**2. Hydrocarbon miscibility**

With decline in overall production levels from mature oil fields, oil companies have turned to enhanced oil recovery (EOR) techniques as a way of maximizing output. The more commonly applied technique is gas injection or miscible flooding. Miscible flooding is a commonly used term used to describe gas injection processes. This involves the displacement of oil that aids in maintaining original reservoir pressures by reducing the interfacial tension that exists between the oil and gas phases. This acts by removing the interphase between the two fluid phases, and commonly used gases include CO2, natural gas, and nitrogen, with CO2 being the most prominently used gas. Research on the use of CO2 has been ongoing since the 1950s, continuing into the 1960s [8–10]. And the advantage that CO2 injection brought about was noticed as the increase in reservoir pressure that resulted in higher oil production due to the driving force provided by it. In its infancy, research on CO2 showed it to be immiscible with oil at reservoir pressures, but it was later discovered that under certain conditions of temperature, pressure, and oil composition, the carbon dioxide becomes enriched and becomes miscible with oil [11]. The pressure required for CO2 gas to attain miscibility in oil is also much lower than methane gas. The term "miscible flooding" has been adopted as the conventional phrase used to describe the process of gas injection.

The main advantages of using CO2 in this process include the following:


From review of various literatures, an indication of the suitability of CO2 as an excellent solvent for EOR in onshore fields of Canada and the USA can be deduced. And from experience garnered worldwide by operators, CO2 flooding increases oil extraction by between 7 and 15% of oil initially in place. And also, it reduces the amount of CO2 in the atmosphere and greenhouse gases in general [12].

*Enhanced Oil Recovery Processes - New Technologies*

of oil, and eventually reduction of the viscosity of oil.

phrase used to describe the process of gas injection.

• The miscibility of CO2 with oil as highlighted earlier.

• It is a cheaper source of gas than other alternatives.

• By injecting it back, CO2 capture is also achieved.

**2. Hydrocarbon miscibility**

maintain the properties of a gas while having the density of a liquid. In this state, oil could be more efficiently mobilized from the depleted reservoir due to the improved volumetric efficiency. Conventionally, CO2 injection method is usually applied to the reservoirs with oil gravity less than 25 [5]. As CO2 is injected into the reservoir, the miscible CO2 will blend thoroughly with the oil in a manner that the interfacial tension between these two fluids becomes zero. The other mechanisms of CO2 by which the oil recovery is improved are the dissolution of CO2 in oil, swelling

Since 2002, as a consequence of Kyoto protocol and imposing of the carbon tax,

With decline in overall production levels from mature oil fields, oil companies have turned to enhanced oil recovery (EOR) techniques as a way of maximizing output. The more commonly applied technique is gas injection or miscible flooding. Miscible flooding is a commonly used term used to describe gas injection processes. This involves the displacement of oil that aids in maintaining original reservoir pressures by reducing the interfacial tension that exists between the oil and gas phases. This acts by removing the interphase between the two fluid phases, and commonly used gases include CO2, natural gas, and nitrogen, with CO2 being the most prominently used gas. Research on the use of CO2 has been ongoing since the 1950s, continuing into the 1960s [8–10]. And the advantage that CO2 injection brought about was noticed as the increase in reservoir pressure that resulted in higher oil production due to the driving force provided by it. In its infancy, research on CO2 showed it to be immiscible with oil at reservoir pressures, but it was later discovered that under certain conditions of temperature, pressure, and oil composition, the carbon dioxide becomes enriched and becomes miscible with oil [11]. The pressure required for CO2 gas to attain miscibility in oil is also much lower than methane gas. The term "miscible flooding" has been adopted as the conventional

The main advantages of using CO2 in this process include the following:

From review of various literatures, an indication of the suitability of CO2 as an excellent solvent for EOR in onshore fields of Canada and the USA can be deduced. And from experience garnered worldwide by operators, CO2 flooding increases oil extraction by between 7 and 15% of oil initially in place. And also, it reduces the

amount of CO2 in the atmosphere and greenhouse gases in general [12].

CO2 sequestration as a method to mitigate the high concentration of CO2 in the atmosphere has received a lot of attention [6, 7]. However, the lack of economic incentives has been the biggest hindrance to industrial field-scale application of CO2 sequestration. Emerging in the last decade, CO2-EOR was proposed as a method to add economic benefits of CO2 injection to mature oil fields to the environmental merits of CO2 sequestration [6]. Therefore, considering the large amounts of research dedicated to CO2-EOR, it is expected that in the near future, more field applications of this technology will be implemented globally. In this chapter, the

phase behavior and hydrocarbon miscibility of CO2 is discussed in detail.

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**Figure 1.** *Pressure composition diagram—gas 1 system for Rangely oil: 95% CO2 and 5% CH4 gas at 160°F [13].*

Challenges associated with CO2-driven EOR include technological, economical, and supply. For example, a long pipe network is usually required to transmit CO2 from source to the field. High-pressure compressors are also another essential requirement in the injection process. Therefore, all these factors have to be assessed and weighed in relation to the extra oil recovered to determine if it is profitable.

In recent times, newer techniques such as water alternating CO2 injection and simultaneous water and CO2 injection have been developed, and they are determined to increase efficiency of oil recovery at lower costs.

A recent study commissioned by the Congressional Research Service shows that, theoretically, carbon-capture technology could remove as much as 80–90% of CO2 from emissions.

The minimum miscibility pressure (MMP) of CO2 and NGL for conventional reservoirs are few hundreds psia higher compared to unconventional reservoir due to difference in pore size according to a study by Teklu et al. [13].

### **2.1 MMP in nanopores, fluid properties, and phase behavior**

The deviation between the nanopore phase behavior from bulk (PVT cell) properties was studied [14–16]. The bubble point and dew point pressure, interfacial tension (IFT), and minimum miscibility pressure (MMP) between injection and reservoir fluid change in nanopores due to small pore confinement effect [13]. MMP was calculated by including capillary pressure and critical property shifts in confined pores using multiple mixing cell (MMC) algorithm of Ahmadi and Johns [17].

Phase behavior is important in the design of a variety of EOR processes, for example, surfactant/polymer processes and gas injection processes. The process of reducing interfaces between oil and displacing phase and hence removing effect of capillary forces between injected fluid and the oil is called miscible displacement. During the gas injection process, the required miscible-displacing fluid is generated by mixing the injected fluid with oil in the reservoir. Phase behavior of gas/oil systems is summarized in the pressure-composition (p-x) diagram. A work by Graue and Zana [18] summarizes the result for CO2 injection in the Rangely field, Colorado. The physical property date was obtained from constant composition expansion (CCE) to determine the phase envelope (bubble point and dew point envelope) and vapor/liquid equilibrium experiment (VLE) to yield vapor/liquid equilibrium constant (K-values). The phase behavior of Rangely reservoir oil with different gases' composition at reservoir temperature of 160°F showed that critical and saturation pressures of the injected gas/reservoir oil system were increased substantially by 10 mol% N2 in the injected gas. The phase behavior data showed solid phase precipitation that amount for 2–5% of the reservoir oil [18]. **Figure 1** illustrates the pressure composition diagram of Rangely oil containing considerable amounts of CO2.
