Page 181 - Drilling Technology in Nontechnical Language

172 Drilling Technology in Nontechnical Language Second Edition

Hydrogen sulfide–related problems. Hydrogen sulfide (H S) may

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enter the mud from a permeable formation, either as a kick or from within
drilled cuttings. Apart from the extreme toxicity of this gas, it causes
hydrogen embrittlement of most steels, which degrades tensile strength.
If H S is anticipated, an excess level of lime in the mud will help. The
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alkaline lime will help neutralize the acidic H S. The reaction forms active
2

sulfides such as CaS or Ca(HS) , which will liberate H S if exposed to a
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mild acid. Once H S has been identified in the mud system, zinc oxide

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(ZnO) may be added. ZnO is an effective scavenger of H S and active
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sulfide salts. This reacts to form stable zinc sulfide. It is not recommended

to use ZnO before H S is identified, as it will mask a slow entry of H S into
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the system. Recommended concentration of ZnO is around 2 lb for each
barrel of mud.
Summary
This chapter covered one of the most important systems on the rig

for safe and efficient drilling—the drilling fluids. First the functions of

a drilling fluid were listed, and then seven distinct classifications of mud

were described. Drilling fluid design was covered in some detail for the
physical, rheological, and chemical requirements.

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