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Implications of Using the Fugacity of the Acid Gases in the Design of Qualification Testing of Oilfield Tubular Materials

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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.

Product Number: 51319-12939-SG
Author: Rudolf Hausler
Publication Date: 2019
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