater with higher temperature and higher concentration of H2S and the presence of
chloride ions and elemental sulfur. Lower temperature hydrogen embrittlement and sulfide stress
cracking (SSC) are also potential failure mechanisms which may be promoted by galvanic
corrosion, acidic conditions, and dissolved H2S.
Resistance to Sulfide Stress Cracking and Hydrogen Embrittlement

In general, resistance to SCC, SSC, and hydrogen embrittlement increases with increasing content
of nickel, chromium, molybdenum, tungsten and niobium in an alloy.
Stress Corrosion Cracking
Alloy strength is a factor in environmental cracking susceptibility. Materials become more prone to
environmental cracking as their strength increases. In order to obtain the optimum level of strength,
ductility and toughness, and cracking resistance, maximum hardness levels are specified for each
alloy in NACE International’s Materials Requirement MR0175'5.

H. R. Copson 6 originally reported the beneficial effect of alloy nickel content on chloride SCC
resistance of austenitic type alloys in 1959. Alloys 825, 925, 718, 625 and 725 all contain 42% or
greater nickel and, as a result, are all very resistant to stress corrosion cracking in water containing

A more severe test in ranking materials performance is the slow strain rate (SSR) test.
Common pass/fail criteria for SSR testing is a ratio of time to failure (TTF), %
reduction of area (%RA) and % elongation (%El) measured in a simulated oil patch
environment relative to the same parameter in an inert environment (gases such as
air or nitrogen). These are referred to as "critical ratios". TTF, %RA and %El ratios of
0.80 typically represent passing behavior in SSR tests. If the ratios are below 0.90,
the specimen is examined under a scanning electron microscope for evidence of ductile
or brittle fracture on the primary fracture surface. Tests exhibiting ductile behavior
are acceptable while those with brittle fracture are not. All specimens are examined
for secondary cracking in the gage length away from the primary fracture. The absence
of secondary cracking is indicative of good SCC or SSR resistance and passes. The
presence of secondary cracks is cause for rejection. One or more inert (air) SSR tests
are conducted along with two or more environmental SSR tests for each test lot of
material 7. The decision to use the critical ratio of 0.80 as the acceptance criterion in
SSR tests was based upon results obtained earlier for cold worked solid solution
nickel-base alloys 8,9. Studies 10 have shown that INCOLOY alloy 925 is consistently more
crack resistant in severe Mobile Bay type sour brine environments than alloy 718, based on SSR
stress corrosion cracking data.
Ultimately, it is the user's responsibility to establish the acceptability of an alloy for a specific
oilfield environment. The data presented here should be helpful in selecting materials for the
corrosive environments of sour oilfields. A group of alloys that represents a range of alternatives
can be selected for testing in an environment simulating the oilfield environment under study. A
final material selection for a specific application should be made based on test results and an
economic analysis of cost-effective alternatives.
INCONEL alloy 725 offers resistance to corrosion in extremely sour brine environments and in the
presence of elemental sulfur at temperatures up to 242C. The maximum permitted hardness under
NACE MR0175 requirements is 40 HRc. The stress corrosion cracking resistance of age-hardened
INCONEL alloy 725 is superior to that of INCONEL alloy 718 in sour environments.
A high-strength grade of alloy 725, INCONEL alloy 725HS, has been assigned a NACE MR0175
maximum hardness level of 43 HRc and can be used for high-strength applications in sour service
up to NACE test level VI at 175C.
The corrosion resistance of age-hardened nickel-base alloys in sour brine environments is as
INCONEL alloy 725> INCONEL alloy 725HS > INCOLOY alloy 925 > INCONEL alloy 718 >
MONEL alloy K-500 and INCONEL alloy X-750
INCOLOY, INCONEL®, MONEL®, 925™ and 725™ are trademarks of the Special Metals
Corporation group of companies.

1. E. L. Hibner and C. S. Tassen, “Corrosion Resistant Oil Country Tubular Goods and Completion
Alloys for Moderately Sour Service,” EUROCORR 2000, paper no. C014/18, London, UK,
September 2000.

2. ASTM G48 ASTM Standard Test Method G48, Annual Book of ASTM Standards, vol. 03.02 (West
Conshohocken, PA: ASTM, 1995)
3. E. L. Hibner, et. al., Effect of Alloy Content vs. PREN on the Selection of Austenitic Oil Country
Tubular Goods for Sour Gas Service,” CORROSIOIN/98, paper no. 98106, NACE International,
Houston, TX, USA, 1998.
4. Standard TM-01-77. “ Testing of Metals for Resistance to Sulfide Stress Cracking at Ambient
Temperatures”. NACE, Houston, TX 1986 revision.
5. NACE Standard Test Method MR0175-2000, “Sulfide Stress Cracking Resistance Metallic Materials
for Oilfield Equipment”.
6. H. R. Copson, T. Rhodin (ed.), Effect of Composition on Stress Corrosion Cracking of Some Alloys
Containing Nickel, “Physical Metallurgy of Stress Corrosion Fracture,” Interscience Publishers, Inc.,
New York, 1959.
7. E.L.Hibner, "Improved SSR Test for Lot Acceptance Criterion," Slow Strain Rate Testing for the
Evaluation of Environmentally Induced Cracking: Research and Engineering Applications, ASTM
STP1210, R.D.Kane, Ed., p.290, American Society for Testing Materials, West Conshohocken, PA,
USA, 1993.
8. H.E.Chaung, M.Watkins and G.A.Vaughn, "Stress-Corrosion Cracking Resistance of Stainless Alloys
in Sour Environments," Corrosion/85, Paper no. 277, NACE International, Houston, TX, USA, 1985.
9. M.Watkins, H.E.Chaung and G.A.Vaughn, "Laboratory Testing of SCC Resistance of Stainless
Alloys," Corrosion/87, Paper no. 0283, NACE International, Houston, TX, USA, 1987.
10. R. B. Bhavsar and E. L. Hibner, “Evaluation of Testing Techniques for Selection of Corrosion
Resistant Alloys for Sour Gas Service,” CORROSION/96, paper no. 59, NACE International,
Houston, TX, USA, 1996.
1. E. L. Hibner and C. S. Tassen, “Corrosion Resistant OCTG’s and Matching Bar Products for a range
of sour gas service conditions,” CORROSION/2001, paper no. 01102, NACE International,
Houston, TX, USA, 2001.

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