Sour well design is often based upon the partial pressure of the acid gases, which have most often been correlated to equivalent partial pressures in laboratory studies conducted at a much lower total pressure. This partial pressure approach is supported by industry standards, notably ISO 151561 / NACE MR01751 . However, total pressure and temperature, as seen in the wells, will impact the solubility and chemical behavior of these acid gases in the water phase, in some instances potentially reducing the corrosive impact by a factor of 3-10, depending upon the potential critical parameters of hydrogen sulfide activity or solubility.

This paper summarizes a test program that studied the physical chemistry of hydrogen sulfide (H2S) and its impact on the sour material properties of low alloy steels. The program investigated various parameters of H2S to determine the most influential environmental factor in sulfide stress cracking (SSC) of alloy steels typically used in mildly sour wells. The parameters assessed included gas partial pressure, gas fugacity, aqueous solubility, and aqueous chemical activity. Statistical data analysis of the results with 95% confidence limits indicated that fugacity was the primary parameter for control of sulfide stress crack propagation for a casing steel.

Incorporating fugacity determinations into well assessments potentially provide opportunities for use of lower grades of materials, especially for high pressure gas wells. However, there are other implications that need to be understood for high pressure liquid systems. In particular, the current approach of using the H2S value at the bubble point of an oil may prove to be non-conservative for high pressure liquid hydrocarbon systems. Lastly, this paper describes some challenges facing ISO 15156 when moving forward to address incorporation of thermodynamic calculations to determine the actual degree of sour severity in the field.

Subject
Hydrogen sulfide, Materials, Correlation, Hydrogen concentration, Hydrostatic pressure, Solubility, Crack propagation, Gas pressure, Mixtures, Partial pressures, Reservoirs, Fugacity, Sulfide stress cracking
Keywords:
Sulfide Stress Cracking, SSC, Fugacity, Sour Wells, Oil Country Tubular Goods
© 2014 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).
2014
Association for Materials Protection and Performance (AMPP)
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