**4. Deterioration effect of crude oil**

*Foams - Emerging Technologies*

toring the foam height over time.

**3.2 Macroscopic sweep experiments**

mobility in porous media.

drop after gas injection [15], as described in Eq. 2:

visual graduated cell [22, 23]. The foam half-life time can be measured by moni-

Additionally, bubble-scale experiments can be conducted under an optical microscope to investigate the foam stability [24]. The foam bubbles are allowed to stabilize and are then placed on a microscope slide. The foam texture and the thin liquid films (lamellae) are then monitored with time to investigate the foam decay rate [23]. **Figure 3** shows a microscopic image for an AOS foam system in contact with a crude oil.

Different flood experiments can be conducted to evaluate the foam performance

\_ kA∆p QL ] \_f

= \_ ∆pf ∆pb

, (2)

[ \_ kA∆p QL ]<sup>b</sup>

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

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

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.

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

> MRF = \_ μf μb = [

that improves the sweep efficiency and increases the oil recovery.

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

**128**

**Figure 3.**

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):

$$E = \sigma\_{\mathcal{S}^{w\flat}} + \sigma\_{aw} - \sigma\_{\mathcal{S}^{p\flat}} \tag{3}$$

$$\mathbf{S} = \sigma\_{\mathcal{S}^{w}} - \sigma\_{ow} - \sigma\_{\mathcal{S}^{p}} \tag{4}$$

$$B = \sigma\_{\mathcal{g}w}^2 + \sigma\_{aw}^2 - \sigma\_{\mathcal{g}v}^2 \tag{5}$$

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

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

**Figure 4.** *Flowchart to predict foam stability from E, S, and B coefficients.*

to more than 5 h in the absence of oil [24]. Two reasons account for the adverse effect of crude oil on foam decay. First, oil droplets tend to spread along with the gas/liquid interface. As a result, the stable gas/liquid interface becomes unstable, which accelerates the rupture of the foam lamellae. Second, oil forms an emulsion in the foam lamellae. The oil droplets in the unstable emulsion agglomerate and accelerate the drainage of the foam lamellae; hence, the foam decays faster [28].

In **Figure 3**, the crude oil forms layers in the interface between the gas and liquid phases, and an emulsion forms inside the AOS foam lamellas. **Figure 5** shows microscope images for AOS foam lamellae in the presence of oil. The oil droplets in the unstable emulsion agglomerate and accelerate the drainage, where the lamella thickness decreases over time. Hence, the foam becomes unstable and decay faster.

**Figure 6** plots the foam height over time that gives indication for the decay rate from a visual cell experiment in which the foam lamellae were in contact with crude oil. At room temperature, the initial AOS foam height was 20 cm; then the foam decayed over time to reach 10 cm after 15 min (foam half-life = 15 min). In contact with oil, the foam decayed faster, and the half-life time decreased to 3 min.

**Figure 7a** shows the change of the normalized foam half-life as a function of temperature. The normalized half-life time is the half-life time for the foam system divided by the half-life time of AOS system at room temperature without crude oil. With increasing temperature, the foam became unstable and decayed more quickly than at room temperature. The half-life for the AOS system in the absence of crude oil at 150°*F* also decreased to 0.13 of its value at 77°*F*. As the temperature increases,

### **Figure 5.**

*Microscopic images for AOS foam to track the unstable emulsion and draining of the foam lamellae over time when in contact with the crude oil (20×) [23].*

**131**

**Figure 7.**

**Figure 6.**

*77°F and 800 psi.*

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

*Normalized foam height for AOS foam system in the absence and presence of crude oil as a function of time at* 

*Effect of 1 vol% crude oil in foam stability, (a) foam stability at different temperatures at 800 psi for 0.5 wt%* 

*AOS foam system and (b) foam half-life time reduction percent for different surfactant systems.*

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

### **Figure 6.**

*Foams - Emerging Technologies*

to more than 5 h in the absence of oil [24]. Two reasons account for the adverse effect of crude oil on foam decay. First, oil droplets tend to spread along with the gas/liquid interface. As a result, the stable gas/liquid interface becomes unstable, which accelerates the rupture of the foam lamellae. Second, oil forms an emulsion in the foam lamellae. The oil droplets in the unstable emulsion agglomerate and accelerate the drainage of the foam lamellae; hence, the foam decays faster [28]. In **Figure 3**, the crude oil forms layers in the interface between the gas and liquid phases, and an emulsion forms inside the AOS foam lamellas. **Figure 5** shows microscope images for AOS foam lamellae in the presence of oil. The oil droplets in the unstable emulsion agglomerate and accelerate the drainage, where the lamella thickness decreases over time. Hence, the foam becomes unstable and decay faster. **Figure 6** plots the foam height over time that gives indication for the decay rate from a visual cell experiment in which the foam lamellae were in contact with crude oil. At room temperature, the initial AOS foam height was 20 cm; then the foam decayed over time to reach 10 cm after 15 min (foam half-life = 15 min). In contact

with oil, the foam decayed faster, and the half-life time decreased to 3 min.

**Figure 7a** shows the change of the normalized foam half-life as a function of temperature. The normalized half-life time is the half-life time for the foam system divided by the half-life time of AOS system at room temperature without crude oil. With increasing temperature, the foam became unstable and decayed more quickly than at room temperature. The half-life for the AOS system in the absence of crude oil at 150°*F* also decreased to 0.13 of its value at 77°*F*. As the temperature increases,

*Microscopic images for AOS foam to track the unstable emulsion and draining of the foam lamellae over time* 

**130**

**Figure 5.**

*when in contact with the crude oil (20×) [23].*

*Normalized foam height for AOS foam system in the absence and presence of crude oil as a function of time at 77°F and 800 psi.*

**Figure 7.**

*Effect of 1 vol% crude oil in foam stability, (a) foam stability at different temperatures at 800 psi for 0.5 wt% AOS foam system and (b) foam half-life time reduction percent for different surfactant systems.*

### **Figure 8.**

*Effect of oil density and viscosity on foam stability [29].*

the liquid viscosity decreases, which leads to faster drainage. As a result, gas bubbles coalesce faster [31]. In contact with crude oil, the AOS was not able to generate stable foam, where the half-life was 3.9 and 0.45 min at 77 and 150°*F*, respectively.

The E, S, and B coefficients for the AOS system at 150°*F* and 800 psi were found to be 7.96, 6.68 mN/m, and 231.74 (mN/m)2 . Based on the flowchart in **Figure 4**, the generated foam is not stable. Increasing the pressure to 1200 psi, the values for the three coefficients slightly decreased to 6.67, 5.38 mN/m, and 141.29 (mN/m)2 , still greater than zero. A positive E value indicates that the oil-entering condition was initially favorable. However, once the oil entered the foam system as an emulsion on the AOS and AOS/SiO2 foam lamellae, it started to spread on the surfaces of the foam bubbles (S > 0) and generated unstable bridges (B > 0) that quickly broke the lamellae and destabilized the foam. The microscopic images of the AOS foam system in contact with crude oil in **Figures 3** and **5** confirm this behavior.

Similar results were observed by Simjoo et al. [27]. Adding 1 vol% crude oil to different surfactant systems decreases the foam stability and the half-life time. **Figure 7b** shows the change in the foam half-life time due to the presence of crude oil. Regardless of the surfactant type, adding crude oil to the foam increases the lamella drainage and decreases the foam stability.

The previous experimental results demonstrate the detrimental effect of crude oil on foam stability [29]. However, regardless of the surfactant used, crude oil with higher hydrocarbon chain lengths has a lower effect on foam stability. **Figure 8** shows the half-life of Coco/SDS (a foam system with a 1:1 mixture of cocobetaine and sodium dodecyl sulfate surfactants) in contact with different crude oils with different densities and viscosities at room temperature. These data show that the higher the density and viscosity, the lower the effect of crude oil on foam stability.
