Low-alloy steels with less than 1% nickel content and having 22 HRC (Hardness Rockwell C) maximum hardness can be used for sour (H2S) service at any temperature in all regions, as per NACE MR0175/ISO 15156-2 guidelines.1 They provide resistance to sulfide stress cracking (SSC) and therefore can be used in downhole completion tools for many oil- and gas-producing environments. However, low-alloy steels can experience general and localized corrosion during medium- to longer-term use in production environments. The corrosion rate is dependent on multiple factors, such as H2S partial pressure, CO2 partial pressure, temperature, in-situ pH, etc. One derived factor that can significantly influence the corrosion rate is the H2S/CO2 ratio. This paper discusses two case histories in which 41XX low-alloy steel, quenched and tempered to 22 HRC maximum hardness, was used in longer-term downhole completion tools. The first case discusses a packer assembly installed near the bottom hole that provided 14 years of good long-term service in an oil- producing environment having high-H2S and -CO2 conditions. The second case discusses a plug assembly installed in a flow control device near the surface during shut-in of a gas well having high-H2S and -CO2 conditions, where the components experienced pitting type corrosion after 18 months of service.
Keywords: Low alloy steel, H2S/CO2 ratio, Case history, Completion tools, NACE MR0175/ISO 15156