1. n. 
A two-phase mixture of liquid water and steam produced from a generator. The latent heat of vaporization for steam is very high, and when the steam condenses in the reservoir a significant amount of heat is transferred from the steam to the formation rock and fluids. Since steam is lighter and more mobile than oil, gravity differences and channeling of the steam through the most permeable parts of the reservoir can create sweep efficiency problems during steam-injection processes. To increase sweep efficiency, there are two categories of improvements. The first is operational changes such as selective completion of injector wells, fracturing operations and constructing horizontal wells, and the second is the use of additives in the steam. For example, water-soluble surfactants modify interfacial properties of the oil-water system, and foams reduce steam mobility.