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Optimizing Sour Qualification Testing – Verification of H2S Solubility From H2S/N2 Gas Mixtures up to 20,000 psig at 70 °F

Picture for Optimizing Sour Qualification Testing – Verification of H2S Solubility From H2S/N2 Gas Mixtures up to 20,000 psig at 70 °F
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Product Number: 51321-16317-SG
Author: Brent W.A. Sherar/Peter F. Ellis II
Publication Date: 2021
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Traditionally, the H2S partial pressure (PH2S) ofthe gas phase has been used as the primary sour severity metric for material qualification and selection under ANSI/NACE MR0175/ISO 15156guidelines. While the PH2Sis appropriate for characterizing low total pressuresystems, 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 ~20timesat 70 °F. Alternatively, the Ensemble Henry’s Law equationcorrectsfor the non-idealphase behavior of H2S at HPHT conditionsandavoidsover-estimation of CH2S. Given the industry’s reliance on using thermodynamic models to evaluatesour HPHT systems, an investigation was initiated to determine the accuracy thesemodel calculations. An empirical program was undertaken to verify CH2Spredictions for the H2S-N2-H2O system. Multiple 2.7-L C-276 lined autoclaves were charged with a fixed inventory of H2S in N2atmultiple total pressuresteps, with increasing N2pressure,between 30 and 20,000 psigat 70 ± 5 °F.Per total pressure step, H2S levels in both the liquid and gas phases were measured using common H2S sampling techniques (H2S-specific colorimetric tubes and iodometric titration, respectively), following ANSI/NACETM0177-2016 guidelines. The results were used to calculate total pressurecorrected (apparent) H2S solubility coefficients(AkH2S). Very good agreement was observed between empiricallyand computationallyderivedAkH2Svalues.

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.

Traditionally, the H2S partial pressure (PH2S) ofthe gas phase has been used as the primary sour severity metric for material qualification and selection under ANSI/NACE MR0175/ISO 15156guidelines. While the PH2Sis appropriate for characterizing low total pressuresystems, 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 ~20timesat 70 °F. Alternatively, the Ensemble Henry’s Law equationcorrectsfor the non-idealphase behavior of H2S at HPHT conditionsandavoidsover-estimation of CH2S. Given the industry’s reliance on using thermodynamic models to evaluatesour HPHT systems, an investigation was initiated to determine the accuracy thesemodel calculations. An empirical program was undertaken to verify CH2Spredictions for the H2S-N2-H2O system. Multiple 2.7-L C-276 lined autoclaves were charged with a fixed inventory of H2S in N2atmultiple total pressuresteps, with increasing N2pressure,between 30 and 20,000 psigat 70 ± 5 °F.Per total pressure step, H2S levels in both the liquid and gas phases were measured using common H2S sampling techniques (H2S-specific colorimetric tubes and iodometric titration, respectively), following ANSI/NACETM0177-2016 guidelines. The results were used to calculate total pressurecorrected (apparent) H2S solubility coefficients(AkH2S). Very good agreement was observed between empiricallyand computationallyderivedAkH2Svalues.

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.

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