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

Gas phase H₂S partial pressure (P_H2S) 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. P_H2S has been accepted historically as the scalable sour severity parameter. However, as the total pressure increases, the relationship between P_H2S and the dissolved H₂S concentration becomes nonlinear. This limits the robustness of P_H2S as the sour severity metric. Thus, ISO 15156-1:2020 now permits the use of H₂S fugacity (f_H2S), H₂S activity (a_H2S), and H₂S aqueous concentration (C_H2S) as alternatives for sour testing. This recent revision is based on evidence that f_H2S and C_H2S each provide better correlations to SSC at elevated total pressures than P_H2S. This paper will address the merits and challenges of using f_H2S or C_H2S to define sour severity: we argue that C_H2S is a practical, experimentally verifiable approach, which can be used to validate ionic-equation of state frameworks used to characterize mildly sour HPHT environments.