Mobility Foam displacement

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. 

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. 

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... 

For cyclic steam-foam injection, it is important that the foam breaks down in the presence of oil or after prolonged exposure to high temperature. In this way, the resistance to the flow of production fluid will not be substantially increased. A concern with a cyclic foam injection process is that the low mobility foam will displace oil further (as compared to steam-only injection) from the well during the injection portion of the cycle. The oil will then have a greater distance to flow to the well during production. Thus, initial water cuts may actually increase in cyclic steam-foam tests (26). The oil recovery may also be low initially but then increase to a level higher than that obtained from a steam-only cycle. 

These tests show that CC -foam is not equally effective in all porous media, and that the relative reduction of mobility caused by foam is much greater in the higher permeability rock. It seems that in more permeable sections of a heterogeneous rock, C02-foam acts like a more viscous liquid than it does in the less permeable sections. Also, we presume that the reduction of relative mobility is caused by an increased population of lamellae in the porous medium. The exact mechanism of the foam flow cannot be discussed further at this point due to the limitation of the current experimental set-up. Although the quantitative exploration of this effect cannot be considered complete on the basis of these tests alone, they are sufficient to raise two important, practical points. One is the hope that by this mechanism, displacement in heterogeneous rocks can be rendered even more uniform than could be expected by the decrease in mobility ratio alone. The second point is that because the effect is very non-linear, the magnitude of the ratio of relative mobility in different rocks cannot be expected to remain the same at all conditions. Further experiments of this type are therefore especially important in order to define the numerical bounds of the effect. 

Achievement of low mobility ratios at the fronts between displacing and displaced fluids is of even greater concern in enhanced oil recovery than in waterflooding owing to the high costs and/or low viscosities of the injected fluids. One response to this concern has been the continuing effort to develop a fundamental understanding of so-called foam flow, which employs aqueous solutions of properly chosen surfactants at relatively low capillary numbers to reduce the effective mobility of low viscosity fluids (see 5,6 and papers on foam flow in this volume). 

In the present paper, pore level descriptions of bubble and bubble train displacement in simple constricted geometries are used in developing mobility expressions for foam flow in porous media. Such expressions provide a basis for understanding many of the previous core flood observations and for evaluating the importance of foam texture and interfacial mobility. Inclusion of the effects of pore constrictions represents an extension of the earlier efforts of Hirasaki and Lawson (1). 

As a final point in this section, we should mention that as the bubble trains advance through the constricted channels, the capillary resistance will assume its maximum value (the mobilization pressure) only when the lamellae in the train assume their most unfavorable positions with respect to displacement. At other times, the capillary resistance will be below this maximum value with the result that the actual work required to maintain foam flow at a given rate will be below that which would be required if the mobilization pressure was operative at all times. This is easily understood if one pushes a bubble through a single constriction in a tube and notes that the pressure in the train builds up to the mobilization pressure as the drainage surface advances into the constriction and then rapidly falls as the front bubble experiences a Haines jump. To account for such effects in the present model, the Km term in Equation 63 would vary with time as the bubble train moved through the constricted channels. 

The surfactant has two important roles in CO2 foam. First, it increases the apparent viscosity of CO2 so that brine and oil are displaced in a stable manner. Second, the surfactant lowers the interfacial tension between CO2 and brine which promotes brine displacement. Reducing the brine saturation below S c allows bulk-phase CO2 to completely access the oil-filled pore network. A high-saturation brine bank also retards CO2 mobility by relative permeability effects. The brine bank carries surfactant and allows oil reconnection and mobilization ahead of the bulk CO2 phase because of the favorable partitioning of CO2 from brine into oil. 

The differences between the miscible CO2 foam process and a stable tertiary miscible solvent process are shown in Figures 2 and 3. In the miscible CO2 foam process, oil mobilization occurs as CO2 partitions into and swells the trapped oil above Sorw allowing it to be displaced by the mobile brine. The carbonated brine in turn is displaced by CO2 foam. In comparison, miscible N2 and LPG do not transfer to oil through solution in the water phase, as CO2 does. Instead of a brine bank, the solvent and oil are separated by a miscible dispersion zone. The brine saturation is not reduced below Swc ... 

It is also interesting to observe that the slope of the fitted lines (that is, the dependence of mobility on overall flow rate) decreases as the surfactant fraction increases. A possible explanation is that the lamellae formed in the pore space between the CO2 and surfactant mixture become more durable as the aqueous fraction is increased. From a macroscopic viewpoint, more uniform displacement would be expected as a result of the decreased mobility. Furthermore, greater scattering of data is observed for the CO2 fraction of 81.1 1.0%. It is possible that this particular mixture may be more thermodynamically unstable than "foams of different quality. 

Tertiary oil was increased up to 41% over conventional CO2 recovery by means of mobility control where a carefully selected surfactant structure was used to form an in situ foam. Linear flow oil displacement tests were performed for both miscible and immiscible floods. Mobility control was achieved without detracting from the C02-oil interaction that enhances recovery. Surfactant selection is critical in maximizing performance. Several tests were combined for surfactant screening, included were foam tests, dynamic flow tests through a porous bed pack and oil displacement tests. Ethoxylated aliphatic alcohols, their sulfate derivatives and ethylene oxide - propylene oxide copolymers were the best performers in oil reservoir brines. One sulfonate surfactant also proved to be effective especially in low salinity injection fluid. 

Table II shows the effect of chain length compatibility on oil recovery, fluid displacement efficiency, breakthrough time and effective gas mobility in porous media. For gas/liquid systems (e.g. Foams), a maximum in fluid displacement efficiency, a...

Mobility control is one of the most important concepts in any enhanced oil recovery process. It can be achieved throngh injection of chemicals to change displacing fluid viscosity or to preferentially rednce specific flnid relative permeability through injection of foams, or even through injection of chemicals, to modify wettability. This chapter does not address a specific mobility control process. Instead, it discusses the general concept of the mobility control requirement in enhanced oil recovery (EOR). 

Proteins are considered to adsorb at the interface, partially unfold and interact to form molecular "gel-like" networks (Wilde et ak, 2004). Collapse of emulsions or foams involves the stretching of the interface and the elasticity of the protein structure is supposed to oppose this effect. For mobile surfactants or lipids stretching of the interface will lead to concentration gradients and rapid diffusion of the molecules to restore the status quo (Wilde et al., 2004). A source of instability for most foods is the presence of both proteins and small mobile molecules at the interface. The incompatibility of the two mechanisms means that mixed interfaces are less stable than interfaces containing pure protein or pure surfactant or lipid (Wilde et al., 2004). If there is sufficient surfactant or lipid present then they will eventually displace the protein. It is the structures formed during the battle for control of the interface that gives rise to instability. 

Flow resistance of foams is much higher than that of the liquid or gas phases alone. This characteristic is utilized for foam-based mobility control in enhanced oil recovery (41—46). The gas mobility in porous medium and the dilatational modulus of a-olefin sulfonates are plotted in Figure 12 (22). The displacement efficiency of the foam in the porous medium is higher if the blocking efficiency of the foam is higher. In the systems... 

The use of surfactant-stabilized foams to counteract these kinds of problems was suggested several decades ago (7, 2) and has recently become actively pursued in laboratory and field tests (3—8). The use of foam is advantageous compared with the use of a simple fluid of the same nominal mobility because the foam, which has an apparent viscosity greater than the displacing medium, lowers the gas mobility in the swept or higher permeability parts of the formation. This lowered gas mobility diverts at least some of the displacing medium into other parts of the formation that were previously unswept or underswept. From these underswept areas, the additional oil is recovered. Because foam mobility is reduced disproportionately more in higher permeability zones, improvement in both vertical and horizontal sweep efficiency can be achieved. 

When C02 foam is used for mobility control, only uneconomic amounts of oil would be expected to remain in the pore space behind a matched-mobility stabilized front. Because of the well-to-well nature of the displacement, however, and because of the presence of reservoir heterogeneities, all segments of the front will not arrive simultaneously at the producers. Consequently, foam will not prevent the breakthrough of the injection fluid before complete recovery. However, the use of C02 foam should increase the recovery that will be obtained from the field before its abandonment is mandated by the rising gas and water cut. 

Steam-based processes in heavy oil reservoirs that are not stabilized by gravity have poor vertical and areal conformance, because gases are more mobile within the pore space than liquids, and steam tends to override or channel through oil in a formation. The steam-foam process, which consists of adding surfactant with or without noncondensible gas to the injected steam, was developed to improve the sweep efficiency of steam drive and cyclic steam processes. The foam-forming components that are injected with the steam stabilize the liquid lamellae and cause some of the steam to exist as a discontinuous phase. The steam mobility (gas relative permeability) is thereby reduced, and the result is in an increased pressure gradient in the steam-swept region, to divert steam to the unheated interval and displace the heated oil better. This chapter discusses the laboratory and field considerations that affect the efficient application of foam. 

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