Dynamic foams

Figure 10. Comparison of the critical-capillary-pressure data of Khatib, Hirasaki and Falls (5) (darkened circles) to the proposed dynamic foam stability theory (solid line). Best fitting parameters for the constant-charge electrostatic model are listed.

For transporting foam, the critical capillary pressure is reduced as lamellae thin under the influence of both capillary suction and stretching by the pore walls. For a given gas superficial velocity, foam cannot exist if the capillary pressure and the pore-body to pore-throat radii ratio exceed a critical value. The dynamic foam stability theory introduced here proves to be in good agreement with direct measurements of the critical capillary pressure in high permeability sandpacks. 

The first method is quite difficult to reproduce due to the strong influence on the results that small contaminations or vibrations can have. The latter two are also difficult to reproduce since the foam generation and collapse is not always uniform, yet these methods are very commonly used. The dynamic foam tests are most suitable for evanescent foams since their lifetimes are transient. For more stable foams the static foam tests are more commonly used. 

Figure 2.17 Illustration of a dynamic foam stability test apparatus in which foam is generated by flowing gas through a porous diffuser. Not drawn to scale.

The foaming capability and foam stability obtained from sparkling wines is usually tested by a dynamic foam stability method, as discussed in Section 2.6.2. Because these foams are evanescent and not really very stable, at least compared with the foams found in other industries, dynamic rather than static foam tests are the most suitable. In one version of the dynamic foam test, the Mosalux method, the foam heights are automatically measured using infrared beams and sensors [848],... 

A measure of the persistence of a foam (the time an average bubble exists before bursting). Ideally independent of the apparatus and procedure used, and characteristic of the foaming solution being tested. See also Dynamic Foam Test. 

Any of several methods for assessing foam stability in which one measures the rate of collapse of a (static) column of foam. See also Dynamic Foam Test, Foaminess. 

More conclusive data about the coalescence kinetics are obtained for dynamic foams where the surface coalescence appears to be the main process (see Chapter 7). 

Dynamic foams are these in which a state of dynamic equilibrium between the rate of foam formation and the rate of foam collapse is reached. The behaviour of such foams as well as the reasons for their stability has been considered in many monographs [e.g. 13,25,39-41,113,114], An analysis of the stability of dynamic foams based on the properties of foam films is presented here. 

According to [116] Hmax IQ remains constant under a dynamic foaming regime when Reynolds number Re < 100 (such a tube diameter is accepted as a characteristic size). When 250 > Re > 100, Hmax/Q first increases and then decreases. Similar dependences have been observed by Pattle [117],... 

As a stability characteristic of dynamic foams Bikerman has proposed the average time of gas retention in the foam (see Eq. (7.19)). In order to determine the foam stability by Bikerman s method along with cylindrical vessels (A = const), conical shaped foam vessels (a... 

The surface elasticity force is considered as the most important factor of stability of steady-state foams [113]. In the model of Malysa [123] it is assumed that a dynamic foam is a non-equilibrium system and phenomena occurring in the solution have an influence on the formation and stability of the foam. The foam collapse takes place only at the top of the foam bubbles at thickness larger than 100 nm, where fl = 0. So, the lifetime of the bubbles at the... 

Results of the Dynamic Foam Tests. Typical plots of pressure drop vs. time for the dynamic foam tests are given in Figure 3. The initial sharp rise in... 

Dynamic foam tests and the displacement tests are needed to complete the screening of the candidate surfactants. Good correspondence was obtained between the two tests. 

Experimental Assessment of Foam Stability. Usually foam stability has been tested by one of three methods (4, 6, 13) (1) the lifetime of single bubbles (2) the steady-state (dynamic) foam volume under given conditions of gas flow, shaking, or shearing or (3) the rate of collapse of a (static) column of foam generated as described. 

In a typical dynamic foam test, foam is generated by flowing gas through a porous orifice into a test solution, as shown in Figure 15. The steady-state foam volume maintained under constant gas flow into the column is then measured. There are many variations of this kind of test (4, 40), including an ASTM standard test (41). This technique is frequently used to assess the stability of evanescent foams. 

Those factors that were previously mentioned that produce finer-textured foams also produce more stable foams. Factors such as surfactant type, concentration, increasing pressure, and higher inputs of mechanical energy generate more stable foams. For higher temperatures such as those that exist downhole, dynamic foam stability relies upon surfactant type and concentration rather than the addition of thickeners (polymer stabilizers). It is not known what rates are necessary to maintain dynamic stability in fractures, or whether those conditions typically exist. 

Steady-state flow is never achieved with foamed fluids rather, the flow is dynamic. Foams flow dynamically because the pressure, which is continually changing, affects the viscosity, flow rate, and density of the foam at any given interval in the tubular. This problem can be accounted for by numerically integrating the mechanical energy balance equation from bottomhole to surface conditions. 

---