Optimizing Sour Gas Qualification Testing: Determination and Verification of H2S Solubility from H2S/N2 Gas Mixtures up to 20,000 psig at 70°F

Abstract

Traditionally, the H<sub>2</sub>S partial pressure (P<sub>H2S</sub>) of the gas/hydrocarbon phase has been used as the primary sour severity metric for material qualification and selection under ANSI/NACE MR0175/ISO 15156 guidelines. While the P<sub>H2S</sub> is appropriate for characterizing low total pressure systems, the strict, or ideal, Henry’s Law approach leads to over estimation of the dissolved H<sub>2</sub>S concentration (C<sub>H2S</sub>) for high-pressure, high-temperature (HPHT) wells by up to ~20 times at 70 °F (21 °C). Alternatively, the Ensemble Henry’s Law equation corrects for the non-ideal phase behavior of H<sub>2</sub>S at HPHT conditions and avoids over-estimation of C<sub>H2S</sub>. Given the industry’s reliance on using thermodynamic models to evaluate sour HPHT systems, an investigation was initiated to determine the accuracy of these model calculations. An empirical program was undertaken to verify C<sub>H2S </sub>predictions for the H<sub>2</sub>S-N<sub>2</sub>-H<sub>2</sub>O system. Multiple 2.7-L C-276 lined autoclaves were charged with a fixed inventory of H<sub>2</sub>S in N<sub>2</sub> at multiple total pressure steps, with increasing N<sub>2</sub> pressure, between 30 and 20,000 psig (3 and 1,380 bar) at 70 ± 5 °F (21 ± 3 °C). Per total pressure step, H<sub>2</sub>S levels in both the liquid and gas phases were measured using common H<sub>2</sub>S sampling techniques (H<sub>2</sub>S-specific colorimetric tubes and iodometric titration, respectively), following ANSI/NACE TM0177-2016 guidelines. The results were used to calculate total pressure corrected (apparent) H<sub>2</sub>S solubility coefficients (<sup>A</sup>k<sub>H2S</sub>). Very good agreement was observed between empirically and computationally derived <sup>A</sup>k<sub>H2S</sub> values. Key words: ANSI/NACE TM0177-2016, ANSI/NACE TM0284-2016, Sour testing, Ensemble Henry’s Law, Ionic-equation of state (EOS) frameworks, H2S solubility, Iodometric titration.