Corrosion-Resistant Alloys for Hot, Deep Sour Wells: Properties, Experience, and Future Trends

ABSTRACT Nickel-base, corrosion-resistant alloys (CRA) have been subjected to extensive corrosion testing and evaluation for potential use in hot, deep sour wells. Of these CRA, the Ni-Cr-Mo-W system has been used in sour gas wells in South Mississippi, Wyoming and Mobile Bay. Corrosion data, mechanical properties and resistance to environmental stress cracking are described for several Oil Country Tubular Goods (OCTG) products. Better understanding of metallurgical factors and of the role of alloying for enhancedresistance to corrosion, along with advances in materials processing, are reviewed in terms of their impact on OCTG product reliability, improved safety and cost effectiveness. INTRODUCTION Decreasing reserves and the energy crisis of the early seventies accelerated ventures in sour gas wells, enhanced recovery, deepwater offshore platforms and arctic explorations. These ventures brought major challenges to materials engineers. In particular, the extremely corrosive and toxic deep sour gas wells highlighted issues related to material reliability and safety of operations. l Conventional steel tubing and other components were rapidly deteriorated by general weight loss, localized pitting/crevice corrosion, stress corrosion cracking and hydrogen embrittlement. Extensive testing and evaluation programs were initiated for a large selection of corrosion-resistant materials, as an alternative to limitations on the use of inhibitors. Of these materials, the high performance alloys of Ni-Cr-Mo-W system offered an optimum combination of corrosion resistance, strength and ductility that is required for deep sour well production tubing. Despite the oil and gas doldrums of the eighties, experimental studies continued and some corrosion-resistant alloys (CRA) were used in sour gas wells both inland and offshore. This paper provides an overview of the corrosion properties of several high-nickel alloys (chemical compositions shown in Table 1), lists the field usage of CRA tubular products, describes further development, covers present issues and illustrates future trends where high performance alloys might be required to enhance reliability and safety in offshore service. PROPERTIES OF CRA PRODUCTS The inability to inhibit corrosion of conventional steel tubing effectively in deep sour gas production led to the testing of CRA products. 2 In addition to the presence of the highly corrosive H2S/brines and to bottom-hole temperatures approaching 500°F (260°C), the CRA tubulars must possess strength levels capable of handling bottom-hole pressures up to 23,000 psiat depths in excess of 15,000 feet.3 To meet these severe stress levels, higher yield strength of CRA tubular products can be achieved by cold working (110 to 180 ksi), by precipitation strengthening (140 to 200 ksi) and by cold working plus aging heat treatments (180 to 280 ksi). Even at strength levels exceeding 140 ksi, the CRA tubulars retain their resistance to various forms of corrosive attack, including hydrogen sulfide stress cracking.2 The corresponding threshold stresses for CRA were much higher than those for stainless steels.