Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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67 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 • Considerations of whether the type of FeS that would form for conditions below the calculated ratio would be mackinawite or troilite/pyrrhotite. For th e hi g h er t emp erature an d higher H 2 S partial pressure condi- tions where troilite or pyrrhotite will form, the value for the calculated ratio w ould be much larger than 2,000 because the troilite and pyrrho- tite of FeS are much more stable and therefore require less H 2 S. • Consideration of the assumptions implicit in the calculation, such as a temperature of 25 °C and a low ionic- strength produced water. The high concentrations of dissolved solids found in many oi lf i el d pro duc ed waters could alter the theoretical ratio by changing the activity coeffi- cients for the dissolved ions. The as- sumption of low ionic strength also means that the effect of bicarbonates from the reservoir or acetic acid is not considered. Conclusions The origin of the value of 500 for the CO 2 /H 2 S ratio appears to be covered in the article by Dunlop, et al. Although the arti- cle did not provide an explanation for the derivation of the value, the reference to Lat- imer did provide the information that was required to develop a derivation of a ratio value that was close enough to 500 to believe that if Dunlop, et al. were not the origin of the value, then at least they were familiar with the source. Although the ratio has historically pro- vided a useful rule of thumb, it has a num- ber of key assumptions, including a tem- perature of 25 °C and low ion-strength aqueous solution . The impact of these limiting assumptions is usually not con- sidered. Although Dunlop's calculated ratio is only valid at 25 °C, temperature was found to have only a minor effect upon the ratio within the range of 25 to 100 °C assuming that the FeS corrosion product is macki- nawite. Temperature increases beyond 100 °C were not investigated because of issues associated with potential phase changes. The ratio would have to increase if the FeS corrosion product changes from mack- inawite to pyrrhotite due to a change in corrosion conditions. Using Latimer's data, the ratio would need to be increased from 500 to at least 2,500 just to account for the change from mackinawite to pyrrhotite. The ratio is also not applicable for the lower temperature and low pH conditions where Fe 2+ is the likely corrosion product rather than FeCO 3 . Although there is a sound scientific basis for the ratio, the error band produced by variations in the thermodynamic data inputs is so large as to make the ratio's use impractical as an engineering guideline. Ratio values ranging from as low as 9 to as high as 2,600 were calculated. However, a change of only 1% to the FeCO 3 free energy increased the value to over 7,600. It is there- fore far too sensitive to the specific thermo- dynamic input values to be of any practical value beyond use as a very rough order of magnitude guide. The use of the CO 2 /H 2 S ratio as a rule of thumb to determine sweet vs. sour corro- sion conditions is recommended for use in only the broadest of terms. The ratio is sim- ply too sensitive to thermodynamic input data quality to be a useful engineering tool. Existing computer tools that model corro- sion chemistr y and can calculate FeCO 3 and FeS formation should be used when- ever possible rather than reliance upon this rule of thumb. If a very quick evaluation is needed, use of the ratio should be limited only to the broadest of evaluations. Where ratios for produced f luids result in values that are below 1 or above 5,000, these conditions might be considered to be sour or sweet, respectively. Any ratio that is between 1 and 5,000 can only be considered to be ques- tionable. For conditions that fall within the questionable range, either laboratory test- ing or a comprehensive thermodynamic modeling program should be used to deter- mine whether the corrosion reactions will be driven by sweet or sour chemistry. References 1 R . Nyborg, "Guidelines for Prediction of CO 2 Corrosion in Oil and Gas Production Systems," IFE/KR/E-2009/003, Sept. 2009. 2 NACE TG 305 Proposed Standard Practice, " Wet Gas Internal Corrosion Direct Assess- ment Methodology for Pipelines," Draft 5 (Houston, TX: NACE International, 2010). 3 A.K. Dunlop, H.L. Hassell, P.R. Rhodes, "Fun- damental Considerations in Sweet Gas Well Corrosion," CORROSION/83, paper no. 46 (Houston, TX: NACE, 1983). 4 C. DeWaard, D.E. Milliams, "Carbonic Acid Corrosion of Steel," Corrosion 31, 5 (1974): pp. 177-181. 5 W.M. Latimer, Oxidation Potentials (New York, NY: Prentice Hall, 1938). 6 S.N. Smith, "Discussion of the History and Relevance of the CO 2 /H 2 S Ratio," CORRO- SION 2011, paper no. 11065 (Houston, TX: NACE, 2011). 7 G.B. Naumov, B.N. Ryzhenko, I.L. Khoda- ko v sky, " Han d b o o k of Th er m o dy n ami c Data," U.S.G.S. Report WRD-74-00 1, January 1974. 8 R.A. Berner, "Stability Fields of Iron Minerals in Anaerobic Marine Sediments," J. Geolog y 72 (1964): pp. 826-834. 9 S.N. Smith, "A Proposed Mechanism for Cor- rosion in Slightly Sour Oil and Gas Produc- tion" (Houston, TX: International Corrosion Congress, 1993). Tis article is based on CORROSION 2011 paper no. 11065, presented in Houston, Texas. STEPHEN N. SMITH, FNACE, is an inde- pendent consultant based in The Wood- lands, Texas, e-mail: Stephen.Smith.PE@ gmail.com. He has an M.S. degree in met- allurgical engineering from the University of Texas at El Paso. A 41-year member of NACE International, he is serving as NACE Institute Policy and Practices Director, is a NACE Fellow, and received a NACE Techni- cal Achievement Award and Distinguished Service Award. He is a Senior Research Fel- low of the Institute of Corrosion and Multi- phase Flow at Ohio University. The Carbon Dioxide/Hydrogen Sulfide Ratio—Use and Relevance

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