Abstract

13Cr-5Ni-2Mo type Super Martensitic stainless steels (SMSS-13Cr) are often chosen for the manufacture of downhole completion equipment within the oil and gas production environment. SMSS-13Cr is primarily used in CO2 rich production environments where carbon/low-alloy steels are subject to high corrosion rates. Currently, different UNS grades of SMSS-13Cr with varying maximum hardness, yield strengths, and heat treatment requirements are listed for use in limited sour service in the NACE MR0175/ISO 15156-3 guidelines1. The hardness limits of the listed grades mainly pertain to the 95 ksi (655 MPa) Specified Minimum Yield Strength (SMYS) level. The environmental limits listed for the 95ksi MYS grade have similar maximum H2S partial pressure and minimum fluid pH values, but vastly different chloride concentration limits. While chloride ions can pit SMSS-13Cr type materials, it is not clear historically why the current limits reflect a difference between the UNS grades within the same alloy type.

This paper discusses a study in which the SSC resistance of two different SMSS-13Cr UNS bar grades, S41425 and S41427 quenched and tempered to 95 ksi MYS were evaluated in multiple simulated downhole production environments to study the main effects of H2S partial pressure, environmental pH, and chloride concentration. The SSC results provide comparison between cracking resistance of S41425-95 and S41427-95 in sour service environments. These results also offer insight for plausible limits on H2S partial pressure, environmental pH, and chloride concentration based on testing per current NACE MR0175/ISO 15156-3 guidelines.

Introduction

13Cr-5Ni-2Mo type Super Martensitic stainless steels referred to as SMSS-13Cr type grades can provide good general corrosion resistance such as in high CO2 environments combined with higher strengths and excellent toughness2 making them a prospective material choice for long term downhole completion equipment depending on actual well conditions. One of the main limiting factors for the use of SMSS-13Cr type grades is the Sulfide Stress Cracking (SSC) resistance in presence of H2S in downhole well conditions. Therefore, a good understanding of this behavior is essential to facilitate the material selection process. SMSS-13Cr is not limited to a single UNS grade and instead refers to a family of grades composed of multiple UNS numbers, including some proprietary grades. SMSS-13Cr grades are mostly available in quenched and tempered condition to 95 ksi SMYS and 110 ksi SMYS and is available as both bar stock and tubing. Bar stock is predominantly used for manufacturing thick-walled completion tool components with UNS S41425-95 and UNS S41427-95 being 95 ksi SMYS examples of prominent bar stock grades both of which are currently listed in NACE MR0175/ISO 15156-3.

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