Steady-state foams,

Here, foam is generated by flowing gas through a porous orifice into a test solution as shown in Figure 2.17. 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 [46,115,116], This technique is frequently used to assess the stability of evanescent foams. 

Comparison between the average lifetime of foam bubbles in a cylindrical and conical vessels for solutions of alkyl glycosides has been carried out by Waltermo et al. [120]. In all cases the results obtained about the lifetimes are close, t = 10-40 s. A more precise characteristic of the stability of steady-state foams has been proposed in [94-97] the retention time (rt). It represents the average time of gas retention in the whole solution+foam system. This characteristic is determined by the slope of the linear segment in the dependence of the total gas volume used in foam formation versus its volumetric rate. 

Krotov [86] has derived an analytical equation involving the maximum height of a steady-state foam column and its lifetime in the case of a border foam . The following expressions are valid for the expansion ratio of a foam layer at height H and for the maximum height //rnax of the steady-state foam column... 

The results presented below from the study of the behaviour of steady-state foams allow to estimate the role of foam films in foam stability. Two types of steady-state foam have been studied 1) wet steady-state foams from aqueous solutions of low surface active surfactants, e.g. normal alcohols [96] and 2) dry steady-state foams [121] from aqueous solutions of micellar surfactants, e.g. NaDoS, in the presence of electrolyte at different concentrations (ensuring different types of foam films). The device employed in this study represents a glass column (of inner diameter 3.4 cm) with a sintered glass filter as a bottom [94-96,121]. The gas volume passing through the column was measured by a rheometer. The total gas volume both in the foam and in the solution was measured when a steady-state was reached, i.e. when the system volume ceases changing. Usually the total gas volume V c as well as the gas rate vc were measured. 

In wet steady-state foams the bubble coalescence in the foam volume can be neglected. Then, it can be assumed that film rupture only occurs at the upper foam layers. In the other foam layers the average number of bubbles remains constant, the volume of the liquid outflow is equal to the liquid volume entering with the bubbles. 

The study of wet steady-state foams has shown that the foam films at the upper layers rupture at very large thicknesses, i.e. before reaching thicknesses at which specific thermodynamic properties begin to appear [96]. Under these conditions the properties of wet steady-state foams are determined mainly by the effects of Marangoni and Gibbs, which stabilise kinetically the whole system [94-97,116,121,122]. 

The dry steady-state foams can be considered the opposite of wet steady-state foams. A difference in the behaviour of steady-state foams from aqueous NaDoS solutions has been observed, depending on the electrolyte content (corresponding to the formation of... 

Fig. 7.22. rt vs. surfactant concentration for NaDoS steady-state foams. 

The difference found in the behaviour of steady-state foams from NaDoS solutions in the presence of various electrolyte concentration reflects the importance of foam films, which can be formed also in such systems. The existence of different types of foam films in the steady-state foams is proved by their destruction by a-particle irradiation [121], Fig. 7.23 shows the dependence of the foam column height on the electrolyte concentration. It is seen that at NaCl concentration higher than 0.35 mol dm 3, H does not change. This concentration is very close to the electrolyte concentration at which there occurs a transition from one foam film type to another. 

Fig. 7.23. Foam column height H of a NaDoS steady-state foam vs. electrolyte concentration under a-...

The analysis presented of the steady-state foam behaviour shows a trend for the further studies which would enable a quantitative description of steady-state foams. 

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... 

The explanation of the change in the stability of steady-state foams in the homologous series of fatty alcohols and acids is based on that correlation. A detailed discussion of the stability and the related to it other properties of steady-state foams can be found in [113,123]. 

The experimental determination of the least residual concentration in the process of foam accumulation was conducted in a separation column of 2 cm diameter [84]. The column height was 35 cm, the volume of the foaming agent solution was 20-25 cm3, and a sintered glass filter (POR-100), was used to disperse the gas. The volumetric rate of gas feed was 10015 cm3 min 1. The process of surfactant extraction begins with formation of a steady-state foam layer that has a constant volume V. It depends on the rate of gas feed when the rate of foam formation equals the rate of its decay. The average lifetime of the gas in the foam is... 

We point out that in steady-state foam (typical of closed but nonisolated systems) the distribution function is Gaussian [273]. 

Second, after foam flooding cores, Bernard et al. (32) flushed with water or brine to estimate trapped-gas saturation. They assumed that water or brine filled the pore space through which gas flowed but did not substantially alter the fraction of gas trapped. Their trapped saturations ranged from 10 to 70% depending upon the surfactant type and the presence of oil in the porous medium during the foam flood. Such measured saturations apply only to trapped gas following a waterflood, and not to dynamic or steady-state foam flooding. 

Figure 10. Comparison of foam lamella rupture pressure and bead pack pressure gradient during steady-state foam flow. Upward directed arrows indicate that the actual rupture pressure is greater than the value indicated...

Our length-effect observations agree with a few observations in the early foam literature (35) that blockage or excessive pressure gradients was obtained in cores of more than 50- to 100-cm length. Steady-state foam... 

The characteristic conditions for generating gas-blocking foam are thus summarized by Table III, and are compared with those from the literature for achieving steady-state foam flow. 

In the presence of oil at high temperature, it is harder to obtain accurate saturation data, but three experiments on 0.5% Fluowet OTN in sea water with crude oil at 70 °C in 8-darcy beads gave Sw between 0.04 and 0.08 (21). Comparable residual nonwetting-phase saturations for the same 8-darcy medium in the absence of foam were 0.13 for the residual water saturation to gas, Swrg, in two gas floods and an average of 0.11 for residual water saturation to oil, Swro, in 24 oil floods with a standard deviation of 0.02 (20). Values of Sw below irreducible were also observed by other investigators (39, 40) in studying foams at comparable conditions. In contrast, steady-state foam flow is consistently associated with 5W values above connate (2—4,18, 34). 

Three prevalent, steady state foam models, all based on modification of the gas phase relative permeability, are reviewed. Two supplemental correlations were derived in order to account for the effect of ambient pressure on foam performance. Thus the reduced mobility of the gas phase, when foam is present, can be... 

Calculated mobility reduction factors are also shown in Figure 6. For the 60% foam quality case an experimental baseline pressure drop was not available, so we used the results of the modelling work described in a later section to estimate the pressure drop expected for gas/brine flow at the appropriate fractional flow. Since the 95% quality foam flood did not reach steady state foam flow conditions, and since the pressure drops in individual sections of the long core were influenced by the pressure drops due to foam flowing in downstream sections of the core, we cannot make exact comparison between the MRFs generated in the long core with... 

The final foam experiments were conducted at 95% quality. In this case reasonably stable pressure drops were achieved across the first two segments of ihe core (Tl 234 kPa and T2 91 kPa) and increasing pressure drops were measured across the remaining segments of the core (ca. 4.8 MPa). Under these conditions, foam was apparently formed, but steady state foam flow across the full core was not achieved. 

Three foam models were investigated in the course of this project. All three models relied on modifying the gas relative permeability in the presence of foam. The foam model by Vassenden and Holt [24 was the most versatile platform to match steady state foam results at various frontal advance rates and foam qualities. With this steady state foam model, it was possible to history match the foaming behaviour investigated on the long and short cores. 

Dynamic Foam Test. Any of several methods for assessing foam stability in which one measures the steady-state foam volume generated under given conditions of gas flow, and shearing or shaking. See also Foaminess. 

The lifetime of bubbles in a foam column can be defined as a significant physical parameter. Bikerman proposed the ratio of steady-state foam volume to gas flow rate as a unit of foaminess, denoted X- A glass frit is used to generate the bubbles in a jacketed glass column [20]. The steady state of... 

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