Foam generation, mechanisms

Leave-behind. Figure 7 illustrates the third foam generation mechanism (60). Leave-behind begins as two gas menisci invade adjacent... 

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. 

Due to the extensive research that has been conducted in the area of foam application in enhanced oil recovery, simulation of foam behaviour has become more feasible. Several methods of foam simulation have been developed population balance models [16, 17], fractional flow models [IS, 19], and models that alter the gas phase permeabilities [20, 21], Although the population balance models treat the foam generation mechanisms in a detailed fashion, they may be impractical to apply on large field scale simulations. Both the fractional flow model and the models that alter the gas phase permeabilities rely on history matching experimental data. The fractional flow model provides insight into onedimensional foam flow, but it may be more difficult to apply in three-dimensional situations. In the following section, the application of relative permeability alterations to model foam flow is investigated. 

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

The properties of a low expansion ratio foam are controlled mainly by changing the rate of the kinetic processes running in it. To decelerate the hydrodynamic processes in order to preserve its structure for a longer time (for instance, in the formation of polymer or frozen foams), the following measures can be recommended i) to use a foam generation mode that allows producing a foam of a uniform expansion ratio such are the stream type generators and some mechanical devices that mix the solutions ii) to produce a foam of maximum dispersity... 

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. 

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. 

In this chapter, we discuss much of the work accomplished since Fried, but without attempting a complete review. Useful synopses are available in the articles and reports of Hirasaki (2, 3), Marsden (4), Heller and Kuntamukkula (5), Baghidikian and Handy (6), and Rossen (7). Our goals are to present a unified perspective of foam flow in porous media to delineate important pore-level foam generation, coalescence, and transport mechanisms and to propose a readily applicable one-dimensional mechanistic model for transient foam displacement based upon gas-bubble size evolution [i.e., bubble or lamella population-balance (8, 9)]. Because foam microstructure or texture (i.e., the size of individual foam bubbles) has important effects on flow phenomena in porous media, it is mandatory that foam texture be accounted for in understanding foam transport. 

This discussion follows the goals listed previously. First, we describe how foam is configured within porous media, and how this configuration controls foam transport. Next, we review briefly pertinent foam generation and coalescence mechanisms. Finally, we incorporate pore-level microstructure and texture-controlling mechanisms into a population-balance to model foam flow in porous media consistent with current reservoir-simulation practice (10). Attention is focused on completely water-wet media that are oil free. Interaction of foam with oil is deferred to Chapter 4. 

The behavior of foam in porous media is intimately related to the connectivity and geometry of the medium in which it resides. Porous media have several attributes that are important to foam flow. First, they are characterized by a size distribution of pore-bodies (sometimes called pores) interconnected through pore-throats of another size distribution. Body and throat size distributions are important as is their possible correlation to determine the distribution of body to throat size ratios. Foam generation and destruction mechanisms in porous media depend strongly on the body to throat size aspect ratio. 

Foam Formation. Three fundamental pore-level generation mechanisms exist snap-off, division, and leave-behind. 

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-... 

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-... 

Lamella Leave-Behind A mechanism for foam lamella generation in porous media. When gas invades a liquid-saturated region of a porous medium, it may not displace all of the liquid, but rather leave behind liquid lamellae that will be oriented parallel to the direction of the flow. A foam generated entirely by the lamella leave-behind mechanism will be gas-continuous. See also Lamella Division, Snap-Off. 

Foaming Agent A material which causes a chemical to form a thick foam often applied to reduce drift or assist in the containment of certain types of chemical spills or fires. Foam Generators Devices for mixing chemical or mechanical foam in proper proportion with a stream of water to produce foam. 

It is possible to generate foam by mechanically tumbling sealed cylinders. Aeration occurs as the liquid falls, affecting the walls of the cylinders. This method is again suitable for monitoring the deactivation of antifoams. 

---