There is currently significant interest in titanium alloys for downhole tubulars and components for corrosive oil and gas wells due to their combination of strength, density and corrosion resistance. These alloys may be able to augment the current list of commonly used materials available to resist corrosion and stress corrosion cracking in hostile service environments, especially for high pressure high temperature (HPHT) well applications. However, there has been only limited service experience with titanium alloys under harsh conditions of H2S, CO2, brine and elemental sulfur. Moreover, little data exists for non-production environments such as those used in concentrated brine workover/completion fluids and strong acids used in well stimulation. This paper will provide a review the results of published studies developed over the past 25 years for titanium alloys and will focus on their composition, metallurgical processing, corrosion and performance.
There is currently significant interest in titanium alloys as a new class of materials for downhole tubulars and components, handling corrosive oil and gas well conditions. These materials have an interesting combination of high strength, low density and high corrosion resistance. This interest is coming from two different perspectives:
• The search for hydrocarbon reserves continues to go deeper where completions have requirements for high tensile loading, high pressure and high temperature. These wells are commonly referred to as high pressure, high temperature (HPHT) wells. Examples of these are wells are shown in Figure 1.1 In many of these applications, the benefits of titanium metallurgy comes from the combination of inherent low density and high strength (i.e. high strength to weight ratio) that has made them attractive materials of construction in aerospace applications. Additionally, in many cases, these materials also have excellent corrosion resistance in many chemical environments.
• Due to the high level of activity in the oil and gas industry, the demand for corrosion resistant stainless and nickel-base alloys for oil country tubular goods (OCTG) has lead to long lead times for these conventional corrosion resistant alloys (CRAs). Until recently, titanium alloys have not been widely utilized in downhole completions and consequently offer an additional source of CRA materials that might be brought to the industry at this time of great need. This paper provides a review of published technical information and data developed over the past 25 years for titanium alloys focusing on the following aspects: (a) composition, (b) metallurgical processing, (c) corrosion and SCC performance, and (d) other unique properties that need to be considered when using these materials in downhole service. METALLURGY The metallurgy of Ti alloys is too comprehensive to be easily summarized in a short paper. However, the following section provides a simplified overview to familiarize those unfamiliar with Ti alloys with the terminology of the Ti alloys and some of their important attributes so it will become clear why Ti alloys are considered the next important step in the application of CRAs to oil and gas production and transportation.
Titanium alloys are grouped depending on their microstructure.
