A specially prepared fluid used to displace matrix acid treatments away from the wellbore at the conclusion of a stimulation treatment. The overflush is typically formulated from a weak acid solution or brine to maintain a low pH environment in the near-wellbore formation that prevents the precipitation of reaction products as the treatment fluids are flowed back.

The overflush is normally a weak acid or brine pumped behind the main treating fluid (mixture of hydrofluoric (HF) and hydrochloric (HCl) or organic acids).
The overflush has several purposes:
路 displacement of the nonreacted mud acid into the formation.
路 displacement of mud-acid reaction by-products such as amorphous silica. The minimum overflush volume should have at least 3 ft (1 m) of radial penetration inside the formation to displace potential problems away from the critical matrix.
路 removal of potential oil-wet relative-permeability problems caused by some corrosion inhibitors.