Macro- and Microemulsions - American Chemical Society

11 Macro- and Microemulsions in Enhanced Oil Recovery M. K. SHARMA and D. O. SHAH

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on March 27, 2016 | http://pubs.acs.org Publication Date: March 27, 1985 | doi: 10.1021/bk-1985-0272.ch011

Departments of Chemical Engineering and Anesthesiology, University of Florida, Gainesville, FL 32611

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 andoilare 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 by varying the concentration and structure of alcohol. It was observed that the formulations consisting of ethoxylated sulfonates and petroleum sulfonates are relatively insensitive to divalent cations. The results show that a minimum in coalescence rate, interfacial tension, surfactant loss, apparent viscosity and a maximum in oil recovery are observed at the optimal salinity of the system. The flattening rate of anoildrop in a surfactant formulation increases strikingly in the presence of alcohol. It appears that the addition of alcohol promotes the mass transfer of surfactant from the aqueous phase to the interface. The addition of alcohol also promotes the coalescence ofoildrops, presumably due to a decrease in the interfacial viscosity. Some novel concepts such as surfactant-polymer incompatibility, injection of anoilbank and demulsification to promote oil recovery have been discussed for surfactant flooding processes. During the past two decades, much attention has been focused on enhanced oil recovery by chemical flooding processes in order to increase the world-wide energy supply. I t is well recognized that the 0097-6156/85/0272-0149$06.75/0 © 1985 American Chemical Society

Shah; Macro- and Microemulsions ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

150

MACRO- AND MICROEMULSIONS

macro- and microemulsion systems play an important role in the oil recovery processes. Recently, several major research findings in t h i s area have been reported in the review a r t i c l e s and books (1 4). Wagner and Leach (5), Taber (6) and Melrose and Bradner (7) have suggested that c a p i l l a r y forces are responsible for entrapping a large amount of oil in the form of oil ganglia within the porous rocks of petroleum r e s e r v o i r s . Foster (8) has also shown that in t e r f a c i a l tension at crude oil/brine i n t e r f a c e , which plays a domi nant role in c o n t r o l l i n g c a p i l l a r y forces, should be reduced by a factor 10,000 times (e.g. 10~3 io-4 dynes/cm) to achieve an e f f i c i e n t displacement of the crude oil. Such low i n t e r f a c i a l ten sion can be achieved by appropriate surfactant formulations. The oil droplets can be deformed e a s i l y in the presence of low i n t e r f a c i a l tension. Figure 1 schematically i l l u s t r a t e s a two dimension a l view of the surfactant-polymer flooding process. A f t e r i n j e c t i n g a surfactant slug into the reservoir, a polymer slug is injected for mobility control. During this process, the displaced oil droplets coalesce and form anoilbank (Figure 2). Once anoilbank is formed in the porous medium, it has to be propagated through the porous medium with minimum entrapment of oil at the t r a i l i n g edge of the oil bank. The maintenance of ultralow i n t e r f a c i a l tension at the oil bank/surfactant interface is neces sary for minimizing the entrapment of the oil in the porous medium. The leading edge of the oil bank coalesces with additional oil ganglia. Moreover, besides i n t e r f a c i a l tension and i n t e r f a c i a l v i s c o s i t y , another i n t e r f a c i a l parameter which influences the oil recovery is the surface charge at the oil/brine and rock/brine in terfaces (9,10). It has been shown that a high surface charge den s i t y leads to a lower i n t e r f a c i a l tension, lower i n t e r f a c i a l v i s cosity and higher oil recovery as shown in Figure 3. In 1959, Wagner and Leach (11) suggested that increased oil recovery could be obtained by changing w e t t a b i l i t y of rock material from oil-wet to water-wet. Melrose and Bradner (7) and Morrow (12) also suggested that for optimal recovery of residual oil by a low i n t e r f a c i a l tension flood, the rock structure should be water-wet. Previous investigators (13,14) have used sodium hydroxide to make the reservoir rock water-wet. Slattery and Oh (15) have shown that intermediate w e t t a b i l i t y may be less desirable than either oil-wet or water-wet rocks. Since, chemical floods s a t i s f y many of these conditions, they have been considered promising for enhanced r e covery of oil. The mechanism of oil displacement in porous media has been reviewed by Bansal and Shah (16) and more recently by Taber (Γ7) .


t

o

Enhanced O i l Recovery By Microemulsion Flooding In t h i s section, several important aspects of microemulsions in r e l a t i o n to enhanced oil recovery w i l l be discussed. I tiswell recognized that the success of the microemulsion flooding process for improving oil recovery depends on the proper s e l e c t i o n of chemicals in formulating the surfactant slug. During the past decade, it has been reported that many surfac tant formulations for enhanced oil recovery generally form m u l t i phase microemulsions (18-20). From these studies, it is evident


11.

SHARMA AND SHAH

Enhanced Oil Recovery

INJECTION

151 PRODUCTION

-SURFACTANT SLUG

ι

* - γ - - ^ — RESIDUAL ^ OIL-WATER OIL-AND" ^ B A N K ^ - - . — -RESIDENT - ^ ^ — BRINE

FLOOD WATER "

-7777777


777777777777%^^^ THICKENED FRESH WATER Figure 1.

Schematic i l l u s t r a t i o n of the surfactant-polymer flooding process.

SURFACTANT SLUG

DISPLACED

OIL G A N G L I A

A CONTINUOUS OIL INTERFACIAL

MUST

BANK.

VISCOSITY

IS

FOR

COALESCE

TO

FORM

THIS A V E R Y LOW

DESIRABLE

Figure 2. Schematic presentation of the r o l e of coalescence of oil ganglia in the formation of theoilbank. High Surface Charge Density - Low Interfacial Tension -Low Interfacial Viscosity -High Electrical Repulsion Between Oil Droplets 8 Sand Low Surface Charge Density - High Interfacial Tension - High Interfacial Viscosity Low Electrical Repulsion Between Oil Droplet â Sand

Figure 3. Schematic i l l u s t r a t i o n of the r o l e of surface charge in the oil displacement process.


152

M A C R O - A N D MICROEMULSIONS


that a variety of phases can exist in equilibrium with each other. Figure 4 shows the e f f e c t of s a l i n i t y on the phase behavior of oil/ brine/surfactant/alcohol systems. The microemulsion slug p a r t i t i o n s into three phases (Figure 4), namely, a surfactant-rich middle phase and a surfactant-lean brine and oil phase (21-23) in the intermediate s a l i n i t y range. This surfactant-rich phase was termed as the middle phase microemulsion (23) . The middle phase microemulsion consists of s o l u b i l i z e d oil, brine, surfactant and alcohol. The 1 m u t r a n s i t i o n of the microemulsion phase can be obtained by varying any of the eight variables l i s t e d in Figure 4. I n t e r f a c i a l Tension. I t is well established that ultralow i n t e r f a c i a l tension plays an important role in oil displacement process (18,20). Figure 5 schematically i l l u s t r a t e s the factors a f f e c t i n g the magnitude of the i n t e r f a c i a l tension. Using this conceptual approach, one can broaden and lower the magnitude of i n t e r f a c i a l tension as well as increase the s a l t tolerance l i m i t of the surfactant formulation. Experimentally, Shah et a l . (24) demonstrated a direct c o r r e l a t i o n between i n t e r f a c i a l tension and i n t e r f a c i a l charge in various oil-water systems. I t was established that the i n t e r f a c i a l charge density plays a dominant role in lowering the i n t e r f a c i a l tension. Figure 6 shows the i n t e r f a c i a l tension and p a r t i t i o n c o e f f i c i e n t of surfactant as a function of the s a l i n i t y . The minimum i n t e r f a c i a l tension occurs at the same s a l i n i t y where the p a r t i t i o n c o e f f i c i e n t is near unity. The same c o r r e l a t i o n between i n t e r f a c i a l tension and p a r t i t i o n c o e f f i c i e n t was also observed by Bavière (25) for p a r a f f i n oil/sodium alkylbenzene s u l f o nate (average MW 350)/isopropyl alcohol/brine system. Chan and Shah (26) proposed a u n i f i e d theory to explain the ultralow i n t e r f a c i a l tension minimum observed in d i l u t e petroleum sulfonate s o l u t i o n / o i l systems encountered in t e r t i a r y oil recovery processes. For several variables such as the s a l i n i t y , the oil chain length and the surfactant concentration, the minimum in i n t e r f a c i a l tension was found to occur when the equilibrated aqueous phase was at CMC. This i n t e r f a c i a l minimum also corresponded to the p a r t i t i o n c o e f f i c i e n t near unity for surfactant d i s t r i b u t i o n in oil and brine. It was observed that the minimum in ultralow i n t e r f a c i a l tension occurs when the concentration of the surfactant monomers in aqueous phase is maximum. Formation and Structure of Middle Phase Microemulsion. The 1 -> m -* u t r a n s i t i o n s of the microemulsion phase as a function of various parameters are shown in Figure 4. Chan and Shah (31) compared the phenomenon of the formation of middle phase microemulsion with that of the coacervation of micelles from the aqueous phase. They concluded that the repulsive forces between the micelles decreases due to the n e u t r a l i z a t i o n of surface charge of micelles by counterions. The reduction in repulsive forces enhances the aggregation of mic e l l e s as the a t t r a c t i v e forces between the micelles become predominant. This theory was v e r i f i e d by measuring the surface charge density of the equilibrated oil droplets in the middle phase (9). It was observed that the surface charge density increases to a maximum at the s a l i n i t y at which the middle phase begins to form. Beyond t h i s s a l i n i t y , the surface charge density decreases in the



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SH ARM A AND SHAH

F i g u r e 4.

F i g u r e 5.


Schematic i l l u s t r a t i o n o f t h e l-»m-»u t r a n s i t i o n o f t h e m i c r o e m u l s i o n phase by s e v e r a l v a r i a b l e s .

Schematic p r e s e n t a t i o n o f t h e t h r e e components o f t h e i n t e r f a c i a l tension.



154


three-phase region. Based on several observations of d i f f e r e n t surf a c t a n t / b r i n e / o i l systems, Chan and Shah (31) proposed the mechanism of middle phase microemulsion formation as shown in Figure 7. In general, the higher the s o l u b i l i z a t i o n of brine or oil in the middle phase microemulsion, the lower the i n t e r f a c i a l tension with the excess phases. The s a l i n i t y at which equal volumes of brine and oil are s o l u b i l i z e d in the middle phase microemulsion is referred to as optimal s a l i n i t y for the surfactant/oil/brine systems under given physicochemical conditions (2^,23) . Previous investigators (22,29, 30) have shown that the oil recovery is maximum near the optimal s a l i n i t y of the system. Therefore, one can conclude that the middle phase microemulsion plays a major role in enhanced oil recovery processes. Using various physicochemical techniques such as high resolution NMR, v i s c o s i t y and e l e c t r i c a l r e s i s t i v i t y measurements, Chan and Shah (31) have proposed that the middle phase microemulsion in three-phase systems at and near optimal s a l i n i t y is a water-external microemulsion of spherical droplets of oil. Extended studies to characterize the middle phase microemulsions by several techniques including freeze-fracture electron micrographs revealed the structure as a water-external microemulsion (31). The droplet size in the middle phase microemulsion decreases with increasing s a l i n i t y . The freeze-fracture electron microscopy of a middle phase microemulsion is shown in Figure 8. This system was extensively studied by Reed and Healy (21-23). I t c l e a r l y indicates that the discrete spherical structure of the droplets in a continuous aqueous phase is consistent with the mechanism proposed in Figure 7. It should be pointed out that several investigators (32-39) have proposed the p o s s i b i l i t y of bicontinuous structure or coexistence of water-external and oil-external microemulsions in the middle phase. For very high surfactant concentration (30-40 %) systems, the existence of anomalous structures which are neither conventional oil-external nor water-external microemulsions, have been proposed to explain some unusual behavior of these systems (32-36). S o l u b i l i z a t i o n . The effectiveness of surfactant formulations for enhanced oil recovery depends on the magnitude of s o l u b i l i z a t i o n . By i n j e c t i n g a chemical slug of complete m i s c i b i l i t y with both oil and brine present in the reservoir, 100% oil recovery can be achieved. The e f f e c t of hydrated r a d i i , valency and concentration of counterions on oil-external and middle phase microemulsions was investigated by Chou and Shah (40). It was observed that 1 mole of CaCl2 was equivalent to 16-19 moles of NaCl f o r s o l u b i l i z a t i o n in middle phase microemulsion, whereas for s o l u b i l i z a t i o n in oil-external microemulsions, 1 mole of CaCl2 was equivalent to only 4 moles of NaCl. For monovalent e l e c t r o l y t e s , the values for optimal s a l i n i t y for s o l u b i l i z a t i o n in oil-external and middle phase microemulsions are in the order: L i C l > NaCl > KC1 > NH4CI, which correlates with the Stokes r a d i i of hydrated counterions. The optimal s a l i n i t y for middle phase microemulsions and c r i t i c a l e l e c t r o l y t e concentration varied in a s i m i l a r fashion with Stokes r a d i i of counterions, which was d i s t i n c t l y d i f f e r e n t for the s o l u b i l i z a t i o n in oil-external microemulsions. Based on these findings, it was



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SHARMA AND SHAH


NaCl

CONCENTRATION

155

(wt.%)

Figure 6. Effect of s a l i n i t y on i n t e r f a c i a l tension and p a r t i t i o n coefficient f o r TRS 10-80/n-octane system.

Figure 7. Schematic i l l u s t r a t i o n for the mechanism of t r a n s i t i o n from lower -> middle -> upper phase microemulsion upon increasing s a l i n i t y . Solid c i r c l e s , oil-swollen micelles (or microdroplets ofoil);open c i r c l e s , reverse micelles (or microdroplets of water).



156


Figure 8.

Freeze-fracture electron micrograph of the middle phase of the Exxon system at the optimal s a l i n i t y .


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SHARMA AND SHAH


157


concluded that the middle phase microemulsion behaves l i k e a water continuous system with respect to the e f f e c t of counterions (40). The effect of alcohol concentration on the s o l u b i l i z a t i o n of brine has been studied in t h i s laboratory (41). I t was observed that there is an optimal alcohol concentration which can s o l u b i l i z e the maximum amount of brine and can also produce ultralow i n t e r * f a c i a l tension. The optimal alcohol concentration depends on the brine concentration of the system. The e f f e c t of d i f f e r e n t alcohols on the equilibrium properties and dynamics of m i c e l l a r solutions has been studied by Zana (42). Phase Behavior. The surfactant formulations for enhanced oil r e covery consist of surfactant, alcohol and brine with or without added oil. As the alcohol and surfactant are added to equal volumes of oil and brine, the surfactant p a r t i t i o n i n g between oil and brine phases depends on the r e l a t i v e s o l u b i l i t i e s of the surfactant in each phase. I f most of the surfactant remains in the brine phase, the system becomes two phases, and the aqueous phase consists of micelles or oil-in-water microemulsions depending upon the amount of oil s o l u b i l i z e d . I f most of the surfactant remains in the oil phase, a two-phase system is formed with reversed micelles or the water-in-oil microemulsion in equilibrium with an aqueous phase. The phase behavior of surfactant formulations for enhanced oil recovery is also affected by the oil s o l u b i l i z a t i o n capacity of the mixed micelles of surfactant and alcohol. For low-surfactant systems, the surfactant concentration in oil phase changes considerably near the phase inversion point. The experimental value of p a r t i t i o n c o e f f i c i e n t is near unity at the phase inversion point (28). The phase inversion also occurs at the p a r t i t i o n c o e f f i c i e n t near unity in the high-surfactant concentration systems (31). Simi l a r results were also reported by previous investigators (43) for pure a l k y l benzene sulfonate systems. Figure 9 shows the e f f e c t of surfactant concentration on the volume of the middle phase microemulsion. It is interesting that the plot goes through the o r i g i n indicating that even at very low surfactant concentrations, a microscopic amount of the midddle phase microemulsion must exist at the interface between oil and brine. S a l i n i t y Tolerance. As the petroleum reservoir s a l i n i t y can be very high, the surfactant formulations should be designed for high s a l t tolerance. The widely used petroleum sulfonates for enhanced oil recovery exhibit r e l a t i v e l y low s a l t tolerance in the range 2-2.5% NaCl concentration, and even smaller for the optimal s a l i n i t y . The presence of divalent cations in the brine decreases the optimal s a l i n i t y of surfactant formulations (44). Since optimal s a l i n i t y leads to a favorable condition for maximum oil recovery, one would l i k e to design methods to adjust the optimal s a l i n i t y of a given surfactant formulation (45,46). Figure 10 shows the optimal s a l i n i t y of a mixed surfactant formulation cons i s t i n g of a petroleum sulfonate and an ethoxylated sulfonate. I t is evident that the optimal s a l i n i t y of the formulation increases with increasing concentration of ethoxylated sulfonate, the optimal s a l i n i t y increases from 1% to 24% NaCl brine. Moreover, it is interesting to note that these formulations, when equilibrated with



158


TRS Figure

9.

10-410, wt. %

E f f e c t o f s u r f a c t a n t c o n c e n t r a t i o n on the volume o f m i d d l e phase m i c r o e m u l s i o n . Key: ο, 1 .57 i s o b u t a n o l ; ., 37o i s o b u t a n o l . 0


11.

SH ARM A AND SHAH


159

oil, produce middle phase microemulsion with very low i n t e r f a c i a l tension (< 10" 3 dynes/cm). Similar results were also obtained using ethoxylated alcohols in surfactant formulations by previous i n v e s t i gators (47). It is i n t e r e s t i n g that the ethoxylated sulfonate alone (curve F in Figure 10) is unable to produce ultralow i n t e r f a c i a l tension. However, when mixed with petroleum sulfonate, it produces very low i n t e r f a c i a l tension. Therefore, the surfactant formulations consisting of mixed petroleum sulfonates and ethoxylated s u l fonates or alcohols are promising for enhanced oil recovery from high s a l i n i t y reservoirs.


Macroemulsions in Enhanced O i l Recovery Processes As the surfactant slug is injected into the reservoir, the mixing of injected slug with reservoir components takes place. The mixing of surfactant with reservoir oil and brine often produces emulsions. Moreover, the reservoir parameters such as porosity, pressure, temperature, composition of connate water and crude oil as well as g a s - o i l r a t i o affect the formation of oil f i e l d emulsions. Usually, the formation of stable macroemulsions in the oil f i e l d s is considered undesirable and can cause severe problems. Previous investigators (48-50) have reported poor oil recovery due to problems associated with stable emulsions. In addition, waterin-oil emulsions should be resolved before the oil r e f i n i n g process because of the presence of considerable amounts of emulsified water in crude oil increases the cost of oil transportation and leads to other maintenance problems. Because of corrosion problems, most pipelines do not accept curde oil with s i g n i f i c a n t amount of emuls i f i e d water. On the other hand, McAuliffe (51) has shown a benef i c i a l e f f e c t of macroemulsions in oil recovery as t h e i r i n j e c t i o n into sandstone cores increased sweep e f f i c i e n c y . According to this concept, the emulsion droplet enters a pore c o n s t r i c t i o n smaller than i t s e l f . For an emulsion to be most e f f e c t i v e , the oil droplets in the emulsion must be larger than the pore-throat constrictions in the porous media. The injected emulsion enters the highly permeable zones, which in turn reduces the channeling of water. Therefore, water s t a r t s to flow into low permeable zones, r e s u l t i n g in a greater sweep e f f i c i e n c y . Based on this concept, a f i e l d test was conducted which showed an improvement in oil recovery by macroemulsion flooding process. In order to understand the e f f e c t of macroemulsions on the oil recovery process, the following parameters should be discussed. Electrophoretic Mobility. The electrophoretic mobility of the crude oil droplets as a function of caustic concentration has been determined in r e l a t i o n to enhanced oil recovery (52) . It was observed that a maximum in electrophoretic mobility corresponds to a minimum in i n t e r f a c i a l tension at the crude oil/caustic interface (Figure 11). The maximum electrophoretic mobility at minimum i n t e r f a c i a l tension can be attributed to the i o n i z a t i o n of carboxyl groups present in the crude oil, which in turn determine the charge density at the crude oil/caustic interface, depending on NaOH concentration. The experimental procedure used for the measurement of i n t e r f a c i a l tension and the electrophoretic mobility to determine the


160


SURFACTANT FORMULATIONS C

P-465 (3%) • EOR-200 (2%)* η PENTANOL (2%)

D: P-465 (2%)• EOR-200 ( 3 % ) t η PENTANOL (2%) E

P-465 (!%)• EOR-200 (4%)f η PENTANOL (2%)


F: EOR-200 (5%)* η PENTANOL (2%)

Tmw

~7 S — d

ΙΟ II

12 13

14 15 16 17

18

£

&

NaCl CONCENTRATION, wt.%

Figure 10. Increase in optimal s a l i n i t y of formulations consisting of a mixture of a petroleum sulfonate (Ρ - 465) and an ethoxylated sulfonate (EOR-200).

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Macro- and Microemulsions - American Chemical Society

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