Selection Of Materials For Optimum Performance Of Wellhead Assemblies In Petroleum Production Operations

Abstract

The influence of well fluids and external environments, including offshore and cold weather, on metallic and non-metallic materials is discussed. Methods of selection based on field data, assisted by accelerated laboratory testing, is dealt with in detail Tabulations or material in everyday use are shown, including carbon steels, low alloy steels, stainless steel and elastomers, with suggestions for specific uses. The selection of materials for petroleum production operations is based primarily on environmental compatibility. cost is an important but secondary consideration. Through the years, users and manufacturers have come to realize that a rather large group of economical materials will serve adequately in a large percentage of fields. Cold weather operations, hydrogen sulfide, carbon dioxide, all influence the need for special materials or routine materials in a different condition. Chosen materials must be compatible with oil and gas environments as encountered in the production cycle. Well streams contain oils, condensates, methane, carbon dioxide, nitrogen, hydrogen sulfide, ethane, and many heavier fractions plus salt water in varying concentrations and quantities. plus salt water in varying concentrations and quantities. Methane is the least harmful as to its effect on metallic materials. Table No. I illustrates the effect of individual fractions on metallic materials. Two basic failure modes are encountered in petroleum production:stress corrosion cracking due to hydrogen production:stress corrosion cracking due to hydrogen sulfide and/or chlorides, andmetal loss in the form of pitting or general corrosion. pitting or general corrosion. Both failure modes are very important but pitting may be more misleading in that if a corrosion monitoring system is based on well stream iron count, the observer may be content, this while his tubing is being perforated in a zone not of his choosing! Sufficient data has been accumulated through field experience to permit users and manufacturers to evaluate the problem and make reasonably accurate predictions as to the performance of a material in specific environments. When new performance of a material in specific environments. When new alloys or new applications for established alloys are considered, accelerated laboratory testing can be helpful for screening purposes prior to field testing. purposes prior to field testing. Internal and external accessory items are made from the various materials shown in the tables on the next page. Table No. 2 and 3 offer several materials in common use for parts wetted by the well stream. parts wetted by the well stream.