Foam Sensitivity to Crude Oil in Porous Media - Advances in

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Foam Sensitivity to Crude Oil in Porous Media Laurier L. Schramm Petroleum Recovery Institute 100, 3512 33rd Street N.W., Calgary, Alberta T2L 2A6, Canada This chapter addresses the sensitivity of foams flowing in porous media to interaction with crude oil. The interaction effects can range from imperceptible to strong and have a corresponding influence on foam stabilities. An outline of predictive models, microvisualization observations, and core-flood measurements is given and used to identify a framework for physical modeling of foam—oil interactions in porous media. A number of comparisons of model predictions with actual core-flood experiments that were conducted under a variety of conditions are presented. In this context, the influence of oil gravity and rock wettability on foam stability is reviewed. Although some gaps in fundamental understanding exist, it is possible to make a number of useful generalizations about the sensitivity of foamsflowingin porous media to crude oil.

Applications of Foam in Enhanced Oil Recovery The production of oil from a petroleum reservoir involves first the primary and secondary production modes, which may recover less than half of the oil originally in place. To recover additional oil, it is necessary to apply enhanced oil recovery (EOR) techniques such as miscible or immiscible gas displacement (C0 , hydrocarbon gases, etc.). However, major problems occur in these EOR methods because the displacing agent has high mobility and low density compared with that of reservoir fluids. Fingering (channeling) and gravity override reduce the sweep efficiency, 2

0065-2393/94/0242-0165$10.34/0 © 1994 American Chemical Society

In Foams: Fundamentals and Applications in the Petroleum Industry; Schramm, L.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.


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contribute to early breakthrough of injected fluid, and thus reduce the amount of oil recovered. The use of surfactant-stabilized foams to counteract these kinds of problems was suggested several decades ago (i, 2) and has recently become actively pursued in laboratory and field tests (3—8). The use of foam is advantageous compared with the use of a simple fluid of the same nominal mobility because the foam, which has an apparent viscosity greater than the displacing medium, lowers the gas mobility in the swept or higher permeability parts of the formation. This lowered gas mobility diverts at least some of the displacing medium into other parts of the formation that were previously unswept or underswept. From these underswept areas, the additional oil is recovered. Because foam mobility is reduced disproportionately more in higher permeability zones, improvement in both vertical and horizontal sweep efficiency can be achieved. There is an abundant literature on the subject of bulk foams and thin films much of which is reviewed in Chapters 1 and 2 of this book. The extension to foams in porous media is discussed in Chapter 3. In the development of suitable foam-forming surfactant solutions for EOR processes, researchers have focused on finding ways to produce foams that have the necessary stability and viscosity to function as diverting and mobility-reducing agents in the reservoir (9, 10). As the increasing number of publications in this area shows, it is crucial to know how to select surfactants or surfactant mixtures to provide effective foam-forming solutions. This selection involves assessing not only foam stability but also such properties as chemical stability, solubility, and adsorption by the rock (11-13). These properties are of particular concern where reservoirs considered for foam injection consist of different rock types and can have very high salinity (e.g., 300 g/L), hardness (e.g., 25 g/L), and temperature. These and other specific considerations that are related to designing foam injection processes that use C 0 , steam, and hydrocarbon gases are discussed in Chapters 5 through 8. 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 afieldapplication, 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 recentfield-scalesimulation 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). 2


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Foam Stability in the Presence of Oil Oils, such as castor oil, are among the earliest chemicals to be used for foam inhibition and foam breaking. Accordingly, considerable literature exists on the subject (15-20). Some theories for the mechanisms of antifoaming action, together with an abundance of qualitative observations, have emerged from this work. In general, matters are not simple. Apparently, foams can be destabilized by oils by several mechanisms, and more than one mechanism may be operative in a given case. Some possibilities for the mechanisms of foam destabilization by a given oil phase include • Foam-forming surfactant may be absorbed or adsorbed by the oil, especially if there is emulsification, causing depletion in the aqueous phase and hence from the gas-liquid interface. • Surfactants from the oil may be adsorbed by the foam lamellae, form either a mixed or replaced adsorption layer, and produce a less favorable state for foaming. • Components from the oil may be adsorbed by the porous medium altering the wettability of the solid phase, and this alteration makes it more difficult for foam to be generated and regenerated. • The oil may spread spontaneously on foam lamellae and displace the foam stabilizing interface. • The oil may emulsify spontaneously and allow drops to breach and rupture the stabilizing interface. Although the physical properties of foams and foam-forming solutions for enhanced oil recovery have been studied intensively, only relatively recently has attention been paid to the effect of the oil on the foams. In the present work, we will not focus attention on the first two mechanisms listed, which concern phase-behavior. The surfactant absorption (partitioning) may be important in some cases but has been shown to be negligible for a number of commercial foam-forming surfactant—crude oil combinations in which quite hydrophilic surfactants were involved (21). Generally, if foam-forming surfactant adsorptions (on the oil and mineral surfaces) have been satisfied, then physical changes such as spreading are responsible for foam destabilization. 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-



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visual apparatus, it was found that all but one of the foams studied were destroyed when brought into contact with oil. Similarly, Kuhlman (22) and Manlowe and Radke (23) observed in micromodel studies that oils were capable of destroying foams. Foam sensitivity to oil is also manifested as an increased difficulty of forming and propagating foams through porous media containing oil (21, 24-28). For example, Novosad and Ionescu (11) found lower mobility reduction factors in foam-floods conducted in cores containing residual oil compared to the same floods conducted in cores that were completely free of oil. The lower mobility reduction factors were interpreted to be due to some kind of foam destruction by the oil. The literature clearly shows that some foams are stable in the presence of oil and others are not. The column foaming tests show a gradual decrease in bulk foam stability as increasing amounts of emulsified oil are contacted, but it is uncertain in these tests how the oil is destabilizing the foam, and whether surfactant is being adsorbed by the increased oil surface area. The foam-flooding experiments conducted in core samples show that the presence of residual oil can significantly lower the mobility reduction that would otherwise have been achieved, but the mechanism by which this happens is not readily judged from such experiments. The microvisualization studies probably provide an intermediate degree of realism between these two kinds of tests and also provide qualitative and semiquantitative information about the possible mechanisms of interaction.

Experimental Studies of Foam—OU Interaction in Porous Media Bulk Foam—Oil Sensitivity Tests. The experimental techniques used to investigate the influence of oil on foam stability in porous media can be categorized broadly as bulk foam stability tests, microvisual simulations, and core-flood tests. The first category, bulk foam tests, are not directly related to the situation of foam flowing in porous media at all. They do have the advantages of simplicity and common availability of suitable apparatus. In a typical dynamic foam test, foam is continuously generated by flowing gas through a porous orifice into a test solution (15, 29, 30). In a typical static foam test, a blender is used to generate a column of foam whose sequential half-lives are determined after shutting off the blender. Again, there are many variations of this kind of test (31-34). One approach to determining the influence of an oil phase involves bulk foam stability tests conducted in the presence and absence of specified volumefractionsof oil. An example of this kind of testing, in the



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context of foam sensitivity to oil in porous media, is the work of Suffridge et al. (35). Bulk foam tests may impart so much shear that artificial mechanisms may dominate. Generally, a considerable concern is that the physical situation in bulk foam tests is too far removedfromwhat happens during foam flowing in porous media, especially regarding the foam quali ty, texture, flow rate, and effective shear rate. For example, Hanssen and Dalland (36) found that their core-flood foam test results were not predictable from bulk foam stability tests. Although the experiments are more difficult, microvisual simulations and core-flood tests have been used to more closely represent the practical physical situation.

Microvisual Simulations. Glass microvisual apparatus of several kinds have been used to visualize the interaction of constrained foam lamellae with oil. One example of such an apparatus is shown in Figure 1 where the glass cell consists of two glass plates, one having a flow pattern etched into it and the other having the fluid entry and exit ports. The etched patterns are generally intended to represent a region of porous medium, although some, such as that depicted in Figure 1, are extremely simplified. The foams are either pregenerated by pumping gas together with surfactant solution through a foam generator or are allowed to form in situ. Somewhere in the cell, foam and oil will be observed as they come into contact with each other. In the pattern shown in Figure 1, advancing foam and residual oil are brought into contact in specific reproducible lo cations in the cell. Kuhlman (22) conducted microvisual experiments for C 0 foams and crude oil and observed foam destruction due to spreading of oil over foam. Manlowe and Radke (23) used microvisual experiments to study foams made from steam-foamers with air in the presence of pure alkane oils and observed foam destruction due to pseudoemulsion film thinning and rupture. Schramm and Novosad (37) used a microvisual apparatus of the design shown in Figure 1 to study foams made from hydrocarbon gas foaming agents in the presence of crude oil. In the work by Schramm and Novosad, a range of foam sensitivities to crude oil was observed, and three qualitative types of behavior were distinguished. 2

1. Type A Foams. These foams showed little interaction with crude oil, although as foam lamellae passed over the oil surface, a cer tain amount of oil would frequently be drawn up and pinched off into large droplets that would be carried along in Plateau borders along cell walls. This behavior is suggested in Figures 2 and 3. 2. Type Β Foams. Other foams showed significant interaction with crude oil. Upon contact with these foams, the oil became emul sified into smaller droplets in the Plateau borders. Some of


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Figure 1. Example of a microvisual apparatus. A microvisual cell plate is shown in the lower portion of thefigureand illustrates an etchedflowpattern and the arrangement of phases at initial contact. (Reproduced with permissionfromreference 37. Copyright 1990 Elsevier Science Publishers.)


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Direction of foam flow


TYPE A

Ruptured lamella

Figure 2. Types of foam behavior after oil contact (Reproduced with permissionfromreference 37. Copyright 1990 Elsevier Science Publishers.)

these smaller size droplets were drawn up and transported into the "bulk" foam (Figure 2). These foams had a moderate stability to collapse; oil-containing lamellae would carry oil droplets some distance before rupturing and releasing their oil. Subsequent lamellae would then sweep up this oil and carry the oil some distance again, and so on. Figure 4 shows an advancing foam lamella sweeping up oil left behind from a previous lamella rupture. 3. Type C Foams. These foams showed a severe interaction effect. Upon contact with foam, the oil emulsified into very small droplets that were drawn up into the foam as described. However, in this case, the smaller oil droplets filled even the thinner lamellar regions, and lamellae were observed to rupture frequently as



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Figure 3. High-speed video photomicrographic sequence that shows microdisplacement of oil by a foam lamella. (Reproduced with per missionfromreference 37. Copyright 1990 Elsevier Science Publish ers.) the oil-filled foam continued to flow in the apparatus. The sequence is illustrated in Figure 2. These foams were quite unstable in the presence of crude oil. In the foregoing, all instances of foam lamella rupture (types Β 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 for ward a third possible mechanism of foam sensitivity to oil, each of which has been observed in microvisual experiments. These will be described further.



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Figure 4. High-speed video photomicrographic sequence that shows microdisplacement (sweep-up) of oil by a foam lamella. In this case the microdisplacement occurs at 90° to that illustrated in Figure 3. (Reproduced with permissionfromreference 37. Copyright 1990 Elsevier Science Publishers.)

Core-Flood Experiments. One example of an apparatus for core-flood testing of foams is illustrated in Figure 5. In general, fluid flow and pressure transducer fittings are secured to sections of porous rock (cores) that are then encased in a constraining sleeve. Foam is prepared either in situ by coinjection of gas and surfactant solution, or the foam is pregenerated by passing gas and surfactant solution through a foam generator prior to injection into the rock. During foam tests, the pressures generated and the oil and aqueous phase productions are measured. These tests are conducted at either constant imposed rates of gas and liquid injection (as in the apparatus shown) or at constant imposed overall pressure drop. In either case, some time elapses before the pseudosteady



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Figure 5. Schematic drawing of a low-pressure core-flood apparatus for studying the performance of foams in porous media in the presence of a residual oil saturation.

state is reached (in which the average pressure drop or average injection flow rate, respectively, become constant). There have been a few studies of foam performance in oil-free cores and cores containing residual oil, but comparisons between the results are difficult. A diversity of parameters have been chosen to measure the efficiency of different foams with regard to their capacity to improve volumetric sweep efficiency. Mobility reduction factors (MRF), permeability reduction factors (PRF), or relative apparent viscosities (RAV) are measured for foams flowing through porous media. The MRF is a ratio of the pseudosteady-state pressure drops across a core with foam and with only gas and brineflowingat rates equivalent to those in the foam. Similar definitions are used for PRFs and RAVs. On the other hand, the experimental procedures adopted by different investigators have been very different, involving for example: • injection of preformed foam into either a surfactant preequilibrated or nonequilibrated core • injection of gas into a surfactant-saturated core • alternate injection of gas and surfactant solution


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• different sequences of foam initiation and flow (increasing or decreasing order of differential pressures) • oil saturations equal or greater than residual oil saturation When this diversity is coupled with the range of surfactants, gases, and oils used, comparisons are difficult. Some examples may be cited as follows. Jensen and Friedmann (25) tested foams in cores at high velocities, a simulation of the phenomena occurring near a wellbore. In cores at residual oil saturation, initial foam injection [10—100 pore volumes (PV)] yielded pressure drops that were about 10 times lower than in the absence of oil. On the other hand, after continued foam injection, the final pressure drops attained were the same as for oil-free cores of the same permeability (but many hundreds of pore volumes had to be injected). The effect of residual oil saturation on foam propagation was strongly surfactant-specific; some foams did not propagate when the oil saturation was above 10-15%. McPhee and Tehrani (38) observed MRFs of 27-30 in cores containing residual oil, and the MRF was about 30 times greater in oil-free cores. Maini (39) conducted foam tests with viscous oils at high temperature and pressure and found a wide range of MRF values for oilfree cores and for cores containing residual oil. Most of the foams studied by Jensen and Friedmann, McPhee and Tehrani, and Maini exhibited sensitivities to residual oil that were about an order of magnitude in MRF (a factor of 10-30). 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 offlowingfoams. Raterman (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. Another little-studied aspect concerns the effect of the amount of oil present in the porous media. Friedmann and Jensen (41) interpreted their foam-flood results in terms of an apparent maximum residual oil saturation above which their foams were not stable. Isaacs et al. (42) tested a number of steam-foamfloodsin sand packs saturated with heavy crude oil and also found that foam effectiveness depended on residual oil saturation. Their foams were effective at saturations below about 10% and were not effective at oil saturations above about 15%. This finding has been incorporated into reservoir foam-flood simulations (14) using the concept of



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"critical foaming oil saturation", residual oil saturations above which a foam will not propagate (14, 41). Yang and Reed (43) also found a residual oil saturation level sensitivity in their C 0 foam-flood studies in sandstone and carbonate rocks. Their foam was effective at residual oil saturations of less than about 5% and virtually nonexistent at oil saturations of about 20%. Similarly, Hudgins and Chung (44) could generate foams in core-floods at very low residual crude oil saturations but not at a higher residual saturation of 23%. The available literature shows that foams can reduce gas mobility in porous media but are sensitive to the presence and saturation level of residual oil. Several of the core-flood studies suggest that incremental oil recovery may be possible because of foam-flooding, and that oil-in-water emulsions may be formed in the process (24, 25, 39, 40). There have also been relatively few attempts to connect core-flood results with micromodel studies and basic physical properties.


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Theories of Foam—Oil Interaction Having reviewed the main experimental approaches, some theories of foam-oil interaction will be examined. Figure 6 illustrates the principal different interaction behaviors that can occur when foam lamellae move along a thin channel and come into contact with an oil phase in the channel. In the first place, there may be no significant interaction, in which case the foam would simply advance unchanged over the oil. As long as the foam spreads over the oil, some oil could become emulsified into the aqueous phase, and films of aqueous solution separating oil from gas (pseudoemulsion films) will be produced. Alternatively, the oil could spread over the foam. The usual starting point for describing foam—oil interactions in porous media has its origins in the basis for destroying bulk foams using defoaming agents. Such agents frequently act by spreading out over the bubble or lamella surfaces or by penetrating and rupturing the surfaces from within the lamellae.

Spreading and Entering Coefficients. From thermodynamics, a defoamer would be predicted to spread as a lens over a foam if its "spreading coefficient" is positive, (17, 45). The spreading coefficient, 5, for an oil—foam system is given by S =

7°

F

-

-

7°

where 7 ° is the foaming solution surface tension, 7 F

(1)

0

O F

is the initial foam-



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Figure 6. Illustration of interaction behaviors that can occur when foam lamellae move along a thin channel (pore) and come into contact with an oil phase. ing solution—oil interfacial tension, and 7 ° is the surface tension of the oil. The process of spreading is illustrated in Figure 7. When oil spreads out over the gas-aqueous interface, a certain amount of both gas-oil and aqueous-oil interface is created, and some gas-aqueous interface is eliminated. For unit surface area, the Gibbs free energy change (AG) is given by —S. The spreading is predicted to be favored when AG is negative and S is positive. Kuhlman (22) suggested a more complex equation to account for complete spreading of oil drops around foam bubbles of different sizes. His equation approaches equation 1 when the foam bubble radii are greater than the oil drop radii. Rapid spreading of a drop of oil that has a low surface tension over the lamella can cause rupture by providing a weak spot (46). The spreading oil lowers the surface tension, increases the radius of curvature of the bubbles, alters the original surface elasticity, and also changes the surface viscosity. Thus the interfacial film loses its foam-stabilizing capability. If S is negative, then the oil should not spread at the interface. Because the interfacial tension can change with time after an initial spreading of oil, S may be time-dependent, and it follows that, in some cases, oils may act as defoamers only for a limited amount of time. The dynamic interfacial tensions were studied (37, 40, 47) for various crude oil and foam-forming surfactant solution combinations. Some of these systems exhibited dynamic interfacial tensions, but typical variations over up Q


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Foam lamella

Figure 7. Spreading entering (pseudoemulsionfilmrupture), and emulsifying processes related to the interaction behaviors shown in Figure 6. (Reproduced with permissionfromreference 37. Copyright 1990 Elsevier Science Publishers.) to 24 h were of the order of 1-2 mN/m. Because the values of S and Ε (the entering coefficient) are frequently less than 10 mN/m, such changes could significantly change the values of these coefficients, but the choice of which time-dependent value to use would not, in most cases, change the sign of either S or E. Another mechanism of rupture involves the drawing up of a droplet of defoamer into the lamellar region between two adjacent bubbles so that it can bridge the two bubbles. The breaching of the aqueous—gas interface by the oil from within the lamellar liquid is termed "entering" (Figure 7). This process is thermodynamically favorable if the "entering coefficient" is positive (originally defined by Robinson and Woods (18) as a rupture coefficient, R). For a foam-oil system, the entering coefficient, E is given as 9

Ε = 7°F

+

7OF -

7ο


(2)


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where the symbols are defined as for equation 1. For unit surface area, the Gibbs free energy change is given by -E. The entering is predicted to be favored and spontaneous when AG is negative and Ε is positive. The terms "penetrates" or "breaches" are used to mean that the pseudoemulsion film thins to the rupture point. The new local oil-gas in terfacial region has an increased radius of curvature. This increase may cause some net increase in radii of curvature of the adjoining bubbles and thereby cause thinning, and again this process alters the original surface elasticity and surface viscosity of the lamellae. Thus the interfacial film can lose its foam-stabilizing capability and thin to the rupture point, although Ross (19) pointed out that entering does not always destabilize a foam. If Ε is negative, the oil should be ejected from the lamellar region and not destabilize the foam by this mechanism. Again, because Ε depends on the same surface properties as S, the value of Ε may be timedependent. In the enhanced oil recovery literature, much use is made of the spreading coefficient, but the entering coefficient (22, 23, 48) is ignored. Many of the correlations with foam behavior attributed to the spreading coefficient would also correlate with the entering coefficient (37, 40, 49). Ross (19) suggested predicting the defoaming action of an insoluble chem ical by considering the possible combinations of values of S and E. These are interrelated through equations 1 and 2 as shown in equation 3. Ε = S + 2 oF 7

(3)

This equation shows that Ε is always greater than or equal to S, but that three combinations of effects can occur: 1. Type A. If Ε is negative, then 5 must be negative, and oil would neither be drawn through nor spread at the foaming solution—gas interface. It would not be expected to act as a defoamer by the previously discussed mechanisms. 2.

Type B. If Ε is positive and S is negative, then the oil would be drawn through but would not be expected to spread at the inter faces. This condition should cause destabilization but may not, depending on whether small enough oil drops are drawn up to seriously reduce the coherence of the lamellae, and whether the oil remains as a lens or is ejected outside the lamella.

3. Type C. If both Ε and S are positive, then the oil will be drawn through and then spread as a film along the lamellae surfaces. This behavior leads to lamellae ruptures.


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Schramm and Novosad (37) showed that the three types of behavior can be represented in a foam "phase-behavior" diagram byfixing7 ° (choosing a single oil) and plotting 7 versus 7 ° This representation is shown in Figure 8 where both S and Ε increase toward the right side of the diagram, and three regions are distinguished by solving equations 1 and 2 for the conditions Ε = 0 and S = 0 to produce the two line boun daries. The four phase-behavior diagrams of Figure 8 correspond to one alkane and three crude oils of increasing gravity, or "heaviness", from top to bottom in the figure legend. In each case, the data correspond to solu tions of a range of foaming agents. Schramm and Novosad's phasebehavior approach has been used with some success by others as well (27). Some authors use a coefficient to describe the spreading of aqueous solution over oil, for example, S = 7 ° — 7 ° — 7 (22, 27). The pro cess described is equivalent to the reverse of entering, and Ε = —S . Thus the region marked "Type A" in Figure 8 could be considered to represent the conditions under which foaming solution will spread over oil, as illus trated in Figure 6a. 0


O F

w

p

0

F

O F

w

Pseudoemulsion Film Model. Although much attention is tra ditionally focused on foam lamellae, the thin liquidfilmsbounded by gas on each side and denoted as air-water-air (A/W/A), a distinct but related thin liquid film, may be the most important for foam stability in the pres ence of oil. That is, the thin liquidfilmsbounded by gas on one side and by oil on the other denoted air-water-oil (A/W/O). These films are illus trated in Figure 6a-6c. They are also referred to as pseudoemulsion films (48). Their importance stems from the fact that it is possible for the pseu doemulsion film to be metastable in a dynamic system, even when the thermodynamic entering coefficient is greater than zero. 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 imbibi tion of oil can actually stabilize foams. Manlowe and Radke's results (23) were interpreted in terms of pseudoemulsion film stabilities depending mainly on electrostatic and dispersion forces. Undoubtedly, interfacial viscosity could also be important. In one approach, the stability of pseudoemulsion films can be pre dicted by calculating the total interaction energy for the thinning of a liquid film bounded by an air—water and a water—oil interface along the lines described earlier in Chapters 1 and 2. From this interaction energy, the disjoining pressure, which is the net pressure difference normal to the surface between the gas and oil phases and the bulk liquidfromwhich the lamellae extend, can be calculated. Estimates of the disjoining and capil-


SCHRAMM



Dodecane

Judy Creek

David

Lone Rock

20

40

80

Foam Surface Tension, mN/m

Figure 8. Foam "phase-behavior" diagrams for a series of foamforming surfactants and four different oils. (Reproduced with permissionfromreference 47. Copyright 1992 Elsevier Science Publishers.)



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lary pressures for foam lamellae in porous media can then be used to make predictions about the probable stabilities of the lamellae (23). Raterman (28) directly measured film disjoining pressures and pseudoemulsion film stabilities and compared these with the results of coreflood tests. His results showed that foam systems containing thermodynamically stable pseudoemulsion films are oil tolerant, but except for thermodynamically stable pseudoemulsion film systems, he found no trend in foam—oil stability and disjoining pressures. On the other hand, Fagan (52) found a good correspondence between high pseudoemulsion film disjoining pressures and good oil tolerance on the part of foams flowing in porous media and vice versa. The disjoining pressure approach has some important limitations. Disjoining pressures typically will be important only for very thin lamellae such asfilmsthinner than 100 nm. There is evidence to suggest that foam lamellae in porous media extend to at least an order of magnitude larger than this. Raterman (28) found a much better correlation between pseudoemulsion film thinning and foam stability for constant gas injection in situ foam generation tests than for constant preformed foam injection tests. The foam lamellae would have been thinner in the foams that are generated by constant gas injection in situ and much thicker in the foams that are generated by constant preformed foam injection. Also, the disjoining pressure itself is usually most significant in systems that contain appreciably charged interfaces, that is, where ionic surfactants are present at the interfaces. When nonionic or zwitterionic surfactants are involved, the disjoining pressures will be significant only for even thinner lamellae or if, as suggested by Wasan et al. (48, 50), the surfactant type and concentration are such as to produce micellar structuring effects.

Emulsification and Imbibition Model. Nikolov et al. (48) concluded, in part, that "Foam destabilization ... may often involve the migration of emulsified oil droplets from the foam film lamellae into the Plateau borders where critical factors, such as the magnitude of the Marangoni effect in the pseudoemulsion film, the pseudoemulsion film tension, the droplet size, and number of droplets may all contribute to destabilizing or stabilizing the three-phase foam structure." Raterman (28) also concluded that oil droplet size should be an important determinant of the relative importance of pseudoemulsion film stability. Further, in typical antifoaming processes, fine emulsions (small drops) make better antifoaming agents (18, 53), and indeed, the colloid chemical description of foam-breaking outlined in equations 1 and 2 has, as a starting point, the introduction of oil droplets as an emulsion that have drop diameters smaller than the thickness of the foam lamellae. In the foams contacting oil in a porous medium, the oil may not be emulsified or may not have such small drop sizes. If this is the case, then the spreading and entering



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coefficients may not directly apply, and any pseudoemulsion films will have a much lower interfacial area. This is one of the reasons there can be a different influence on foam stability between oil as a macrophase and oil as emulsified droplets. The observations of a number of researchers (22, 24, 25, 37, 39, 40, 48) indicate that emulsification of oil into the foam, as depicted in Figure 6b, may be a very important factor. To describe the process of an oil phase emulsifying into foam lamellae, one could adopt an equilibrium thermodynamic approach analogous to that employed to obtain S and E. Such an approach produces the result that emulsification is only favored (negative AG) for negative values of the interfacial tension. In an alternative approach, the tendency of an oil phase to become emulsified and imbibed into foam lamellae has been described through a simplified balance of forces by the lamella number, L (37). For foam lamellae flowing in porous media, it is predicted that oil will be drawn in and pinched off to produce emulsified drops inside foam lamellae when L > 1, where AP

(7°F) r

C

L =

Q

= AP

R

v

J

(4) K

(7QF) r

}

p

Here AP and AP are the pressure difference between inside the Plateau border and inside the laminar part of the lamella, and the pressure difference across the oil—aqueous interface, respectively, and r and r are the radii of the oil surface with which the lamella comes into contact and that of the lamella Plateau border, respectively (Figure 7). If L is scaled by substituting one-half the lamella thickness for r , then L > 1 indicates that small enough oil droplets will be produced to permit their movement within a lamella. Experimental work with a microvisual cell of fixed dimensions has shown that equation 4 can be simplified to a useful approximate form. For a wide variety of foams, oils, flow rates, and foam qualities in the range 85-95%, all studied in the microvisual cell depicted in Figure 1, equation 4 was found to simplify to (37, 40, 47) C

R

Q

p

Q

0.146(7° ) F

(7OF)

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


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of the foam lamellae through the pores, the stretching and contracting of lamellae as they pass through pores and throats, and any Marangoni flows within the foam lamellae. None of these sources of shear are accounted for in the simplified description given by equations 4 or 5. Some impor tant limitations of the lamella number are that it will be important only for foam lamellae that are thick enough to accommodate realistic emul sion droplet sizes, and that an explicit factor for the effect of mechanical shear is missing.

Comparisons between Theory and Experiment

Microvisual Experiments and the Proposed Mechanisms. The spreading and entering coefficients have correlated with foam sensi tivity 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 exper iments have been compared to the thermodynamic predictions based on 5 and Ε 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 onehalf of the surfactant solutions predicted to produce foams of type C actu ally 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 fre quency in the microvisual experiments showed that satisfactory correla tions with S or Ε were not obtained (37, 40, 47). Although a very large negative spreading coefficient can be used to distinguish the type A foams from the rest, neither S nor Ε could be used to distinguish the Β and C foams from each other. Thus it appears that even the most stable foam-oil systems cannot reliably be predicted from the thermodynamic relationships for S and E. In Kuhlman's work (22), spreading of oil over the foam lamellae was observed and correlated with foam stability. In this work, foam sensitivity to the crude oil was also as sociated with emulsification of the oil into small droplet sizes. Converse ly, Hanssen and co-workers (36, 55) could not find a consistent link be tween oil spreading and foam sensitivity to oil in porous media. In Wasan et al. (48, 50, 51) and Manlowe and Radke's (23) studies, spreading and entering were not found to be important causes of foam destruction. Instead, thin film (pseudoemulsion film) drainage between solution and oil were proposed. The microvisual experiments of Roberts et al. (53) and Schramm et al. (37, 40, 47) showed that foam destruction was associated with the abili ty or degree of emulsified oil droplets to travel within the foam lamellae. For example, on the basis of the work of Schramm et al., thefrequencyof


4.

SCHRAMM


185

foam lamella ruptures, / , a measure of foam stability in microvisual exper iments, is shown plotted versus the logarithm of the lamella number from equation 5 for each of four different oils in Figure 9. In each case, there is a fairly good correlation between stability and lamella number, especial ly considering the uncertainty in the measured values of f (±14%). The qualitative and quantitative results can be related as in the regions marked A, B, and C. In region A (L < 1), it was predicted and observed that oil would not be emulsified to a size that would allow ready access to the foam lamellar structure, and type A foam behavior (Figure 2) would result. In region Β (1 < L < 7), it was predicted and observed that the emulsification of oil into smaller droplets would become more favorable, and type Β foam behavior would result. In region C (L > 7), it was predicted and observed that emulsification into smaller drops would occur that would allow ready access to the foam structure and result in type C b


h

Figure 9. Relation between foam breakage frequencies in a microvisual cell and the lamella number for four different oils. (Repro duced with permission from reference 47. Copyright 1992 Elsevier Science Publishers.)



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FOAMS: FUNDAMENTALS & APPLICATIONS m THE PETROLEUM INDUSTRY

behavior. The quantitative measurements and the qualitative observations matched in 65 of the 67 foam-oil systems investigated (47). Regarding the limitation of the lamella number description, that it applies to foam lamellae that are thick enough to accommodate realistic emulsion droplet sizes, the microvisual studies reported are consistent with this result. Schramm et al. (37, 40, 47) observed emulsification into lamellae that have thicknesses on the order of 50-80 μπι, as did Kuhlman (22) for foam lamellae on the order of 40-μπι thick. Nikolov et al. (48) argued that the presence of emulsified oil droplets themselves has a thick ening effect on foam lamellae, and can even have a stabilizing effect on foams if the droplets sizes are small enough. (Foams that are stabilized by micellar structuring can apparently be further stabilized when emulsion droplets are incorporated into the micelles, making them swell.) Another possible stabilizing mechanism is the existence of a strong electric doublelayer repulsion between oil drops and aqueous-gas foam interfaces, which causes a large disjoining pressure. Such stabilization by disjoining pres sure would be expected only for ionic surfactant systems. On the other hand, some (52) have suggested that the thickness of foamfilmsin porous media is less than about 100 nm. If this is true for some foams, then in these cases, it is unlikely that oil droplets would be emulsified into such thin films and move at typical reservoir injection flow rates unless the interfacial tensions were extremely low.

Core-Flood Experiments and the Proposed Mechanisms. 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 con ditions and found that the presence of residual oil could reduce the effec tiveness 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. Raterman's (28) core-flood studies for foams flowing in Berea sand stone cores at moderate pressure and in the presence of a residual oil phase led to the conclusion that oil spreading is of only secondary impor tance to foam sensitivity to oil. For gas injection that causes in situ foam generation and flow, he found a good correlation between foam stability and pseudoemulsion film stability. However, for constant quality pre formed foam injection, he found pseudoemulsion film stability to be less directly connected with foam sensitivity. Thisfindingis probably due to the fact that the foam lamellae in the constant quality preformed foam in jection floods were too thick for disjoining pressures to be the most im portant factor. Fagan (52) found a good correspondence between high



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pseudoemulsion film disjoining pressures and good oil tolerance on the part of foams flowing in porous media and vice versa. 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. Raterman (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. Some of these results can be brought together in Figure 11. Hanssen and Dalland (36) measured gas permeabilities for different foams (variable quality) in synthetic seawater flowing in relatively high-permeability (K = a

Micromodel breakage frequency (s"') 1

Figure 10. Mobility reduction factors for foamsflowingin Berea sandstone at residual oil saturation versus the breakage frequencies of foam lamellaeflowingin a microvisual cell when in contact with the same oil. (Reproduced with permissionfromreference 40. Copyright 1990 Society of Petroleum Engineers.)


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FOAMS: FUNDAMENTALS & APPLICATIONS IN THE PETROLEUM INDUSTRY 50 Raterman, Soltrol, MRF Raterman, C16, MRF Schramm MRF Hanssen, 10 Log PRF Holt, App Viscosity

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