lime mud

A type of water-base mud that is saturated with lime, Ca(OH)₂, and has excess, undissolved lime solids maintained in reserve. Lime muds are classified according to excess lime content: (1) low-lime, 0.5 to 2 lbm/bbl, (2) medium-lime, 2 to 4 lbm/bbl and (3) high-lime, over 4 lbm/bbl. All lime muds have pH in the range of 12, and the filtrate is saturated with lime. Fluid-loss additives include starch, HP-starch, carboxymethylcellulose (CMC) or polyanionic cellulose (PAC). Prehydrated bentonite can improve the fluid loss and rheology of a lime mud. A maltodextrin in lime muds has been used as a clay deflocculant, a shale stabilizer and to increase calcium solubility. KCl in lime muds has been another innovation for successful drilling of hydratable shales. The ability to carry very high mud alkalinity (as excess lime) to neutralize acid gases is one reason lime muds are used. H₂S zones can be drilled with more safety and copious amounts of CO₂ can be neutralized by a large excess of lime.