Salinity gradients negative

Nelson and Pope concluded that chemical flood design should be such as to maintain as much surfactant as possible in the type III phase environment. This condition can be accomplished by designing the micellar fluid such that the initial phase environment of the immiscible displacement is type II(+). A negative salinity gradient is imposed, and it moves the phase environment to type III and, eventually, to II(-). 

To the best of our knowledge, the work by Gupta and Trushenski (1979) and experimental data from Nelson (1982) are the only data published so far to support the concept of a negative salinity gradient. Gupta and Trushenski, and Nelson used the same kind of surfactant with a special phase behavior. My explanation to their observation on salinity effect is that for the surfactant they used, the IFTs for both microemulsion/oil and microemulsion/water in the type III system were high. Therefore, when a lower salinity was in the drive water, low IFT was obtained because the lower salinity matched the lower optimum salinity of surfactant as the surfactant concentration was diluted. 

Hirasaki also explained why the negative salinity gradient works from the point of phase velocity. He stated that overoptimum salinity ahead of the surfactant bank is desirable because surfactant that mixes with high-salinity water will partition into the oleic phase, and because the phase velocity (phase cut/... 

Negative salinity gradient was proposed based on very limited core flood data. 

In Cases kr9 and krlO, we use the negative salinity gradient and positive salinity gradient, respectively. Their recovery factors are shown Table 8.3. Now the RF from Case kr9 of SG(-) is higher than that from Case kr6 of type III,... 

Experiments by Gupta and Trushenski (1979) were the first that supported the concept of the negative salinity gradient. Later, Nelson (1982) conducted... 

This section further discusses the effects of k, curves, optimum phase type, and phase viscosity. The effect of negative salinity gradient is further discussed under conditions where different relationships between optimum salinity and surfactant concentrations occur. 

Hirasaki (1981) explained why the negative salinity gradient works from the point of phase velocity. He stated that an overoptimum salinity ahead of the surfactant bank is desirable because surfactant that mixes with the high-sahnity water will partition into the oleic phase, and because the phase velocity (fa/S) of the oleic phase is less than unity, the surfactant will be retarded. An underoptimum salinity is not desirable ahead of the surfactant bank because the surfactant partitions into the aqueous phase, which has a phase velocity greater than unity. However, an underoptimum salinity is desirable in the drive to propagate the surfactant. Thus, a system with overoptimum salinity ahead of the surfactant bank and underoptimum salinity in the drive will tend to accumulate the surfactant in the three-phase region where the lowest interfacial tensions generally occur. 

According to Hirasaki s statement, if the oil velocity is reduced, then negative salinity gradient would work better. To verify his statement, we reduce oleic phase velocity by increasing oil viscosity. In Cases Yisl to Vis5, which are based on Cases krl to kr5, we increase oil viscosity from 5 to 25 mPa s. The oleic phase velocity is reduced about live times. The results are shown in... 

Table 8.15. The RF in Case Vis4 [SG(-)] is higher than that in Case Yis5 [SG(+)], and also higher than that in Case Visl (type III). These results indicate that reducing oleic phase velocity does improve the performance of negative salinity gradient relatively. However, the recovery factor from type II(+) is stiU the highest.

FIGURE 8.15 Salinity profiles in a negative salinity gradient. 

---