Page 97 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

CHAPTER 4 Hybrid Chemical EOR Using Low-Salinity and Smart Waterflood 89

separation is observed and the solution becomes solubility in aqueous phase and negligible solubility
cloudy. The nonionic surfactants have polyoxyethylene in oleic phase. The microemulsion system of low
chains, which exhibit a reverse solubility in water salinity condition is the water-external microemulsion
depending on the temperature. Because the hydrophilic and excess oil system. The microemulsion in aqueous
group of nonionic surfactants has an oxygen, the solubi- phase is denser than excess oil and it resides below
lity into water is mainly attributed to hydrogen-oxygen the oleic phase. In the intermediate salinity condition,
bond. Because the high temperature increases the sur- the microemulsion system is involved with three phases
factant molecular activity, the bond becomes weak. of excess oleic phase, microemulsion, and excess
Therefore, the high temperature condition makes the aqueous phase. In this system, the density of microe-
cloudy solution because of the separated surfactant mulsion is between the densities of oleic and aqueous
molecules. phases. The microemulsion is located above the
aqueous phase and below the oleic phase. The phase
behavior of the microemulsion system at different
Phase behavior of microemulsion salinity conditions is described in Fig. 4.19. There are
It is necessary to distinguish the microemulsion from a couple of terminologies to indicate the microemul-
the macroemulsion beforehand. The macroemulsion sion type. The Winsor type Ⅰ or type$Ⅱ ( ) indicates
is the mixture of two or more immiscible liquids. The the water-external microemulsion in low salinity condi-
one liquid phase is dispersed in the other phase. The tion. The Winsor type Ⅱ or type Ⅱ (þ) indicates the oil-
dispersed phase in the other continuous phase is external microemulsion in high salinity condition. In
commonly appeared to be cloudy. In contrast, the the intermediate salinity condition, Winsor type Ⅲ or
microemulsion system has clear transparency because type Ⅲ forms.
of the small size of the dispersed emulsion. The aggrega- There are terminologies to determine the phase
tion of micelles in the microemulsion system often behavior of the microemulsion and type of microemul-
shows some cloudy transparency. It is thermodynami- sion: (1) solubilization ratio, (2) R-ratio, and (3) pack-
cally stable; therefore, the simple mixing of two or ing factor. The solubilization ratio is defined as the ratio
more immiscible liquids forms the microemulsion. of the solubilized volume of oil or water to the surfac-
The microemulsion is of interest to the surfactant EOR tant volume in the microemulsion system as repre-
process. The two immiscible phase systems, i.e., oleic sented in Eqs. (4.14) and (4.15). Healy, Reed, and
and aqueous phases, can be solubilized by the micelles, Stenmark (1976) developed the relationship between
and the system is termed as the microemulsion system. solubilization ratios and interfacial tension between
The microemulsion system is beneficial to the EOR pro- the microemulsion and excess phase. It is also suggested
cess by recovering trapped oil. that the equal amounts of oil and water are solubilized
The phase behavior of microemulsion is sensitive to in the microemulsion Winsor type Ⅲ, indicating the op-
a number of factors including the surfactant type, sur- timum solubilization ratio, at optimal salinity. Huh
factant concentration, cosolvent, oil composition, pres- (1979) theoretically developed the correlation between
ence of alkali, salinity, temperature, and to much lesser the interfacial tension and optimum solubilization ra-
degree, pressure. Because no universal equation exists to tio as shown in Eq. (4.16). When the optimum solubi-
model the phase behavior of microemulsion consid- lization ratio is higher than 10, the IFT at the optimum
ering the factors, the phase behavior has to be identified salinity is on the order of 10 3 dyne/cm or less and it
with experiments. Among the factors, the salinity is of sufficiently mobilizes the residual oil saturation.
importance because of the huge influence to change
phase behavior of the system. Conventionally, the solu- S o ¼ V o (4.14)
bility of the anionic surfactant in the brine decreases V surf
with an increase in the salinity of brine, i.e., the surfac- V w
tant is separated from the aqueous phase as the electro- S w ¼ V surf (4.15)
lyte concentration increases. In turn, the surfactant C
easily dissolves in oleic phase rather than in aqueous s ¼ S 2 (4.16)
phase at high salinity condition. The microemulsion
system of high salinity condition is the oil-external where S o is the oil solubilization ratio; V o is the solubi-
microemulsion and excess water system. Because the lized volume of oil in microemulsion; V surf is the surfac-
microemulsion in oleic phase has less density than wa- tant volume in microemulsion; S w is the water
ter, the microemulsion is located above the water. At the solubilization ratio; V w is the solubilized volume of wa-
low salinity condition, the surfactant shows the higher ter in microemulsion; C is the constant and,