Lamella leave-behind

Lamella Division A mechanism for foam lamella generation in porous media. Typically, when a foam lamella reaches a branch point in a flow channel, then the lamella may divide into two lamellae (bubbles) rather than simply follow one of the two available pathways. See also Lamella Leave-Behind, Snap-Off. 

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. 

Snap-Off A mechanism for foam lamella generation in porous media. When gas enters and passes through a constriction in a pore, a capillary pressure gradient is created and causes liquid to flow toward the region of the constriction, where it accumulates and may cause the gas to pinch-off or snap-off to create a new gas bubble separated from the original gas by a liquid lamella. See also Lamella Division, Lamella Leave-Behind. 

For this reason, lamellae created by leave-behind offer considerably less resistance to flow. A second important feature of this mechanism is that each site can form only a single lamella, unless the pore body is refilled by the wetting fluid once the first lamella ruptures or leaves the site. 

Figure 2. Schematic of lamella formation by the "leave-behind" mechanism. (Reproduced with permission from Ref. 40. Copyright I986 SPE-AIME.)...

A key factor in the commercialization of surfactant-based mobility control will be the ability to create and control dispersions at distances far from the injection well (TJ ). Capillary snap-off is often considered to be the most important mechanism for dispersion formation, because it is the only mechanism that can form dispersions directly when none are present (39,40). The only alternative to snap-off is either leave-behind, or else injection of a dispersion, followed by adequate rates of thread breakup and division to maintain the injected lamellae. 

Effects of Capillary Number, Capillary Pressure, and the Porous Medium. Since the mechanisms of leave-behind, snap-off, lamella division and coalescence have been observed in several types of porous media, it may be supposed that they all play roles in the various combinations of oil-bearing rocks and types of dispersion-based mobility control (35,37,39-41). However, the relative importance of these mechanisms depends on the porous medium and other physico-chemical conditions. Hence, it is important to understand quantitatively how the various mechanisms depend on capillary number, capillary pressure, interfacial properties, and other parameters. 

Ransohoff and Radke found that for C <10 lamella generation occurred only by the leave-behind mechanism. The lamellae moderately increased the resistance to flow, raising it by about a factor of five (40). (Here p is the viscosity of the nonwetting phase, U is the total superficial velocity, R is the bead radius, K is the absolute permeability, k is the relative permeability, L is length, and Y is the interSacial tension.) The use of this definition of the capillary number, instead of the usual C = jU/y, with J the viscosity of the wetting phase, was justified in the heoretical analysis ( ). 

For capillary numbers greater than the critical value, snap-off occurred even in homogenous bead packs (40). The resulting dispersions caused much greater resistance to flow than the resistance produced by leave-behind lamellae (which do not disperse the nonwetting phase). 

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). 

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-... 

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