pH prediction represents a crucial step before selecting materials for use in sour oil and gas wells as regards weight loss corrosion and H2S cracking. Among the numerous parameters which determine the equilibrium pH, important ones are CO2 and H2S partial pressures (PCO2 and PH2S 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 CO2 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 CO2 and H2S as determined by the Soreide and Whitson formalism. The influence of water and CH4 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 H2S activity, with strong implications for the evaluation of SSC risks.

Subject
Water, Hydrogen concentration, Solubility, Gas pressure, Partial pressures, Oil and gas, Acidity, Liquids, Constants, Experimental data, Fugacity, Case studies, High temperature
Keywords:
pH, CO2, H2S, high pressure, high temperature
© 2013 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2013
Association for Materials Protection and Performance (AMPP)
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