Macroemulsion flooding

Macroemulsion and Microemulsion Flooding If a suitable surfactant is injected into the reservoir, it can form macroemulsions and/or microemulsions with the reservoir oil depending on the composition and reservoir conditions. Several articles have been published on the recovery of oil by microemulsion and macroemulsion flooding processes.Among various factors, the most important factor of surfactant flooding in the form of an emulsion is the lowering of the interfacial tension (IFT) at the oil/water interface. Microemulsions are more effective in oil displacement as compared to macroemulsions because microemulsions can provide low IFT systems. 

The physicochemical aspects of micro- and macroemulsions have been discussed in relation to enhanced oil recovery processes. The interfacial parameters (e.g. interfacial tension, interfacial viscosity, interfacial charge, contact angle, etc.) responsible for enhanced oil recovery by chemical flooding are described. In oil/brine/surfactant/alcohol systems, a middle phase microemulsion in equilibrium with excess oil and brine forms in a narrow salinity range. The salinity at which equal volumes of brine and oil are solubilized in the middel phase microemulsion is termed as the optimal salinity. The optimal salinity of the system can be shifted to a desired value hy varying the concentration and structure of alcohol. 

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. 

Emulsions formed in crude oil and bitumen during extraction operations are usually water-in-oil (W/O) macroemulsions (>0.1 to 100 om in diameter). Macroemulsions are kinetically stable, unlike microe-mulsions, which are thermodynamically stable. In conventional oil recovery (high-energy process), the crude is often in contact with formation water or injection water, as in secondary recovery. In tertiary or enhanced oil recovery, surfactants are used purposely in water floods to make microemulsions for enhancing the flowability of the crude. Crude-oil macroemulsions are produced when two immiscible liquid phases such as oil and water are mixed via the input of mechanical or thermal energy into the processes. Conventional crudes held under high pressures and temperatures amidst... 

Emulsions are commonly produced at the wellhead during primary (natural pressure driven) and secondary (water-flood driven) oil production. For these processes the emulsification has not usually been attributed to formation in reservoirs, but rather to formation in, or at the face of, the wellbore itself [154]. However, at least in the case of heavy oil production, laboratory [162] and field [156,157] results suggest that W/O emulsions can be formed in the reservoir itself during water and steam-flooding. Macroemulsions, as opposed to microemulsions, can be injected or produced in situ in order to either for blocking and diverting [158,159], or for improved mobility control [160]. 

In summary, various phenomena occurring at an optimal salinity in relation to enhanced oil recovery by macroemulsion and microemulsion flooding are schematically shown in Figure 6. It has been demonstrated that a maximum in oil recovery correlates well with several equilibrium and transient properties of surfactant flooding in the form of macroemulsion and microemulsion systems. Results have shown that a maximum in oil recovery, a minimum in surfactant adsorption, a minimum in apparent viscosity of the emulsion, a minimum in phase separation time, an equal solubilization of oil/brine phases in middle phase microemulsion, and a minimum in interfacial tension occur at an optimal salinity of the system. 

The chemical flooding systems discussed in this section are without alcohol present as cosnrfactant. As pointed ont by Sanz and Pope [J 7], a major difficulty was to preclude gels, liquid crystals, macroemulsions, and precipitates along the compositional path during a chemical flood if cosurfactants/alcohols are not part of the chemical formnlation. Phase trapping and blockage of the porous medium must be avoided. 

Three-Phase Displacement. Provided that the alcohol free surfactant system shows a classic phase behavior, II( —) -> III II( -f) by increasing the salinity without forming gels or stable macroemulsions during the phase transitions, the chemical flood can be performed as a three-phase flood using a negative salt gradient. 

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