Gas phase H<sub>2</sub>S partial pressure (P<sub>H2S</sub>) 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 H<sub>2</sub>S at high pressure may place the well beyond the 0.05 psia (0.3 kPa) sour service threshold. P<sub>H2S</sub> has been accepted historically as the scalable sour severity parameter. However, as the total pressure increases, the relationship between P<sub>H2S</sub> and the dissolved H<sub>2</sub>S concentration becomes non-linear. This limits the robustness of P<sub>H2S</sub> as the sour severity metric. Thus, ISO 15156-1:2020 now permits the use of H2S fugacity (f<sub>H2S</sub>), H<sub>2</sub>S activity (a<sub>H2S</sub>), and H<sub>2</sub>S aqueous concentration (C<sub>H2S</sub>) as alternatives for sour testing. This recent revision is based on evidence that f<sub>H2S</sub> and C<sub>H2S</sub> each provide better correlations to SSC at elevated total pressures than P<sub>H2S</sub>. This paper will address the merits and challenges of using f<sub>H2S</sub> or C<sub>H2S</sub> to define sour severity: We argue that C<sub>H2S</sub> is a practical, experimentally verifiable approach, which can be used to validate ionic-equation of state (EOS) frameworks used to characterize mildly sour HPHT environments.

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