Enhanced oil recovery (EOR) is used to recover stranded oil that remains in reservoirs after primary production.

Oil is first produced due to the high pressures of buried reservoirs pushing the hydrocarbons to the surface. This pressure difference can be aided by pumps. Secondary recovery generally occurs by a waterflood, in which 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 the viscosity of crude oil. Chemical injection decreases surface tension and capillary pressure within the reservoir through the interaction of various chemicals with oil. Gas injection is currently the most common EOR technique. The gas—usually carbon dioxide (CO₂)—displaces and mixes with oil to enhance production. CO₂ injection is commonly referred to as CO₂-EOR, and the term is often used interchangeably with EOR.