Improved High-Pressure, High-Temperature (HPHT) Materials Qualification Using Dissolved H2S Concentration as the Sour Service Scalable Metric

Gas phase H2S partial pressure (PH2S) is associated 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. 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 nonlinear. This limits the robustness of PH2S as the sour severity metric. Thus, ISO 15156-1:2020 now permits the use of H2S fugacity (fH2S), H2S activity (aH2S), and H2S aqueous concentration (CH2S) as alternatives for sour testing. This recent revision is based on evidence that fH2S and CH2S each provide better correlations to SSC at elevated total pressures than PH2S. This paper will address the merits and challenges of using fH2S or CH2S to define sour severity: we argue that CH2S is a practical, experimentally verifiable approach, which can be used to validate ionic-equation of state frameworks used to characterize mildly sour HPHT environments.