Enhanced oil recovery is used to recover stranded oil that remains in reservoirs after primary depletion.

Oil is first produced from reservoirs under primary recovery due to in-situ reservoir pressure aided by pumps. Secondary recovery generally occurs by a waterflood. Water is injected into the reservoir to physically displace the residual oil, which is subsequently recovered by adjacent production wells. The success of waterfloods depends on the permeability of the reservoir and the properties of the oil.

Tertiary recovery techniques are also referred to as enhanced oil recovery, or EOR. The three most common EOR techniques are heat injection, chemical injection, and gas injection. Heat injection enhances reservoir productivity by reducing crude oil's viscosity. Chemical injection, through the interaction of various chemicals with oil, decreases surface tension and capillary pressure within the reservoir. Gas injection is currently the most common EOR technique. The gas, usually carbon dioxide (CO2), displaces and mixes with oil to enhance productivity. CO2 injection is commonly referred to as CO2-EOR, and the term is often used interchangeably with EOR.