Abstract
The use of Cr-Mn austenitic stainless steels in non-magnetic drill collars (NMDCs) has evolved over the last few decades as drilling operations have become more challenging, driving the demand for improved materials. Throughout the life of NMDC tools, localized corrosion (i.e., pitting corrosion, crevice corrosion) and stress corrosion cracking (SCC) are among the most common damages observed on NMDCs in between drilling runs. In some of the worst drilling scenarios, these two forms of corrosion have led to downhole NMDC component failures such as “washouts” and “twist-offs”. This paper first reviews the major forms of corrosion-related failures on NMDCs and the factors that influence NMDC corrosion (grades, Pitting Resistance Equivalent Number - PREN, mud type, drilling conditions, run life, etc.). It then describes a few selected field failures along with examples of preventive measures and improved best practices for reduced field failures. This paper also emphasizes the importance of rigorous inspection and maintenance on new and repaired NMDC components, as is the need for improved methods to detect sensitization among the many proprietary grades of NMDC materials.
