Due to their high strength and relatively low cost, martensitic stainless steel (MSS) well tubulars are attractive. However, sour applications of MSS are limited by their susceptibility to Sulphide Stress Cracking (SSC) under shut-in conditions, where the pressure is the highest and the temperature the lowest. This currently restricts their use to a greater extent than high temperature pitting and Stress Corrosion Cracking (SCC).
A comparison between field experience in mildly sour gas wells and laboratory test data on 13Cr MSS tubulars showed that in the laboratory SSC tests, cracking is observed under conditions that are significantly less severe than the field conditions in which these tubulars are operated without reported failures. This raised the question whether the current method of SSC testing is too conservative. To investigate this, a detailed assessment of field-relevant exposure conditions for SSC was carried out.
Susceptibility to SSC requires a near to ambient temperature. Therefore, only the top section of the well can be susceptible, after closing in the well and a subsequent cooling down period. Because the liquids will drain away after closing in a gas well, the tubular will be exposed to a thin water film (or water droplets) only. This thin water film exposure was evaluated and modelled. Results indicate a lower susceptibility to SSC than fully immersed conditions, attributable to pH elevation in the thin water film, which is induced by very slow corrosion under passive conditions. Existing SSC test methods were adapted to simulate the thin water film exposure in a realistic way, which resulted in a new fit-for-purpose SSC test method. The results obtained with the thin water film test demonstrate that for mildly sour gas wells, the safe operating envelope of martensitic stainless steels can be extended.
Martensitic stainless steel (MSS) well tubulars are favorable due to their high strength and relatively low cost and are therefore widely applied in the Oil & Gas industry. This is especially the case for 13Cr and Super13Cr grades, which are often selected for mildly sour gas fields, where a relatively low content of H2S is present. When selecting martensitic stainless steels for sour service, the susceptibility to Stress Corrosion Cracking (SCC) and Sulfide Stress Cracking (SSC), determined by standard laboratory tests, are the most important selection criteria.