An extraordinarily poisonous gas with a molecular formula of H2S. At low concentrations, H₂S has the odor of rotten eggs, but at higher, lethal concentrations, it is odorless. H₂S is hazardous to workers and a few seconds of exposure at relatively low concentrations can be lethal, but exposure to lower concentrations can also be harmful. The effect of H₂S depends on duration, frequency and intensity of exposure as well as the susceptibility of the individual.
Hydrogen sulfide is a serious and potentially lethal hazard, so awareness, detection and monitoring of H₂S is essential. Since hydrogen sulfide gas is present in some subsurface formations, drilling and other operational crews must be prepared to use detection equipment, personal protective equipment, proper training and contingency procedures in H₂S-prone areas.
Hydrogen sulfide is produced during the decomposition of organic matter and occurs with hydrocarbons in some areas. It enters drilling mud from subsurface formations and can also be generated by sulfate-reducing bacteria in stored muds. H₂S can cause sulfide-stress-corrosion cracking of metals. Because it is corrosive, H₂S production may require costly special production equipment such as stainless steel tubing.
Sulfides can be precipitated harmlessly from water muds or oil muds by treatments with the proper sulfide scavenger. H₂S is a weak acid, donating two hydrogen ions in neutralization reactions, forming HS^- and S^-2 ions. In water or water-base muds, the three sulfide species, H₂S and HS^- and S^-2 ions, are in dynamic equilibrium with water and H⁺ and OH^- ions. The percent distribution among the three sulfide species depends on pH. H₂S is dominant at low pH, the HS^- ion is dominant at mid-range pH and S² ions dominate at high pH. In this equilibrium situation, sulfide ions revert to H₂S if pH falls. Sulfides in water mud and oil mud can be quantitatively measured with the Garrett Gas Train according to procedures set by API.