**3.2 Macroscopic sweep experiments**

Different flood experiments can be conducted to evaluate the foam performance in porous media. Glass bead packs or cores can be used to represent the porous media. Three distinctly different modes are used for foam injecting: (1) alternative injection of gas and liquid with foaming agents, (2) co-injection of the gas and the liquid phase at the same time, and (3) injection of pregenerated foam. Foam stability usually quantified by the oil recovery and MRF. The MRF can be calculated by comparing the pressure drop across the core during foam injection to the pressure drop after gas injection [15], as described in Eq. 2:

\*\*a-cesses me core aurring roam injection to me pressure as described in Eq. 2:

\*\*MRF =  $\frac{\mu\_{\rm f}}{\mu\_{\rm b}} = \frac{\left[\frac{\text{kA}\Delta p}{\text{QL}}\right]\_{\rm f}}{\left[\frac{\text{kA}\Delta p}{\text{QL}}\right]\_{\rm b}} = \frac{\Delta \text{p}\_{\rm f}}{\Delta \text{p}\_{\rm b}},\tag{2}$ 

**129**

**Figure 4.**

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

*B* = σ*gw*

between both surfaces of the lamellae (B is positive).

*Flowchart to predict foam stability from E, S, and B coefficients.*

The instability effect of crude oil on the CO2-foam system is another challenge for the use of this foam in EOR applications [26]. Crude oil composition, especially the presence of light components, decreases foam stability [27]. Foam stability decreases in contact with crude oil as a result of direct surface interactions between oil and foam. These interactions are governed by three main mechanisms: entry of oil droplets into the gas–liquid interface, spreading of oil on the gas–liquid interface, and formation of an unstable bridge across lamellae [27–30]. These three mechanisms can be quantified as a function of the interfacial tensions between oil, gas, and water by evaluating the entering coefficient (E), spreading coefficient (S), and bridging coefficient (B) [26]. E, S, and B can be calculated as follows (Eqs. 3–5):

*E* = σ*gw* + σ*ow* − σ*go* (3)

*S* = σ*gw* − σ*ow* − σ*go* (4)

where σ*gw*, σ*ow*, and σ*go* are the interfacal tensions between CO2 and water, oil and water, and oil and CO2, respectively. **Figure 4** presents a flowchart to predict the foam stability when in contact with oil, as indicated by the E, S, and B coefficients [27]. The oil droplets should be able to enter the gas-water interface to destabilize the foam. Once the entry condition is achieved (E is positive) and the oil droplets spread on the gas–liquid interface (S is positive), the gas/water interface will expand. As a result, the foam lamellae become thin and rupture, thus weakening the foam. If there is no spreading (S is negative) and the oil droplets form an emulsion at the gas/water interface, the foam film may rupture once oil droplets bridge

Ibrahim and Nasr-El-Din [23, 24] found that the AOS foam in contact with oil became unstable and decayed very fast, dissolving completely in 30 min compared

(5)

2 + σ*ow* 2 − σ*go* 2

**4. Deterioration effect of crude oil**

where Q is the total flow rate, k is the absolute core permeability, A is the cross-section area of the core, L is the core length, μ is the viscosity, ∆p is the pressure drop across the core, and the subscripts "f " and "b" represent the experiments with and without foam, respectively. The pressure buildup along the porous medium indicates foam-generation and gas mobility reductions [25]. A higher pressure drop signifies viscous foam and considerable resistance to gas mobility in porous media.

Dual coreflood experiments can be conducted to evaluate the divergent ability of foam systems within heterogeneous systems [23]. The foam is injected in two parallel cores with different permeabilities. The stable foam will be generated in the high-permeability formation and divert the flow toward the low-permeability core that improves the sweep efficiency and increases the oil recovery.

Macroscopic sweep experiments are usually combined with X-ray computed tomography (CT) measurements. CT scan analysis can be used to determine the porosity, oil distribution, and foam propagation inside the porous medium.

**Figure 3.** *Microscopic image of AOS foam in contact with a crude oil (5×) [23].*
