BAKER HUGHES - Drilling Fluids Reference Manual

CORROSION
1. Source – Formations are the primary source with bacterial and thermal degradation of
fluid additives contributing minor amounts.
2. Reaction – H 2 S + Fe ++ + H 2 O → FeS + 2H +
3. Attack Type – Severe pitting, hydrogen embrittlement or sulfide stress corrosion
cracking, also generalized pitting and black sulfide coating. Black sulfide coating is
apparent as a black powder or scale on the drillstring.
4. Treating Agents
• Caustic soda – NaOH
• Lime – Ca(OH) 2
• MIL-GARD ® – 2ZnCO 3 × 3Zn(OH) 2
• MIL-GARD ® L – Zinc ammonium NTA
• MIL-GARD ® FE – Iron based scavenger designed to be used where zinc compounds are
not environmentally acceptable
5. Treating Methods – The most effective method of controlling H 2 S involves the use of
MIL-GARD ® (to scavenge sulfides) in conjunction with pH control, using caustic soda
and/or lime, to minimize embrittlement. 1
MIL-GARD ® preventative treatment is recommended at 2-3 lb m /bbl (1 lb m /bbl = 500
mg/L sulfide)
MIL-GARD ® FE preventative treatment is recommended at 0.05 gal/bbl (0.12 vol%).
Treatments of 0.1 gal/bbl (0.24%) will treat approximately 100 ppm H 2 S.
6. Caustic soda and/or lime should be used in the fluid system to establish an alkaline
environment. In an alkaline environment, H 2 S is reduced to soluble sulfides which have
less of a tendency to cause hydrogen embrittlement (see Figure 8 - 6). However, since
soluble sulfides present in the system are corrosive and will revert to H 2 S if a reduction in
pH occurs, do not rely on this approach for total control.
1 Chemical Scavengers for Sulfides in Water-Base Drilling Fluids, Garrett, R. L., Clark, R. K., Carney, L.L.,
and Grantham, C.K., SPE 7499, Journal of Petroleum Technology, June 1979.
BAKER HUGHES DRILLING FLUIDS
REVISION 2006 8-5
REFERENCE MANUAL