Bulk foam

Foam generated in porous media consists of a gas (or a liquid) dispersed in a second interconnected wetting liquid phase, usually an aqueous surfactant solution (1). Figure 1 shows a micrograph of foam flowing in a two-dimensional etched-glass porous medium micromodel (replicated from a Kuparuk sandstone, Prudhoe Bay, Alaska (2)). Observe that the dispersion microstructure is not that of bulk foam. Rather discontinuous... 

T.W. Patzek Self-Similar Collapse of Stationary Bulk Foams. AIChE J. 39, 1697 (1993). 

Foam Stability Measurements. The average lifetime (LF) is a property of bulk foams that is useful in ranking foams in order of stability. LF is defined as the area under the drainage profile divided by the initial foam height The apparatus used to determine these drainage profiles is shown in Fig. 9. 

Major changes in all three measured parameters in Figure 15 occur at R = 0.9 - 1.0. It is significant that this is also the point where instability is first observed in the bulk foam [13,22]. Thus, there is strong evidence that suggests a link between changes in the adsorbed... 

Figure 17. A summary of the bulk foam stability ( ), equilibrium thin film thickness (o), and FITC-a-la surface diffusion (A) as a function of molar ratio of Tween 20 to protein (R). The concentration of a-la was 0.5 mg/ml (35.4 piM). Reproduced from reference [41] with the permission of VCH Verlagsgesellschaft.

Bikerman [46] describes studies of the specific conductivity of individual foam lamellae. For bulk foams, the specific conductivity of the foam, kf, is proportional to the volume fraction of liquid in the foam and its conductivity, kl ... 

For bulk foams, some researchers have used a variation of the turbidity Eq. (2.9) by fitting the intensity of light transmitted through a sample of foam (It) to an equation of the form ... 

The stability of foams in constraining media, such as porous media, is much more complicated. Some combination of surface elasticity, surface viscosity and disjoining pressure is still needed, but the specific requirements for an effective foam in porous media remain elusive, partly because little relevant information is available and partly because what information there is appears to be somewhat conflicting. For example, both direct [304] and inverse [305] correlations have been found between surface elasticity and foam stability and performance in porous media. Overall, it is generally found that the effectiveness of foams in porous media is not reliably predicted based on bulk physical properties or on bulk foam measurements. Instead, it tends to be more useful to study the foaming properties in porous media at various laboratory scales micro-, meso-, and macro-scale. 

There are several mechanisms by which bulk foams can be destabilized by oils, and more than one mechanism may be involved in any given situation. 

The thin liquid films bounded by gas on one side and by oil on the other, denoted air/water/oil are referred to as pseudoemulsion films [301], They are important because the pseudoemulsion film can be metastable in a dynamic system even when the thermodynamic entering coefficient is greater than zero. Several groups [301,331,342] have interpreted foam destabilization by oils in terms of pseudoemulsion film stabilities [114]. This is done based on disjoining pressures in the films, which may be measured experimentally [330] or calculated from electrostatic and dispersion forces [331], The pseudoemulsion model has been applied to both bulk foams and to foams flowing in porous media. 

Figure 6.23 Shearing stress applied to a bulk foam causing distortion (frames 2-4) and eventually bulk flow once the yield stress is exceeded (frames 5 and 6). Dots in a constellation of cells show how the relative positions of those cells have changed. From Aubert et al. [403]. Scientific American Inc.

The bubble size distribution in a foam. For foams in porous media, it can be expressed in terms of the length scale of foam bubbles as compared with that for the spaces confining the foam. When the length scale of the confining space is comparable, to or less than, the length scale of the foam bubbles, the foam is sometimes termed lamellar foam , to distinguish it from the opposite case, termed bulk foam . 

In contrast to bulk foam, in a porous medium the majority of films move separately so their rupture under certain critical pressure does not occur simultaneously, i.e. the films are not affected by the rupture of other films (there is no collective effect). On the other hand, moving films are subjected to the oscillation of thickness as well as to other mechanical effects. Hence, their critical pressure should be lower than that of the static foam and should depend also on the rate of movement due to the dynamic effects. Analogous reduction in the critical pressure is observed when a bulk foam advances [47] and when a foam is placed in a centrifugal field [183]. The influence of the dynamic effects on the critical pressure has been explained by the model of Jimenez and Radke [175]. 

A third, related limit on the capillary pressure is created by the existence of an upper critical capillary pressure above which the life times of thin films become exceedingly short. Values of this critical capillary number were measured by Khistov and co-workers for single films and bulk foams (72). The importance of this phenomenon for dispersions in porous media was confirmed by Khatib and colleagues (41). Figure 5 shows the latter authors plot of the capillary pressures required for capillary entry by the nonwetting fluid and for lamella stability versus permeability of the porous medium. 

Measurement of the viscoelastic properties of foams and dense emulsions is also complicated by slip at the rheometer surfaces (Yoshimura and Prud homme 1988). The liquid in an aqueous foam lubricates flat rheometer fixtures, reducing the strain imposed on the bulk foam. This lubrication is a desirable feature of some foam products such as shaving cream, but it complicates rheological studies. The use of roughened surfaces, such as sandpaper bonded to the rheometer fixtures, seems to be an effective countermeasure (Khan et al. 1988). ... 

Bulk foams are characterised by global parameters which are often measured by... 

Foam densities typically vary from about 0.02 to about 0.5 g/mL (8). However, it should be borne in mind that bulk foams are not necessarily homogeneous and usually exhibit a distribution of densities throughout the vertical direction due to gravity-induced drainage. 

Sufffidge et al. (103) found that the effect of crude oil on bulk foams can vary from relatively mild (in a fluorinated surfactant) to essentially catastrophic (in C10-a-olefin sulfonate). 

Correlation between Pseudoemulsion Film Stability and Improved Oil Recovery. As the pseudoemulsion film controls the stability of bulk foams, it plays an important role also in the stability of foams in porous media containing crude oil. 

Marsden et al. (19) were apparently the first to determine the relevant size scale characteristic of foam in porous media. They measured mean bubble sizes for foam exiting sand packs, and concluded that foam bubbles were roughly the same size as pore bodies. Despite the equivalence of bubble and pore sizes, they treated foam as a continuous, non-Newtonian fluid (i.e., a bulk foam). Holm (15), at roughly the same time,... 

Initially several researchers measured the effective viscosity of bulk foam using rotational or capillary viscometers (1, 49) with the hope of applying their results to porous media. On the basis of the earlier discussion of foam morphology in porous media, such data are inappropriate (50). Interaction of elongated bubbles and pore-spanning lamellae with pore walls determines the effective viscosity of the flowing portion of foam. Such interactions are simply not mirrored in bulk foam viscometry. 

This chapter will focus on the stability of foams flowing in porous media when in the presence of crude oil. Many laboratory investigations of foam-flooding have been carried out in the absence of oil, but comparatively few have been carried out in the presence of oil. For a field application, where the residual oil saturation may vary from as low as 0 to as high as 40% depending on the recovery method applied, any effect of the oil on foam stability becomes a crucial matter. The discussion in Chapter 2 showed how important the volume fraction of oil present can be to bulk foam stability. A recent field-scale simulation study of the effect of oil sensitivity on steam-foam flood performance concluded that the magnitude of the residual oil saturation was a very significant factor for the success of a full-scale steam-foam process (14). 

Ample evidence suggests that crude oil can have an effect on foams applied to enhanced oil recovery. Rendall et al. (21) investigated the behavior of several commercial surfactant-stabilized foams in the presence of crude oils. On the basis of dynamic bulk foaming tests, gas mobility reduction factors measured in reservoir cores, and observations in a micro-... 

---