Crude oil emulsions in high electric fields as studied by dielectric spectroscopy. Influence of interaction between commercial and indigenous surfactants

COLLOIDS AND ELSEVIER

Colloids and Surfaces A: Physicochemicaland Engineering Aspects 106 (1996) 33-47

A

SURFACES

Crude oil emulsions in high electric fields as studied by dielectric spectroscopy. Influence of interaction between commercial and indigenous surfactants Harald Fordedal a,1, Yannick Schildberg u, Johan Sj6blom a'*, Jean-Luc Voile u a Department of Chemistry, University of Bergen, All~gt. 41, N-5007 Bergen, Norway b Elf Aquitaine Production, F-64000 Pau, France Received 10 March 1995; accepted 20 July 1995

Abstract

The behaviour of different types of water-in-oil emulsions in high electric fields as investigated by means of timedomain dielectric spectroscopy (TDS) is reported. The studied emulsions include true crude-oil-based ones as well as model systems stabilised by indigenous crude oil fractions or, alternatively, by commercial nonionic surfactants. It is seen that the developed TDS equipment gives a good quantitative measure of the emulsion stability. The emulsion stability in crude oil systems can be modelled by the separated asphaltene fraction as far as coalescence is considered. Although the resin fraction might be even more interfacially active than the asphaltenes, it cannot alone stabilise the w/o emulsions. The importance of the interplay between the asphaltenes and resins is clearly revealed. When commercial surfactants (ethoxylated nonyl phenols, NP-EO or monoalkyl sorbitan esters) are combined with the separated crude oil fractions, different levels of compatibility are displayed. The addition of a tetraoxyethylene nonyl phenol ether (NP-4), for instance, completely destabilises the original emulsion, although a high level of interfacial activity is retained in the system.

Keywords: Crude oils; Dielectric spectroscopy; High electric fields; Interfacially active fractions; Interfacial tension; W/o emulsions

1. Introduction An agitation of brine and crude oil, can result in a stable system where the water is dispersed in the crude oil. Naturally occurring stabilisers present in the oil adsorb at the water-crude oil interface to stabilise the aqueous droplets against destabilisation mechanisms like coalescence. The total interfacial area of the dispersed droplets is very large compared with the volume of the phases,

*Corresponding author. E-mail:[email protected] i

E-mail: [email protected]

0927-7757/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03354-8

Hence, processes taking place at the w/o interface are of crucial importance. In order to understand the stability of emulsions it is important to understand these processes. If the surface membranes are stable against coalescence the water can be dispersed in the oil for a long time. For several reasons it is of interest to dehydrate the crude oils. Under the right conditions, water can form gas hydrates [1], and it is also responsible for scaling and corrosion problems. A stable w/o emulsion increases the viscosity of the emulsified system tremendously in comparison with the neat crude oil, which increases the pumping costs during transport.

34

I-L Fordedal et al./Colloids Surfaces A." Physicochem. Eng. Aspects 106 (1996) 33 47

There exist several different techniques for extracting the water from the oil, i.e. heat treatment techniques, centrifugation, addition of demulsifiers, application of high electric fields, etc. [2]. Dielectric measurements on emulsified systems have been reported previously in the k H z - G H z region [3]. With time-domain dielectric spectroscopy the interfacial polarisation mechanism that occurs in w/o emulsions in the MHz region, can be fully characterised, all in one measurement. This implies that the changes in the dielectric parameters can be monitored as the electric field is applied to the emulsified system. From the levels of the measured response signals, the macroscopic conductivity of the sample can also be determined. In previous papers we investigated the stability of model emulsions in high electric fields I-4,5]. In high electric fields an irreversible rupturing of the stabilising membranes can occur, while at lower electric fields a reversible mechanism can be observed. At lower fields the water droplets in the emulsion attain a linear chain-like configuration, but the electric field is not high enough to induce coalescence. When the electric field is switched off the system returns to a random distribution of the aqueous droplets, which explains the reversibility. When the external electric field exceeds a critical value the surfactant membrane protecting the aqueous droplet will rupture and the droplets coalesce, which explains why this process is irreversible, In this paper we investigate the stability of different kinds of water-in-crude oil emulsions. These are true crude-oil-based emulsions, model emulsions stabilised by indigenous components separated from crude oil, and finally emulsions based on completely synthetic components. From our background in characterisation of natural sur-

factants, and knowing their influence upon the emulsion stability, we decided to compare these emulsions by characterising the emulsion stability by means of dielectric measurements at high external electric fields. This study also includes interfacial tension measurements and gravity tests on the emulsions and molecular weight (Mw) characterisations on the extracted interfacially active fractions.

2. Experimental

2.1. Chemicals Crude oil B1 is from an Elf production field in France, while B2 is an Elf crude from the North Sea, and G is a Statoil crude from the North Sea. The commercial surfactants used are tetraoxyethylene nonylphenolether (CgPhE4, NP-4), and octaoxyethylene nonylphenolether (C9PhEs, NP-8) from Berol Nobel Industries, Sweden, and sorbitan monolaurate (Span20) and sorbitan monooleate (Span80) from Sigma, and they were used as supplied. The pentane (>97%, Riedel de Hahn), decane (> 95%, Merck AG), octanoic acid (>98%, Fluka AG), decanol (97%, Fluka AG), methanol (> 99.8%, Merck AG) and dichloromethane (> 99.8%, Fisons) were also used without further purification. The electrolyte solution was prepared by dissolving 50 g NaC1 in 1000 ml of distilled water, giving a salinity of 5%. The adsorption of the crude oil components was performed onto Porasil ® Silica 125 ,~ from Waters Millipore Corp.

2.2. Methods The procedure for extracting the active fractions from the crude oils is presented in Fig. 1. More details of this procedure can be found in Refs. [6,7]. The totalactive fraction consists o f a m i x t u r e of the precipitated and adsorbed fractions.

2.2.1. Precipitated active fraction (asphaltene) 40 ml of the crude was mixed with 200ml of pentane at room temperature. The mixture was centrifuged for 10min at 2000 rpm, in order to Crude oil

,---p~n~e

centrifugation

]

I

I

Precipitated fraction

soluble adsorpt on onto s ica particles

]

I

I Remaining crude

Adsorbedfraction Fig. 1. S e p a r a t i o n

process.

H. Fordedal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 33-47

precipitate the heaviest fractions. The precipitate was filtered from the mixture. In the following this heavy fraction is called asphaltene. The supernatant is used in the adsorption procedure for recovering the other active fractions as described below. 2.2.2. Adsorbed active fraction (resin) The supernatant from the centrifugation step was agitated with added silica particles until the black colour of the supernatant disappeared. The silica with the adsorbed fractions was filtrated. The adsorbed fractions were dissolved in a mixture of 7% methanol in dichloromethane, and the solid silica was removed by filtration. The methanoldichloromethane mixture was then evaporated; in the following the remaining fraction is called resin,

As stabilisers for the w/o emulsions, we used different ratios of these precipitated and adsorbed fractions.

2.2.3. Interfacial tension measurements The interfacial tension between the aqueous solution and decane-containing dissolved inter-

facially active fractions was measured with the ring method with a KSV Sigma 70 Tension meter (KSV Chemicals, Finland). 2.2.4. Molecular weight measurements Filter papers with active fractions were immersed in T H F at room temperature. The extract was evaporated, weighed and redissolved in THF. The molecular weights were determined with Size Exclusion Chromatography (HPSEC) on Varian TSK gel columns using T H F as eluent, and with a RI detector (refractive index). The molecular weight calibrations were based on polystyrene M W standards, 2.2.5. Emulsification The emulsions were prepared by dissolving the stabilisers in 25 ml of decane, and mixing with equal amounts of water solution at 1000 rpm on a Silverson Laboratory Mixer Emulsifier Model STD 1, using an emulsor screen head. The components were mixed for 3 min before being transferred to graded cylinders, where the separation of the dispersed phase and/or the decantation of oil from

35

the w/o emulsions were studied under normal gravity conditions. 2.2.6. Dielectric spectroscopy The dielectric spectra were obtained by using the time-domain dielectric spectroscopy (TDS) technique [-8,9]. A Hewlett Packard pulse generator (HP54123A) generates a fast rising ac step pulse ( ~ 2 0 0 m V ) which propagates through a coaxial line and is reflected from the cell placed at the end of the coaxial line. The response signals reflected from the sample and from the reference liquids were observed on a H P digitising oscilloscope (HP54120B), and transferred to a PC. A software program carries out a Fourier transform, from which the dielectric spectrum is obtained:

e*(og) = e'(o~)-ie'(e~).

(1)

The bilinear calibration procedure of Cole [10] was used to calibrate the measuring cell. This procedure takes into account mismatches both along the coaxial line and at the terminating cell. Two reference liquids are required to determine the calibration parameters; in this case heptane and methanol were used. The pulse shapes were observed in a time window of 25 ns. The pulse shapes were Fourier transformed and the permittivities were calculated at 75 frequencies between 40 MHz and 4 GHz. In this study the sample cell was arranged in the form of a lumped capacitance at the end of the coaxial line, as shown in Fig. 2. A mylar film 190 lam thick separates the electrodes, and determines the cell length. The critical electric field was measured using a Metrix AX 322 power supply connected to the Hewlett Packard pulse generator and the digitising oscilloscope, as shown in Fig. 3. In addition to the fast rising measuring step pulse, a dc electric field is applied through a bias tee. The bias tee includes a dc block which allows the application of a voltage over the sample cell, while it passes the fast rising step pulse used to measure the permittivity spectrum without degrading the step pulse. Ecr is the critical electric field at which the first signs of macroscopic conductivity are observed. The total permittivity e*ot(~o) calculated from the Fourier transform of the pulse shapes, in addition

H. Fordedal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 33-47

36

Sample

~Z/Jf_Z/////~ KeI-F

1

7 mm line to APC7--SMA adapter Fig. 2. Dielectric measuring cell.

~

AC+DCout c~

"

Bias Tee

AC in

Es - - 6oo (i(.oz) 1 - e '

e*(o)) = eoo + 1 +

l'~eg~etator [

--1

DC in

the sample and the reference liquids. For a sample showing conductivity, the level of the reflected pulse will not reach the level of the incident pulse. The spectra obtained were fitted to a Cole-Cole model function [ 11 ],

I-IP

Oscilloscope

(--~ ~A..~

-

'

(3)

where ~ is the mean relaxation time, es is the static permittivity, and e~ is the permittivity at high frequencies, c~ gives the distribution of relaxation times around a mean relaxation time

Computer

Fig. 3. Setup for the time domain dielectric spectroscopy measurements, based on the high electric field principle.

to the dielectric part e*(o)), includes a contribution from dc conductivity, a, i.e. ia E't*ot = ff*((D) -- - - , (2) e)eo where eo is the permittivity of free space. In timedomain dielectric spectroscopy a can be obtained from the final levels of the pulses reflected from

3. Results The procedure for extracting the active fractions from the crudes is presented in Fig. 1. The extracted fraction is divided into precipitated fractions (asphaltenes) and adsorbed fractions (resins). The molecular weights (Mw) of the fractions from the three crudes are listed in Table 1. The asphaltenes from the crude B1 have higher Mw than those from B2 and G, respectively. The same trend is observed for the resins, which is presented in Fig. 4.

H. Fordedal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 33-47

Table 1 Molecular weightsfor resins and asphaltenesfor the crudes Mw B1 asphaltene

4200

B1 resin B2 asphaltene B2 resin G asphaltene G resin

1700 1300 1200 1400 900

A typical dielectric spectrum of a w/o emulsion is presented in Fig. 5, showing the dielectric dispersion typical for w/o emulsions with a conducting aqueous phase. A schematic illustration of the "apparent dipoles" as a function of the frequency of the alternating electric field is included. The dielectric dispersion is characterised by a high permittivity at low frequencies and a low permittivity at high frequencies. The volume fractions separated and decanted in the bottle tests, of the disperse and continuous phases, respectively, for model emulsions with commercial surfactants are listed in Table 2. The corresponding data for emulsions based on the natural surfactants extracted from the crudes B1, B2 and G are listed in Tables 3, 4 and 5, respectively. As seen from the tables, some emulsions are stable over long periods of time, while others separate immediately. Large differences are observed in the data, varying with the amount and the nature of the stabiliser used. When only natural surfactants, i.e. asphaltenes and resins, are used as stabilisers, the interfacial tension is at a relatively high level, 24-32 mN m -1. As commercial surfactants are added, the interfacial tension is reduced to a level of 0.3-5 mN m-1. For the mixed systems containing both natural and commercial surfactants, the bottle tests also reveal large differences in emulsion behaviour. The volume decanted/sedimented of the emulsion varies with the stabiliser used. For NP-4, 90% of the continuous phase decants after 12 h, while for Span 80 the corresponding value is only 5%. When the commercial surfactant NP-4 is added to an emulsion stabilised by natural surfactants, the stability of the system completely disappears, At the same time the interfacial tension measured

37

is very low. This effect occurs in both asphaltene

and resin systems, as well as in systems with both asphaltenes and resins. Such phenomena are not observed for any of the three other commercial surfactants tested. As the length of the hydrophilic part of the nonylphenol-based surfactant is increased from 4 to 8 ethoxy units, the emulsion inverts from a w/o to an o/w emulsion. In these o/w emulsions macroscopic conductivities are observed. The conductivities calculated from the final level of the reflected pulses are listed in Table 6. These conductivities are 1/2-1/3 of those calculated in corresponding bulk electrolyte solutions. Some of the emulsions prepared show a slight macroscopic conductivity, even though they are not o/w emulsions. The values of these macroscopic conductivities are listed in Table 6, together with the values for the o/w emulsions.

4. Discussion

4.1. Behaviour ofw/o emulsions in high electric fields When w/o emulsions are placed in an electric field they form droplet chains as a result of polarisation of the electrolyte in the water domains. This phenomenon was first observed by Cottrell early in this century [12-14]. If the electric field is sufficiently high, an immediate coalescence of the water droplets occurs, and the water phase settles out under gravity. In the oil industry much effort has been focused on the design of new coalescers, in order to obtain optimal conditions for the resolution of the dispersed water with respect to the electric field, such as pulse frequency and electric field intensity [15-17]. Although many authors have reported on the demulsification of emulsions utilising the electrostatic coalescence method, only a limited number of papers have been related to the emulsion characteristics and the processes taking place at the interfaces. However, more recent publications have tried to contribute to a deeper knowledge of the interfacial properties of the membranes separating the water domains, and the actual processes involved in

O. 000

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30.00

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.o.oo-

--

. OOO

---7

50.O0!

~o.oo~

,

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!

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r

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B1

i

~

I

~

l~--r----7

Log Mw

F

G

BI

I 4. 000

--~---

\\

'

!

i

Fig. 4. Molecular weight distribution of the resins extracted from the crudes B1, B2 and G.

!--

G

B

-

i

~

i

;

__ .............................. ~--i

Cum

S. 000

]

Area

Mw

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H. Fordedal et al./Colloids Surfaces A. Physicochem. Eng. Aspects 106 (1996) 33-47

39

COMPLEX PEFIMITTIVITY I

i

i

i iiiiii

i

i i iiiiii

2o

':

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]

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0

!

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i

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FREQUENCY [MHz] I

) increasing frequency Fig. 5. Dielectric spectrum of w/o emulsion stabilised by asphaltenes and resins extracted from crude B1. The different subpictures show the different "dipoles" as a function of the frequency of the ac field,

g

=

N ,~

~ az

"O

breaking the oil-continuous emulsion in electric fields• Mohammad et al. have conducted fundamental studies of dewatering of crude oil emulsions [ 18-21]. Their investigations include rheological and interfacial conditions of the emulsions, water resolution by chemical means and an application of an ac external electric field. In Ref. [21] they show that their experimental findings from optical microscopy studies of crude oil emulsions in electric fields, can be modelled with computer simulations based on a hard sphere model describing the droplets stabilised by a rigid asphaltene film. Forces arising from electrostatic, thermal (Brownian) and hydrodynamic effects are discussed. Taylor reports on the resolution of water from water-in-crude oil emulsions 1-22,23], using conduction current profiles and optical microscopy. The coalescence behaviour was highly dependent on the properties of the interfacial

~ ~ ~ ~ ~

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~ ~ ~.~ ~ ~

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= .~ ~, ~ ~ 2

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Span 80

Span 20

NP-8

w/o w/o w/o

w/o w/o o/w

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w/o

w/o w/o w/o w/o o/w o/w ojw

Crude 2% asph. 1% asph. 2% res. 1% res. 1% asph. + 1% res 1% asph. + 1% NP-4 1% res. + 1% NP-4 0.75% asph.+0.75% res.+0.5% 1% asph. + 1% NP-8 1% r e s . + l % NP-8 0.75% asph,+0,75% res.+0.5% 1% asph.+ 1% Span 20 1% res.+ 1% Span 20 0.75% asph,+0.75% res.+0.5% 1% asph.+ 1% Span 80 1% res.+ 1% Span 80 0.75% asph,+0.75% res.+0.5% 1% a s p h . + l % decanol 1% res. + 1% decanol 0.75% asph.+0.75% res.+0.5% 1% asph. + 1% octanoic acid 1% res. + 1% octanoic acid 0.75% asph,+0,75% res.+0.5% acid NP-4

w/o or o/w

50 : 50 emulsion (by volume) + additives to the oil phase

0 0 0 100 i00 0 100 I00 100 2 2 12 0 8 0 0 0 0 73 100 0 0 100 0

0 0 0 100 i00 0 100 100 100 2 2 12 0 15 0 0 0 0 75 100 0 0 100 0

0 5 15 100 1013 15 100 100 1013 8 12 35 0 0 3 0 0 0 0 100 16 60 100 20

0 20 25 100 100 30 100 100 100 48 55 65 1 3 20 2 2 5 0 100 32 70 100 40

after 12 h

after I h

after 1 h

after 12 h

Separation continuous phase [%)

Separation dispersed phase (%)

Table 3 Parameters measured for emulsions stabilised by interfacially active fractions from Crude B1

20.1 26.5 29.3 28.0 32.7 30.3 0.3 0.3 2.1 0.4 0.5 0.6 1.9 2.1 1.5 3.7 4.2 3.6 31.6 27.4 29.7 26.8 27.7 23.7

,/ (raN m 1)

7.8 9.1 8.8

c~

120 180 200

r (ps)

0.15 0.12 0.11

~

20.7

10.5 245 0.09 . . . . . . . . . . 9.1 7.4 120 0.10 11.0 7.7 115 0.15 11.6 7.8 130 0.11 10.1 7.8 105 0.11 9.4 7.6 100 0.11 17,0 9.5 150 0.12 . . . . 19.0 9.3 270 0.12 23,0 8.4 410 0.26 14.9 8.9 180 0.11

10,0 15,9 15.1

c

0.42

0,37 0.03

0.13 0.03 0.05 0,37 0.37 0.34

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0.84 0.81 0.32

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H, Fordedal et al. 'Colloids Surfaces A: Physicochem. Eng, Aspects 106 (1996) 33 47

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41

m e m b r a n e s , a n d two distinctly different types of coalescence were identified. One, where m o b i l e interfacial films lead to low e m u l s i o n c o n d u c t i v i t y due to i m m e d i a t e d r o p l e t - d r o p l e t coalescence, a n d a n o t h e r where i n c o m p r e s s i b l e interfacial films lead to an increase in the e m u l s i o n c o n d u c t i v i t y due to d r o p l e t chain f o r m a t i o n , leading to a b r i d g i n g of the electrodes. In these studies dilute w/o emulsions were characterised, i.e. the water c o n t e n t was only 5%. H o w e v e r , if the a m o u n t of the disperse phase increases, the i n t e r d r o p l e t distance decreases due to v o l u m e t r i c reasons. In a d d i t i o n , a high degree of flocculation occurs. In a previous p a p e r we investigated the b e h a v i o u r of m o r e c o n c e n t r a t e d w/o emulsions in high electric fields by m e a n s of t i m e - d o m a i n dielectric s p e c t r o s c o p y [ 4 ] . In these type of emulsions the response to the external electric field is d e p e n d e n t on the b e h a v i o u r of the floc in this field. The coalescence p a t t e r n d e p e n d s on several factors, such as the i n t e r d r o p l e t distance, the p o t e n t i a l difference over the stabilising film, the a p p l i e d electric field, the d r o p l e t sizes a n d the direction of the electric field relative to the orientation of the droplets. The highest p o t e n t i a l d r o p over the stabilising m e m b r a n e is found for d r o p l e t s o r i e n t e d parallel to the electric field. In a flocculated system the emulsion d r o p l e t s are no longer discrete droplets, but in m o r e densely p a c k e d clusters where the stabilising m e m b r a n e can be d e f o r m e d into thin p l a n a r films, built up by bilayers of the stabilising molecules. An exact thickness of such a thin film c a n n o t be given, but a g o o d estimate is twice the length of the fully e x t e n d e d h y d r o p h o b i c p a r t of the stabilising molecule. A p o t e n t i a l difference of 0.5 1 V over a distance of 4 0 A , results in a field of a p p r o x i m a t e l y 103 kV cm ~ in the film. In o r d e r to d e t e r m i n e h o w several significant factors r e g a r d i n g e m u l s i o n stability influence the critical electric field, a m u l t i v a r i a t e screening of the factors was p e r f o r m e d [ 5 ] . This technique p r o v i d e s i n f o r m a t i o n a b o u t the general trends of the variables investigated. The investigation d e m o n s t r a t e d that a m o n g the variables the level of the dispersed phase, the salinity a n d the a m o u n t of the surfactant were the three m o s t i m p o r t a n t in d e t e r m i n i n g the emulsion stability.

octanoic

decanol

Span 80

Span 20

NP-8

NP-4

w/o w/o

Crude 2% asph. 1% asph. 2% res. 1% asph.+ 1% res 1% asph.+ 1% NP-4 1% res.+ 1% NP-4 1% res. + 1% NP-4 0.75% asph. + 0.75% res. + 0.5% 1% asph.+ 1% NP-8 1% res. + 1% NP-8 0.75% asph.+0.75% res.+0.5% 1% asph.+ 1% Span 20 1% r e s . + l % Span 20 0.75% asph.+0.75% res.+0.5% 1% asph. + 1% Span 80 t% res.+ 1% Span 80 0.75% asph.+0.75% res.+0.5% 1% asph. + 1% decanol 1% res. + 1% decanol 0.75% asph.+0.75% res.+0.5% 1% asph.+ 1% octanoic acid 1% res. + 1% octanoic acid 0.75% asph. + 0.75% res. + 0.5% acid o/w o/w o/w w/o w/o w/o w/o w/o w/o w/o* w/o* w/o

w/o

w/o or o/w

50 : 50 emulsion (by volume) + additives to the oil phase

0 0 100 100 100 0 100 100 100 5 10 5 0 5 0 0 3 0 100 100 0 0 100 0

0 0 100 100 100 0 100 100 100 5 10 5 0 5 2 0 25 0 100 100 0 0 100 0

0 50 100 100 100 45 100 100 100 5 10 12 0 12 2 0 2 0 100 100 70 85 100 85

0 90 100 100 100 90 100 100 100 15 15 20 8 60 10 8 10 8 100 100 70.3 85 100 85

after 12 h

after 1 h

after 1 h

after 12 h

Separation continuous phase (%)

Separation dispersed phase (%)

31.5 28.5 2.6 0.3 3.2 0.8 0.5 0.7 1.6 1.9 1.5 3.5 4.7 4.3 30.4 30.3 0.3 29.4 30.2 29.4

10.6 24.7 32.7

~ (mN m 1)

. 10.1 .

8.9 9.7

eo~

. 145 .

120 140

r (ps)

25.4

9.9

355

. . . . 14.0 8.5 190 16.0 8.6 180 11.3 8.3 170 12.2 8.4 170 15.0 8.9 170 10.6 7.9 160 . . . . . . . . . . . _

. . 16.3 . .

13.3 15.1

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0.18

0.11 0.13 0.09 0.07 0.08 0.07

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0.04 0.05 0.03 0.26 0.05 0.13

-

0.13

0.26 0.24 -

Eor (kV cm 1)

Table 5 Parameters measured for emulsions stabilised by interfacially active fractions from crude G (*, response signal as for w/o emulsion but at the same time a slight macroscopic condictivity, or)

--.a

I

o,

'~ ~

.~.

~'~

4~

H. Fordedal et al./Colloids Surfaces A. Physicochem. Eng. Aspects 106 (1996) 33-47

Table 6 Macroscopic conductivity calculated from the level of the reflected pulses, for o/w emulsions and emulsions with nondispersed water a

(S/m) 5 wt.% NaC1 (bulk)

B1 B1 B1 B1 B2a G G G Ga G~

NP-8 1% a s p h . + l % NP-8 1% res.+ 1% NP-8 0.75% res.+0.75% asph.+0.5% NP-8 1% a s p h . + l % decanol crude 1% asph.+ 1% NP-8 1% res.+l% NP-8 0.5% Be267+0.75% res.+0.75% asph. 1% a s p h . + l % octanoic acid

0.75% res.+0.75% asph.+0.5% decanol

6.426

2.642 2.505 2.669 2.376 2.475 0.515 2.219 2.823 2.438 1.790 0.317

In a series of papers Sjrblom et al. have performed systematic characterisations of water-incrude oil emulsions from the Norwegian continental shelf. The aim was to obtain information on the properties of the naturally occurring interfacially active components, and the formation, stability and destabilisation of the water-in-crude oil emulsions. In these works different experimental techniques were utilised, like interfacial tension [24,25], Fourier transform infrared spectroscopy [7,26], Langmuir-Blodgett [27], dielectric spectroscopy measurements [26,28], and droplet size distribution based on N M R and videoenhanced microscopy characterisations [29]. Based on these data for the natural surfactants, and knowing their stabilising influence, we decided to characterise the emulsion stability by means of dielectric measurements at high external electric fields, Crude oils are mixtures of numerous aliphatic and aromatic hydrocarbons, and oxygen, nitrogen and sulphuric compounds. Some of these compounds are surface-active in nature, i.e. they can adsorb to water--crude interfaces and stabilise the system against coalescence. Two such classes of compounds are asphaltenes and resins. They are polymeric in nature and have structural similarities, as observed from infrared spectroscopy [6,30].

43

There exist different definitions of asphaltenes and resins, depending on the scientific point of view of the investigation, msphaltenes are often defined as the pentane-insoluble and benzene-soluble portion of the crude oil [31]. However, in this work we chose to call the precipitated fraction from pentane treatment asphaltenes, and the silica adsorbed fraction resins, as shown in Fig. 1. The nature and the relative amount of asphaltenes and resins from a crude varies, depending on the origin of the crude. The differences depend on several factors such as the nature of the source rock, the thermal evolution of' the sedimentary organic material, the migration processes and the efficiency of the petroleum trap [32]. Some production fields contain oil with a high degree of light components, while other fields contain high content of heavy fractions, waxes and sand, depending on the combinations of all the factors listed above. The interfacially active fractions extracted from the crude oils B1, B2 and G have different amounts and ratios between asphaltenes and resins. The differences in molecular weight between the asphaltenes and the resins are listed in Table 1. The molecular weights of the asphaltenes extracted from B1 are as much as three times higher than those extracted from B2 and G. The corresponding molecular weight .measurements of the resins are shown in Fig. 4, and show the same consecutive order. From Fig. 4 it is clearly seen that the resins from B1 have both a smaller portion of light compounds as well as a higher portion of heavier compounds than B2 and G. On the other hand, these two crude oils show small differences in Mw between the asphaltene and the resin fractions. This indicates, in good accordance with our previous work on similar constituents, that the distinction between the asphaltene and resin fractions is not so well defined [33]. Since the resins from B1 have approximately the same Mw as the asphaltenes from B2 and G, these asphaltenes can more or less be defined as heavy resins, which is also in accordance with [34,35]. This fact influences the ability of these fractions to stabilise the emulsions. In this work we have investigated the influence of a high electric field on the water-in-crude oil emulsions, in order to classify the ability of natural

44

H. Fordedal et aL/Colloids SurJaces A: Physieochem. Eng. Aspects 106 (1996)33-47

surfactants to stabilise and prevent coalescence, Our systems include true crude-oil-based emulsions, model emulsions stabilised by indigenous components separated from crude oils and emulsions based on completely synthetic components, For crude oil emulsions the highest E~r is obtained for crude B1. For this crude no visual separation of the aqueous phase is observed within 12 h, and a relatively high electric field is required in order to irreversibly break this emulsion. The corresponding values of the electric field for crude oil G emulsions are distinctly lower. The emulsion based on B2, on the other hand, shows a slight macroscopic conductivity, indicating that the interfacially active fractions present in the crude are not capable of dispersing efficiently as much as 50% water, Emulsions stabilised by 2% of asphaltenes show similar stabilities as the original crude oil emulsions. An emulsion stabilised by 2% of asphaltenes breaks at an electric field close to that required to break the crude oil emulsion. This seems reasonable, since the actual amounts of asphaltenes in the crudes often lie around 2 3%. This is observed for both the B1 and G crude oil. The level of the critical electric field is much lower for the G emulsions than for B1 emulsions. Similarly, 1% of asphaltenes from B1 is capable of stabilising the emulsion, while 1% of asphaltenes from G is not. This can again be explained by the difference in polarity of the compounds, as revealed from the Mw measurements. The interfacially active fractions extracted from B2 are poor stabilisers. Even 2% of asphaltenes are not capable of stabilising the emulsion. Of the systems investigated, only the emulsion stabilised by 1% of asphaltenes and 1% resins from B2 is stable, indicating that the interplay between the asphaltenes and the resins is also important. However, a relatively low electric field is required to irreversibly break this emulsion, A general trend observed for all crudes is that the resins alone are not capable of stabilising the emulsions. After being agitated the phases separate within seconds, The experimental data clearly indicate that the stabilising fraction is the asphaltenes, and not the resins, although the resins can show a high interfacial activity as judged from interfacial tension measurements in Tables 3-5. However, when these

fractions are combined, in most cases the result is a stabilisation of the water-in-crude oil emulsion. When 1% of asphaltenes is combined with 1% of resins the resulting emulsions show lower stability than the original crude emulsions and those stabilised by 2% of asphaltenes. This also rules out the possibility that these emulsions should be stabilised by the asphaltenes alone. Hence the active interaction between asphaltenes and resins will give rise to stability. Obviously, due to their higher interfacial activity, the resins will be the first species to reach the w/o interface. After this the asphaltene molecules will accumulate at the interface, building up a complex macromolecular constellation at the interface. In order to preserve the stability level of the emulsions this new interface must have some essential properties with regard to rigidity of the original asphaltene interface. Hence one can anticipate a critical ratio between asphaltenes and resins for the interface to have necessary rigidity. In the petroleum industry numerous different additives are added in the production, transportation and separation steps in order to inhibit different processes, like crystallisation, foaming, bacteria growth, corrosion, emulsion formation, etc. These species have specific purposes, but in many cases can interact with ionic as well as nonionic compounds in the crude oils. In order to study the effects of different additives upon the emulsion stability we have chosen to introduce different surface-active molecules as well as polar molecules to our emulsions. The commercial surfactants used in this study represent different categories with regard to molecular structure. The Berol surfactants used are nonylphenol-based, with 4 (NP-4) and 8 (NP-8) ethoxy units, respectively. The EO4 derivate is oil soluble, while EO8 is water soluble. Their cloud points are 44 and 18°C, respectively [36]. The Span molecules are monoalkyl sorbitan esters. Here we have chosen to vary the chain length of the molecule, going from lauryl (C12, Span 20) to oleyl (C18, Span 80). We have also chosen to introduce other polar additives like octanoic acid and 1-decanol, both of which have a marked preference for location in the oil phase. However, when combined with surface-active agents the alcohol and the fatty acid tend to pack at the w/o interfaces.

H. Fordedal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 33-47

Similar investigations for the commercial surfacrants as for the crude-oil-based systems, also reveal large differences in the critical electric field. The Ecr required to break an emulsion stabilised by Span 80 is distinctly higher than for Span 20 and NP-4. The difference in stabilising behaviour can to some extent also be observed from the decantation/sedimentation data. Emulsions stabilised by NP-4 and NP-8 sediment until only 10% of the continuous phase separates the droplets in the emulsion. The percentage of the dispersed phase in the rest-emulsion thus increases from 50% to approximately 90%. Span 80, on the other hand, stabilises emulsions that do not readily sediment, since only 5% of the continuous phase separates, Decantation/separation is mainly a result of flocculation processes taking place in the emulsion, and the decantation data in Tables 2-5, clearly show that there are distinct differences among the commercial surfactants, and among the different interfacially active fractions. Microscopy pictures show that Span 80gives small, rather monodisperse droplets under our mixing conditions, while Span 20 and NP-4 give substantially larger and more polydisperse droplets, When two or more stabilisers are present in the same mixture a competitive adsorption between the different emulsifiers can occur, which can be of crucial importance for the emulsion stability. A stabiliser can be removed from the interface if another stabiliser is added in sufficiently high concentrations, resulting in a loss of functionality for the removed stabiliser. The addition of Span surfactants to an emulsion stabilisedby asphaltenes and/or resins still give stable w/o emulsions, but the level of Ecr is distinctly lower than those observed for the pure emulsions stabilised by either commercial surfactants or natural surfactants. However, a conspicuous behaviour is observed when NP-4 is added to an emulsion stabilised by interfacially active fractions. As a consequence of the addition, the stability of the emulsion vanishes, regardless of how the system is stabilised. At the same time the interfacial tension is at a very low level, which indicates a high interfacial activity, This effect is not observed for the other three surfactants. This might be due to the conditions in the continuous media between the droplets. Phase

45

diagram studies of the components involved might reveal interesting details. Many of the effects referred to essentially reflect differences in droplet sizes and their distributions. Obviously the sorbitan esters will create basically smaller droplets than the nonylphenol-stabilised emulsions. In addition to the pure droplet sizes, the flocculation level of the system will of course dictate the sedimentation (decantation). Also in this respect the NP-based systems show less protection. A lengthening of the hydrophilic part of the NP surfactants, from 4 to 8, gives rise to o/w emulsions. This is simply a result of the changes in the interfacial conditions as the hydrophilic-lipophilic balance is altered. Changes in the length of the hydrocarbon chain of the Span surfactants also influence the emulsion stability. The Span 80 emulsions are more stable than the Span 20 emulsions. The polar head group is the same for these two surfactants. The longer oleate chain in Span 80 seems to better stabilise the emulsion than the shorter laurate chain in Span 20, which must be due to a more favourable packing of the surfactants in the stabilising membrane. This indicates that the lack of stabilising effect of the NP-4 can be due to the packing conditions when the surfactants and the interfacially active fractions are acting at the interface simultaneously. Some of the emulsions prepared are o/w emulsions, revealing a macroscopic conductivity. The level of the conductivity is far lower than that in the water solution with sodium chloride. This reduced conductivity must be due to a droplet obstruction. Jrnsson et al. [37] investigated the self-diffusion of small molecules in colloidal systerns, and found that the level of the diffusion was 1/3-2/3 of free diffusion, depending on the shape of the aggregates. Based on experiments with latex particles and computer simulations, their investigation clearly demonstrated that the shape of the aggregates strongly influences the self-diffusion. From Table 6 it can be seen that the conductivities calculated from the final level of the reflected pulses are reduced to 1/2-1/3 of that observed in bulk electrolyte solution. This reduction in conductivity must be due to the obstruction effect caused by the emulsion droplets, as they impede the

H. Fordedal et al./Colloids Surfaces A: Physicochem. Eng. Aspects 106 (1996) 33-47

46

diffusion of the ions, and increase the path length of the ions. The conductivities calculated must be highly dependent on the amount of disperse phase, degree of flocculation, degree of particle deformation, sedimentation/creaming, etc. All of these factors influence the conductivity, and make a detailed comparison with J6nsson's models and results difficult.

5. Conclusions In this work we have demonstrated that the value of the critical electric field is a good quantitative parameter for characterising emulsion stability. Changes in the amount and nature of the stabilising compounds reveal changes in the interfacial properties of the emulsions. These are also reflected as changes in the interfacial tension, the decantation/ sedimentation process, the dielectric parameters, as well as in the critical electric field. The differences

that are visually observed in the decantation/sedimentation data are quantified through the value of the critical electric field. The observed differences in the properties of the interfacially active fractions can be explained by differences in polarity and molecular weights of the interfacially active fractions extracted from the different crude oils, together with their interactions. The results include stabilities of emulsions based o n different commercial surfactants, and combinations of commercial and natural surfactants.

Acknowledgements We thank Jarl Hemming, Abo Akademi in Finland, for performing the molecular weight Y.S. acknowledges Elf Aquitaine for financial support. H.F. acknowledges the Norwegian Research Council for a PhD grant. Financial support from Statoil, Saga Petroleum measurements.

and Elf Aquitaine is also highly appreciated. Statoil a n d Elf Aquitaine are also thanked for providing us with the crudes.

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[33] Y, Schildberg, J. Sj6blom, A.A. Christy, J.L. Voile and O. Rambeau, J. Dispersion Sci. Technol., in press, 1995. [34] K.J. Leontaris and G.A. Mansoori, Soc. Petr. Eng. J., 16258 (1987)149. [35] H. Lian, J.R. Lin and T.F. Yen, Fuel, 73 (1994) 423. [36] Datasheets from the manufacturer, Berol Nobel Industries, Sweden. Cloudpoint for NP-4 determined from 5 g product in 25 ml of 25% butyldiglycol solution, and cloud point for NP-8 determined from 1% product in water. [37] B. J6nsson, H. Wennerstr6m, P.G. Nilsson and P. Linse, Colloid Polymer Sci. 264 (1986) 77.