**1. Introduction**

Gas injection has been used widely for enhanced oil recovery (EOR) application. Carbon dioxide (CO2) gas injection was started for EOR applications in the 1950s to improve oil recovery and provide for carbon sequestration in underground formations [1, 2]. However, CO2 injection proved impractical at that time due to its low viscosity compared to formation fluids, leading to viscous fingering and early breakthrough [3]. Hence, the sweep efficiency and recovery factor were low [4]. Polymer flooding was introduced to reduce the mobility ratio of the displacing fluid to the displaced fluid [5]. The mobility ratio can be calculated using Eq. (1): *<sup>M</sup>* = \_ = \_

$$\mathbf{M} = \frac{\lambda\_{dișplacing}}{\lambda\_{dișplaccal}} = \frac{\left(k\_e/\mu\right)\_{dișplacing}}{\left(k\_e/\mu\right)\_{dișplaccal}},\tag{1}$$

where M is the mobility ratio of the displacing fluid to the displaced fluid, λ is the fluid mobility, *ke* is the effective fluid permeability, and *μ* is the fluid viscosity. Using polymer flooding increases the displacing fluid viscosity. As a result, the mobility ratio decreases, and the viscous fingering is reduced. Hence, sweep efficiency improved, as shown in **Figure 1**. However, polymer flooding is associated with formation damage due to physical adsorption of the high-molecular-weight

### **Figure 1.**

*Areal sweep efficiency, (a) low sweep efficiency and early breakthrough due to viscous fingering at unfavorable mobility ratio values (M < 1) and (b) high sweep efficiency at favorable mobility ratio (M > 1).*

polymer on the rock surface and mechanical trapping within the smaller-diameter pore throats [6, 7].

CO2 foam was introduced in the 1960s as a replacement for polymers to avoid formation damage [8]. Foam has low water content, which reduces formation damage in water-sensitive formations and allows fast cleanup [9]. Foam is a dispersion of a gas (nitrogen, carbon dioxide, or methane) as a non-wetting fluid in a continuous wetting phase. The wetting phase is water that contains surfactant at a particular concentration that is above the critical micelle concentration (CMC). The liquid film separates the gas phase from each other, the outer membranes of the gas bubbles, called *foam lamella*e. The first surfactant families selected for EOR method were petroleum and synthetic aromatic sulfonates [such as alpha-olefin sulfonate (AOS)] because of their availability, lower adsorption on porous rocks, high compatibility with hard water, and good wetting and foaming properties [10, 11].

Bulk foam can be characterized by several properties such as quality, texture, stability, and foam density [12]. Foam quality is the volume percent of gas within foam at a specified pressure and temperature [13]. Foam quality for EOR applications is typically 75–90%. Foam texture is a measure of the average gas bubble size. Foam stability depends on the chemical and physical properties of the surfactant-stabilized water film separating the gas bubbles (lamellae). Foams are metastable systems; accordingly, all foams will eventually break down. Foam stability is measured by the half-life time, which is the time required to lose 50% of the foam volume [14]. In general, as a foam texture becomes finer, the foam will be more stable and will have greater resistance to flow in matrix rock. Foam flow resistance in porous media is measured by the mobility reduction factor (MRF). MRF is defined as the ratio of total mobility of CO2/brine to foam mobility. When foams become more stable, more resistance to flow is expected and leads to a higher mobility reduction [15].
