Photograph of Garrett gas trainX-Y plot of sulfide ion species
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1. n. [Drilling Fluids]
A compound of sulfur that contains the S-2 ion. H2S is the gaseous and highly toxic molecular form often found in the subsurface. Sulfide, S-2, and bisulfide, HS-, are the corresponding ionic forms. Sulfides can be generated from soluble iron sulfide minerals or from sulfate-reducing bacteria. The term "active sulfide" is used to denote compounds that revert to H2S gas when acidified with 2-molar citric acid solution, as opposed to inert sulfides, which are stable. Active sulfides include calcium sulfide and bisulfide formed when H2S reacts with lime in an oil mud. Their accumulation constitutes a safety concern at the rig because of the risk of reverting to H2S gas should an acidic influx occur. They may be converted to inert sulfides by adding zinc oxide.

Garrett RL, Carlton LA and Denekas MO: "Methods for Field Monitoring of Oil-Based Drilling Fluids for Hydrogen Sulfide and Water Intrusions," SPE Drilling Engineering 3, no.3 (September 1988): 296-302.