Oil-ganglia

In the next run, a core pack was saturated with 8.6 cp (at 50° C) Ranger-zone crude oil and water flooded to residual oil saturation. Polymer flood was then initiated and about 1.2% of the original oil in place (OOIP) was recovered. The results are shown in Figure 4. The pressure profiles show behavior essentially similar to the previous run except that the pressure drop across the core increased to 100 psi within 4 PV of injection of polymer. The steady state values of pH and viscosity were 7.0 and 0.7 cp. respectively. The oil ganglia retained in larger pores resisting displacement probably reduced the amount of polymer adsorbed and reduced the number of pores that the polymer molecules needed to seal off in order to block the core. This could explain the more rapid plugging of the core. Effluent pH and viscosities remained much lower than influent values. 

Figure 11.6 Illustration offlowing oil ganglia, in water-wet porous media, coalescing to form a continuous oil bank.

The net rate of bubble generation, H, describes redistribution of mass in bubble-bubble interactions. Thus, H is a nonlinear functional of F(x,m,t) and Equations (2) and (3) are a pair of coupled, nonlinear, integro-differential equations in the bubble number density, similar to Boltzmann s equation in the kinetic theory of gases (26,27) or to Payatakes et al (22) equations of oil ganglia dynamics. 

DISPLACED OIL GANGLIA MUST COALESCE TO FORM A CONTINUOUS OIL BANK -. FOR THIS A VERY LOW INTERFACIAL VISCOSITY IS DESIRABLE... 

Figure 2. Schematic presentation of the role of coalescence of oil ganglia in the formation of the oil bank.

Transient Processes. There are several transient processes such as formation and coalescence of oil drops as well as their flow through porous media, that are likely to occur during the flooding process. Figure 12 shows the coalescence or phase separation time for hand-shaken and sonicated macroemulsions as a function of salinity. It is evident ithat a minimum in phase separation time or the fastest coalescence rate occurs at the optimal salinity (53). The rapid coalescence could contribute significantly to the formation of an oil bank from the mobilized oil ganglia. This also suggests that at the optimal salinity of the system, the interfacial viscosity must be very low to promote the rapid coalescence. 

In oil recovery processes, the formation of an oil bank is very important for an efficient oil displacement process in porous media. This was established from studies on the injection of an artificial oil bank followed by the surfactant formulation which can produce ultralow interfacial tension with the injected oil. We observed that the oil recovery increased considerably and the residual oil saturation decreased with the injection of an oil bank as compared to the same studies carried out in the absence of an injected oil bank (54). Figure 17 schematically represents the oil bank formation and its propagation in porous media, which is analogous to the snowball effect. If an early oil bank is formed then it moves through the porous medium accumulating additional oil ganglia resulting in an excellent oil recovery, whereas a late oil bank formation will result in a poor oil recovery. 

After water flooding, residual oil is believed to be in the form of discontinuous oil ganglia trapped in the pores of rocks in the reservoir. The two major forces acting on an oil ganglion are viscous forces and capillary forces, the ratio of which is represented by the capillary number. At the end of the secondary oil recovery stage, the capillary number is around 10 . To recover additional oil, the capillary number has to be increased to around 10" —10, which can be achieved by decreasing the interfacial tension at the oil/brine interface. Surfactants are used for this purpose. 

Figure 1 Three-dimensional view of a petroleum reservoir and the displacement of oil by water or surfactant solutions. Prior to EOR, oil in the reservoir is trapped within the rock in the form of oil ganglia (A). Injection of a surfactant solution mobilizes the oil ganglia (B) and forms an oil bank. The oil bank approaches (C) and subsequently reaches (D) the production wells.

In the high surfactant concentration systems, a middle-phase microemulsion forms that is in equilibrium with excess oil and brine. The basic components of this microemulsion are surfactant, water, oil, alcohol, and salt. High surfactant concentrations in the injected plug result in a relatively small pore volume (about 3-20%) compared to micellar solutions (15-60%). Rgure 1 schematically shows a three-dimensional view of a petroleum reservoir. At the end of water flooding, the oil that remains in the reservoir is believed to be in the form of oil ganglia trapped in the pore structure of the rock as shown in Fig. lA. These... 

Figure 2 Two-dimensional view of the surfactant-polymer flooding process. Injection of a surfactant solution to coalesce the oil ganglia is followed by injection of a polymer slug to push the oil to production wells.

Figure 3 Schematic diagram of coalescence of oil ganglia due to low interfacial viscosity during the surfactant-polymer flooding process. Displaced oil ganglia must coalesce to form a continuous oil bank. For this, a very low interfacial viscosity is necessary.

After secondary oil recovery (water flooding), the oil ganglia are trapped inside the pores by viscous and capillary forces, the magnitude of which can be accounted for by the capillary number, which is defined as follows ... 

Surface charge density is another parameter that can strongly and favourably affect the displacement efficiency of oil ganglia by changing the interfacial tension, surface viscosity and electrical repulsion of the ganglia entrapped in the porous media (Figure 11.14) (6,21,22, 33). 

Figure 7 illustrates the propagation of the oil bank and its subsequent coalescence with additional oil ganglia. If ultralow interfacial tension is not maintained at the surfactant slug/oil bank interface, considerable oil would be lost due to entrapment process (6). Hence, an efficient oil recovery process requires the presence of ultralow interfacial tension at the trailing edge of the oil bank. 

In addition to the factors mentioned earlier (e.g., ultralow interfacial tension and coalescence of oil ganglia), mobility control of the oil bank and surfactant slug is an important requirement for a successful oil recovery process. As shown in Figure 8, pol3rmer solutions are used as drive fluids for proper mobility control of the oil recovery process. The dispersion of surfactant, polymer, oil and brine during the flow could lead to emulsion formation and/or phase-separation due to surfactant-polymer incompatibility (7). Efforts should be made to minimize the formation of... 

Fig. 5. The effect of interfacial tension on the movement of oil ganglia through narrow neck of pores. For the movement of the oil ganglia a very low oil/water interfacial tension is desirable 0.001 dynes/cm.

An oil/brine/surfactant/alcohol system often forms a middle phase microemulsion in an appropriate salinity range. The salinity at which the middle phase microemulsion contains an equal volume of oil and brine is defined as the optimal salinity (9). At the optimal salinity, the interfacial tension is in the millidynes/cm range at both oil/microemulsion and microemulsion/brine interfaces, and the oil recovery is maximum (6,9). Moreover, we have shown (10) that at optimal salinity, the coalescence time or phase-separation time is minimum for oil/brine/surfactant/alcohol systems. When these systems are pumped through porous media, a minimum pressure drop or apparent viscosity is observed at the optimal salinity (10). All these phenomena occurring at optimal salinity are summarized in Figure 11. In a recent study, we have also found that the surfactant loss in porous media is minimum at the optimal salinity. Therefore, besides ultralow interfacial tension, a favorable coalescence process for mobilized oil ganglia and the minimum apparent viscosity (or minimum AP) of the oil bank and the minimum surfactant loss are the other factors contributing towards the maximum oil recovery at the optimal salinity. 

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