One of a number of methods used to increase the amount of crude oil that can be recovered from an oil reservoir. These include the use of water, gas, chemical, and steam injection into the reservoir. Gas injection is the most commonly used method and involves the use of natural gas, carbon dioxide, or nitrogen to displace the oil. The method is dependent on the temperature and pressure of the reservoir and composition of the oil within it. Carbon dioxide is miscible in light oil and can alter the viscosity and (p. 127) surface tension of the oil. The use of chemical injection involves dilute alkaline solutions to alter the surface tension and enhance oil recovery. Surfactants are also used. Steam injection also reduces the viscosity of the oil and can also partially vaporize it, aiding recovery.