1. n. [Drilling Fluids]

An oil-base mud with diesel oil as its external phase. Diesel-oil mud is the traditional oil mud and has a history of excellent performance for drilling difficult wells. It has been used because the base oil is low-cost and widely available motor fuel. In-gauge holes can be drilled through all types of shales, salt, gypsum and other difficult strata using diesel-oil mud systems. It is often the mud of choice for drilling high-pressure, high-temperature zones. Diesel-oil muds usually contain from 5 to 40 vol.% emulsified brine water (except those that are specially designed to have none). The water phase usually contains 20 to 40 wt.% dissolved calcium chloride for shale control. Diesel-oil muds have been replaced in land drilling by mineral-oil muds and offshore by synthetic-fluid muds. These newer muds have fewer health, safety and environmental concerns compared to diesel oil.

See: aniline point test,  balanced-activity oil mud,  external phase,  isomerized olefin,  linear alphaolefin,  oil content,  oil mud,  oil-base mud,  polyalphaolefin,  pour point,  synthetic-base mud,  water-in-oil emulsion