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.
Drill collars are thick-walled pipes that provide stiffness and concentration of mass at or near the bit and are among the main components of the bottom-hole assembly (BHA).1 The non-magnetic drill collars (NMDCs) represent a sub-category of proprietary drill collars that enable magnetic surveying and directional drilling. Due to their cross-section, NMDCs are inherently heavy and can convey a strong push on the drill bit itself, minimizing cutting instability problems, while their strength is sufficient to prevent buckling while drilling. NMDCs are critical to modern directional drilling operations and serve as a structural platform to a variety of measurements-while-drilling (MWD) and logging-while-drilling (LWD) tools that carry sensors for formation evaluation.2,3 NMDC pipes are typically 30 ft (9 m) long, with nominal outer diameter (OD) between approximately 4 to 11 in (0.10 to 0.28 m).3-5 Directional drilling demands durable and cost-effective materials capable of long service in fields. The Cr-Mn austenitic stainless steels have established themselves in directional drilling. They have high minimum yield strengths (i.e., 110 to 140 ksi or 759 to 965 MPa), elevated toughness (> 100 ft.lb or 134 J typical), high fatigue resistance (80 ksi or 552 MPa at 100,000 cycles), and overall present good resistance against corrosion, galling, and a low magnetic permeability. NMDC materials are manufactured per the minimum requirements of API(1) Specification 7-1 and per specifications exceeding API requirements through proprietary compositions and manufacturing routes.1,3,5 Today these specifications describe a number of grades with chemistries, microstructures, properties, and manufacturing processes that often overlap and have evolved based on field experience and new advancements in drilling technologies.6 NMDC materials are most commonly austenitic stainless steels with chromium (Cr) and manganese (Mn) as the main alloying elements, and with nickel (Ni), nitrogen (N) and molybdenum (Mo) all added to various percentages to balance manufacturability, mechanical properties, corrosion resistance, and repairability.3,4 NMDC materials are strengthened through some form of warm-forging, thus they have various levels of residual stresses. When fusion welding is needed, these materials have various tendencies to lose their initial strength and corrosion resistance, important factors that are accounted for in the selection of proprietary grades of NMDC materials.
