Testing Titanium-Alloy Tubing for HP/HT Applications
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 115708, "Titanium- Alloy Tubing for HP/HT Applications," by Manuel Gonzalez, SPE, Krystian Maskos, and Robert Hargrave, SPE, Chevron Energy Technology Company; John Kuberry, SPE, and Doyle Reeves, Hunting International; Jim Grauman, Titanium Metals Corporation; Jim Skogsberg, Consultant; and Syed Ali, SPE, Schlumberger, prepared for the 2008 SPE Annual Technical Conference and Exhibition, Denver, 21-24 Septem ber. The paper has not been peer reviewed.
Flow streams from high-pressure/high-temperature (HP/HT) wells usually contain H2S, CO2, and chlorides. Such wells with temperatures greater than 350°F require corrosion-resistant-alloy (CRA) tubing/casing. Titanium alloys can provide for an alternative. A testing program was developed to qualify Ti-6246 alloy for use in Chevron's sour-gas oil fields.

Introduction
The test program's objective was to qualify a 135-ksi Ti-6246 production-tubing alloy. The alloy would be suitable for production tubing as an alternative to nickel-based alloys and super-duplex stainless steels, depending upon the application. This alloy, Ti-6Al-2Sn-4Zr-6Mo, was selected for qualification testing. Adding molybdenum gives the alloy exceptional corrosion resistance to chlorides and H2S. The two-phase structure allows the alloy to be heat treated to optimize desired properties.

Environmental Material Test
This qualification program was designed to evaluate environmental-cracking susceptibility (e.g., stress-corrosion cracking and sulfide stress cracking) and overall corrosion performance of alloy tubulars exposed to several simulated oilfield-service environments including packer fluids, simulated production fluids, methanol, and mineral acid. The production environment was formulated to show suitability for most HP/HT wells. C-ring, slow-strain-rate testing (SSRT), and fracture tough-ness were methods used in this test program. Crevice-corrosion coupons and galvanic couples also were used in some tests.

Packer-Fluid Tests.
Saturated 15-lbm/gal noninhibited CaBr2 brine at 500°F was used to test C-rings stressed at 90% average-yield strength (AYS), crevice specimens, and corrosion coupons. Test duration was 90 days. All C-rings passed with no cracking, pitting, or crevice corrosion. Corrosion did occur on the carbon steel.
Saturated noninhibited 19-lbm/gal ZnBr2 brine at 500°F was used to test C-rings stressed at 90% AYS, crevice specimens, and corrosion coupons. Testing duration was 90 days. All C-rings passed with no cracking, pitting, or crevice corrosion. Carbon-steel wastage was very significant compared with specimens exposed to CaBr2 brine.
Production-Environment Tests. C-Ring Tests in Production-Environment A. C-rings stressed to 90% AYS were tested in Production-Environment A (without elemental sulfur) for 5 months. Production-Environment A was at 500°F with 1,500-psia H2S, 425-psia CO2, and 25% NaCl brine (150,000 ppm Cl). All C-rings in the test passed the Production-Environment-A test with no cracking, pitting, or crevice corrosion.
Some titanium alloys are sensitive to hydrogen embrittlement, and because carbon steel corrodes, there is the possibility of hydrogen charging of the titanium alloy. The charging in these tests was not significant, and the C-rings did not crack. Subsequent environmental tests measured very low hydrogen levels in the alloy after galvanic coupling with carbon steel.