High Strength Proprietary Low Alloy OCTG Applicable in Severe Sour Environment

Abstract

Abstract Environmental cracking susceptibility of high strength low alloy steel oil country tubular goods (OCTG) (110 ksi SMYS proprietary sour grade and 110 ksi SMYS non sour grade) in severe sour environment was investigated. Environmental cracking resistances of materials were investigated in high H2S pressures using four-point bend beam tests in autoclaves. The H2S pressures and testing temperatures were varied from 10 bar H2S – 100 bar H2S to 24 °C - 150 °C. 110 ksi sour grade remained crack free in all tested conditions. Some localized corrosion occurred after one month exposure on sour grade material in higher H2S in the higher temperature conditions, but these corrosion initiations did not propagate after long term (3 months) immersion test. It was concluded that the 110 ksi proprietary sour grade tested in this research would be used up to 100 bar H2S and up to 150 °C, as opposed to API P110 material (110 ksi non sour grade) which suffered SSC (Sulfide Stress Cracking) below 66 °C when exposed to 10 bar H2S. Consequently the proprietary 110 ksi sour grade evaluated in this research can be considered as a validated high H2S environment SSC resistant material usable for casing applications anywhere within Regions 1, 2 & 3 of ISO 15156-2 Figure-1 (pH versus H2S partial pressure diagram) and more than 10 bar H2S environment, that is in any combination of H2S partial pressure and temperature under in site pH occurring in oil & gas production environments. Additionally, the SSC susceptibility of newly developed 125 ksi SMYS proprietary sour grade was investigated under SSC region 3 environments of ISO 15156-2 Figure-1 by NACE TM0177-2005 Method A. New 125 ksi proprietary sour grade survived at 0.05 bar H2S - pH2.7, 0.1 bar H2S - pH3.5 and 1 bar H2S - pH4.0. On the other hand, new 125 ksi proprietary sour grade showed SSC at 0.1 bar H2S - pH3.0, 1bar H2S - pH3.5. The SSC susceptibility of new 125 ksi proprietary sour grade increased with an increase in H2S partial pressure and a decrease in pH. From these results, the SSC resistance limitation for newly developed 125 ksi proprietary sour grade has been clarified.