Qualification of UNS N08028 and UNS N08825 Material as a Cost-Effective Solution for Extreme Sour Environment

In recent years, Oil and Gas operators have faced new challenges with higher H2S partial pressure wells, especially in the Middle East. NACE MR 0175 part III provides guidance on the material selection for Corrosion Resistant Alloys; however, the well conditions considered by the Operator (total pressure 5526 psi, 1044 psi H2S, 279°F) are on the border of the UNS N08028 / UNS N08825 application domain, and therefore the recommendation would be to use a higher alloy, UNS N06985 material. This paper describes a methodology to perform fine material selection for such critical conditions considering the representativeness of the testing condition and specific laboratory testing to qualify a suitable material for the intended application. The objective was to evaluate the suitability of UNS N08028 and UNS N08825 material 110 ksi in the conditions described above. Based on actual reservoir conditions and fluid analysis, the test solution was defined to represent actual corrosiveness of the environment. Solution components, gas phases and in situ pH were defined using OLI® Stream Analyzer Thermodynamic software to reach the specified conditions at the test temperatures. Then, Stress Corrosion Cracking was assessed based on SSRT (Slow Strain Rate Test) as per NACE TM 0198-2016 [9], a rather conservative test as the material is stressed in its plastic domain; and C-ring as per NACE TM 0177-2016 [12], performed on rolled material to be representative of the final material surface finish. All SSRT specimens showed a high ductility ratio and the absence of secondary cracking on the gauge section on both grades. All SCC C-ring specimens exposed to the environment successfully completed 720 hours exposure on both grades. Both UNS N08028 and UNS N08825 materials successfully passed the SSRT and C-ring tests, allowing the Operator to optimize the material selection by choosing a grade specifically qualified for their application and reducing the overall tubular cost by 30%.