Foam displacement

An important application of foams arises in foam displacement, another means to aid enhanced oil recovery. The effectiveness of various foams in displacing oil from porous media has been studied by Shah and co-workers [237, 238]. The displacement efficiency depends on numerous physicochemical variables such as surfactant chain length and temperature with the surface properties of the foaming solution being an important determinant of performance. 

Reservoir Sandstone (RS) Micromodel. The RS micromodel was used in a variety of experiments examining the effects of surfactant, foam quality, injection scheme and pressure level on foam displacement efficiency and flow patterns. Various gases and brines or surfactant solutions were used, primarily field injection gas and brine, and the surfactant AES (trade name Alipal CD-128) at a concentration of 0.5 wt.%. 

Figure 8. Final Fluid Saturations for Foam Displacement of Brine...

Effect of Injection Scheme on Foam Displacement. If the main interest in using foam is for controlling gas mobility, then it is necessary to have a criterion to judge the effectiveness of a foam... 

Extensive mobility control applications of foams are limited by inadequate knowledge of foam displacement in porous media, plus uncertainties in the control of foam injection. Because of the importance of in situ foam texture (bubble size, bubble size distribution, bubble train length, etc.), conventional fractional flow approaches where the phase mobilities are represented in terms of phase saturations are not sufficient. As yet, an adequate description of foam displacement mechanisms and behavior is lacking, as well as a basis for understanding the various, often contradictory, macroscopic core flood observations. 

Numerous investigators have conducted foam displacement tests in... 

In most foam displacement applications, the gas and liquid phases (the latter containing the foaming agent) are injected simultaneously or intermittently with the foam being formed in situ. In particular, as gas passes through the porous medium in the presence of foaming agent, several pore level events contribute to the formation of the foam bubbles. Such processes are similar to those observed with oil-water systems (16). 

Figure 1. Illustration of foam displacement (a) For bubble 1 to be displaced through constriction E, the bubble train behind it must first advance through constriction C, form a continuous train, and then push bubble 1 through constriction E. (b) The displacement of bubble 2 first requires the advancement of bubble 4 through E, bubble 5 through C, etc., to form a continuous train. Once this train pushes bubble 2 through F, the train momentarily breaks with 3 trapped at F and 5 trapped at E.

Figure 7. Bead pack apparatus for foam displacement tests.

Although the current permeability model properly reflects many of the important features of foam displacement, the authors acknowledge its limitations in several respects. First, the open pore, constricted tube, network model is an oversimplification of true 3-D porous structures. Even though communication was allowed between adjacent pore channels, the dissipation associated with transverse motions was not considered. Further, the actual local displacement events are highly transient with the bubble trains moving in channels considerably more complex than those used here. Also, the foam texture has been taken as fixed the important effects of gas and liquid rates, displacement history, pore structure, and foam stability on in situ foam texture were not considered. Finally, the use of the permeability model for quantitative predictions would require the apriori specification of fc, the fraction of Da channels containing flowing foam, which at present is not possible. Obviously, such limitations and factors must be addressed in future studies if a more complete description of foam flow and displacement is to be realized. 

Recently, use of a surfactant in the injected water such that a foam or emulsion is formed with carbon dioxide has been proposed (20.21) and research is proceeding on finding appropriate surfactants (22-24). The use of such a foam or emulsion offers the possibility of providing mobility control combined with amelioration of the density difference, a combination which should yield improved oil recovery. Laboratory studies at the University of Houston (25) with the same five-spot bead-pack model as used before show that this is so, for both the relatively water-wet and relatively oil-wet condition. We have now simulated, with a finite-difference reservoir process computer program, the laboratory model results under non-WA3, WAG, and foam displacement conditions for both secondary and tertiary recovery processes. This paper presents the results of that work. 

In this chapter, we discuss much of the work accomplished since Fried, but without attempting a complete review. Useful synopses are available in the articles and reports of Hirasaki (2, 3), Marsden (4), Heller and Kuntamukkula (5), Baghidikian and Handy (6), and Rossen (7). Our goals are to present a unified perspective of foam flow in porous media to delineate important pore-level foam generation, coalescence, and transport mechanisms and to propose a readily applicable one-dimensional mechanistic model for transient foam displacement based upon gas-bubble size evolution [i.e., bubble or lamella population-balance (8, 9)]. Because foam microstructure or texture (i.e., the size of individual foam bubbles) has important effects on flow phenomena in porous media, it is mandatory that foam texture be accounted for in understanding foam transport. 

The core is initially completely saturated with aqueous foamer solution with rock adsorption satisfied. Nitrogen and aqueous surfactant solution are then injected at fixed flow rates until steady state is achieved. Transient pressure and aqueous saturation profiles are monitored for a wide range of gas and liquid flows. Only one transient foam displacement is reported here. Additional results are available elsewhere (78-80). 

To model the measured transient foam displacements, equations 2 through 12 are rewritten in standard implicit-pressure, explicit-saturation (IMPES) finite difference form, with upstream weighting of the phase mobilities following standard reservoir simulation practice (10). Iteration of the nonlinear algebraic equations is by Newton s method. The three primitive unknowns are pressure, gas-phase saturation, and bubble density. Four boundary conditions are necessary because the differential mass balances are second order in pressure and first order in saturation and bubble concentration. The outlet pressure and the inlet superficial velocities of gas and liquid are fixed. No foam is injected, so Qh is set to zero in equation... 

Nine additional parameters are demanded to predict foam displace-... 

Only the case of steady coinjection of surfactant solution and gas into a one-dimensional core initially filled with surfactant solution is addressed. Calculated transient foam displacement well represents both the measured wetting liquid saturations and pressure profiles with physically meaningful parameter values. It is predicted and experimentally verified that foam moves in a piston-like fashion through a linear porous medium presaturated with surfactant solution. Moreover, the proposed population-balance predicts the entire spectrum of unique steady foam-flow behavior in the capillary-pressure regime. 

The population-balance is a powerful tool for modeling foam displacement and flow in porous media because it correctly predicts the evolution of foam microstructure from well documented pore-level events, and because it merges with current reservoir simulation practice. Perhaps the main power of the population-balance approach is its general framework. As understanding of mechanistic detail improves, this information may be incorporated in the modeling effort. 

The e q>eriment was conducted under steady-state conditions. The results would thus represent conditions w behind a foam-displacement front where properties ate changing. 

Foams in steam systems can be generated at the low fluid velocities that occur at relatively large distances away from a wellbore. That is, very little mechanical energy is required to develop a foam if a system is properly designed. In a foam displacement, however, the foam requires continued regeneration to maintain high resistance. 

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