Mechanisms of foam generation

Researchers have investigated the nature of the foam flow by examining the mechanisms of foam generation (l ). An extensive study (1 ), that is quite relevant to the mechanism of foam flow in porous media, has shown that the apparent viscosity of foam in a capillary tube decreases rapidly as the ratio of bubble radius-to-tube radius is increased. 

A porous medium shapes foam to its own liking as confined, porefilling bubbles and lamellae. Foam in porous media is not a continuous fluid. The three mechanisms of foam generation (snap-off, division, and leave-behind) are all pore geometry specific. Snap-off is a mechanical process that occurs in multiphase flow without surfactant. For successful gas-bubble snap-off, the pore-body to pore-throat constriction ratio must be sufficiently large (roughly 2) and gently sloped. Otherwise stable wet-... 

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). 

As shown in the studies commented below, the most important about the mechanism of foam in EOR applications are the connectivity and geometry of medium (a size distribution of pore bodies of the order of 100 pm in diameter and pore-throats of the order of 10 pm in diameter) the distribution of the two-phase systems (liquid-gas) in pores which depends on the wetting of pore walls and the volume ratio of the liquid and gas phases the regulating capillary pressure the mode of foam generation and foam microstructure. 

Foam Destruction. Net foam generation cannot continue unchecked. It is balanced by foam destruction processes. Chambers and Radke (26) enunciated two basic mechanisms of foam coalescence capillary-suction and gas diffusion. Because capillary-suction coalescence is the primary mechanism for lamellae breakage, we focus on it, and only briefly touch upon foam coarsening by gas diffusion. 

The rate of bubble division, the second mechanism for creating foam, is proportional to the flux of lamellae into division sites (20). Thus, the rate of foam generation by division is formally identical to equation 6. Further, both rate constants share the property of being small when Sw is high because more division sites become available as Sw drops. It is diffi-... 

Nevertheless, it is important to point out that a lamella cannot be created directly at a pore-throat. Rather, a lens forms first with lamella creation occurring upon expansion into the adjacent pore-body, provided surfactant is available (see the discussion of foam-generation mechanisms). During two-phase flow without stabilizing surfactant present, lenses are still created by snap-off in Roof sites (54, 60) followed by expansion and rapid coalescence in the downstream pore-body, once the lens thins to a film. If stabilized lamellae are pictured to rupture before exiting the immediate downstream pore-body, they are not much longer lived than unstable lenses. Such processes are accounted for in measurements of continuum relative permeabilities. 

Eurther specific outgassing studies are necessary to understand the mechanisms of gas generation during the foaming process, even though this investigation presents several difficulties because of the large number of parameters involved. 

Barring direct measurement of foam texture, we adopt the following reasoning. Because of the generation of foam bubbles by the snap-off and division mechanisms (4), bubble sizes are expected to be approximately that of pore bodies. Thus, the linear bubble density should scale roughly as n 6/Dwhere... 

Foam Generators Devices for mixing chemical or mechanical foam in proper proportion with a stream of water to produce foam. 

Polyurethanes are important synthetic macromolecules. They are manufactured, for example, in the form of foams. Such a foam is obtained during the formation and solidification of the polyurethane when a gas escapes from the reaction mixture, expanding the material. An elegant possibility for generating such a gas uniformly distributed everywhere in the reaction medium is as follows besides the diol, one adds a small amount of H20 to the diisocyanate. H20 also adds to the C=N double bond of the diisocyanate. According to the uncatalyzed addition mechanism of Figure 8.12, this produces an A-arylated free carbamic acid Ar—NH— C(=0)—OH. However, such a compound decomposes easily in perfect analogy to the decom-... 

It can be considered from the scheme that one has to distinguish between the foam kinetics, i.e. the rate of generation of foam under well defined conditions (air input and mechanical treatment) and the stability and lifetime of a foam once generated. The foam kinetics is also sometimes termed foamability in the literature. These quantities can be related to interfacial parameters such as dynamic surface tension, i.e. the non-equilibrium surface tension of a newly generated surface, interfacial rheology, dynamic surface elasticity and interfacial potential. In the case of the presence of oily droplets (e.g. an antifoam, a... 

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