ABSTRACT

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.

INTRODUCTION

The effect of the H2S/CO2 ratio on the corrosion of carbon steels has been studied by numerous researchers. These works mainly focus on performing laboratory studies2,3,4 and models for prediction5 to obtain a better understanding on the mechanisms involved. The studies show that in addition to the H2S/CO2 ratio, other factors, such as temperature, in-situ pH, and gas-oil ratio (GOR), play a role in the corrosion rate and mechanisms observed. Other researchers provide a broad review of weight-loss corrosion in sour well conditions for tubing, pipelines, etc.7 and design guidelines for material selection.8

Chromium-molybdenum-based low-alloy steels, such as 41XX type, are popular grades used for completion tools because they have better hardenability than carbon steels for manufacturing thick-walled components. These low-alloy steels, when quenched and tempered to 22 HRC maximum hardness, can provide good SSC resistance in H2S environments in compliance with NACE MR0175/ISO 15156-2 guidelines.1 While low- alloy steels with the addition of chromium-molybdenum have better general corrosion resistance than carbon steel, they are typically subject to similar corrosion mechanisms in H2S and CO2 environments downhole.

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