Microemulsions capillary numbers

This method is also known as micellar flooding, microemulsion flooding, or low tension water fiooding. The primary effect of the use of surfactants is the lowering of the interfacial tension between the driving fluid and the oil. More formally the capillary number, Ac, is... 

This chapter covers the fundamentals of surfactant flooding, which include microemulsion properties, phase behavior, interfacial tension, capillary desaturation, surfactant adsorption and retention, and relative permeabilities in surfactant flooding. It provides the basic theories for surfactant flooding and presents new concepts and views about capillary number (trapping number), relative permeabilities, two-phase approximation of the microemulsion phase behavior, and interfacial tension. This chapter also presents an experimental study of surfactant flooding in a low-permeability reservoir. 

This section discusses how to select the parameters to calculate capillary number. Initially, capillary number was proposed to correlate the residual saturation of the fluid (oil) displaced by another fluid (water) in the two-phase system. In surfactant-related flooding, there is multiphase flow (water, oil, and microemulsion), especially at the displacing front. If we use up/a to define the relationship between capillary number and residual oil saturation, which phase u and p and which o should be used then To the best of the author s knowledge, this issue has not been discussed in the literature. The following is what we propose. 

In general, when we discuss a parameter between two phases, we must define the conjugate phases. In any two-phase flow, it is obvious that both of the phases are conjugate phases. In a three-phase flow—for example, involving water, oil, and microemulsion three conjugates exist water-oil, water-microemulsion, and oil-microemulsion. There are also three interfacial tensions Owo, ctwm, and Oom. Each phase has two capillary numbers. Eor example, the oil phase has two capillary numbers (Nc)ow for the water phase displacing the oil phase and (Nc)om for the microemulsion phase displacing the oil phase. 

Similarly, the water phase also has two capillary numbers (Nc)wo for the oil phase displacing the water phase and (Nc)wm for the microemulsion phase displacing the water phase. When the definition Nc = up/o is nsed to calculate the capillary number, Uo and Po of the oil phase and 0 0 should be used for (Nc)wo and Um and Pm of the microemulsion phase and Owm should be used for (Nc)wm- When the definition Nc = k(Ap/L)/o is used, ko, Apo, and o o should be used for (Nc)wo and k , Apm, and Owm should be used for (Nc) - Two residual oil saturations are calculated S ro (residual water saturation in the water-oil conjugates) calculated using (Nc)wo, and Swm (residual water saturation in the water-microemulsion conjugates) calculated using (Nc) - The final residual water saturation should be saturation-weighted, as in... 

A similar discussion can be applied to (Nc)om and (Nc)mo, and (Nc)wm and (Nc)mw hi practice, a further approximation may be made. For instance, in measuring three-phase relative permeabilities, Delshad et al. (1987) used Nc = k(Ap/L)/o, where o was the average of the two IFTs Omo (IFT between the microemulsion and oil phases) and Omw (IFT between the microemulsion and water phases). In this case, only a single form of capillary number was calculated for the three-phase flow. 

Two-phase flow could be either a low capillary number waterflooding case (water and oil phases), type I system (excess oil and microemulsion phases), or type 11 system (excess water and microemulsion phases). For one two-phase flow, Eq. 7.126 becomes... 

Generally, it is assumed that Sm, Om, and k m for the microemulsion phase at the low capillary number, (Nc) would be the same as those for the water at (Nc)c. According to the preceding discussion, if the microemulsion is type II, kmi is close to km. Then Sm, Um, and km for the microemulsion phase at (Nc)c should be close to those for the oil at (Nc)c- In other words, Sm. Rm, and km for the microemulsion phase at (Nc)c should also be composition-dependent, as defined by Eq. 7.147, for example. The current version of UTCHEM does not take into account the composition-dependent km. Instead, Sm. and km at (Nc)c and (Nc)t are required input, and the interpolation is performed in between based on the capillary number. 

A microemulsion can exist in three types of systems—type II(-), type III, or type II(+)—depending on salinity. Below a certain salinity Cs i, the system is type II(-). Above a certain salinity Cse , the system is type II(+). If the salinity is between Csei and Cseu, the system is type III. In a type III system, the interfacial tension (HT) of microemulsion/brine is lower than that in a type II(-i-) system, and the IFT of microemulsion/oil is lower than that in a type II(-) system. Thus, both IFTs are collectively low. At optimum salinity, which is defined as the middle of Cjei and Cse , the two IFTs are equal. IFT is a very important parameter, with a lower value resulting in a higher capillary number (Nc). A higher capillary number will lead to lower residual oil saturation, thus higher oil recovery. Therefore, optimum salinity seems to be an obvious choice. Another area of contention is whether a type II(-) or type II(-i-) system is better for oil recovery (Larson, 1979). 

Low Tension Polymer Water Flood. In oil reservoirs, where the critical capillary number is relatively low, a significant amount of waterflooded residual oil can be displaced by surfactants of high efficiency even at two-phase flood conditions. This was demonstrated by the snccessfnl second Ripley surfactant flood pilot test in the Loudon field where approximately 68% of waterflooded residual oil was recovered by injecting a 0.3 PV microemulsion bank [63]. The microemulsion bank was followed by I.O PV of higher viscosity polymer drive. The chemical formnlation consisted of a blend of two PO-EO sulfates. 

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