Mechanisms of Foam Generation in Glass-Bead Packs

Abstract

Summary The fundamental, pore-level mechanisms of foam generation are investigated in monodisperse bead packs. First, direct visual observations identify the following generation mechanisms: lamella leave-behind, gas-bubble snap-off, and lamella division. Then, to ascertain the relative importance of these mechanisms, quantitative experiments are pursued on the role of bead-pack permeability (bead sizes from 0.25 to 1 mm [0.01 to 0.04 in.]), gas-phase velocity (0.001 to 0.8 cm/s [0.0004 to 0.3 in./sec]), gas-phase fractional flow (0.60 to 1.0), permeability variations, and surfactant type [sodium dodecyl benzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), Chevron Chaser SD1000™, and Suntech IV 1035™]. We discover a critical velocity, above which a "strong" foam is generated and below which only "weak" foam is formed. The snap-off mechanism is the primary mechanism responsible for the formation of the strong foam. A simple model, based on the concept of a "germination site," is developed to predict the onset of snap-off at higher gas velocities. New experimental data obtained in the homogeneous glass-bead packs for the critical capillary number necessary to form a strong foam are in excellent agreement with the proposed germination-site model. Introduction Steamflooding is a common thermal technique to recover oil that is too viscous to be displaced by standard waterflooding techniques. Two of the problems associated with steamflooding are gravity override and viscous fingering. Gravity override occurs because the steam is much less dense than the oil that it is displacing, causing the steam to "ride over" the oil bank. Viscous fingering occurs because the displacing steam is much less viscous than the oil phase, causing channels or "fingers" to form. Thus, steam can bypass most of the oil. Fried1 suggested that these mobility problems might be ameliorated by injecting the steam in the form of a foam. Field tests have in fact demonstrated that foam can significantly increase the efficiency of a steam drive.2,3 Foam also holds promise for CO2 flooding and as a general mobility-control fluid. To eliminate confusion over the meaning of "foam" in the context of this paper,4 we define foam in porous media to be a gs dispersed in an interconnected liquid comprised of stable lamellae. This definition is similar to the one presented earlier by Falls et al.5 The important points to the definition are that the gas may exist in either continuous or discontinuous form and that the foam is not a "bulk" foam; in other words, the size of the bubbles is generally on the order of the size of the pore channels. Consequently, bubble interactions with pore walls dominate foam flow behavior in porous media. The success of foam as a displacing fluid is caused in part by its high apparent viscosity when forced to flow through porous media.6 Increased resistance to gas flow caused by the foam reduces the rate at which gravity override occurs and improves the mobility ratio, thus lessening the extent of viscous fingering. It has been shown by Hirasaki and Lawson7 that the flow properties and apparent viscosity of foam in porous media are highly dependent on the texture (i.e., bubble size and bubble-size distribution) of the foam. Foam texture is, in turn, a strong function of the way in which the foam is generated. Therefore, an understanding of the generation step is crucial in predicting the efficiency of a foam drive. Foam flooding is an excellent example of a process with macroscopic or overall properties, such as pressure drop and displacement efficiency, that depend on microscopic or pore-level events that are currently not well understood, such as gas-bubble formation and lamella breakage. For this reason, we attempt to understand the mechanisms and physical processes involved in foam generation at the pore level. First, the mechanisms of foam generation are identified in glass-bead packs. Then the quantitative effects of bead size, gas velocity, and surfactant type on the foam generation process are studied. Experimental Method Two general types of experiments are performed. In the first set, the "visual" experiments, 16-mm movies record the primary mechanisms of foam generation in bead packs. In the second set, the "parametric" experiments, the effects of gas velocity and fractional flow, bead size, and surfactant type on the relative importance of the generation mechanisms are determined. The parametric experiments are further classified as quantitative or qualitative. Results from the quantitative experiments are tested against the theory for the critical-velocity onset of snap-off; results from the qualitative experiments provide insight into the foam-generation process. All experiments are carried out in a transparent plexiglass bead pack of rectangular cross section as illustrated in Fig. 1. The dimensions of the bead-filled space are 6×25×165 mm [0.2×1×6.5 in.]. In a pack of height h=6 mm [0.2 in.] filled with beads on the order of 1 mm [0.04 in.] or smaller, gravity forces are small in relation to capillary forces (i.e., the Bond number, NBo=??gRgh/s, is less than unity). A liquid inlet, a gas inlet, a pressure transducer port, and an exit piece are attached to the bead pack. The pressure transducer (Validyne, Model DP-15) measures the gas-phase pressure drop across the entire pack. Packing consists of plugging the inlet ports, filling the cavity about halfway with the liquid solution, loading and settling the glass beads, and securing the screen and exit piece to ensure a tight pack. Settling is usually aided by a Bransonic (Model 220) ultrasonic bath. Three different sizes of glass beads are used, having nominal diameters of 0.25, 0.5, and 1 mm [0.01, 0.02, and 0.04 in.]. The absolute permeability to water, k, and porosity, f, of three representative bead packs are listed in Table 1. Either pure water or an aqueous surfactant solution is used as the liquid phase in the foam-generation experiments. The surfactants used include SDBS (Sharpe Chemical Co.), SDS (Eastman Kodak Co.), Chaser SD1000, and Suntech IV 1035. The surface tensions of 1 wt% aqueous solutions of the above surfactants are measured using a Rosano surface tensiometer with a Wilhelmy plate. Their values are presented in Table 2.