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INVESTIGATION OF EMULSIFICATION AND COALESCENCEPHENOMENA IN CRUDE OIL/ALKALINE WATER SYSTEMSVIVIEN J. CAMBRIDGE a , JOANNE M. WOLCOTT a & W. DAVID CONSTANT aa Petroleum Engineering Department , Louisiana State University , Baton Rouge, LA, 70803,|Published online: 28 Mar 2007.

To cite this article: VIVIEN J. CAMBRIDGE , JOANNE M. WOLCOTT & W. DAVID CONSTANT (1990) INVESTIGATION OFEMULSIFICATION AND COALESCENCE PHENOMENA IN CRUDE OIL/ALKALINE WATER SYSTEMS, Chemical EngineeringCommunications, 92:1, 121-137, DOI: 10.1080/00986449008911426

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Chem. Eng. Comm. 1990, Vol. 92, pp. 121-137 Reprints available directly from the publisher. Photocopying permitted by license only. 0 1990 Gordon and Breach Science Publishers S.A. Printed in the United States of America

AN INVESTIGATION OF EMULSIFICATION AND COALESCENCE PHENOMENA IN CRUDE

OILJALKALINE WATER SYSTEMS

VIVIEN J. CAMBRIDGE,? JOANNE M. WOLCOTTQ and W. DAVID CONSTANTS

Petroleum Engineering Department Louisiana State University Baton Rouge, L A 70803

(Received April 20, 1989; in final form December 15, 19W)

Emulsification and coalescence processes in crude oillalkaline water systems were examined and their influence on alkaline flood enhanced oil recovery was assessed. Emulsification mechanisms were investigated under static and dynamic conditions using microvisual techniques. Coalescence rates were measured using the inclined spinning drop tensiometer. The relative impact of interfacial viscosity on coalescence processes was determined through measurements of interfacial shear viscosities. In addition, the influences of chemical composition on ease of emulsification, coalescence rate, and interfacial shear viscosity were examined.

Ease of emulsification was influenced by the composition of the crude oil, the electrolyte concentration, and the partitioning coefficient of surfactants. Coalescence was primarily affected by processes which disrupted the crude oillwater interface. Alkaline flood oil recovery efficiency was promoted by emulsification followed by rapid coalescence to form a stable oil bank. KEYWORDS Alkaline flooding Emulsification Coalescence Petroleum chemistry

Surfactant behaviour Liquid-liquid interfaces.

INTRODUCTION

The production of surfactants through the interaction of acidic crude oils with alkalihe water has found numerous applications in the petroleum industry. Reservoirs that have been "depleted" by conventional means of oil production may be stimulated to recover additional-oil through the use of alkaline flooding techniques (Mayer et al., 1983). The emulsions formed through the interaction of alkaline water with acidic crudes have been utilized as mobility control agents in enhanced oil recovery (EOR) techniques which employ low-density fluids such as steam or COP (Robinson er al., 1977; Okoye and Tiab, 1982). In addition, alkaline chemicals have been used to break crude oillwater emulsions which are formed during oil production (Strassner, 1963).

Emulsification and coalescence phenomena assume paramount roles in all of

t Present address: Sverdrup Technology Inc. Stennis Space Center, MS 39529. $Present address: Hazardous Waste Research Center Louisiana State University Baton Rouge, LA

70803. 5 Author responsible for inquiries.

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122 V.J. CAMBRIDGE, J.M. WOLCOTT AND W.D. CONSTANT

these applications. Relative emulsification and coalescence rates are important considerations in the design of emulsions for mobility control; while, destabiliza- tion of emulsions is central to oillwater separation processes. In alkaline flooding procedures, oil recovery has been found to improve when emulsions are intially formed but rapidly coalesce to produce a stable oil bank (Wasan et al., 1979).

The phenomena of emulsification and coalescence are complex and not yet fully understood even though considerable research has been dedicated to the topics. Numerous studies concerning emulsification and coalescence behavior in surfactant systems have appeared in the literature (Shah, 1985; Manev et al., 1984; Davies and Rideal, 1963; Rosen, 1978; Flumerfelt et al . , 1981; Kim and Li, 1985); however, few researchers have addressed the specialized case of crude oil/alkaline water systems (Wasan et al . , 1979; Plegue et al., 1986; Chang and Wasan, 1980). A more detailed study of emulsification and coalescence in crude oil/alkaline water was needed especially in light of the fact that crude oillalkaline water systems display quite different interfacial tension behavior from that observed in traditional surfactant systems (Cambridge et al., 1986; England and Berg, 1971; Rubin and Radke, 1981; Trujillo, 1983).

This paper examines emulsification and coalescence processes in crude oil/alkaline water systems. Emulsification mechanisms are investigated under static conditions using three different experimental designs. Thin flow cells are employed to determine the effects of flow through porous media on the emulsification processes. The impact of emulsification on alkaline flood recovery efficiency is also assessed.

Coalescence rates were measured using the procedure described by Flumerfelt er al. (1981) which employs an inclined spinning drop tensiometer. Interfacial shear viscosity measurements were used to determine the relative impacts of film drainage rate and collapse distance on coalescence. In addition, correlations of both coalescence rates and interfacial shear viscosities with alkaline flood oil recovery efficiencies were examined.

Finally, the influence of chemical composition on ease of emulsification, coalescence rate, and interfacial shear viscosity was determined. Synthetic crudes and natural crude components as well as water phase additives were examined in the study.

EXPERIMENTAL

General Procedures

Acid numbers were determined using ASTM procedure D-664. Bulk fluid viscosities were measured at room temperature (22°C) using kinematic viscometer tubes. Liquid-liquid interfacial shear viscosities were measured using the method described in Cambridge et al. (1988). Interfacial tension (IFT) measurements were conducted on a SITE LP 12 Spinning Drop Interfacial Tensiometer using the procedure described by Cayias et al. (1975). All IFT measurements were conducted at 30°C and employed an aqueous solution containing 0.1 NaOH and

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EMULSIFICATION AND COALESCENCE PHENOMENA 123

1.0% NaCI. Microscopic observations were conducted using an Olympus Model PM-10AD Microscope equipped with a video camera and recorder. Magnifica- tions of 100 x , 500 x , and 1000 x were employed in the studies. Recovery efficiency studies were conducted as described in Dahmani, et al. (1988).

MG#3 crude was obtained from the 703-708 m sands of the Matilda Gray field located near Vinton, LA. Tullos crude was obtained from the Tullos field at Tullos, LA. Synthetic crudes were prepared by dissolving an equimolar mixture of decanoic, lauric, pentadecanoic, octadecanoic, and oleic acids in 20% toluene and diluting with decane. The effect of phenols was determined by the addition of 0.5% 2-naphthol to the preceding mixture.

Thin Flow Cell Studies

Thin flow cells were similar to those described by Dahmani, et al. (1988) except that they were modified to reduce distortion at higher magnifications as illustrated in Figure 1. The cells were initially saturated with brine, flooded with oil to irreducible water saturation, and waterflooded with brine before injecting an alkaline solution. The crude oil fractions were not waterflooded prior to alkaline flooding. Watertloods and alkaline floods were performed at a rate of approxi- mately 1.3cmJhr. All floods were conducted at room temperature and atmos- pheric pressure.

Emulsification Mechanism Studies

In all of the experiments described in this section, emulsification processes were observed with a microscope and recorded using a video camera and recorder. The equipment and are described elsewhere (Cambridge, 1987). Briefly, Marangoni turbulence was observed by placing a one microliter drop of oil on a

~ -

microscope slide and covering with a microscope cover slip. he drop was allowed to spread, and an oillalkaline water interface was created by allowing the alkaline solution to imbibe between the glass surfaces. Spontaneous emulsifica- tion processes were also observed using the pendant drop apparatus illustrated in Figure 2 and the cell described in Figure 3. Initially, the cell described in Figure 3 was partially filled with distilled water, then two glass tubes were inserted slightly below the water's surface to allow replacement of distilled water by alkaline

MICROSCOPE COYER CLASS /k%lk

TOP GLASS

INLET

NICHROME WIRE SEPARATING PLATES

FIGURE 1 Modified thin flow cell.

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124 V.J. CAMBRIDGE, J.M. WOLCOTT AND W.D. CONSTANT

---L- a MICROSCOPE LIGHT

U SYRINGE

FIGURE 2 Pendant drop apparatus.

solution without undue agitation of the fluids contained in the cell. The alkaline solution employed in the emulsification studies consisted of 0.1% NaOH and 1.0% NaCl dissolved in distilled water.

Fractionation of Crude

The acidic crude oil components were isolated and characterized as previously described (Wolcott, et al. , 1989). Briefly, the fractions were isolated from deasphaltenated crude using a quaternary amine bonded-phase (J.T. Baker t7043) . The bonded-phase was converted to the hydroxide form by washing with a solution containing NH,OH dissolved in methanol. Deasphaltenated crude was placed on top of the bonded-phase, and the contents were washed in succession with the following solvents: pentane (fraction B), toluene (fraction B), 10% isopropyl alcohol/toluene (fraction C), 25% methanol/toluene (fraction D), and 0.1 N HCI in 50% methanol/toluene followed by 0.2 N HCI in methanol (fraction El.

ALKALI SOLUTI

INLE'

N E ION1 f SUCTION

MICROSCOPE LIGHT

FIGURE 3 Interface emulsification experiment.

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EMULSIFICATION AND COALESCENCE PHENOMENA 125

The fractions were analyzed using elemental analyses, NMR spectroscopy, IR spectroscopy, interfacial tension reduction on contact with alkaline water, and acid numbers. Molecular weights were determined from freezing-point depres- sions of esterified samples in benzene. For IFT measurements and thin cell floods, fractions B, C, D, and E were dissolved in toluene, and fraction A was used without dilution. Mixtures of fractions B and/or E in fraction A were prepared so that the concentrations of the fractions approximated those found in the whole crude.

Base Extraction of Crudes

The crude oil was dissolved in excess hexane and shaken with 0.2% aqueous hydroxide in a separatory funnel. The emulsion that resulted was allowed to sit for several hours to achieve partial separation of phases. The aqueous phase was drained; fresh hydroxide solution was added; and the extraction procedure was repeated until the resulting water phase was clear. The oil phase was acidified with dilute HCI, and washed with distilled water until all traces of acid were removed. Hexane was evaporated, and the acid number of the oil residue was measured. The MG#3 residue had an acid number of 0.345gKOHlg oil as compared to the Tullos residue which had an acid number of 0.438 g KOH/g oil.

Coalescence Rates

Drop-drop coalescence rates were measured using the method developed by Flumerfelt et al. (1981). A detailed description of the procedure is presented by Cambridge (1987). A value of six was used for the Hamaker in the calculations (Flumerfelt, 1986). The measureable quantities t,, F, and R are reported as a variable r, the normalized coalescence time, which is normalized with respect to F and R:

where F is the force driving the drops together; R is the contact radius of the thin film; and t, is the coalescence time as defined in Flumerfelt et al. (1981). It is assumed in the calculations that the interface is fluid. Alkaline water solutions contained 1.0% NaCl and 0.1% NaOH.

RESULTS AND DISCUSSION

EmuLFijication Process

Two crude oils with similar chemical and physical properties (Table I) exhibited quite different behavior in laboratory studies of the alkaline flooding process (Dahmani, et al., 1988). The alkaline recovery efficiency of MG#3 using aqueous 0.2% NaOH and 1.0% NaCl was twice that of Tullos crude (Table 11). Since MG#3 and Tullos waterflood recoveries were comparable, factors such as bulk

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126 V.J. CAMBRIDGE, J.M. WOLCOTT AND W.D. CONSTANT

TABLE I

Physical and chemical properties' of MG#3 and Tullos crude oils

Density Viscosity Acid no. Oil (g cm- ) (Pa. s) (mg KOH/g oil) Molecular wt.

MG#3 0.931 0.170 3.50 309 Tullos 0.930 0.250 1.84 360

nThe elemental analysis are given in Table IV.

TABLE I1

Alkaline flood recovery efficiency studies

WaterRood Alkaline flood Oil Flood composition recovery, % OOIPa recoveryb, % OOIP - -

MG#3 1% NaCI, 0.2% NaOH 58.6 Tullos 1% NaCI, 0.1% NaOH 55.9 Tullos 0.04 NaCI, 0.1% NaOH 53.4

- - -

" Percent of Original Oil-in-Place (001P). Percent additional oil recovered after waterflood.

viscosity and API gravity which have similar influences on waterflood and alkaline flood recoveries could not account for the differences observed.

The higher recovery efficiency of the MG#3 flood was attributed to emulsion formation. Microscopic examinations of the flooding process in thin flow cells showed that MG#3 emulsified; whereas, Tullos did not. Tullos oil recovery was found to be aided by a combination of interfacial tension reduction and wettability reversal. When the brine concentration of the flood solution was reduced to 0.04% NaCI, Tullos oil did emulsify, and the recovery efficiency improved dramatically. It was apparent that, for these oils, emulsion formation provided a more efficient mechanism for the release of trapped oil.

Although interfacial tension (IFT) reduction is known to promote emulsifica- tion, I F T reduction alone cannot account for the differences in emulsification behavior of the two oils. As can be seen in Figure 4, Tullos oil produced a slightly lower and more stable IFT minimum than MG#3. A closer examination of the factors influencing emulsification in crude oil/alkaline water systems was needed to explain the observed behavior.

Emulsification processes in the MG#3 and Tullos/alkaline water systems were investigated under a variety of experimental conditions. Spontaneous emulsifica- tion phenomena were examined by microscopic observation of crude oillwater interfaces under static conditions using three different experimental designs. In the first experiment, a drop of oil was placed on a microscope slide and covered with a thin glass cover slip. The drop was allowed to spread between the glass surfaces. Then, the alkaline solution was imbibed between the glass surfaces to create an oillwater interface as described by Wasan, er al. (1979). Microscopic

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EMULSIFICATION AND COALESCENCE PHENOMENA 127

TULLOS 0 M G # 3

FIGURE 4 IFT behavior of Tullos and MGX3 crudes against alkaline water.

examination of the MG#3/alkaline water interface revealed kicking motions which are indicative of Marangoni turbulence (Davies and Rideal, 1963). Oil droplets "pinched off' from the interface and rapidly moved into the water phase. Similar behavior was noted by Wasan, et al. (1979) during a study of crude oil/alkaline water interactions. They described this emulsification process as interfacial turbulence-induced spontaneous emulsification. The Tullos oil/alkaline water interface was calm, and interfacial turbulence was not observed.

The configuration of the microscope slide experiment only allowed disturbances in a direction perpendicular to the field of gravity. Gravity, however, may induce interfacial instabilities through density fluctuations (Davies and Rideal, 1963; Kim and Li, 1985) and thus cause emulsification. It has also been suggested that, in certain cases, gravity may stabilize the interface and hinderemulsification processes (Bupara, 1964). In order to observe emulsification in a three dimen- sional environment, a pendant drop apparatus (Figure 2) was constructed. Pendant drops of either MG#3 or Tullos oils in alkaline water remained stationary and did not kick. Since kicking motions of the pendant drop are commonly observed in systems with a strong Marangoni turbulence (Davies and Rideal, 1963), it is apparent that Marangoni turbulence was of relatively minor importance in these systems.

A third experimental design was developed for the observation of interfacial phenomena at a horizontal oillwater interface (Figure 3). Microscopic examina- tion of the MG#3/alkaline water interface showed the formation of a cloud of finely dispersed emulsion that moved way from the interface into the water phase. The formation of larger emulsion droplets was not noted. There was evidence of diffusion-induced flow as the movement of the emulsion cloud followed a definite pattern with respect to the interface. Wasan, et al: (1979) observed similar behavior in a crude oil/surfactant system, and they labeled this emulsification

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128 V.J. CAMBRIDGE, J.M. WOLCO'IT AND W.D. CONSTANT

process as diffusion and stranding. In contrast, the Tullos oil/alkaline water interface was clear, and' no emulsification was observed.

In addition to studying emulsification processes in static systems, emulsification behavior in a dynamic environment during fluid flow was investigated. For this purpose, thin flow cells containing a porous medium were constructed (Figure 1) to simulate the flow environment of a reservoir. The cells were initially saturated with brine, flooded with oil to irreducible water saturation, and then watertlooded prior to alkaline flooding. No significant changes in behavior were noted upon initial contact of an entrapped MG#3 oil drop with alkaline water (0.2% NaOH, 1.0% NaCI). The drop did not deform or emulsify. Somewhat later in the course of the flood, oil drops in certain regions were observed to elongate as they flowed through pore throats forming thin strings of oil in the water phase. The formation of these thin strings was attributed to the development of localized regions of ultralow interfacial tension. Small droplets of oil broke off the leading edge of the string and became entrained in the water phase to form an emulsion.

Droplets of emulsion moved very rapidly through the thin cell to a region of higher oil saturation where they coalesced with entrapped oil to form an oil bank. The oil bank lost oil as it progressed across the cell, but it was constantly replenished by oil overtaken at the leading edge and by coalescence of emulsion droplets at the trailing edge. Oil bank stability depended on the rate of oil loss versus the rate of oil gain. Coalescence phenomena, therefore, played an important role in sustaining propagation of the oil bank.

The interfacially active region, where emulsion formation was observed, was located behind the oil bank and flood front (Figure 5). Emulsion formation was noted in certain localized regions while neighboring areas showed no signs of interfacial tension reduction, indicating that surfactant concentration was not uniform throughout the porous medium. The emulsion cloud observed in the horizontal cell indicated that surfactant partitioned into the water phase. Since the aqueous surfactant concentration would not be expected to be uniform throughout the cell due to circuitous flow patterns, it is conceivable that elevated concentration of surfactant could develop in localized regions.

Entrapped Tullos oil drops also did not show any indication of IFT reduction upon initial contact with alkaline water (0.2% NaOH, 1.0% NaCI). Later in the course of the alkaline flood, some deformation of oil drops was observed but not to the extent noted with MG#3. The reduction in interfacial tension eased the

ALKALINE ROW LOW IFT REGION OIL-IN-WATER 8 OIL BANK WATER FLOOD RESIDUAL OIL I I EMULSION "W:i~~-Ayi;L 1

Z

rn

FIGURE 5 MG#3 alkaline flood.

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EMULSIFICATION AND COALESCENCE PHENOMENA 129

flow of oil drops through some pores but the drops became reentrapped later. Partial coating of the rock surface with oil was also noted and appeared to accommodate oil flow along the grains of rock.

It was obvious from these 'studies that MG#3 oil encountered a lower interfacial tension environment during alkaline flooding than Tullos oil. Since Tullos oil exhibited a slightly lower IFT than MG#3 in the spinnning drop tensiometer, surfactant concentrations in the thin cell and spinning drop tensiometer must have been quite different. This observation may be explained by the fact that the water:oil ratio in the spinning drop tensiometer is much higher than that encountered during flooding (500: 1 as compared to approxim- ately 2: I , respectively). Since surfactants that diffuse from the oil phase into the water phase are less likely to affect the IFT when the water: oil ratio is very high, the interfacial tensiometer value may be much higher than the actual IFT that develops during flooding. If the MG#3 surfactant species partitioned more readily into the water phase than those of Tullos oil, the differences in alkaline flood behavior could be explained.

The relative water solubilities of the MG#3 and Tullos surfactant species were compared by exhaustive extraction of the oils with alkaline solution. 3.16meq acidlg MG#3 oil were extracted as compared to 1.41 meq acidlg Tullos oil which is equivalent to 90% of the original carboxylic acid content of MG#3 as compared to 76% of that of Tullos. The surfactant content of the water phase during alkaline flooding would, therefore, be much higher for MG#3 as compared to Tullos which is consistent with the observed recovery mechanisms.

The fact that Tullos oil emulsified at relatively low brine concentrations, but not at higher brine concentrations, indicates that the interfacial tension depended on brine concentration. Chan and Shah (1979) have shown that salt concentration affects the partitioning of surfactants between the oil and water phases with elevated brine concentration inhibiting the partitioning of surfactant into the water phase. The higher surfactant content of the water phase at low brine concentrations would favor production of the ultralow interfacial tensions necessary for emulsification.

Coalescence Processes

Since the rate of oil drop-drop coalescence was found to affect oil bank stability during alkaline flooding, the oil drop-drop coalescence rates of MG#3 and Tullos oils in alkaline solution were compared. Coalescence rates of MG#3 and Tullos oil drops in alkaline solution were measured using the method proposed by Flumerfelt, et al. (1981). The normalized coalescence times for MG#3 and Tullos oils were 15.22 and 63.50 dynes crK2, respectively. Since this difference in coalescence rates may have contributed to the observed differences in oil recovery efficiencies, the factors influencing oil drop coalescence were examined.

The rate of coalescence has been found to depend primarily on two factors: interdroplet film thinning and interdroplet film rupture with the film thinning step considered to be rate limiting (Rosen, 1978; Flumerfelt, 1981; Hazlett and Schecter, 1986). Flumerfelt, et al. (1981) related the initial stage of film drainage

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130 V.J. CAMBRIDGE, J.M. WOLCOTI AND W.D. CONSTANT

or thinning to the sum of the interfacial shear and dilatational viscosities, the coalescence force (i.e. the force pushing the drops together), the continuous phase viscosity, and the interfacial tension. In a later paper, Flumerfelt, et al. (1982) extended the analysis to the latter stages of film thinning by taking into account the effects of London and van der Waals forces. The rate of film drainage was found to decrease with increasing surface viscosities. Since film drainage rate is directly related to interfacial viscosity, the shear viscosities of MG#3 and Tullos/alkaline water interfacial films were measured to determine the impact of film drainage rates on coalescence behavior in these systems. The interfacial shear viscosity (IFSV) for MG#3/alkaline water was 0.056mNsm-'; while Tullos/alkaline water yielded an IFSV value of 0.062mNsm-'. Since the interfacial shear viscosity values were similar, the film drainage rates would also be expected to be very close. Film drainage rates, therefore, could not account for the differences in the observed coalescence rates of these systems.

Since film drainage rates exerted a relatively minor influence, the film rupture process was assumed to be the factor controlling coalescence. This conclusion is supported by the earlier observation that mass transfer of surfactant species from the oil phase to the water phase occurred to a greater extent for MG#3 as compared to Tullos. The transport of surfactant species across the interfacial film would cause IFT fluctuations which could disrupt the interface thus causing film rupture at a greater collapse distance and enhancing the rate of coalescence.

Influence of Oil Chemistry on Emulsification and Coalescence Processes

The influences of crude oil composition on emulsification and coalescence processes were examined by studying chemical fractions isolated from MG#3 and Tullos crudes. The crude oils were separated into five fractions using ion exchange chromatography. Deasphaltenated crude was placed on an ion ex- change medium, and fractions were eluted with pentane (Fraction A), toluene (Fraction B), 10% isopropyl alcohol (Fraction C), 25% rnethanol/toluene (Fraction D) and 0.1 N HCI in 50% methanol/toluene followed by 0.2 N HCI in methanol (Fraction E). Typical weights for each fraction (per 20g deasphalten- ated crude) are listed in Table 111. The IR and NMR spectra of equivalent MG#3 and Tullos fractions were nearly superimposable indicating that a given solvent

TABLE 111

Typical weights of MG#3 and Tullos fractions isolated from 20g deasphaltenated crude

Fraction MG#3, g Tullos, g

"approximate value

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EMULSIFICATION AND COALESCENCE PHENOMENA 131

3.00

M G C 3 0 Froction 0 n Fraction C . Fraction 0 r Froclion E

TIME lminl

FIGURE 6 IFT behavior of MG#3 crude fractions against alkaline water.

extracted similar functional groups from each crude. In addition, Tullos and MG#3 fractions displayed similar IFT behavior in alkaline solution so only one set of IFT curves is shown (Figure 6). A detailed analysis of the fractions is given elsewhere (Wolcott, et al., 1989). Briefly, the fractions were characterized as follows:

Fraction A: The IR and NMR spectra were similar to those of the whole crude except that there were no carbonyl signals in the IR. The elemental analyses (Table IV) were also similar to the whole crudes except the oxygen content was reduced. In addition, acid numbers were negligible and the A fractions displayed no interfacial tension reduction on contact with alkaline water. It was concluded that the A fractions consisted of the bulk of the crude minus the surface active species.

Fraction B: Acid numbers were negligible. Elemental analyses (Table IV) showed elevated C/H ratios and enrichment in N, S, 0-content as compared to the whole crude. The IFT behavior against alkaline water (Figure 6) showed some I l T reduction but not the extent observed with the whole crude. These chemical analyses in conjunction with the IR and NMR spectra implied that the B fractions were predominately phenols. It was assumed that traces of carbonyl compounds were present since Seifert and Howells (1969) found that pure phenolic fractions isolated from natural crude oils did not contribute to IFT reduction on contact with alkaline solution. Carboxylic acids were the only components responsible for IFT reduction under these conditions.

Fraction C: Elemental analyses (Table 4) were similar to those of the B

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132 V.J. CAMBRIDGE, J.M: WOLCOTT AND W.D. CONSTANT

TABLE IV

Elemental analyses of crudes and fractions

Sample %C %H %N %S %O

Tullos Crude 86.40 12.88 0.03 0.17 0.72 Tullos A 86.38 13.07 0.00 0.17 0.66 Tullos B 84.82 8.64 0.82 0.78 5.56 Tullos C 81.26 9.40 0.88 0.63 9.15" Tullos D 81.52 12.16 0.33 0.32 6.67" Tullos E 80.10 12.17 0.08 0.33 8.04

-

'These values were obtained from samples collected at a different time than those used for the rest of the measurements.

fractions; however, the acid numbers were significantly higher. The interfacial tension in alkaline water was lower than that of the B fractions. IR spectra were also similar to those of the B fractions except that carbonyl signals were more intense. It was concluded that the C fractions contained substantial quantities of both phenols and carboxylic acids.

Fractions D and E: The chemical analyses of these fractions were very similar to each other but differed dramatically from those of the other fractions. IR spectra showed intense carbonyl signals, while NMR spectra showed mainly aliphatic hydrocarbon protons with very small multiplets in the aromatic region and broad acid proton signals. The elemental analyses (Table 4) showed reduced C/H ratios and N, S content as compared to fractions B and C; however, oxygen concentrations remained high. In addition, acid numbers were dramatically higher than those of other fractions. Interfacial tensions against alkaline water were initially very low but rose rapidly (Figure S ) , indicating the presence of water-soluble surfactants which are rapidly lost from the interface to the water phase (Cambridge, et al., 1989). It was concluded that these fractions contained carboxylic acids connected primarily to saturated hydrocarbons having some aromatic substitution.

The emulsification behavior of crude oil fractions during alkaline flooding was studied using thin flow cells (fractions B-E (0.020 g) were dissolved in toluene (5 ml); fraction A was not diluted). The thin cells were initially saturated with 1.0% NaCl brine, flooded with the appropriate oil fraction to irreducible water saturation and then flooded with an aqueous solution containing 0.2% NaOH and 1.0 NaCI. MG#3 and Tullos fractions A and B did not emulsify during alkaline flooding. This behavior was expected since these fractions contained little, if any, carboxylic acids which form active surfactant species on contact with alkaline water. Fractions C, D, and E of both crudes did emulsify so that differences in Tullos and MG#3 ease of emulsification could not be explained by behavior of the individual fractions.

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EMULSIFICATION AND COALESCENCE PHENOMENA 133

Since fractions A and B did not emulsify, Tullos fractions A and B were combined with Tullos fraction E to determine if either or both of these fractions inhibited emulsification. Concentrations of Tullos fractions B and E dissolved in fraction A approximated those found in the whole crude. The mixture containing Tullos fractions A and E emulsified during alkaline flooding; however, when fraction B was added to the previous mixture, emulsification was not observed. Fraction B, therefore, appeared to inhibit emulsification.

Ease of emulsification is obviously influenced by a combination of factors. Earlier, in the Emulsification Process section, a relatively high concentration of surfactant in the water phase was found to be necessary to produce the ultralow interfacial tension needed to precipitate emulsification during alkaline flooding. In this section, it was found that Tullos fraction A which contains the bulk of the crude emulsified even with a reduced surfactant content (fraction E alone). The ease of emulsification appeared to be related to the ratio of the carboxylic acid content (fraction D plus E) to the phenolic fraction content (fraction B). The ratio of carboxylic acid to phenol content for MG#3 is 11.54 as compared to 3.05 for Tullos indicating that additional surfactant may overcome the inhibitory effect of the phenol fraction. This fact alone, however, does not explain the propensity of Tullos to emulsify at low salt concentrations (0.04% NaCI) but not at higher salt concentrations (1.0% NaCI). Ease of emulsification must also be influenced by electrolyte concentration.

As mentioned earlier, electrolyte concentration influences the partitioning of surfactant between the oil and water phases (Chan and Shah, 1979). Electrolyte concentration also affects the concentration of surface-inactive, undissociated acid salt, NaA (Eq. (1)).

As the sodium ion concentration increases, the equilibrium in Eq. (1) is shifted to the right thus reducing the concentration of the surface-active, dissociated acid anion, A-. Chan and Yen (1982) found that higher molecular weight acids were more prone to formation of the undissociated acid salt than their lower molecular weight counterparts. Since the molecular weight of Tullos fraction E was higher than that of MG#3 fraction E (272.2 as compared to 258.0, respectively), the Tullos acids would be expected to be more sensitive to electrolyte concentration. At elevated electrolyte concentrations, the concentration of active surfactant species would be reduced leaving insufiicient active surfactant to overcome the inhibitory effects of the phenol components.

The influence of the phenolic fraction on drop-drop coalescence rates was determined by comparing the coalescence rates of the Tullos mixture containing fractions A and E and the corresponding mixture containing added fraction B. The normalized coalescence times were 17 and 81 dynes s cm-', respectively, indicating that the phenolic fraction B retards coalescence. Since, as noted earlier, coalescence rate depends on interfacial viscosity, interfacial shear viscosities of Tullos and MG#3 fractions against alkaline solution were measured to determine the effect of chemical composition. The phenolic fractions of both Tullos and MG#3 produced significantly higher interfacial shear viscosities than comparable carboxylic acid fractions (Table V). The higher coalescence rate of

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134 V.J. CAMBRIDGE, J.M. WOLCOlT AND W.D. CONSTANT

TABLE V

Interfacial shear viscosities of crude oil fractions against alkaline water

Sample JFSV, mN sm-'

Tullos Fractions A + B 0.247 Tullos Fractions A + D + E 0.054 MG#3 Fractions A + B 0.125 MG#3 Fractions A + D + E 0.037

TABLE VI

Interfacial shear viscosities of synthetic crude oillwater systems

Oil phase Water phase IFSV, nM sm-'

synthetic crude Distilled water 0.019 synthetic crude w/2-naphthol Distilled water 0.020 synthetic crude 1.0% NaCI; 0.1% NaOH 4.410 synthetic crude w/2-naphthol 1.0% NaCI; 0.1% NaOH 0.028

the Tullos mixture containing fraction B can, therefore, be explained as arising from the formation of a more viscous interface which retards film thinning and rupture.

The influence of phenols on IFSV was further examined by the use of synthetic crudes. Synthetic crudes containing a variety of pure carboxylic acids, similar to those found in natural crudes (Seifert, 1979) were prepared. 2-naphthol was added to determine the effect of added phenols. The IFSV values (Table VI) show little effect of added phenol on the synthetic crude oil/distilled water interface; however, a large difference is noted when alkaline brine solution is used as the aqueous phase. Contrary to the results obtained with the natural crude oil fractions, added phenol, in this case, drastically reduced the IFSV of the carboxylic acid mixture. Crystalline films were observed to form at the crude oil/water- interface during earlier studies of similar systems (Cambridge, et al., 1989). The crystalline film was presumed to consist of the surface-inactive undissociated acid salt which preferentially partitioned at the interface. Presum- ably, 2-napthol, which is a much smaller molecule than the majority of phenols isolated from natural crudes (Seifert and Howells, 1969) disrupted the crystalline structure of the interface producing a more fluid interfacial film. From this and previous studies (Cambridge, et al., 1989), it has become apparent that synthetic crudes frequently do not reflect the behavior of natural crude oils on contact with alkaline water.

Alcohol Additives

In an earlier study (Dahmani, et al. , 1988), alcohol additives were found to improve alkaline flood recovery efficiency. Microscopic observations of alcohol-

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EMULSIFICATION AND COALESCENCE PHENOMENA 135

TABLE VII

Comparison of MG#3 recovery efficiencies of alcohol-assisted alkaline floods with IFSV and coalescence rates

Total recoveryb Coalescence rate IFSV Alcohol' (% OOIP) (dyne. s cm-') ( m ~ sm-')

1-Pentanol 68.6 None 72.1 2-Propanol 81.9 t-Butanol 84.6

- -

'The alcohol concentration was 0.5%. bTotal oil recovered from plain waterflood in terms of original oil-in-place (OOIP) (initial oil

saturation = 87.9%, recovery from plain waterflood = 58.6% OOIP).

assisted alkaline floods in thin flow cells showed that alcohol additives improved oil-bank stability by apparently enhancing oil drop-drop coalescence rates. A comparison of oil-drop coalescence times and interfacial shear viscosities with oil recovery efficiencies is given in Table VII. Interfacial shear viscosities were found to directly correlate with recovery efficiency; while, the correlation with coales- cence data was poorer but showed the same general trend. Alcohols apparently assisted coalescence by reducing the viscosity of the interfacial film which promotes film thinning and drainage. In addition, alcohols may have increased the solubility of surfactants in the water phase (Baviere, et al., 1981) which would enhance mass transfer across the interfacial film and destabilize the interface.

CONCLUSIONS

1. Alkaline recovery was found to be promoted by emulsification followed by rapid coalescence to form a stable oil bank.

2. Emulsions were generated through a combination of ultralow interfacial tension and oil flow through pore throat restrictions. Spontaneous emulsification processes were limited to the formation of a diffuse emulsion cloud at the crude oil water interface.

3. Ease of emulsification was influenced by the aqueous surfactant concentra- tion which was governed by the composition and concentration of crude oil carboxylic acids, electrolyte concentration, and partitioning of the surfactant between the oil and water phases.

4. The phenolic fraction of the crude oil was found to inhibit emulsification. Although the mechanism responsible for the inhibitory effect is not known at this time, it is presumed to be related to the high viscosity of this fraction and its propensity to accumulate at the crude oillwater interface.

5. Coalescence rates of crude oils in alkaline water were found to be influenced primarily by film rupture with film drainage assuming a minor role. The transport of surfactant across the crude oillwater interface appeared to disrupt the interface thus enhancing coalescence. The crude oil phenolic fraction was found to inhibit

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136 v.r. CAMBRIDGE, 1.M. WOLCOTI AND W.D. CONSTANT

coalescence by increasing the viscosity of the interfacial film, thus retarding filmthinning and drainage . Alcohol additives enhanced coalescence by reducing theviscosity and destabilizing the interfacial film.

ACKNOWLEDGEMENTS

The authors are indebted to Shell Oil Co. for partial support of this project andM. Amine Dahmani for technical assistance .

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