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Improving pH Prediction for High Pressure and High Temperature Applications in Oil and Gas Production

Abstract : pH prediction represents a crucial step before selecting materials for use in sour oil and gas wells as regards weight loss corrosion and H 2 S cracking. Among the numerous parameters which determine the equilibrium pH, important ones are CO 2 and H 2 S partial pressures (P CO2 and P H2S respectively), the total pressure, the ionic strength and the chemical composition of the solution, and the temperature. Most models used by oil and gas operators present a too narrow range of validity for these parameters, which makes them inappropriate for high pressure and high temperature (HPHT) fields or for CO 2 reinjection. This paper presents modeling improvements which allows extending the prediction validity in temperature and pressure to respectively 200 °C and 1,000 bar of total pressure, and for an ionic strength up to 5 mol.kg-1. These improvements take into account the fugacities in gas phase of CO 2 and H 2 S as determined by the Soreide and Whitson formalism. The influence of water and CH 4 pressure is also taken into account up to several hundred bars. Activity coefficients in the water phase are calculated using the Pitzer model. The consistency of the model is verified by comparison with experimental measurements of pH under high pressure. It is then applied to oil and gas applications at high pressure and high temperature. The impact of the new calculation method is discussed both for pH evaluation and also for H 2 S activity, with strong implications for the evaluation of SSC risks.
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C. Plennevaux, Thierry Cassagne, M. Bonis, Nicolas Ferrando, J. Kittel, et al.. Improving pH Prediction for High Pressure and High Temperature Applications in Oil and Gas Production. Corrosion 2013, Mar 2013, Orlando, United States. ⟨hal-02464643⟩

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