*CO2 Foam for Enhanced Oil Recovery Applications DOI: http://dx.doi.org/10.5772/intechopen.89301*

*Foams - Emerging Technologies*

pore throats [6, 7].

**Figure 1.**

wetting and foaming properties [10, 11].

mobility reduction [15].

**2. Foam generation**

polymer on the rock surface and mechanical trapping within the smaller-diameter

*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).*

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

Foam generates in porous media through three different mechanisms: (a) snap-

In the snap-off mechanism, lamellae are created in gas-filled pore throats as a result of the capillary pressure difference between the pore body and the pore neck [16]. **Figure 2a** shows the foam-generation process by the snap-off mechanism. As

off, (b) lamella division, and (c) leave-behind [16, 17].

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

**126**

the gas phase displaces the liquid zone, the difference in capillary pressure between the pore body and the pore throat forces the wetting phase (water) to flow back and then snap-off the gas phase.

Lamella division generally occurs when a foam lamella that is larger than that of the pore body approaches a "branching point" and divides into two or more bubbles (**Figure 2b)**. If the lamella is at a branching point with more than one path that requires the same pressure for the lamella to flow, the lamella divides into two bubbles or lamellae [18].

Leave-behind occurs when the gas enters a porous medium that is initially saturated with a liquid or when two gas fronts approach a pore space that is filled with liquid; these processes squeeze the liquid into a lamella (**Figure 2c**). The leavebehind mechanism typically forms a weak foam because the generated lamella is parallel to the flow direction [17, 18].
