(1) (n.) Drilling Fluids. A chemical that reacts with dissolved oxygen (O2) to reduce corrosion, such as sulfite (SO3-2) and bisulfite (HSO3-) ions that combine with oxygen to form sulfate (SO4-2). This is a redox reaction and requires a nickel or cobalt catalyst. Removal of air from a mud by defoaming and mechanical degassing is an essential first step before a scavenger can lower the dissolved oxygen content. See: precipitation, redox, scavenger, sequestering agent, sulfide scavenger. (2) (n.) Well Completions. A chemical agent used in some brines and completion fluids to reduce corrosion resulting from, or exacerbated by, dissolved oxygen. Oxygen scavengers capture the dissolved oxygen in a harmless chemical reaction that renders the oxygen unavailable for corrosive reactions. The use of oxygen scavengers is more critical in applications in which fluids are to be circulated in the wellbore. Additional oxygen may be dissolved during agitation at surface