3. Fundamentals and mechanism of CO2 flooding

Improvement of oil recovery occurs through different techniques, one of which is CO2 flooding in low permeable and light-oil reservoirs [35, 36], as it can increase recovery factor from 10 to 20% [37]. Moreover, it reduces atmospheric gas emissions through CO2 storage [38]. Gas miscible flooding implies that the displacing gas is miscible with reservoir oil either at first contact or after multiple contacts, which in turn improve the volumetric sweeping and displacement efficiencies (Ev and Ed) respectively [39, 40]. A transition zone will develop between the reservoir oil and displacing gas, where the miscibility of the injected gas depend on reservoir pressure, temperature, and oil properties [41, 42]. CO2 miscible flooding comprises two mechanisms.

#### 3.1. Miscible flooding

decrease through its disposal in the petroleum reservoir [1, 4]. This chapter aims to provide basic

Owing to increased oil demand, improved oil recovery become a challenging task [3], since fossil fuels are the dominant source of the global energy supply [2] and represent about 85% of energy needs. Crude oil production occurs through three distinct phases [5–9]. The first stage is known as primary recovery, in which oil is recovered by natural reservoir energy including expansion of rock, fluid and dissolved gases, gravity drainage, and aquifer influx, or combination of these factors, which drive the hydrocarbon fluids from the reservoir to the wellbores. Primary oil recoveries range between 5 and 20% [10] of the original oil-in-place (OOIP). As reservoir pressure declines with the sustained production process, so the reservoir pressure must be built-up by injecting either water or natural gas, which drive reservoir fluid to wellbore [11]. This stage is known as secondary oil recovery, in which the recovered oil estimated to be in the range of 20– 40% of the OOIP [10]. At the end of secondary recovery, a significant amount of residual oil remains in the reservoir and becomes the target for additional recovery using tertiary recovery or enhanced oil recovery (EOR) methods. EOR refers to the displacement of the remaining oil in the reservoir through injection of materials not normally present in the reservoir [10, 12–17]. Gener-

Injection of steam has historically been the most widely applied EOR method. Heat from steam or hot water dramatically reduces heavy oils viscosity, thus improving its flow. The process involves cyclic steam injection ("huff and puff," where steam is first injected, followed by oil production from the same well); Continuous steam injection (where steam injected into wells drives oil to separate production wells); hot water injection, and steam assisted gravity drainage (SAGD) using horizontal wells. Another set of thermal methods include, in situ combus-

Miscible EOR employs supercritical CO2 to displace oil from a depleted oil reservoir. CO2 improve oil recovery by dissolving in, swelling, and reducing the viscosity of the oil. CO2 is a cheap injection source for increasing recovery factor by the rate of 1–2\$/Mscf [21]. Most CO2 flooding processes occur in United States [22]. Hydrocarbon gases (natural gas and flue gas) in addition to compressed nitrogen used for miscible oil displacement in high deep reservoirs. These displacements may simply amount to "pressure maintenance" in the reservoir [23–25].

Chemical flooding was, up to 2000s, a less common EOR method than thermal and gas flooding, but now, huge projects are retrieved. The chemical flooding processes involve the

technical information concerning enhanced oil recovery by CO2 flooding.

2. Enhanced oil recovery (EOR) processes

80 Carbon Capture, Utilization and Sequestration

ally, EOR processes comprise the following three categories:

tion or fire flooding are currently implemented [18–20].

2.1. Thermal EOR

2.2. Miscible EOR

2.3. Chemical EOR

Miscible flooding depends on mobilizing the oil light components, reduction of oil viscosity, the vaporization and swelling of the oil, and the lowering of interfacial tension [41]. The injected CO2 completely dissolve through crude oil at the minimum miscibility pressure (MMP) which determined experimentally through slim-tube tests or by mathematical correlations [3, 43, 44] and defined as, the pressure at which more than 80% of original oil-in-place (OOIP) is recovered at CO2 breakthrough [45]. However, on an industrial scale, an oil recovery of at least 90% at 1.2 pore volume of CO2 injected is used as a rule-of-thumb for estimating MMP [46, 47]. When the reservoir pressure is above the MMP, miscibility between CO2 and reservoir oil is achieved through multiple-contact or dynamic miscibility, where the intermediate and higher molecular weight hydrocarbons from the reservoir oil vaporize into the CO2 (vaporized gas-drive process) and part of the injected CO2 dissolves into the oil (condensed gas-drive process) [48]. This mass transfer between the oil and CO2 allows the two phases to become completely miscible without any interface and helps to develop a transition zone [49] that is miscible with oil and CO2. CO2 miscible flooding comprises; 1 first contact; vaporizing gas drive, and condensing gas drive [10].

A. First contact: in which miscible solvents mix with reservoir oil in all proportions and the mixture remains in one phase. Either through single or multiple contacts, and resulting in much improved oil recovery [50].

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

remains quite low (0.05–0.08 cp). This dense CO2phase can extract hydrocarbon components

CO2 Miscible Flooding for Enhanced Oil Recovery http://dx.doi.org/10.5772/intechopen.79082 83

Depending on the reservoir geology, fluid and rock properties, the CO2 flooding involves the

This process requires continuous CO2 injection with no other fluid. Sometimes a lighter gas,

This process is the same as the continuous CO2 injection process except for chase water that

In this process, a predetermined volume of CO2 is injected in cycles alternating with equal volumes of water. The water alternating with CO2 injection helps overcome the gas override and reduces the CO2 channeling consequently, improving overall CO2 sweep

This design is similar in concept to the conventional WAG but with a gradual reduction in the

This process is a conventional WAG process followed by a chase of less expensive gas (e.g., air

Several literatures stated about implementation of CO2 and carbonated water to improve oil recovery since 1951 [45, 54, 58, 59], owing to its availability in adequate amounts from both natural and industrial sources [60]. The first field-wide application occurred in 1972 at the SACROC (Scurry Area Canyon Reef Operators Committee) unit in the Permian Basin, where the CO2 was transported via a 200-mile-long pipeline from the Delaware-Val Verde Basin

such as nitrogen, follows CO2 injection to maximize gravity segregation.

5.3. Conventional water-alternating-gas (WAG) followed with water

from oil more easily than gaseous CO2 [49].

5.1. Continuous CO2 injection

following;

efficiency.

5.4. Tapered WAG

5.5. WAG followed with gas

5. CO2 flooding and injection designs

5.2. Continuous CO2 injection followed with water

injected CO2 volume relative to the water volume.

or nitrogen) after the full CO2 slug volume has been injected.

6. CO2-EOR flooding projects and case studies

follows the total injected CO2 slug volume.

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 reservoir oil through condensation process [52].

#### 3.2. Immiscible flooding

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 and decreases with temperature [55, 56].
