Titanium Alloy Tubing for HPHT OCTG Applications

Titanium alloys are currently being considered for use as downhole tubulars in high pressure, high temperature (HPHT) wells to provide an alternative to nickel-base alloys. Chevron has qualified Ti-6Al-2Sn-4Zr-6Mo for OCTG use in corrosive and highly sour wells. Ti-6246 is a 135 ksi YS alloy that demonstrates excellent resistance to environmental stress cracking and corrosion in production, workover and completion environments that may occur in severe HPHT and deepwater fields.  This paper documents the qualification test program for sour production environments, completion brines, and acids.  Qualification test methods includes C-rings per NACE TM0177-2005, slow strain rate per NACE TM0198-2004, fracture toughness per ASTM E399 using SENB geometry, sand erosion and erosion corrosion test loops, and corrosion coupons.  As with any CRA materials selection, project-specific testing should always be considered to define material performance with respect to the environments and stresses assumed for the completion.