1. n. [Enhanced Oil Recovery]

A thick, viscous emulsion containing oil, water, sediment and residue that forms because of the incompatibility of certain native crude oils and strong inorganic acids used in well treatments. Use of certain additives, such as surfactants, or the presence of dissolved iron can promote sludge formation, especially if asphaltenes are present in the crude oil. Therefore, it is important to test a sample of crude with the treating fluid before injecting a treatment into a reservoir.

See: crude oil,  surfactant,  treatment fluid