Gas-Liquid Foams

10-1 FOAM FLOODWG

Gas-liquid foams offer an alternative to polymers for providing mobility control in micellar floods. In addition, and perhaps more importantly, foams can be used as mobility control agents in miscible floods and well treatment and have been both proposed and field tested as mobility control agents in thermal floods. Because most of the properties of foam stem from adding surfactants to an aqueous *use, and the background of these agents was given in Sec. 9-2, 'we devote a section in this chapter to discussing foams for all EOR applications.

Foams are dispersions of gas bubbles in liquids. Such dispersions are normally quite unstable and. Will quickly break in less than a second. But if surfactants are added to the liquid, stability is greatly improved so that some foams can persist indefinitely. Surfactants used as foaming agents have many of the attributes scribed for micellar-polymer flooding. For aqueous foams, it is usually desirable for the surfactant to have a somewhat smaller molecular weiÊ1t to enhance solubility. Fried (1960) and Patton et al. (1981) give extensive lists of that have mobility control potential. Of course, we should that such surfactants also possess many of the undesirable features of the micellar-polymer surfactants, particularly with respect to sensitivity to highly saline brine, temperature, oil type and retention.

10-2 FOAM STABIUTY

The stability of a foam may be understood by viewing the liquid film separating two gas bubbles in cross section, as in Fig. 10-1 (b). The polar head groups of the surfactant are oriented into the interior of the film, and the nonpolar tail toward the bulk gas phase. Except, perhaps, in the case of dense, hydrocarbonlike gases, the tails are not actually in the exterior fluid as shown, but most surfactant molecules are oriented as shown just inside the film boundaries. Figure 10-I (a) shows that the gas - liquid surface tension is a decreasing function of surface adsorption, which is defined as the difference between surface and bulk concentration. Suppose some external force causes the film to thin as shown in Fig. 10-I (b). The film surface area increases locally, causing the surface concentration to decrease and the surface tension to increase. The increase promotes a surface tension difference along the film boundaries and causes the film to regain its original configuration. This restoration is the Gibbs-Marangoni effect (Overbeek, 1972). Clearly, the surface tension at the gas - liquid interfaces plays an important role in film stability. Very Iow surface tensions would not be favorable; fortunately, surface tensions are rarely Iower than 20 mN,'m even with the best foaming surfactants. In the absence of external forces, the film is in a state of equilibrium caused by a balance between the repulsion forces of the electrical layer just inside the film boundary and the attractive van der Wall forces between the molecules in the film (Fig. 10-2). –