1. n. [Drilling Fluids]
An outdated distinction between two types of oil muds. In the past, invert-emulsion oil muds were those with more than 5 vol.% emulsified water, and oil-base muds were those with less than 5 vol.% water. Today, this distinction is not pertinent because the general term oil mud covers all water concentrations.
See: asphalt, emulsion, fatty-acid soap, hectorite, interfacial tension, invert emulsion, mud, oil content, oil mud, oil-base mud, oil-in-water emulsion, oil-mud emulsifier, organophilic clay, organophilic lignite, rheology modifier, surface tension, synthetic-base fluid, synthetic-base mud, water-in-oil emulsion