1. n. 
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: · displacement of the nonreacted mud acid into the formation. · 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. · removal of potential oil-wet relative-permeability problems caused by some corrosion inhibitors.