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

Traditionally, the H2S partial pressure (PH2S) of the gas/hydrocarbon phase has been used as the primary sour severity metric for material qualification and selection under ANSI/AMPP (NACE) MR0175/ISO 15156 guidelines. While the PH2S is appropriate for characterizing low total pressure systems, the strict, or ideal, Henry’s Law approach leads to over estimation of the dissolved H2S concentration (CH2S) for high-pressure, high-temperature (HPHT) wells by up to approximately 20 times at 70°F (21°C). Alternatively, the Ensemble Henry’s Law equation corrects for the non-ideal phase behavior of H2S at HPHT conditions and avoids over-estimation of CH2S. 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 CH2S predictions for the H2S-N2-H2O system. Multiple 2.7 L C-276 lined autoclaves were charged with a fixed inventory of H2S in N2 at multiple total pressure steps, with increasing N2 pressure, between 30 psig and 20,000 psig (3 bar and 1,380 bar) at 70±5°F (21±3°C). Per total pressure step, H2S levels in both the headspace gas and liquid phases were measured using common H2S sampling techniques (H2S-specific colorimetric tubes and iodometric titration, respectively), following ANSI/NACE TM0177-2016 guidelines. The results were used to calculate total pressure corrected (apparent) H2S solubility coefficients (AkH2S). Very good agreement was observed between empirically and computationally derived AkH2S values.