Foam control mechanisms

Although silicone oils by themselves or hydrophobic particles (e.g., specially treated silica) are effective antifoams, combinations of silicone oils with hydrophobic silica particles are most effective and commonly used. The mechanism of film destruction has been studied with the use of surface and interfacial tensions, measurements, contact angles, oil-spreading rates, and globule-entering characteristics for PDMS-based antifoams in a variety of surfactant solutions.490 A very recent study of the effect of surfactant composition and structure on foam-control performance has been reported.380 The science and technology of silicone antifoams have recently been reviewed.491... 

The objective is to reduce volatiles to below 50-100-ppm levels. In most devolatilization equipment, the solution is exposed to a vacuum, the level of which sets the thermodynamic upper limit of separation. The vacuum is generally high enough to superheat the solution and foam it. Foaming is essentially a boiling mechanism. In this case, the mechanism involves a series of steps creation of a vapor phase by nucleation, bubble growth, bubble coalescence and breakup, and bubble rupture. At a very low concentration of volatiles, foaming may not take place, and removal of volatiles would proceed via a diffusion-controlled mechanism to a liquid-vapor macroscopic interface enhanced by laminar flow-induced repeated surface renewals, which can also cause entrapment of vapor bubbles. 

A study of the effect of pore geometry on foam formation mechanisms shows that snap-off" bubble formation is dominant in highly heterogeneous pore systems. The morphology of the foams formed by the two mechanisms are quite different. A comparison of two foam injection schemes, simultaneous gas/surfactant solution injection (SI) and alternate gas/surfactant solution injection (GDS), shows that the SI scheme is more efficient at controlling gas mobility on a micro-scale during a foam flood. 

Extensive mobility control applications of foams are limited by inadequate knowledge of foam displacement in porous media, plus uncertainties in the control of foam injection. Because of the importance of in situ foam texture (bubble size, bubble size distribution, bubble train length, etc.), conventional fractional flow approaches where the phase mobilities are represented in terms of phase saturations are not sufficient. As yet, an adequate description of foam displacement mechanisms and behavior is lacking, as well as a basis for understanding the various, often contradictory, macroscopic core flood observations. 

Silicones are well known for their versatility, which makes them ideally suitable for a variety of applications. The fluids can be used as solvents, as foam-control systems, or as release agents (20% of the total volume). High-molecular-weight silicones are mainly used in mbber applications such as High Temperature Vulcanisable (HTV) and Room Temperature Vulcanisable (RTV) (43%), resins (4%), or specialties (15%). Other applications for silicones are masonry protection (8%), textiles (7%), and paper coatings (3%). Silicones can be uniquely tailored for each application area by substitution by reactive groups, allowing them to be cured by different mechanisms. 

Over few last decades, applied problems have arisen which are related to the behavior of foams in porous media. It turns out that aqueous surfactant-stabilized foams can drastically reduce the gas mobility in porous media [334], This fact is of great applied significance in petroleum and gas industry. In [106, 189, 233], mechanisms of foam control of gas migration through porous media are presented. 

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. 

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. 

In most cases, these active defoaming components are insoluble in the de-foamer formulation as well as in the foaming media, hut there are cases which fimction hy the cloud-point mechanism (15). These products are soluble at low temperature and precipitate when the temperature is raised. When precipitated, these defoamer-surfactants fimction as defoamers when dissolved, they may act as foam stabilizers. Examples of this type are the block polymers of poly(ethylene oxide) and poly(propylene oxide) and other low HLB (hydrophilic-lipophilic balance) nonionic surfactants. The use of soluble foam control agents has increased in recent years (7). 

Zemeth, Z., Racz, G. and Koezo, K., Foam control by silicone polyethers - mechanism of cloud point antifoamers, J. Colloid Interface ScL, 207, 386-394 (1998). 

Sparging/bubble Mechanical foam control better than column chemical control for mass transfer. 

---