Enhanced Oil Recovery (EOR) is a technique for increasing the amount of oil that can be extracted from an oil field. It is also called improved oil recovery or tertiary recovery. This extraction is achieved by either gas injection, thermal recovery, or chemical injection. Gas injection is the most commonly used EOR technique. Gas such as carbon dioxide, natural gas, or nitrogen is injected into the reservoir whereupon it expands and thereby pushes additional oil to a production wellbore, and dissolves in the oil to lower its viscosity and improves the flow rate of the oil. Thermal recovery uses heat to improve flow rates. In chemical injection, polymers are injected to increase the effectiveness of waterfloods, or the use of detergent-like surfactants to help lower the surface tension that often prevents oil droplets from moving through a reservoir.