fluids containing surfactants and hydrophobic particles are effective media for cleaning non - aqueous fluids out of a subterranean wellbore . the fibers and surfactants are may be added to a drilling fluid , a spacer fluid , a sacrificial spacer fluid , a chemical wash , a cement slurry or combinations thereof . nafs , such as an oil - base mud or a water - in - oil emulsion mud , are attracted to the fibers as the treatment fluid circulates in the wellbore .