Continuous-gas foam

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 3. Schematic of a continuous-gas foam in porous media. Rock grains are hatched gray shading represents gas trapped by stationary lamellae, shown as thick dark lines. A continuous-gas channel is pictured unshaded.

Continuous-gas foams are readily accommodated within the previously mentioned model. During the later stages of gas-only injection, flowing lamellae may collapse and not be regenerated. Thus, some, or all, of the flowing bubble-trains are replaced by continuous gas. Trapped lamellae... 

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). 

Foams can also be produced during primary production because pressure is greater in the reservoir at the locations from which oil is being drained, and lower near and in the well-bore. As oil moves toward a producing well and then into the bottom of the well, the reduced pressure it experiences can cause dissolved gas to be released. When this happens to a light oil, the gas normally separates from the oil. In the case of some heavy oils, however, the gas remains dispersed in the oil as an in situ oil foam [339,698], This is called foamy-oil production, and can be associated with increased primary-oil production compared to what would be expected from non-foamy-oil production. It is thought that the formation of foamy-oil delays the formation of a continuous gas phase (increases the trapped gas saturation) and contributes a natural pressure-maintenance function [339,698],... 

A quantitative dependence of the foam expansion ratio on the surfactant concentration, solution viscosity, surface viscosity and height of the foam column in a continuously generated foam has been reported in [83], Lowering the rate of gas supply led to an increase in... 

Gas mobility depends on the permeability of the porous medium. In the presence of foam gas mobility is the mobility of the continuous gas phase through the free channels and the mobility of the confined gas along with the liquid. Formally the relative permeability of each phase (liquid or gas) can be expressed by Darcy s equation. 

In a third paper by the Bernard and Holm group, visual studies (in a sand-packed capillary tube, 0.25 mm in diameter) and gas tracer measurements were also used to elucidate flow mechanisms ( ). Bubbles were observed to break into smaller bubbles at the exits of constrictions between sand grains (see Capillary Snap-Off, below), and bubbles tended to coalesce in pore spaces as they entered constrictions (see Coalescence, below). It was concluded that liquid moved through the film network between bubbles, that gas moved by a dynamic process of the breakage and formation of films (lamellae) between bubbles, that there were no continuous gas path, and that flow rates were a function of the number and strength of the aqueous films between the bubbles. As in the previous studies (it is important to note), flow measurements were made at low pressures with a steady-state method. Thus, the dispersions studied were true foams (dispersions of a gaseous phase in a liquid phase), and the experimental technique avoided long-lived transient effects, which are produced by nonsteady-state flow and are extremely difficult to interpret. 

Viscous Pressure Drop. For continuous bubble trains with perfectly mobile interfaces moving through a given Dm channel, the dynamic pressure drop in the gas (foam) phase over a single tube segment D follows from Equation 16 ... 

Foam (5) is a collection of gas bubbles with sizes ranging from microscopic to infinite for a continuous gas path. These bubbles are dispersed in a connected liquid phase and separated either by lamellae, thin liquid films, or by liquid slugs. The average bubble density, related to foam texture, most strongly influences gas mobility. Bubbles can be created or divided in pore necks by capillary snap-off, and they can also divide upon entering pore branchings (5). Moreover, the bubbles can coalesce due to instability of lamellae or change size because of diffusion, evaporation, or condensation (5,8). Often, only a fraction of foam flows as some gas flow is blocked by stationary lamellae (4). 

Foaming. This is the formation of a stable gas-in-liquid dispersion, in which the bubbles do not coalesce with each other or with the continuous gas phase. It is a defect commonly encountered in application by a roller, particularly with latex paints. The remedy is the addition of an antifoam agent in the manufacture of the paint, and/or a reduction in the speed of the roller. ... 

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). 

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. 

Pseudoemulsion film thinning probably comes into independent prominence when the system is quite dynamic and where the foam lamellae are thin. Pseudoemulsion film thinning, causing entering and subsequent lamella ruptures, is more likely to be a dominant mechanism when continuous gas injection into surfactant flooded media at residual oil saturation is practiced. 

In this chapter the properties of nonaqueous hydrocarbon foams will be reviewed and the effects of foam formation on flow of oil—gas mixtures in porous media will be discussed A laboratory technique for investigating the role of foamy-oil behavior in solution gas drive is described, and experimental verification of the in situ formation of non-aqueous foams under solution gas drive conditions is presented The experimental results show that the in situ formation of nonaqueous foam retards the formation of a continuous gas phase and dramatically increases the apparent trapped-gas saturation. This condition provides a natural pressure maintenance mechanism and leads to recovery of a much higher fraction of the original oil in place under solution gas drive. 

Results of the experimental study suggest that the formation of an oil-continuous foam may be involved in flow of heavy oil under solution gas drive. Such foam formation can be very beneficial for increasing the oil recovery. It delays the formation of a continuous gas phase, and thereby acts as a natural pressure-maintenance mechanism. In terms of the conventional solution gas drive theory, it serves to greatly increase the apparent trapped-gas saturation. 

These surfactant sticks (often referred to under a variety of common and trade-marked names such as soap sticks ) can be dropped manually or with automatic stick launchers. The gas-releasing capability from so-equipped sticks are not very large this feature is intended for use in stimulating completely dead wells. Alternatively, one can continuously inject foaming surfactant solution through tubing in the annulus. This consumes more surfactant but can lead to a more consistent unloading of the water with less pressure fluctuation. The surfactants that have been used are mostly anionic or anionic/non-ionic blends [53, 60]. 

We first consider a polydisperse foam under mechanical equilibrium where drainage is absent. The upper region of such a foam is shown schematically in Figure 1.13. The continuous gas-liquid surface at y = 0 covers the dome-shaped tops of bubbles and the upper Plateau borders. The Laplace pressure jiunp at the gas-liquid surface of the Plateau borders is the difference between the atmospheric pressure, and... 

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