Dewatering of crude oil emulsions 2. Interfacial properties of the asphaltic constituents of crude oil

C&ids and Surfaces A: Physicochemical and Engineering Aspects, 80 (1993) 237-242 0927-7757/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

Dewatering of crude oil emulsions 2. Interfacial properties of the asphaltic constituents crude oil R.A. Mohammed”,

A.I. Bailey”**, P.F. Luckham”,

237

of

S.E. Taylorb

‘Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BY, UK bColloid Science Branch, BP Research Centre, Chertsey Road, Sunbury-on-Thames, Middlesex, TW16 7LN, UK (Received

3 February

1993; accepted

27 April 1993)

Abstract The importance of the interfacial rheology in determining the stability of water-in-Buchan crude oil emulsions has been demonstrated in part 1 of this series of papers (R.A. Mohammed, AI. Bailey, P.F. Luckham and S.E. Taylor, Colloids Surfaces A: Physicochem. Eng. Aspects, 80( 1993)223). In part 2, interracial tensions of crude oil, and solutions of asphaltenes and resins in a model oil have been investigated. Surface pressure vs. area (U-A) curves of monolayers of asphaltenes, resins and their mixtures have been established. In its dependence on the ratio of resins to asphaltenes, the pseudostatic dilatational modulus has high values for low resin-to-asphaltene ratios and low values for high resinto-asphaltene ratios. This is expected to throw light on the cause of the enhanced stability of water-in-crude oil emulsions. Key words: Asphaltic

constituents;

Crude oil emulsions;

Dewatering;

Introduction The properties of interfacial films between crude oil and water play an important role in determining the efficiency of oil recovery [l-3]. During the displacement of oil that is trapped in the pores of the rock by water injected into the well, the interface may be subjected to expansion and compression, causing phase changes. These effects depend on many factors, such as temperature, pressure gradient, water salinity, and the type and concentration of asphaltics present in the crude. Many workers [l-4] have demonstrated the existence of rigid interfacial films at the water-crude oil interface by retracting an aged pendant drop of crude oil in water. Reisberg and Doscher [l] have shown *Corresponding

author.

Interfacial

properties

experimentally that these films still exist at temperatures close to 90°C. These authors have obtained values for the equilibrium interfacial tension between 27.5 and 34 mN m- ’ for different crude oil-water systems. Strassner [2] used the same technique to investigate the behaviour of pentaneprecipitated asphaltene and resin films when the interfacial area of the pendant drop was varied. He described three types of film and film behaviour: solid films that form a relatively insoluble skin; highly mobile films that pack under compression to give a momentary distortion of the drop but rapidly redistribute and return the drop to the asymmetrical shape; and transition films that showed no distortion under drop contraction. The first group of workers who employed the Langmuir balance to study interfacial films between water and crude oil were Kimbler et al.

238

R.A. Mohammed et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 237-242

[3]. The surface pressure vs. area (H-A) isotherms of these films showed the usual phase transitions: gaseous, expanded, and condensed liquid phases followed ultimately by a collapse of the film. They also investigated the effect of adding 0.1% by volume of the non-ionic surfactant Triton X-100 o n the interfacial film and found that only expanded films were formed when the crude was aged for 2 h after the addition of the surfactant. Their data suggest that the film was gradually penetrated by the surfactant molecules. It is not possible to correlate their calculated compressional moduli values with the asphaltic content as there is no indication of the composition of the films in the crudes that they have used. Jones et al. [5] used a Langmuir trough to study interracial film behaviour at the water-crude oil interface for Kuwait, Iranian Heavy, Ninian and Forties crudes. The equilibrium interfacial tension values were between 22.5 and 25 mN m -1. These workers distinguished three types of film and film behaviour: incompressible non-relaxing films (Iranian heavy and Ninian crudes); incompressible relaxing films (Kuwait and Forties); and compressible relaxing films (Forties crude at a short contact time with water). They indicated that the dynamics of the film relaxation process dictates the extent of the barrier to emulsion stability. Temperature effects on the interfacial films were also investigated. Incompressible non-relaxing films start to relax upon heating. However, at 65°C there still exists a kinetic barrier to coalescence. This has been indicated by the rate of fall of film pressure (relaxation process) for films held at a constant compressed area. Pasquarelli and Wasan I-6] measured interfacial tensions between crude oil and alkaline brine solutions using a spinning drop tensiometer. They fractionated crude oil by employing a vacuum distillation technique to three different temperature cuts: a low boiling cut consisting mainly of saturated oils; a medium boiling temperature cut that is rich in resins and had an interfacial tension value of 29.86 mN m ~ against the brine solution and a value of 0.729 m N m ~ against the alkaline solu-

tion; and a high boiling temperature cut that is rich in asphaltenes and had an interracial value of 26.72 mN m 1 against brine and a value of 0.014 mN m 1 against the alkaline solution. The interfacial tension of the whole crude oil against brine was 28.09 mN m 1 and 0.045 mN m 1 against the alkaline solution. They concluded that asphaltic material greatly contributes to surface activity. Blair [7] concluded that initially adsorbed interfacial films may be augmented by a secondary adsorption of large particles originally suspended in the crude. Sj6blom et al. [8] believe that the interface between water and crude oil is built up by non-specified polar compounds, waxes and asphaltenes. Van der Waarden [9] demonstrated that asphaltenes are potential stabilisers of waterin-mineral oil emulsions when the oil phase contains asphaltenes that are near the condition of incipient flocculation. Mackay et al. [10] concluded from chemical analysis of a compound isolated from asphalt that the emulsifying agent responsible for the formation of water-in-crude oil emulsions was an asphaltenic-type substance. In this paper the behaviour of monolayers of asphaltenes, resins and their mixtures when subjected to high surface pressures have been explored with the view of providing some indications concerning the contribution of asphaltenes and resins to the stability of crude oil emulsions.

Experimental The materials used in this study were those mentioned in part 1 of this series [11].

Interfacial tension measurements Interfacial tensions of water-crude oil and water-model oil systems were measured using a hydrophobic Wilhelmy plate (platinum plate coated with carbon black). This was necessary to ensure a stable zero contact angle between the plate and the oil phase. The model oil was prepared by dissolving a known weight of heptane-precipitated asphaltenes,

R.A. Mohammed

et al./Colloids

Surfaces A: Physicochem.

Eng. Aspects

80 (1993) 237-242

239

and resins extracted from Buchan crude oil in xylene. Heptane was added in a ratio of 1 : 3 xylene/heptane

in order

to simulate

the aliphatic 25

part of the crude. <

Langmuirfilm

balance experiments

>-

Surface layers

pressure-area

of Buchan

mixtures

(U-A)

asphaltenes,

adsorbed

curves

of mono-

resins,

and

their

interface

were

at the air-water

obtained using a Joyce-Loebl Langmuir film balance Model 4. A Lauda Langmuir film balance (Messterate-werk Lauda, Lauda Konigshosen, Germany-) was used to obtain U-A isotherms for the asphaltics at temperatures above room temperature. The materials were spread (5.9 mg m-‘) using toluene as the spreading solvent. The trough was cleaned by wiping it with a clean, wet tissue. Preparation emulsions)

of water-in-model

oil emulsion (model

Emulsions stabilised by asphaltics were prepared by mixing distilled water with model oil containing asphaltics (asphaltenes, resins or their mixtures) using an air-driven emulsifier for a period of 40 s. Results Interfacial were

tensions

measured

as

between a function

water and crude oil of time

at

the

to the interface. The equilibrium value yes is temperature dependent, and is 18.5 mN m _ ’ at 25 “C, 12 mN m-l at 3O”C, and 8.5 mN m-’ at 35°C. The time taken to reach effective equilibrium reduces as the temperature increases: about 2.8 h at 25”C, about 2.2 h at 30°C and about 0.28 h solutions of asphaloil and water were

l5 10 5 ’ 0

1

2

3 4 Time hours

I 6

5

Fig. 1. Interfacial tension between water and Buchan as a function of time at different temperatures.

i 7

crude oil

measured as a function of time, and are shown in Fig. 2. The two curves for asphaltene concentrations of 0.008 and 0.02 g ml-’ show that the equilibrium interfacial tension has not been reached yet. The yes values for the other curves are 26.8 mN m-l for 0.01 g ml ’ of asphaltenes and 25.0 mN m-l for 0.05 g ml-’ of resin in the model oil. The interfacial tension for the water-model oil interface without the presence of asphaltics is 31.2 mN m-l. Following the addition of asphaltenes, the surface tension decreases rapidly with time within the first 30 min, after which there is a reduction in the rate. The first measured value of the interfacial tension was obtained 30 s from the beginning of each experiment. These values are 29.6 mN m-l for 0.008 g ml-‘, 27.8 mN m-l for 0.01 gml-‘, and 25.6 mN m-l for 0.02 gml-’ of

following temperatures: 25, 30, and 35 “C. The data are plotted in Fig. 1. The results show that the interfacial tension decreases slowly as the time increases before attaining an equilibrium value, indicating that asphaltene diffuses at a slow rate

at 35°C. Interfacial tensions between tenes dissolved in the model

20

f

30

)_

24

22

’ 0

I 2

6 4 Time hours

6

10

Fig. 2. Interfacial tension between water and the model oil as a function of time at different asphaltics concentrations at 25°C.

R.A. Mohammed et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 237-242

240

asphaltenes

in the model

for 0.05 g ml

27.0 mN mm 1

oil, and

1 of resin in the model oil. The error

in measuring y was kO.05 mN m- ‘. It is perhaps important to point out that asphaltenes have no definite molecular weight (see Refs. 8, 10 and 12). Surface pressure-area of asphaltene,

isotherms

for monolayers

resin and their mixtures

spread

the monolayer, each isotherm

which shown

the resins/asphaltenes withstand

pressures

value decreases

have

been

ratio. greater

Asphaltene

distilled water (pH 6.2) at 25°C were obtained at the air-water interface. The ratios 0 : 1, 1 : 1, 2 : 1, 3 : 1, 7 : 1, 10 : 1 and 1 :0 of the resins/asphaltenes were investigated. Typical isotherms of asphaltene and resin monolayers are shown in a linear form in Fig. 3(a) and in a logarithmic form in Fig. 3(b). The figures show that asphaltene monolayers form solid films upon compression. Table I presents the percentage reduction in area for different states of

to 7 mN m

this

compressional

from Eq. (1)

-dll

KS =

of

’ for the case of resin

K" can be calculated

modulus

for

films can

than 45 mN m-‘;

films. The values of the pseudostatic

on

calculated

in Fig. 3(a) as a function

(1)

d lnW&)

where A is the area of the trough after the compression and A, is the area of the trough

before the

compression. Isotherms of asphaltene monolayers at the following temperatures: 30, 40 and 55°C were

7 tesin:Asphaltene

(a)

-2.0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 1.0 A/Ao

Fig. 3. (a) The II-(A/A,)

curves for monolayers

Table I The collapse pressure Buchan asphaltics

values

for

different

Resins/asphaltenes ratio

Collapse pressure” (mN m-‘)

0:l 1:l 2:l 3:l 7:l IO:1 l:o

>45.0 >45.0 31.5 35.0 26.0 19.5 7.0

Collapse pressure values are given at 25°C. “Values are kO.05 mN m-r.

W

of asphaltenes, resins and their mixtures the curves shown in Fig. 3(a).

monolayers

-1.0 In A/Ao

0.0

1.0

at 25°C. (b) A plot of n versus ln(A/A,) for

of

20 -

0.0

0.2

Fig. 4. The IG(A/AJ different temperatures.

0.4 0.8 */*0 isotherms

0.1

,.0

for asphaltene

monolayers

at

R.A. Mohammed et al./Colloids Surfaces

A: Physicochem. Eng. Aspects 80 (1993) 237-242

obtained. The &(A/&) isotherms for asphaltene monolayers at these temperatures are shown in Fig. 4. The isotherms demonstrate that asphaltene monolayers can withstand pressures close to 60 mN m-i. There is no marked difference in the shape of these isotherms at different temperatures, and hence, the pseudostatic compressional modulus K” is apparently constant too. Some 10% water-in-model oil emulsions stabilised by asphaltenes (asphaltene concentrations 0.001, 0.004, 0.008, 0.01 and 0.02 gml-‘), resins (concentration 0.08 g ml-‘) and their mixture (0.06 g of both asphaltenes and resins, at a 13 : 1 resimasphaltene ratio in 1 ml of the model oil) were prepared. Emulsions prepared with asphaltenes as the stabilising agent were stable against coalescence for over 18 months, and due to the light viscosity of the model oil and the drop size, sedimentation of the dispersed drops had occurred. In contrast, the emulsion that was stabilised by resins separated within a few hours. The mixture of asphaltenes and resins in the model oil gave a stable emulsion. This emulsion appeared to be slightly more viscous than the others. Discussion

The interfacial tension of the water-crude oil system decreases slowly with time before attaining an equilibrium value, indicating that the asphaltene moieties diffuse at a slow rate to the interface. The equilibrium value yeq is temperature dependent, as would be expected. This reduction may be attributed to an increase in the amount of asphaltenes adsorbed at the interface. Alternatively, these data may suggest that there is an increase in the bulk aggregation concentration with temperature. In this case, y will still decrease as a result of an increase in the activity coefficient of the asphaltenes. The increase in the rate of reduction of the interfacial tension with temperature is due to the increase in the rate of diffusion of asphaltenes to the interface as a result of a drop in the bulk viscosity of the crude with temperature increase. Asphaltene monolayers form solid films upon compression, as was shown in Fig. 3(a). As asphal-

241

tene films are able to withstand high pressures before they collapse, the addition of small amounts of resins does not significantly affect the collapse pressure. At high resin ratios, however, the collapse pressure is markedly reduced and therefore, the compressional modulus K” is dependent on the resin concentration, as is shown in Fig. 5. The results show an increase in KS with resin content to 340 mN m-l at a weight ratio of 1 : 1 resin to asphaltene. It is likely that a high value of KS is caused by the ultimate interaction between resin and asphaltene particles to give well-packed, strongly interacting films at a 1 : 1 resimasphaltene ratio (see below). This value drops as the ratio of resins/asphaltenes decreases, revealing that the film has become more compressible due to the presence of more resins in the asphaltenes’ environment. In the case of monolayers of resin only, the value of K” is about 9 mN m-i. In agreement with the observations of Reisberg and Doscher [l], and Strassner [2], these results reveal that the rigidity of the crude oil-water interface is due in principle to the adsorption of asphaltenes, and may be enhanced by the coadsorption of resins at the interface. However, the presence of weak films at crude oil-water interfaces is an indication that the interface is mainly built up by adsorbed resins. A possible picture of a mixed layer of both asphaltenes and resins at the air-water interface is that the resins redistribute

400

I

4:l 6:l ResimAsphaltene

8:l ratio

Fig. 5. The effect of different resin-to-asphaltene ratios pseudostatic compressional modulus K” at 25°C.

on the

R.A. Mohammed et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 237-242

242

among

the asphaltene

moieties,

causing

asphal-

tene-resin association, in order to reduce the forces of self-association of asphaltenes. The ultimate geometric packing of this system is probably responsible for the highest resin-to-asphaltene ratio.

value

of KS at a 1 : 1

depend

very much on the type and content

asphaltic combined resin

and

strong

species. Langmuir surface balance work with model emulsions with asphaltene, their

mixtures

have

films are predominantly

character.

of the

These

strong

established

that

of an asphaltene

films are responsible

for

Asphaltenes can withstand high surface pressures (see Figs. 3(a) and 4) possibly due to the strength

stabilising emulsions weak films contain

of the aromatic

do not form stable emulsions. It is therefore expected that crudes that consist mainly of

ring

structure

lying

flat on the

surface of the water. A picture of resins having a low degree of aromaticity is possible since they act as deflocculants for the asphaltenes according to the theory concerning the association of asphaltenes in the crude [12]. This would provide a smooth transition from a high aromaticity region where asphaltene particles are abundant, to the aliphatic region. The effect of temperature on the compressibility of asphaltene monolayers is hardly significant. This is supported by the fact that there is only a slight difference in the shape of these isotherms at different temperatures. This result suggests that asphaltenes form rigid, temperature insensitive films at the airwater interface. This also coincides with the observations of Reisberg and Doscher [l] on the existence of the natural films at the water-crude oil interface at temperatures close to 90°C. Experimental work on model emulsions stabilised by asphaltics showed that only asphaltenes, and their mixtures with resins, were able to stabilise water-in-model oil emulsions. Resins alone, however, produced unstable model emulsions. These results confirm

that asphaltenes

resins would emulsions.

unstable

water-in-crude

oil

Acknowledgements We would like to acknowledge the support from the Arabian Gulf University in Bahrain for the award of a scholarship to RAM, and BP International for financial support and permission to publish. We are grateful to Dr. J. Clint for his assistance in processing the ZSA data.

References 1 2 3 4 5 6

are mainly contrib-

uting to the stability of water-in-crude oil emulsions and that they form strongly interacting films. Resins, by contrast, form weak films, unable to stabilise water-in-oil emulsions against coalescence.

form

against coalescence, whereas more resinous material and

7 8 9 10

Conclusion

11

The strength of the interfacial films formed between crude oil and water has been shown to

12

J. Reisberg and T.M. Doscher, Prod. Mon., 21 (1956) 43. J.E. Strassner, J. Pet. Technol., 20 (1968) 303. O.K. Kimbler, R.L. Reed and I.H. Silberberg, Sot. Pet. Eng. J., 6 (1966) 153. S.E. Taylor, Chem. Ind. London, Ott 19 (1992) 770. T.J. Jones, E.L. Neustadter, and K.P. Whittingham, J. Can. Pet. Technol., 17 (1978) 100. C.H. Pasquarelli and D.T. Wasan, in D.O. Shah (Ed.), Surface Phenomena in Enhanced Oil Recovery, Plenum Press p. 231. CD. Blair, Chem. Ind. London, May 14 (1960) 538. J. Sjoblom, H. Siiderlund, S. Lindblad, E.J. Johansen and I.M. Skjlrvii, Colloid Polym. Sci., 268 (1990) 389. M. Van der Waarden, Kolloid-Z., 156 (1958) 116. G.D.M. Mackay, A.Y. McLean, O.J. Betancourt and B.D. Johnson, J. Inst. Pet., 59 (1973) 164. R.A. Mohammed, A.I. Bailey, P.F. Luckham and SE. Taylor, Colloids Surfaces A: Physicochem. Eng. Aspects, 80 (1993) 223. J.Ph. Pfeiffer and R.N. Saal, J. Phys. Chem., 43 (1940) 139.