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An outdated distinction between two types of oil muds.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.

Alternate Form: invert-emulsion oil mud