Foam flow

Rheology. The rheology of foam is striking it simultaneously shares the hallmark rheological properties of soHds, Hquids, and gases. Like an ordinary soHd, foams have a finite shear modulus and respond elastically to a small shear stress. However, if the appHed stress is increased beyond the yield stress, the foam flows like a viscous Hquid. In addition, because they contain a large volume fraction of gas, foams are quite compressible, like gases. Thus foams defy classification as soHd, Hquid, or vapor, and their mechanical response to external forces can be very complex. 

Consistent with this model, foams exhibit plug flow when forced through a channel or pipe. In the center of the channel the foam flows as a soHd plug, with a constant velocity. AH the shear flow occurs near the waHs, where the yield stress has been exceeded and the foam behaves like a viscous Hquid. At the waH, foams can exhibit waH sHp such that bubbles adjacent to the waH have nonzero velocity. The amount of waH sHp present has a significant influence on the overaH flow rate obtained for a given pressure gradient. 

The simplest devices have rakes mounted on the stirrer shaft located on the surface of the liquid. A more sophisticated device is the Funda-foam system , in which the foam is destroyed by centrifugal forces. The nutrient solution held in the foam flows back into the bioreactor, and the air released from the foam leaves the vessel. 

Figure 1.2 Normal and focused multilamination flow patterns, slug flow composed of gas/liquid segments (Taylor flow ), and ordered foam flow ( hexagon flow ) (from top to bottom).

Laboratory studies of foam flow in porous media suggest that the relative foam mobility is approximately inversely proportional to the permeability. This means that foam has potential as a flow-diverting agent, in principle sweeping low-permeability regions as effectively as high-permeability regions [716]. 

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... 

Accordingly, for a given capillary pressure in a porous medium there will be a specific foam flow rate at which the lamellae rupture. This is the proposed origin of the flow-rate dependence... 

In order to understand the nature and mechanisms of foam flow in the reservoir, some investigators have examined the generation of foam in glass bead packs (12). Porous micromodels have also been used to represent actual porous rock in which the flow behavior of bubble-films or lamellae have been observed (13,14). Furthermore, since foaming agents often exhibit pseudo-plastic behavior in a flow situation, the flow of non-Newtonian fluid in porous media has been examined from a mathematical standpoint. However, representation of such flow in mathematical models has been reported to be still inadequate (15). Theoretical approaches, with the goal of computing the mobility of foam in a porous medium modelled by a bead or sand pack, have been attempted as well (16,17). 

These tests show that CC -foam is not equally effective in all porous media, and that the relative reduction of mobility caused by foam is much greater in the higher permeability rock. It seems that in more permeable sections of a heterogeneous rock, C02-foam acts like a more viscous liquid than it does in the less permeable sections. Also, we presume that the reduction of relative mobility is caused by an increased population of lamellae in the porous medium. The exact mechanism of the foam flow cannot be discussed further at this point due to the limitation of the current experimental set-up. Although the quantitative exploration of this effect cannot be considered complete on the basis of these tests alone, they are sufficient to raise two important, practical points. One is the hope that by this mechanism, displacement in heterogeneous rocks can be rendered even more uniform than could be expected by the decrease in mobility ratio alone. The second point is that because the effect is very non-linear, the magnitude of the ratio of relative mobility in different rocks cannot be expected to remain the same at all conditions. Further experiments of this type are therefore especially important in order to define the numerical bounds of the effect. 

The water pumped to the underside of the piston of the rodded cell can also be delivered to the test cell, to fill the jacket surrounding the test unit(Figure 1). The contents of the jacket can be discharged via a back-pressure relief valve(BPRV1 in Figure 1) which is arranged to open when the water pressure exceeds that of the foam flowing to the inlet of the test un i t.. 

The Viscosity-Quality Spectrum of Foam Flowing in Straight Capillary Tubes" (in press)... 

Foam—stable aggression of small bubbles of lower density than oil or water, which shows tenacious qualities in covering and clinging to vertical or horizontal surfaces. Foam flows freely over a burning liquid surface, forming a tough, air-excluding continuous blanket to seal volatile combustible vapors from access to air. 

The micro-mixer then generates a continuous foam flow composed of small bubbles of hydrogen (ca. 200 pm in diameter) in the liquid (ethylene glycol-water... 

A continuous foam flow was generated by the micro mixer, which was composed of small bubbles of hydrogen with a diameter of about 200 pm in the liquid [ethylene glycol/water, 60 40 (w/w) sodium dodecyl sulfate as surfactant] (see Figure 3.77). The reaction rate was proportional to the catalyst concentration and decreased with increasing surfactant concentration [6],... 

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. 

Chambers, K.T. Radke, C.J. Capillary Phenomena in Foam Flow Through Porous Media in Interfadal Phenomena in Petroleum Recovery, Morrow, N.R. (Ed.), Dekker New York, 1991, pp. 191-256. 

Table 4.4 Benchmarking of figures of merit for the gas-liquid screening using a well-behaved minibatch and a continuous foaming-flow microreactor operation.

This aim fully describes the target quantity of a foam centrifuge, s. sketch in Fig. 4. One has to determine the minimum rotational speed, nmin, required to set the foam flowing. (According to [17], a resulting foam density of ca. p = 0.50 kg/1 should suffice.)... 

To control features of the flow itself (examples include drag reduction by addition of polymer or microbubbles, magnetic stabilization of fluidized beds, foam flow in porous media for mobility control, antimisting or cavitation suppression via polymer additives) and, finally,... 

The concept of the critical capillary pressure considered in Sections 6.5.2. and 6.5.3. has been used in [86-88] for the explanation of the behaviour of a foam flowing through porous media. 

Values of To [34] close to those reported in [5,33], are obtained for foams with expansion ratio in the range of 86 to 300, and consisting of bubbles with 0.3-1 mm diameter, i.e. To,si = 2-12 Pa. It was established also that the value of To., depends on the viscosity of the dispersion medium, which does not conform with the theory presented in [10]. Probably, this is related to the slip at the tube walls during foam flow. 

The yield stress of a foam depends to a considerable extent on the character of foam interaction with the tube walls or the cylindrical surface of the viscometer, used in the study of its rheological properties. At low flow rates and smooth tube walls the maximum shear stress of the foam layers contacting the wall can be less than the shear stress of the foam matrix (shear of bubble layers). Hence, the foam flow will occur as a movement of a continuous medium in a cylinder covered with a thin lubricating layer of thickness 2-10 pm [9,16], In this case t0 is ca. 1 Pa, that is, much less than its theoretical value. 

The critical analysis of the results on foam rheology, proposed by Heller and Kuntamukkula [16], has shown that in most of the experiments the structural viscosity depends on the geometrical parameters of the device used to study foam flow. This means that incorrect data about flow regime and boundary conditions, created at the tube and capillary walls, etc., are introduced in the calculation of viscosity (slip or zero flow rate). Most unclear remains the problem of the effect of the kind of surfactant and its surface properties on foam viscosity and on the regime of foam flow (cross section rate profile and condition of inhibition of motion at the wall surface). 

When n > 100, the Q(At) dependence becomes non-linear. Low expansion ratio foams can be measured also in a regime of foam flow. 

EOR process requires a detail study not only of foam behaviour in porous media but also of the options to control it. Foam flow in porous media during EOR is a complex, multifaceted process. A number of papers are dedicated to that topic, including some reviews [e.g. 13,14,18] which describe the experimental set-up used in the study of foams in porous media. We will focus on those illustrating the efficiency of EOR from oil pools and the role of some important factors, involving the effect of foam properties, especially of the critical capillary pressure. 

Mast, in a pioneering 1972 paper, reported visual observations of foam flow in etched glass micromodels (37 ) His observations showed that some of the conflicting claims about the properties of foam flow in porous media were probably due simply to the dominance of different mechanisms under the various conditions employed by the separate researchers (37). Mast observed most of the various mechanisms of dispersion formation, flow, and breakdown that are now believed to control the sweep control properties of surfactant-based mobility control (37,39-41). 

The wettability of the porous medium was found to have a significant effect on foam flow as early as 1966 (D. C. Bond and G. G. Bernard, AICh 58th Annual Meeting, Dallas, February 7-10, 1966). Later, Kanda and Schechter showed that a foam produced a large reduction of permeability only if the aqueous phase wet the porous medium (64). Thus, various flow studies confirm the importance of wettability. 

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