5.1. Continuous CO2 injection

B. The vaporizing gas-drive process (high-pressure gas drive): achieves dynamic miscibility by in situ vaporization of the intermediate-molecular-weight hydrocarbons from the

C. The condensing gas-drive process (enriched gas drive): achieves dynamic miscibility by in situ transfer of intermediate molecular weight hydrocarbons from rich solvent to lean

Immiscible flooding depends on oil viscosity reduction, oil phase swelling, the extraction of lighter components, and the fluid drive [53]. When the reservoir pressure is below the MMP or the reservoir oil composition is not favorable, the CO2 and oil will not form a single phase (i.e., immiscible). However, CO2 will dissolve in the oil causing oil swelling, viscosity reduction and solution gas derive which in turn improve sweeping efficiency and facilitate further oil recovery [54]. Like hydrocarbon gases, CO2 miscibility through crude oil increases with pressure

In order to fully understand CO2-EOR flooding, it is important to look at the properties of CO2 and the fundamentals of the CO2-EOR process. Under ambient conditions, CO2 is a colorless, odorless gas and about 1.5 times heavier than air. CO2 is (2–10 times) more soluble in oil than in the water. CO2 increases the water viscosity and forms carbonate acid, which has a beneficial effect on shale and carbonate rocks [57]. Its properties under standard and critical conditions are summarized in Table 1. Above critical pressures and temperatures, CO2 is in the supercritical state and forms a phase whose density is close to that of a liquid, even though its viscosity

Molecular weight 44.010 g/mol Specific gravity 1.529 Density 1.95 kg/m<sup>3</sup> Viscosity 0.0137 mPa/s

Critical pressure (Pc) 1070.6 psia Critical temperature (Tc) 87.98F Critical volume (Vc) 94 cm3

Critical viscosity (μc) 0.0335 cp

/mol

reservoir oil through injection of lean gases or CO2 [51].

reservoir oil through condensation process [52].

3.2. Immiscible flooding

82 Carbon Capture, Utilization and Sequestration

4. Properties of CO2

and decreases with temperature [55, 56].

Properties at standard conditions (14.7 psia, 0C)

Table 1. Properties of CO2 under standard and critical conditions.

Critical properties

This process requires continuous CO2 injection with no other fluid. Sometimes a lighter gas, such as nitrogen, follows CO2 injection to maximize gravity segregation.
