Foam-forming surfactant adsorption

Although many factors, such as film thickness and adsorption behaviour, have to be taken into account, the ability of a surfactant to reduce surface tension and contribute to surface elasticity are among the most important features of foam stabilization (see Section 5.4.2). The relation between Marangoni surface elasticity and foam stability [201,204,305,443] partially explains why some surfactants will act to promote foaming while others reduce foam stability (foam breakers or defoamers), and still others prevent foam formation in the first place (foam preventatives, foam inhibitors). Continued research into the dynamic physical properties of thin-liquid films and bubble surfaces is necessary to more fully understand foaming behaviour. Schramm et al. [306] discuss some of the factors that must be considered in the selection of practical foam-forming surfactants for industrial processes. 

Mannhardt, K. Novosad, J.J. Adsorption of Foam-Forming Surfactants for Hydrocarbon-Miscible Flooding at High Salinities in Foams, Fundamentals and Applications in the Petroleum Industry, Schramm, L.L. (Ed.), American Chemical Society Washington, 1994, pp. 259-316. 

Novosad, J.J. On Minimizing Adsorption of Foam-Forming Surfactants in Porous Media in Chemicals in the Oil Industry Developments and Applications, Ogden, P.H. (Ed.), Royal Society of Chemistry Cambridge, 1991, pp. 159-173. 

Adsorption of Foam-Forming Surfactants for Hydrocarbon-Miscible Flooding at High Salinities... 

This section has demonstrated that some commercially available surfactants are soluble in brines of extreme salinity and hardness and also form effective mobility control foams under these conditions. The remainder of this chapter is devoted to the development of a better understanding of the adsorption properties of foam-forming surfactants, mainly those for high-salinity conditions. It is hoped that this discussion will contribute to the development of a systematic approach for selecting or formulating surfactants with minimal adsorption levels. 

Some anionic foam-forming surfactants do not adsorb appreciably on sandstone from low-salinity (0.5 mass %) brine (7). However, adsorption increases steeply when the salinity is increased to several mass percent. 

Figure 6 shows that adsorption levels of foam-forming surfactants can vary widely from near-zero (measurements numbered 1 and 2) to almost... 

Figure 6. Adsorption levels measured in 96 core-flood experiments with foam-forming surfactants.

Most foam-forming surfactants, particularly those suitable for high-salinity conditions, are very hydrophilic and do not partition into oil, eliminating the first of the listed mechanisms. The amount of surfactant adsorbed at the oil—water interface depends on the surface excess of surfactant at this interface and on the amount of interface present. Although the surface excess of surfactant at the oil—water interface can be estimated from interfacial tension data using the Gibbs adsorption equation, the amount of interface that is present is not easily accessible to measurement. 

This section discusses the adsorption behavior of a number of foamforming surfactants under a wide variety of conditions. Surfactant adsorption data that appear in the petroleum literature are often difficult to compare because they are measured by different methods (batch or coreflood experiments), and experimental conditions may vary widely and are frequently not completely specified. The data discussed in this section, which are taken from references 7—12, 34, and 82 and some unpublished results, constitute the most extensive set of adsorption data for commercial foam-forming surfactants that were measured in a consistent manner by performing core-floods. 

Dependence of Adsorption on Temperature. Figures 11 and 12 show the temperature dependence of adsorption for several foam-forming surfactants on sandstone or unconsolidated sand. Physical adsorption is an exothermic process and is expected to decrease with increasing temperature. This trend is observed for the anionic surfactants (Figures 11a and 12) Adsorption decreases up to an order of magnitude when the temperature is raised from 50 to 150 °C. In contrast, adsorption of the amphoteric surfactants is affected very little by temperature and may even show a slight increase with temperature in some cases (Figure lib). An increase in adsorption with temperature has sometimes been taken as an indication of chemisorption (36). 

Dependence of Adsorption, on Rock Type. Table I shows that gas injection EOR projects are being conducted in sandstone and carbonate pools. Hydrocarbon- and C02-misdble projects are run largely in carbonate reservoirs. With the exception of several studies that report adsorption levels of EOR surfactants on carbonates (4,11,12, 24, 33, 62—64, 86), the petroleum literature has dealt almost exclusively with anionic, and sometimes nonionic, surfactant adsorption on sandstones, because most studies have been carried out with surfactants used in low-tension flooding. These surfactants are not considered suitable for application in carbonate reservoirs because of their low salinity and hardness tolerance. Foam-forming surfactants suitable for high-salinity environments include amphoteric surfactants (2). The adsorption behavior of this surfactant type has also rarely been studied (10—12, 87, 88). 

Clays are considered detrimental to EOR processes that are based on the injection of chemicals, such as foam-forming surfactants, because clays provide a large amount of surface area for adsorption. Table VII shows a comparison of specific surface areas of some clays (97, 117, 118) and of the solids used in the adsorption experiments of Figure 15 (12, 119, 120). Figure 15 allows comparison of adsorption levels in Berea sandstone, which consists mainly of quartz and 6-8% clays, with adsorption on clean quartz sand. 

The potential of surfactant mixtures in lowering adsorption has been discussed in the literature (129—134). However, experimental work with surfactant mixtures has not advanced sufficiently to establish predictive capabilities for the formulation of surfactant mixtures with low adsorption levels. Two examples of experimental results obtained with commercially available foam-forming surfactants are shown next to illustrate the concepts. 

The second example involves a mixture of two different types of commercial foam-forming surfactants anionic and amphoteric (7). Unlike the mixture of the previous example, an anionic—amphoteric surfactant mixture probably does not follow ideal mixed micelle behavior (138). The results of three core-floods, performed separately with each surfactant and with a mixture of the two surfactants, are summarized as follows. The anionic surfactant adsorbs negligibly when used either by itself or when mixed with the betaine (at least at the low salinity used in these particular core-floods). Betaine adsorption is lowered by about an order of magnitude by mixing it with the anionic surfactant, from 1.7 down to 0.2 mg/g. 

---