Foam sensitivity to oil

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],... 

Foams stabilized with proteins, such as egg white, can be quite sensitive to the presence of oil (fat) droplets (see also Section 5.6.7). Just as is the case with foam sensitivity to oil in other industries, the presence of even small amounts of oil (0.03 mass % in foods [814]) can destabilize a foam. Oils such as lipids are thought to interfere with foaming by displacing proteins from the air-aqueous interface. One approach to improving the foam stability involves combining acidic proteins, such as whey or serum albumin, with basic proteins [814]. 

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. 

The spreading and entering coefficients have correlated with foam sensitivity to oil in a number, but not in all, of the cases. As already noted, the degrees of foam sensitivity to oil (Figure 2) observed in microvisual experiments have been compared to the thermodynamic predictions based on S and E in Figure 8 (37, 40, 47). In these comparisons, the predictions were not always borne out Depending on the oil studied, on the order of one-half of the surfactant solutions predicted to produce foams of type C actually produced foams of type B. The predictions of which surfactants would produce foams of type A were much better for the heavier oils than for the lighter oils. Even quantitative measurements of lamella rupture frequency in the microvisual experiments showed that satisfactory correlations with 5 or were not obtained (37, 40, 47). 

The mechanisms for foam sensitivity to oils can also be compared to the results from core-flood experiments in which foams were made to flow through porous rock in the presence of residual oil. Holt and Kristiansen (26, 27, 56) studied foams flowing in cores under North Sea reservoir conditions and found that the presence of residual oil could reduce the effectiveness of flowing foams. They compared their results with the spreading and entering coefficients and found foam sensitivity to be correlated with the (oil) spreading coefficient. 

Most of the research on foam sensitivity to oils in porous media, whether in microvisual or core-flood tests, has been concerned with water-wetted pore and throat surfaces. Because petroleum reservoirs are frequently of intermediate, mixed, or oil wettability, it is of considerable interest to understand how rock wettability influences foam stability. 

These results have been expressed in terms of the influence of the wetting condition of the porous medium as foam is flowing through it. A complication is that the foam-forming surfactant may adsorb onto the solid surfaces and may alter the wettability. In the microvisual experiments of Schramm and Mannhardt (60), some of the foaming systems investigated appeared to change the wettability back to water-wet, in which case the foam sensitivities to oil reverted back to those appropriate to the... 

Schramm, L.L. Foam Sensitivity to Crude Oil in Foams, Fundamentals and Applications in the Petroleum Industry, Schramm, L.L. (Ed.), American Chemical Society Washington,... 

Increasing the aqueous phase salinity appeared to increase foam sensitivity to the presence of a hydrocarbon phase. This behavior may be due to increased surfactant partitioning into the oil phase. This can be quantified by determining the ratio of foam volume in the presence of decane to that in the absence of an added hydrocarbon (Table II. Figure 3). With few exceptions, this ratio decreased with increasing aqueous phase salinity. The values of this ratio for AEGS surfactants declined less with increasing aqueous phase salinity than for other surfactants. 

Schramm Foam Sensitivity to Crude Oil in Porous Media... 

Holt and Kristiansen (26, 27) obtained similar results for foams flowing in cores under North Sea reservoir conditions in that the presence of any of a number of residual oils (including a crude oil and a variety of pure hydrocarbon oils) reduced the effectiveness of flowing foams. Rater-man (28) measured the pressure drops obtainable for several foams flowing in sandstone cores under moderate pressure and in the presence of a residual pure alkane oil phase and found that the foams were destabilized by the oil. Schramm et al. (40) conducted foam-floods in sandstone cores and found a range of sensitivities to residual crude oil from oil-tolerant foams through to oil-sensitive foams. 

Foams flowing in porous media can be very sensitive to contact with any crude oil that may be encountered. Furthermore, the degree of sensitivity depends upon both the nature of the foam and the nature of the oil. Whereas many foams are quite sensitive to oil contact, it is also true that some foams are very resistant to oil contact. 

Some evidence supports the concept that even foams that are sensitive to oil contact in porous media can still be effective if the residual oil saturation is quite low, less than about 10%. The literature suggests that these same foams will be significantly destabilized at higher residual oil saturations, higher than about 20%. 

Among the many available defoamers, crude oil has been used to prevent the formation of foams, or destroy foams already generated, in a variety of industrial processes [43,46,327]. Crude oil can also destabilize foams applied in petroleum reservoirs, i.e., foams in porous media [3,306,328-331] (see Section 11.2.2). Although crude oils tend to act as defoamers, foams actually exhibit a wide range of sensitivities to the presence of oils, and some foams are very resistant to oil [3,332,333]. Many system variables influence the oil tolerance of a given foam and many attempts have been made to correlate foam-oil sensitivity with physical parameters [307,332-337]. These have met with mixed success [114,338],... 

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