Long- and Short-Term Laboratory Testing of UNS N06985 for OCTG in Extreme Sour Environments

UNS N06985 is a nickel-based alloy with additions of Mo, Cu and W. Because of its high strength and high resistance to stress corrosion cracking (SCC), cold worked UNS N06985 has been extensively used for OCTG (Oil Country Tubular Goods). To meet deeper and more challenging environments, tube products manufactured in high quality corrosion resistant alloys (CRAs) are required.

Recently, cold worked UNS N06985 tubes have been developed and produced for OCTG in specified minimum yield strength 110 ksi and 125 ksi in two dimensions. Laboratory testing has provided mechanical properties and corrosion resistance to localized corrosion and SCC. The SCC resistance has been verified using both short-term SSRT and long-term autoclave exposures using C-ring and mass loss for 90 days. The SSRT was performed in the maximum boundary conditions of environmental limits for 4d type nickel-based alloys per ISO15156-3, whereas the long-term autoclave exposures were performed in a condition outside of the limits for the 4d type materials.

With comparison to the commercial tube products of UNS N06985 in literature, the newly developed tube materials of grades 110 ksi and 125 ksi showed high SCC resistance and stable mechanical properties. The tube materials fulfill ISO13680/API-5CRA requirement and ISO15156-3.

UNS N06985 is a nickel-based alloy with additions of Mo, Cu and W. Because of its high strength and high resistance to stress corrosion cracking (SCC), cold worked UNS N06985 has been extensively used for OCTG (Oil Country Tubular Goods). The SCC initiation has been studied in Cl-H₂S-CO₂ containing sour environments; and the SCC resistance was often evaluated using slow strain rate testing (SSRT) for screening by short-term testing and using tensile or C-ring for verification by long-term autoclave testing exposures¹. UNS N06985 showed high resistance to SCC at high temperatures in elemental sulfur containing environments with moderate chlorides². Acceptable environments were reported based on testing results³. The testing conditions were up to 25% NaCl (180 000 mg/l Cl-), 100 psi H2S, 200 psi CO₂, at 218 °C for 60-120 days⁴. On another hand, 52 000 mg/l Cl-, 1709 psi H2S, 556 psi CO₂, 100 g/l elemental sulfur at 149 °C for 90 days have been used for SCC testing to simulate a more severe reservoir in the Middle East, typically with high H2S contents, high pressures, high temperatures and high salinity brines, by which UNS N06985 products from different material suppliers through distinct manufacturing routes showed acceptable SCC resistance in a fit-for-purpose testing program⁵. All these testing conditions were chosen within the environmental limits for UNS N06985 as 4d type nickel-based alloys per ISO⁽¹⁾ 15156-3⁶/ NACE⁽²⁾ TM0175⁷. Moreover, long-term exposure testing is necessary to ensure the SCC resistance of the high grade CRAs in the deep and sour wells with high concentrations of chloride and hydrogen sulfide at high temperature for at least 90 days, according to the ongoing rewriting of ISO 15156 committee activity.