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The Implications and Practicalities of Adopting the Dissolved H2S Concentration as the Sour Service Scalable Metric to Improve HPHT Materials Qualification Testing

Product Number: 51321-16451-SG
Author: Brent W.A. Sherar/Peter F. Ellis II/Jing Ning
Publication Date: 2021
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Gas phase H2S is synonymous with sulfide stress cracking (SSC) and is routinely used as the ‘scalable’ parameter to qualify materials for high-pressure, high-temperature (HPHT) wells. Candidate materials for HPHT wells routinely require ANSI/NACE MR0175/ISO 15156 compliance because a few mole ppm of H2S at high pressure may place the well beyond the 0.05 psia (0.3 kPa) sour service threshold. H2S partial pressure (PH2S) has been accepted historically as the scalable sour severity parameter. However, as the total pressure increases, the relationship between PH2S and the dissolved H2S concentration becomes non-linear. This limits the robustness of PH2S as the sour severity metric. Thus, ISO 15156- 2:2015/Cir.1:2017 now permits the use of H2S fugacity (fH2S), H2S activity (aH2S), and H2S concentration (CH2S) as alternatives for sour testing. This recent revision is based on evidence that fH2S and CH2S each provide better correlations to SCC at elevated total pressures than PH2S. This paper will address the merits and challenges of using fH2S or CH2S to define sour severity: We will argue that CH2S is the preferable parameter because it can be measured directly, and it has a direct correlation to hydrogen ingress through the metallurgy that is a precursor to cracking.

Key words: Fugacity, Partial pressure, Sulfide stress cracking (SSC), ANSI/NACE MR0175/ISO15156, H2S activity, Ensemble Henry’s law, HPHT wells, Ionic-equation-of-state (EOS), Non-ideal thermodynamics, material qualification

Gas phase H2S is synonymous with sulfide stress cracking (SSC) and is routinely used as the ‘scalable’ parameter to qualify materials for high-pressure, high-temperature (HPHT) wells. Candidate materials for HPHT wells routinely require ANSI/NACE MR0175/ISO 15156 compliance because a few mole ppm of H2S at high pressure may place the well beyond the 0.05 psia (0.3 kPa) sour service threshold. H2S partial pressure (PH2S) has been accepted historically as the scalable sour severity parameter. However, as the total pressure increases, the relationship between PH2S and the dissolved H2S concentration becomes non-linear. This limits the robustness of PH2S as the sour severity metric. Thus, ISO 15156- 2:2015/Cir.1:2017 now permits the use of H2S fugacity (fH2S), H2S activity (aH2S), and H2S concentration (CH2S) as alternatives for sour testing. This recent revision is based on evidence that fH2S and CH2S each provide better correlations to SCC at elevated total pressures than PH2S. This paper will address the merits and challenges of using fH2S or CH2S to define sour severity: We will argue that CH2S is the preferable parameter because it can be measured directly, and it has a direct correlation to hydrogen ingress through the metallurgy that is a precursor to cracking.

Key words: Fugacity, Partial pressure, Sulfide stress cracking (SSC), ANSI/NACE MR0175/ISO15156, H2S activity, Ensemble Henry’s law, HPHT wells, Ionic-equation-of-state (EOS), Non-ideal thermodynamics, material qualification