Enhanced Oil Recovery (EOR):  The set of technologies that permits production of a greater share of the oil that remains after primary and secondary recovery.

IEA defined 5 classes of EOR

• Thermal EOR: steam is used to heat the oil in the ground, reducing its viscosity and making it easier to move. This is most often applied used in heavy oil reservoirs.
• CO2 EOR: CO2 is injected into the subsurface, and production enhanced either through a miscible or immiscible process. In a miscible process, the CO2 mixes with, or dissolves into, the oil, reducing its viscosity and increasing its susceptibility to being pushed by water. In an immiscible process, the gas does not dissolve into the oil but rather pushes the remaining oil; this is often combined with water injection.
• Other gas injection EOR: similar to CO2-EOR, but with other gases injected such as natural gas or nitrogen
• Chemical EOR: water soluble polymers or surfactants are added to the injected water. This polymer-loaded water has a high viscosity and can push more oil out of the pores in the subsurface.
• Other EOR: this class contains all other technologies such as microbial EOR, in which micro-organisms are injected in the reservoir, or combustion EOR, which involves in-situ burning of some of the oil to generate both heat and gases that help the rest of the oil move more easily.

Source : IEA 2018,