(n.) Drilling Fluids. A water-base mud designed for offshore drilling whose make-up water is taken from the ocean. Sea water contains relatively low salinity, about 3 to 4 wt. % NaCl, but has a high hardness because of Mg⁺² and Ca⁺² ions. Hardness is removed from sea water by adding NaOH, which precipitates Mg⁺² as Mg(OH)₂, and by adding Na₂CO₃, which removes Ca⁺² as CaCO₃. Mud additives are the same as those used in freshwater mud-bentonite clay, lignosulfonate, lignite, carboxymethylcellulose or polyanionic cellulose and caustic soda. XC polymer may also be used in place of bentonite. Higher concentrations of each additive are required because of salinity effects. Bentonite (if used) should be prehydrated in fresh water. See: Acrylamide-Acrylate Polymer, Calcium Contamination, Calcium Sulfate, Caustic Soda, Chloride Test, Hydration, Make-Up Water, PHPA Mud, Prehydrated Bentonite, Saltwater Flow, Water-Base Drilling Fluid, XC Polymer