Non-Magnetic drill collars have been developed to meet requirements of magnetic permeability, strength and galling resistance. Until recently, corrosion resistance has been treated as a secondary consideration. This paper describes recent developments in corrosion resistance in these applications, and the results of comparative testing, using Critical Pitting Temperature testing. Corrosion resistance generally correlates well with chemistry, and the CPT identifies exceptional cases quite clearly. Recent developments in drill collar materials have markedly improved their corrosion resistance.
INTRODUCTION
Non-magnetic drill collars have unusual design requirements, and as a result pose unique problems of corrosion resistance. Drill collars are the bottom components in a drill string, where their weight generates the force on the drill bit allowing it to drill and their stiffness allows the force to be transmitted without buckling. Conventional drill collars are made of alloy steel, like the rest of the drill string, and their corrosion usually poses no problems except to their threaded connections. A typical drill collar is 30 feet long and 4 to 9 inches in diameter.
Non-magnetic drill collars were developed to allow magnetic surveying of the well trajectory using instruments inside the drill string. This was originally introduced as a regulatory requirement, but has become an essential tool with the widespread use of directional drilling. These drill collars must be completely non-magnetic, and must resist galling when screwed together. The typical yield strength requirement is a minimum of 110ksi, which is quite demanding for non-magnetic steels, and these mechanical and magnetic properties have been the primary alloy design requirements. The first non- magnetic collars were made of Monel, N05500, and they are still commonly known within the industry as monels. However, as their use became widespread, it became clear that nickel-based materials were much too expensive and they were replaced by stainless steels. It is doubtful that a copper-nickel drill collar could be made to meet the current mechanical requirements for collars.
Since the beginning, stainless steels for this application have been proprietary alloys, with specific compositions and processing paths. The first generation were chrome-nickel stainless steels, similar to commercial 300 series grades. In order to get the required strengths, they had to be cold worked (or more commonly warm-worked), and they had to resist transformation of austenite to magnetic phases as a result of the work. This was achieved by judicious additions of carbon and nitrogen, which enhance cold-work response, as well as stabilizing austenite. A typical formulation at this time closely resembled 302 stainless: 18Cr, 8Ni, 0.15C, with probably an unreported component of nitrogen. These alloys performed quite well until the late 1970's, when they started showing severe cracking. It is reported that the change was due to the elimination of chromate-based corrosion inhibitors for environmental reasons.
The environmental cracking was found to be of two types. The first was classical stress corrosion cracking. The forging process generates large, tensile, residual stresses on the inside of drill collars, and this stress frequently exceeded the threshold stress for SCC in salt muds. Drilling muds usually contain significant chloride concentrations, whether from the base water (which offshore is seawater), deliberate additions to increase density, or pickup from formation water. Hindsight showed that the 18- 8 composition is one of the worst possible for SCC resistance ~. This prompted the development of the second generation of
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