A fluid stage, normally hydrochloric acid (HCl), pumped ahead of the main treating fluid (mixture of hydrofluoric (HF) and hydrochloric (HCl) or organic acids) in a sandstone matrix-stimulation treatment. One of the purposes of a preflush is to displace formation brines that contain K, Na, Ca ions away from the wellbore, decreasing the possibility of crystallizing alkali-fluosilicates that could plug the pores. The other purpose of a preflush is to dissolve calcareous materials to minimize calcium fluoride (CaF₂) precipitation, and to dissolve iron scale or rust to avoid the precipitation of the gelatinous, highly insoluble ferric hydroxide (Fe(OH)₃).

Multiple preflush stages using brines such as ammonium chloride (NH₄Cl) or solvents are used when multiple damage types are present. A preflush is sometimes called a spearhead.