Emulsification and imbibition

Alcohol ethoxysulfates have been used in field tests as nitrogen (177) and carbon dioxide (178) foaming agents. Field use of alcohol ethoxysulfates is restricted to low temperature formations owing to its limited hydrolytic stabihty at low pH and elevated temperature (179). It has been reported that some foams can reduce residual oil saturation, not by oil displacement, but by emulsification and imbibition of the oil into the foam (180). 

Figure 1.5 shows an example of an aqueous foam with emulsified and imbibed crude oil droplets residing in its plateau borders. Using a simple model the degree of emulsification and imbibition has been found to correlate quite well with foam sensitivity to oil for a wide variety of foams, oils, and conditions [114]. A limitation of emulsification/imbibition models is that they will only be important for foam lamellae that are thick enough to accommodate realistic emulsion droplet sizes. Typical foam lamellae in porous media appear to have thicknesses on the order of tens of nm [70,71,114,328],... 

In the foregoing, all instances of foam lamella rupture (types B and C foams) appeared to result from the imbibition (after emulsification) of oil droplets into the foam lamellae. Together with oil spreading (e.g., Kuhlman s observations) and pseudoemulsion film thinning (e.g., Manlowe and Radke s observations), the emulsification and imbibition brings forward a third possible mechanism of foam sensitivity to oil, each of which has been observed in microvisual experiments. These will be described further. 

Wasan and co-workers (48, 50, 51) have found pseudoemulsion film stability to be influenced by micelle structuring effects, Marangoni surface effects, and the presence of oil droplets, (see the discussion in Chapter 2) They have also found that in some systems, the emulsification and imbibition of oil can actually stabilize foams. Manlowe and Radke s results (25) were interpreted in terms of pseudoemulsion film stabilities depending mainly on electrostatic and dispersion forces. Undoubtedly, interfadal viscosity could also be important. 

The physical basis for the lamella number is that a combination of capillary suction in the Plateau borders and the influence of mechanical shear cause the oil-phase distortion and pinch off into droplets. This result is in accord with the observations of emulsification and imbibition by Lobo et al. (50), French et al. (54), and Schramm and Novosad (37). The mechanical shear may come from several sources, including the flow... 

In these homogeneous cores, any small changes in sweep efficiency due to the differences in MRF among the foams should not have had a significant influence on oil recovery, surfactant solutions alone had no significant influence on oil recovery, so the additional oil recovered by the type B foam is interpreted as being caused by the transport of oil droplets by the emulsification and imbibition mechanism. As pointed out earlier in this chapter, quite a number of authors have speculated on some role for oil-in-water (O/W) emulsions produced by foams. Maini (39) and Farrell and Marsden (58) suggested that improved oil recoveries could be achieved by foam injection if some kind of emulsification—microdisplacement of oil by foam can occur such as that just described. The selection of foams for this purpose is described in a recent patent (59). 

Oil droplet size is thought to be quite important to the effectiveness of crude oils at destabilizing foams, with smaller droplets being the more effective [44,114,330,342]. A number of microvisual and core-flood studies suggest that the emulsification/ imbibition of oil into foam can be a very important factor [44,114,328,330,339,342]. 

The emulsification—imbibition of oil into foam, which the lamella number is intended to describe, has been noticed or predicted by a number of authors and was illustrated in Chapter 2. Lobo et al. (50) emphasized the importance of this phenomenon for foam stability. Rater-man (28) predicted that emulsification—imbibition would be important in constant quality preformed foam injection floods. In the core-flood studies of French et al. (54), they observed that the contacting of foam with crude oils produced emulsified droplets of oil within the foam lamellae. Schramm et al. (40) determined MRFs for a number of foams flowing in Berea sandstone cores containing residual light crude oil and found a strong correspondence with the micromodel results (Figure 10). This work was the first to show that foams that are quite stable to oil in the micromodel are also quite effective in core-floods and vice versa. 

A single model of foam-oil interaction cannot account for all situations. Certain foam—oil sensitivity models can be reconciled with both microvisual studies and core-flood foam effectiveness measurements, all for a wide variety of foams, oils, porous media, and other experimental conditions. However, exceptions are readily found. In an earlier section, the models of emulsification—imbibition, pseudoemulsion film thinning, entering, and spreading were introduced. Cases in favor of, and exceptions to, the applicability of each of these can be found in the literature. Although this situation prompts some inclination to search for additional mechanisms, the truth may be that all the models presented have some validity and that one or another valid mechanism is most significant in a given situation. 

Emulsification—imbibition greatly increases the oil—aqueous contact area in the foam and is possibly an important process when constant quality foam injection into waterflooded media at residual oil saturation is practiced, and when the amount of interfadal area constituted by pseudoemulsion films is a rate-determining factor for foam sensitivity. This emulsification—imbibition can act in favor of or against foam stability, depending upon whether the pseudoemulsion films, once formed, are quite stable or quite unstable. 

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