Discontinuous-gas foam

Figure 2 illustrates what is coined a discontinuous-gas foam (2, 9), in that the entire gas phase is made discontinuous by lamellae, and no gas channels are continuous over sample-spanning dimensions. Gas is encapsulated in small packets or bubbles by surfactant-stabilized aqueous films. These packets transport in a time-averaged sense through the porous medium (20). 

Lenses created by leave-behind are generally oriented parallel to the local direction of flow (i.e., the pore-level flow that created them), and do not make the gas phase discontinuous. If leave-behind is the only form of lens or lamella generation, a continuous-gas foam results. Ransohoff and Radke (60) found that foam generated solely by leave-behind gave approximately a five-fold reduction in steady-state gas permeability, whereas discontinuous-gas foams created by snap-off resulted in a several hundred-fold reduction in gas mobility (20, 61). 

Not all confined foams are discontinuous (9). A continuous-gas foam is illustrated schematically in Figure 3. In continuous-gas foam the medium contains one, or several, interconnected gas channels that are uninterrupted by lamellae over macroscopic distances. As in discontinuous-gas... 

Figure 4. Pore-level schematic of fluid distribution for a discontinuous-gas flowing foam. Flowing bubbles are unshaded, and trapped gas is darkly shaded. (Reproduced with permission from reference 38. Copyright 1990 Society of Petroleum Engineers.)...

Porous materials are two-phase structures composed of a continuous solid/liquid phase and either continuous or discontinuous gaseous phase [2,4,6,7], As explained before, it is possible to find various materials with a porous structure (polymers, wood, metals, ceramics, etc). Porous structures can be classified according to their topology on one hand would be the open pore foams, which show continuity of the gas and solid phases, and on the other hand would be the closed pore foams, in which the gas is enclosed in the pores (ie, continuous solid phase and discontinuous gas phase). 

Flexible foams are three-dimensional agglomerations of gas bubbles separated from each other by thin sections of polyurethanes and polyureas. The microstmetures observed in TDI- and MDI-based flexible foams are different. In TDI foams monodentate urea segments form after 40% conversion, foUowed by a bidentate urea phase, which is insoluble in the soft segment. As the foam cures, annealing of the precipitated discontinuous urea phase... 

Foam is gas-liquid dispersion in which the liquid is the continuous phase and the gas is the discontinuous phase. The first use of foam in drilling was reported in 1964. 

Foam formation in a boiler is primarily a surface active phenomena, whereby a discontinuous gaseous phase of steam, carbon dioxide, and other gas bubbles is dispersed in a continuous liquid phase of BW. Because the largest component of the foam is usually gas, the bubbles generally are separated only by a thin, liquid film composed of several layers of molecules that can slide over each other to provide considerable elasticity. Foaming occurs when these bubbles arrive at a steam-water interface at a rate faster than that at which they can collapse or decay into steam vapor. 

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... 

The primary factor controlling how much gas is in the form of discontinuous bubbles is the lamellae stability. As lamellae rupture, the bubble size or texture increases. Indeed, if bubble coalescence is very rapid, then most all of the gas phase will be continuous and the effectiveness of foam as a mobility-control fluid will be lost. This paper addresses the fundamental mechanisms underlying foam stability in oil-free porous media. 

Typically, the gas-drive of surfactant solution require a higher P than the gas-drive of brine. One possible explanation is that the foam formation in the "surfactant-present" gas-drives was poor, such that very little gas was present in a discontinuous state. 

When gas alone is injected into a porous medium where foam had been flowing, the liquid saturation can be reduced below the irreducible liquid saturation. It would be expected that the liquid phase becomes discontinuous at this point and the further reduction of the liquid saturation occurs as a result of liquid flowing from the core as bubble train lamellae. This does not occur in conventional gas-liquid displacement, and the lower limit of the liquid saturation corresponds to the irreducible value. 

It is well known that a foam is a composite solid-gas material. The continuous phase is the polyurethane polymer and the discontinuous phase is the gas phase. Polyurethanes are an extremely versatile group of polymers, produced in a wide range of densities, crosslink densities and stiffnesses, from very soft to very hard structures, as shown in Figure 1.6. 

Flumerfelt and Prieditis (40) performed a similar gas-only injection into a 7-pm2 bead pack. A foam was first generated under conditions of simultaneous injection of gas and surfactant solution at a variety of gas rates but at fixed liquid rates. After steady state was reached, liquid flow was discontinued, and the foam was allowed to decay until continuous gas was produced. It was demonstrated that the permeability of the bead pack to gas at the first appearance of effluent continuous gas was 2 orders of magnitude less than the foam-free case, and that this permeability was independent of gas and initial liquid flow rates. It was concluded that the number of channels available to carry gas was 100 times less in the presence of foam than in the foam-free case. 

The foam of everyday experience, though different in several ways from the foam that occurs and is used in reservoirs, is worthy of some examination. Such everyday foam is a two-phase mixture of gas and liquid, in which the liquid is the continuous fluid and the gas is held in separate cells. See also the discussions in Chapters 1—3 of this book.) To display the distinctive foamlike characteristics, the volume fraction of the discontinuous phase must be greater than about 70%. At this high gas volume fraction (the so-called quality), the bubbles of gas are closely crowded together so that they cannot move independently. They also change in shape, and the walls of the cells become approximately planer, polygonal surfaces that are called lamellae or bubble-films. 

A further point of difference between C02 foam and everyday foam is the fact that in C02 foams, the gas, which is a dense fluid, acts more like a liquid than like the gas encountered in everyday foams. This means that the discontinuous phase of the foam is also capable of carrying other materials, such as surfactants, in solution. The capability imposes a new constraint on the surfactant, as we shall see. Perhaps surprisingly, however, the dense state of the gas does not cause any difficulty in the formation of the foam. In fact, it is possible to make analogous foams at low pressure with the same surfactants in water or brine, and with a light hydrocarbon like isooctane substituted for the dense C02. Despite this fact, it is common in the foam literature to refer to the discontinuous phase as gas . Similarly, the brine—surfactant solution that forms the continuous phase is often referred to simply as water . 

An almost irrelevant point of difference in the behavior of foam was whether the discontinuous phase was a gas, a dense gas, or a low-viscosity liquid. The presence of the lamellae across some of the pores gives foam its distinctive properties in porous media. 

Steam-based processes in heavy oil reservoirs that are not stabilized by gravity have poor vertical and areal conformance, because gases are more mobile within the pore space than liquids, and steam tends to override or channel through oil in a formation. The steam-foam process, which consists of adding surfactant with or without noncondensible gas to the injected steam, was developed to improve the sweep efficiency of steam drive and cyclic steam processes. The foam-forming components that are injected with the steam stabilize the liquid lamellae and cause some of the steam to exist as a discontinuous phase. The steam mobility (gas relative permeability) is thereby reduced, and the result is in an increased pressure gradient in the steam-swept region, to divert steam to the unheated interval and displace the heated oil better. This chapter discusses the laboratory and field considerations that affect the efficient application of foam. 

Investigations to determine the leak-off control mechanisms of foam have shown (26—29) that the effective permeability of a porous medium is greatly reduced in the presence of foam. Some basic assumptions were used during the testing to determine the leak-off control mechanisms of foamed fracturing fluids. The first assumption was that the liquid or continuous phase moves freely, and permeability reduction is a function of the liquid saturation. The other assumption was that the gas or discontinuous phase flows only by rupture and reformation of the foam film. The resistance of foam to flow through porous media is a function of the stability of the foam. 

A porous material consists of at least two immiscible phases of which one is usually a continuous sohd material, the matrix, which surrounds the second phase of finely dispersed voids, the pores, containing a liquid, gas, or vacuum. If the void phase is discontinuous and comprises individually separated cavities filled with gas (bubbles), the material represents a foam structure. On the other hand, if both phases form two interpenetrating continua with the matrix as well as the pores being continuous, the material represents a sponge structure or a so-called porous network. Such porous networks with interconnected voids are the focus of this article as they may have funda-... 

Foam with large and less stable bubbles is less likely to flow as a single fluid. Mast and Fried deduced that foam is propogated inside a porous medium by the breaking and reforming of foam bubbles. The gas flows as a discontinuous phase while toe liquid is transported as a free phase via toe film network. Nahid proposed that toe gas flow could be treated according to Darcy s law if a correction factor for the gas permeability is used. 

A foam is a composite consisting of a skeleton of solid materials, and, in the case of closed-cell types, voids filled with various gases. The term "closed cell" is used to describe a foam where the voids form discrete, noninterconnected chambers. The resulting discontinuity in the gas phase is important in establishing the thermal conductivity, as is illustrated in Fig. 2. The fraction of open cells also affects permeability of gases. The effects of permeability will be considered in the appropriate sections to follow. 

However, a downcomer is more complicated than a chemical reactor. Indeed, the gas bubbles only disappear on the flee smface of the foam. The downcomer then behaves like an inverted decanter in which the discontinuous phase gathers in the upper part. According to the decanter theory, the downcomer must provide the foam with a sufficient decantation surface Ap where ... 

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