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The Implications and Practicalities of Adopting the Dissolved H2S Concentration as the Sour Service Scalable Metric to Improve HPHT Materials Qualification Testing

Product Number: 51321-16451-SG
Author: Brent W.A. Sherar/Peter F. Ellis II/Jing Ning
Publication Date: 2021
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Gas phase H2S is synonymous 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. H2S partial pressure (PH2S) has been accepted historically as the scalable sour severity parameter. However, as the total pressure increases, the relationship between PH2S and the dissolved H2S concentration becomes non-linear. This limits the robustness of PH2S as the sour severity metric. Thus, ISO 15156- 2:2015/Cir.1:2017 now permits the use of H2S fugacity (fH2S), H2S activity (aH2S), and H2S concentration (CH2S) as alternatives for sour testing. This recent revision is based on evidence that fH2S and CH2S each provide better correlations to SCC at elevated total pressures than PH2S. This paper will address the merits and challenges of using fH2S or CH2S to define sour severity: We will argue that CH2S is the preferable parameter because it can be measured directly, and it has a direct correlation to hydrogen ingress through the metallurgy that is a precursor to cracking.

Key words: Fugacity, Partial pressure, Sulfide stress cracking (SSC), ANSI/NACE MR0175/ISO15156, H2S activity, Ensemble Henry’s law, HPHT wells, Ionic-equation-of-state (EOS), Non-ideal thermodynamics, material qualification

Gas phase H2S is synonymous 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. H2S partial pressure (PH2S) has been accepted historically as the scalable sour severity parameter. However, as the total pressure increases, the relationship between PH2S and the dissolved H2S concentration becomes non-linear. This limits the robustness of PH2S as the sour severity metric. Thus, ISO 15156- 2:2015/Cir.1:2017 now permits the use of H2S fugacity (fH2S), H2S activity (aH2S), and H2S concentration (CH2S) as alternatives for sour testing. This recent revision is based on evidence that fH2S and CH2S each provide better correlations to SCC at elevated total pressures than PH2S. This paper will address the merits and challenges of using fH2S or CH2S to define sour severity: We will argue that CH2S is the preferable parameter because it can be measured directly, and it has a direct correlation to hydrogen ingress through the metallurgy that is a precursor to cracking.

Key words: Fugacity, Partial pressure, Sulfide stress cracking (SSC), ANSI/NACE MR0175/ISO15156, H2S activity, Ensemble Henry’s law, HPHT wells, Ionic-equation-of-state (EOS), Non-ideal thermodynamics, material qualification

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Product Number: 51319-12939-SG
Author: Rudolf Hausler
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The transition from partial pressure to fugacity in the assessment of acid gas activity (concentration) for the design of qualification testing of metals to be used in sour service according to MR0175/ISO 15156 entails a number of important consequences. This transition came about in the wake of oil and gas production moving off-shore to ever higher pressures and temperatures. It was recognized that multiplying total pressure by the mol fraction of H2S in the “gas phase” could no longer reflect the physicochemical realities with respect to the reactions between H2S and the metal surfaces. As a consequence it was proposed that the activity of H2S in the gas phase should be replaced by the activity (concentration) of H2S in the aqueous phase. This change in paradigm had already been accepted in the ISO Standard but not implemented. Nevertheless it stands to reason that the dissolved H2S is the active corrosion vector rather than the H2S in the gas phase.An unintended consequence of this shift in thinking lies in the fact that a very large number of Heritage Metals have been qualified for partial pressure criteria as specified in MR0175/ISO-15156 by the use of the Crolet Diagram i.e. as function of pH vs. pH2S. In order to overcome this difficulty it is proposed to generate an array of look-up tables preferably in electronic form to translate the experimental conditions from pH2S to cH2S. This translation has to be made as a function of the test parameters (to the extent they may be known) as well as the field parameters. In parallel the pH2S axis in the Crolet diagram will need to be changed to a cH2S axis. In this manner it will be possible to assign to existing test data corresponding field conditions or vice versa specific field conditions can be used to select the appropriate metal from existing test data.An additional outcome of this methodology will be a quantitative assessment of the excess conservatism practiced in the past.

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