For most of oil based mud, lime (Ca(OH)2) is used in the system in order to perform a chemical reaction with fatty acid emulsifiers. Typically, 3 to 5 lb/bbl of lime is added in the drilling mud so that there is enough hydroxide (OH-1) ions to keep the emulsion stability in good shape.
Moreover, lime (Ca(OH)2) will control acid gases such as H2S and CO2. The following chemical equations demonstrate how lime reacts with H2S and CO2, respectively.
Ca(OH)2 + H2S -> CaS + 2(H2O)
Ca(OH)2 + CO2 -> CaCO3 + H2O
If drilling into zones where CO2 or H2S exists, the amount of excess lime should be increased to around 5 to 10 lb./bbl., because some lime is used for emulsion stability and others react with acid gases to maintain the mud’s properties.
What’s more, when the well is drilled into formations containing H2S, the excess lime must be kept constantly at 5 to 10 lb/bbl all the time. Do not try to reduce the amount of excess lime because the chemical reaction between H2S and lime is reversible. Therefore, if the level of excess lime is not maintained, the H2S gas can be released at the surface from the reversible chemical process.
Andy Philips, 2012. So You Want to be a Mud Engineer: An Introduction to Drilling Fluids Technology. Edition. CreateSpace Independent Publishing Platform.
Ryen Caenn, 2011. Composition and Properties of Drilling and Completion Fluids, Sixth Edition. 6 Edition. Gulf Professional Publishing.