**3.2 CO2 enhanced oil recovery**

CO2-EOR is a tertiary oil recovery technique where CO2 and usually other fluids such as water or brine is injected into the reservoir to achieve miscibility with the oil and recover residual oil. In addition to extraction of residual oil, the injected gas provides pressure support and could remain stored permanently after the recovery process. Under subsurface conditions, CO2 mixes with oil above a certain minimum miscibility pressure (MMP), reducing the capillary effect that retain the oil in place [26–28]. There are four main underlying mechanisms of CO2-EOR as outlined by Rojas and Ali [29] and Tunio et al. [30] which include (1) oil swelling; (2) reduction of oil viscosity; (3) reduction of oil and water density; and (4) extraction of oil components.

Alternative forms of CO2-EOR have been developed over the past years, including continuous CO2 injection, continuous CO2 injection followed by water, water-alternating gas (WAG) and WAG followed by gas or water [31–33]. To improve sweep efficiency, carbonated water injection has also been used as a viable alternative [34, 35]. Other emerging injection schemes include CO2 low salinity water alternating gas (CO2-LSWAG) injection under miscible CO2 displacement conditions [36–38]. Depleted oil and gas reservoirs are also attractive candidates for CO2 storage due to the potential to reuse some of the production equipment and geological data collected over the producing life of the reservoirs to lower exploration cost and reduce the risk associated with CO2 storage. It has been reported that depleted oil and gas reservoirs could hold about 45% of the projected CO2 emissions by 2050 [9].
