Assessing the Magnitude and Consequences of Reservoir Souring
catastrophic failures of steel Christmas trees on onshore wells in West Texas. Later there were also failures of valve components and
production tubing in other locations. In each case, H2S was released resulting in fatalities.7
At the time, the mechanism of failure was not
understood, but it is now known that it is a form of hydrogen embrittlement in which atomic hydrogen (created during the
The information in MR01752 is derived from laboratory SSC testing and over 60 years’ experience of sour service operations.
corrosion process) is encouraged to enter the steel due to the presence of sulphide on the steel surface. This cracking is now known as sulphide stress cracking (SSC).
SSC is the most dramatic cracking mechanism in metallic materials
caused by H2S, but there are others, including HIC, stress-oriented HIC (SOHIC) and soft zone cracking (SZC). For all these mechanisms the causative agent is the atomic hydrogen that is formed in the corrosion process. Sulphide on the surface of the steel inhibits the process by which hydrogen atoms combine to create hydrogen gas molecules; this encourages them to enter the steel. When hydrogen enters steel it has the potential to create different types of damage, depending on the material’s microstructure and the stress level in the component.
The information in MR01752 is derived from laboratory SSC testing and over 60 years’ experience of sour service operations. A steel’s hardness is the dominant factor in determining its susceptibility to SSC, and in-service failures are commonly associated with hard material including hard heat-affected zones in steel with high ‘carbon equivalent’. SSC is a very rapid mechanism; therefore, even a short time of wetness, e.g. arising from a process upset condition, in an otherwise dry sour system can be sufficient to cause this form of cracking.
SSC affects CRAs (when it is more properly called H2S-induced SCC) as well as carbon steel. In practice, it is usually the resistance to SCC,
rather than the resistance to corrosion, that governs the selection of CRAs for use in the oilfield.
HIC affects carbon and low-alloy steels, but not CRAs. The dominant factor that determines the susceptibility of a steel to HIC is the presence of laminar inclusions in its microstructure; this is a function
1.
Eden B, Laycock PJ, Fielder M, Oilfield Reservoir Souring, Health and Safety Executive Offshore Technology Report OTH 92 385, Sudbury: HSE Books, 1993.
2. NACE MR0175/ISO 15156-2:2009, Petroleum and Natural Gas Industries - Materials for Use in H2S-Containing Environments in Oil and Gas Production, NACE International, Houston, 2009.
3.
Ligthelm DJ, de Boer RB, Brint JF, Schulte WM, Reservoir Souring: An Analytical Model for H2S Generation and
96
data on a whole range of alloys at a time when there are fewer materials engineers (and particularly metallurgists) coming into the industry and fewer corrosion testing facilities exist in steel companies, which means that operating companies and even fabrication contractors are having to take on the task of generating materials susceptibility data. n
Transportation in an Oil Reservoir Owing to Bacterial Activity, SPE 23141, 1991.
4. 5. 6.
Sunde E, Thorstenson T, Torsvik T, et al., Field Related Mathematical Model to Predict and Reduce Reservoir Souring, SPE 25197, 1993.
Vance I, Thrasher DR, Reservoir Souring Mechanisms and Prevention. In Oliver B, Magot M (eds), Petroleum Microbiology, Washington: ASM, 2005.
Stott JFD, Dicken G, Rizk TY, NACE, Corrosion Inhibition in 7. 8.
PWRI Systems that Use Nitrate Treatment to Control SRB Activity and Reservoir Souring, Paper 08507, Presented at: Corrosion 2008, New Orleans, 16–20 March 2008.
Patrick DH, NACE, MR0175 – A History and Development Study, Paper 418, Presented at: Corrosion 1999, San Antonio, 25–30 April 1999.
Canadian Association of Petroleum Producers, Recommended Practice for Mitigation of Internal Corrosion in Sour Gas Gathering Systems, 2003.
EXPLORATION & PRODUCTION – VOLUME 9 ISSUE 2
of metal composition and processing detail. Atomic hydrogen created on the corroding surface passes through the material and discharges into these inclusions, combining to form molecular hydrogen. The pressure exerted by molecular hydrogen is very high and can exceed the through-thickness tensile strength of the steel, causing cracks. These cracks can link, resulting in stepwise cracking through the thickness of the steel. This was a common failure mode of pipelines in the 1970s and 1980s; nowadays, improvements in steel-making practice have resulted in a reduced incidence of HIC in steels from reputable steel makers. HIC is a slow mechanism; therefore, it is usually practical to monitor the development of these cracks and to schedule equipment replacement. The authors have witnessed monthly monitoring (by ultrasonic techniques) on pressure vessels fabricated some 25 years ago, this option being cheaper than vessel replacement in the specific circumstances.
Despite the significant advances in the understanding of H2S-related degradation mechanisms, they still cause many failures every year. For
example, over the period 2000–1, failures of sour gas pipelines accounted for 35 (4 %) of the 952 pipeline failures in Alberta, Canada.8
While it may
be understandable that such failures continue to occur in operating environments where reservoir souring is a new phenomenon, failures are also occurring in areas where the industry is mature. In a recent example in the UK, a carbon steel pipeline transporting sour hydrocarbons on-shore failed by SSC after some six weeks of operation necessitating
replacement at a cost of some £100 million. The H2S in this particular case was known to be present; it did not arise from unexpected reservoir souring.
As an increasing number of high-temperature, high-pressure (HTHP) reservoirs are exploited in the future, this will require more widespread use of CRAs, hence increasing the costs of wells and downstream equipment. For this reason we need more corrosion
Despite the significant advances in the understanding of
H2S-related degradation mechanisms, they still cause many failures every year.
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