Research Council; and N.R. Morrow,* New Mexico Petroleum Recovery Research Ctr. *SPE Members ..... figures, symbols represent measured data, and the.
Societ;g o~ Pelmoleum Engineers
SPE 16964 Influence of Electrical Surface Charges on the Wetting Properties of Crude Oils by J.S. Buckley,* New Mexico Petroleum Recovery Research Ctr.; K. Takamur.a, Alberta Research Council; and N.R. Morrow,* New Mexico Petroleum Recovery Research Ctr. *SPE Members Copyright 1987, Society of Petroleum Engineers This paper was prepared for presentation at the 62nd Annual Technical Conference and Exhibition of the Society of Petroleum Engineers held in Dallas, TX September 27-30, 1987. This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Write Publications Manager, SPE, P.O. Box 833836, Richardson, TX 75083-3836. Telex, 730989 SPEDAL.
ABSTI~ACT
areas of rock surface which are contacted by the crude oil, there is the potential for adsorption of water-insoluble polar components from the crude oil. Thus in situ wettability may depend directly on the initial distribution of oil and connate water with respect to the rock surface. The results shown in Fig. 1 illustrate the importance of performing laboratory waterfloods at properly representative reservoir conditions, but maintaining or establishing the correct conditions is a major difficulty. Much improved understanding of the effects of crude oils on wettability is needed to give confidence in core recovery and handling procedures9,10 aimed at preservation or restoration of reservoir wettability.
Reservoir wettability is important to oil recovery by waterflooding and many other processes. The difficulties associated with determination of in situ wettability together with uncertainties about application of laboratory observations to field conditions necessitate a more basic understanding of factors that control wettability. In this work, conditions under which oil adheres to a particular solid surface are demonstrated for several crude oils. For a .given oil, pH and ionic strength were varied to obtain a mapping of conditions under which adhesion occurs. Results were satisfactorily explained by double layer calculations in combination with the Ionizable Surface Group model. Lack of adhesion signifies the presence of a stable water film that results from double layer repulsion between the crude oil and the solid surface. Flow visualization of displacement of crude oil by waterflooding in micromodels confirmed the important effect of brine composition on oil recovery.
Contact between crude oil and rock is dependent on the stability of water films between rock surface and the crude oil. The existence of stable water films in the range of 1-100 nm thickness has been shown to depend on electrical double layer repulsion that results from surface charges at the solid-water and water-oil interfaces being of the same sign.~1-~3 In the regions of contact, thin films contour the solid surface, except as modified by surface roughness. Water held essentially as a skin at the rock surface by electrical double layer or shorter range forces will be referred to as pellicular water.~ Equilibrium with the bulk water, which will be at some capillary pressure, is satisfied by the disjoining pressure~5,~s acting in the pellicular water. A schematic of the distribution of crude oil, and bulk and pellicular water in pores of trianular crosssection is shown in Figs. 2a and b for smooth and rough pore walls, respectively. In this paper discussion of film stability will refer to pellicular water unless otherwise stated.
INTRODUCTION Results of laboratory waterfloods show that wettability can have a profound effect on the efficiency of displacement of oil by water.l,2 Departure from strongly water-wet conditions, which are often taken as a convenient standard, can result in either decrease or increase in oil recovery efficiency, reflecting a range of possible wettability changes. Laboratory waterfloods in which wettability change involved use of or exposure to crude oil are compared in Fig. 1 with results for strongly wetting conditions.3-s Results are presented as percent of initial oil recovered, ED, vs. PV of water injected.
As long as water soluble surfactants from the crude or in the formation water do not alter wettability, the stability of water films between crude oil and the rock surface and their ability to prevent adsorption of water insoluble components over geologic time are key factors in maintainingi
There is growing opinion that mixed wettability conditions pertain in many oil reservoirs.~,7,s At References and illustrations at end of paper. 3"17
SPE 16964 on the Wetting Properties of Crude Oils [H~] = [~] exp(yo) (3a) reservoirs at strongly water-wet conditions. If the film is unstable, polar components from the oil will where have the opportunity to adsorb directly on the rock surface. If the adsorbed components cannot migrate (3b) from the region of contact, a mixed wettability Yo = e~o/kT condition can be expected with the distribution of where, e is the electron charge, ~o is the surface oil-wet surfaces complementing the regions overlain potential, k is the Boltzman constant and T is the by bulk water. Whether migration occurs or not, absolute temperature. instability of the wetting film followed by adsorption probably results in departure from a strongly The surface charge density, So, for this type of water-wet condition in the region of contact. interface can be written as It has been shown ~hat film stability, as sO = eNal ’ indicated by adhesion of crude oil at mineral +(Ka=/[ ]) exp(vo) surfaces, is dominated by pH, brine concentration and (4) composition,zT,z8 In the present study, adhesion i behavior observed for crude oils is related through - (l+([~]/Kal) exp(-yo))] electrical double layer theory to the properties of the oil/water and water/solid interface. Changes in where fractional coverage, f, of the surface by wettability with brine composition and their effects acidic basic groups is expressed as on oil recovery mechanisms are demonstrated through f = and Na=/Na (5) ~ flow visualization experiments in two dimensional pore networks. Influence of Electrical Surface Charges
and Na and Na are the total number of acidic and basic ~ites at ~he interface, respectively. Electric Properties of the Oil/Water Interface
The surface charge density must be balanced by the charge density in the electric double layer, ~d,
It has been previously demonstrated using bitumen that an oil/water interface has a negative electric charge which can be adequately explained by the Ionizable Surface Group (ISG) model. In application of this model it is assumed that the negative charge of the interface is caused by the dissociation of carboxylic acids, which are naturally occurring surfactants.17,1s
Od = -(8nbvkT)~ sinh(-e~o/kT)
where, nb is the ion density in the bulk solution and ~ is the permittivity of the medium. The pH vs ~o relationships which satisfy the condition of electrical neutrality (~o + Od = 0) can be determined numerically for a given set of values of Na1 , Na2 , pKa and. pKa 2 in various electrolyte . ¯ ¯ solutions. The ~eta~led description of the numerical 2~ method has been discussed by Mealy et al. and Healy and White.~2
In contrast to observations reported previously for bitumen, electrophoresis measurements for three conventional crude oils studied in the present work ~howed a change from negative to positive interfacial charge at low pH indicating the presence of basic as well as acidic surface active groups at the oil-water interface. The theoretical model applied to bitumen was, therefore, extended to take into account the ~witterionic nature of the crude oil-water interface using a method described by Harding and Healy2°
A~ Z A- + H+
Zeta potential vs. pH relationships, rather than the surface potential, are required to compare theoretical predictions with experimentally determined electrophoretic mobilities. Therefore, the zeta potential, ~, is calculated using the GouyChapmann relation of
(la)
and BM+Z B + H+
~(x) =__2kT in 1 + ~o exp (-~x) e i - 7o exp (-~x)
(lh)
Here, A- and BH+ represent acidic and basic groups at the interface. Their dissociation constants are defined by Kal =
[A-] [~s]
_ exp (yo/2) -1 exp (Yo/2) +1
(7b)
(2a)
[AH]
[B] [H~]
(7a)
where
assuming that the shear plane is located 0.6 nm from the surface. Evidence in support of this value has been discussed previously,z?’~s
and
Ka2 =
(6)
Electric Properties of the Glass/Water Interface (2b)
The amphoteric nature of mineral oxides/water interface can be described by
[BH+]
where [H~] is the hydrogen ion concentration in the vicinity of the surface, which can be related to the bulk concentration, [H~] using the Boltzman relationship
AH=+ ~ AH + H+ and
318
(8a)
SPE 16964
J.S. Buckle~~ K. Takamura~ and N.R. Morrow
AH ~A-+H+ Dissociation constants for given by:
1.0, 1.4 and 1.6x10-i M NaCI at pH 8.0 are shown in Fig. 3.
(Sb) these reactions are
EAH~]
(9a)
and
K_ = [A-] [~] [A~]
(gb)
Eqs. 6, 8 and 9 can be combined to give an expression for the surface charge density: oo = eN~ ([H~b]/K+) exp(-Yo) - (K-/[H~b]) exp(Y0) (i0) I+([~]/K+) exp(-yo) + (K_/[H~])exp(yo) Numerical solution of Eqs. 6 and i0 gives the relationship between ~o and pH.
Results were used to predict adhesion behavior on a glass surface for three crude oils. It was assumed that adhesion of the crude oil occurs when ~max < 0, where ~max is the maximum value of the disjoining pressure (see Fig. 3). Resulting adhesion diagrams for three crude oils are shown in Fig. 4. In the figure, the ~max values between 0 and 10 kPa are indicated as a transition zone from adhesion to non-adhesion. This range of Hmax was arbitrarily chosen because in practice the transition from adhesion to non-adhesion is not sharply defined. The transition zone also serves to reflect the uncertainty in calculated values of FA(h) and FD(h) because of uncertainty in the value of the Hamaker constant used and approximations involved in the theory. Discrepancies between the experimentally determined adhesion behavior and the theoretical predictions based on calculations of disjoining pressures indicate the conditions under which short range forces, presumably hydration, come into play.
Dis~oinin~ Pressure Calculations
EXPERIMENTAL METHODS The disjoining pressure, H, acting between the crude oil and solid surfaces is a composite of three types of interactions: London-van der Waals attractive force, electric double layer force and short range repulsive forces, such as hydration forces:2~ H(h) = FA(h) + FD(h) + FH(h)
(ii)
where h is the separation distance between the surfaces. The retarded van der Waals attractive force between two parallel plates can be expressed by25 FA(h) = -A (15.96 h/l + 2) 12 ~ h3 (i + 5.32 h/X)2
Three conventional crude oil samples from Texas (Moutray), North Sea (ST-86) and Alberta (Leduc) were tested for their interracial electric properties, stability of those properties with emulsion storage time and adhesion behavior as a function of brine concentration and pH. Physical and chemical properties of the oils are given in Table i. Adhesion Tests Tests were performed with each of the three oils to determine whether or not oil would adhere to brine-covered glass. Microscope slides (soft glass) were used as the glass surface. Ultrasonic cleaning in hydrogen peroxide was followed by soaking in the desired brine for periods from several days to a week or more.
(12)
where A is the Hamaker constant (approximately 1.0xl0-21J for oil/glass in water)i~ and I is the London wavelength of roughly 100 nm.25
Adhesion tests were performed in a rectangular glass cell with a contact angle goniometer for observations of oil drop-water-glass contacts,s An oil drop was introduced to the system from a microburette and allowed to contact the glass surface for varying amounts of time. The oil was then retracted slowly back into the burette. Two fairly distinct types of behavior were observed at this point: in one case, the oil came cleanly away from the surface, and in the other, oil adhered to the glass to ~ form a capillary bridge between the burette and the glass surface. As the oil was withdrawn further, the bridge became unstable and broke, leaving a drop of oil on the glass surface.
The double layer interaction for interfaces possessing different potentials, ~i and ~2, can be approximated under the constant potential approximation2~
FD(h) = nbkT{[2y~y2 cosh(~h)-yi2-y22]/sinh(~h)} (13) where Yi is the reduced potential (Yi = e~i/kT) and ~ is the reciprocal Debye-Huckel double layer length, which is given by ~= = 2e~nb/EkT
Crude Oil Samples
(14)
The magnitude and decay length of the hydration force is not well understood except for two mica surfaces immersed in an aqueous electrolyte solution.2~-2s In the present study, the disjoining !pressure was calculated from Eqs. 11-13 by assuming FH(h) = 0 for various values of pH and electrolyte concentrations. Zeta potentials predicted from the ISG model were used to estimate the double layer interaction. Examples of H(h) relations for Moutray crude oil in 10-2 M NaCI at pH 4, 5 and 6, andin
Addition of salts of weak acids and bases to the aqueous phase allows a well-defined pH to be maintained despite ionization of polar groups from the oil at the oil/water interface. Several buffers have been tested to span different pH ranges and to determine whether specific ions (other than H+ and OH-) influence the adhesion/non-adhesion behavior or whether the results can be attributed primarily to pH. Table 2 shows compositions of buffer solutions.
319
4
Influence of Electrical Surface Charges on the Wetting Properties of CrudeOils
SPE 16964
figures, symbols represent measured data, and the lines show the ~ vs. pH relationships predicted from the ISG model using the constants listed in Table 3. Values of N’s and pK’s were determined by a procedure similar to that described previously. Results for all three crude oils show an isoelectric point (IEP), at which the dissociated acid and base groups present at the interface have equal effect on the surface charge.
Electrophoresis Measurements Crude Oil Emulsions
Emulsions of crude oil in water were formed at room temperature using a 4 ml sample of crude oil in 100 ml of water with pH adjusted to i0 using NaOH. A hand-held homogenizer (Chase Logman Corp., Hixville, NY) was used to disperse the oil with each sample being passed through the homogenizer twice.
For the ST-86 emulsion, the IEP was lowered from 4.3, measured after one week, to 3.4 after storage for two weeks in the aqueous solution at pH i0.0. The ~ vs. pH relationships measured after storage were matched by changing the value of Na2 from 2.08xi0x8 to 1.97xi0z8 m-~ (see Fig. 6b). This suggests that some of basic groups were leached from the crude oil during storage.
The resulting oil d~ops ranged in radii from 0.5 to 8 ~m with mean radii of approximately 1.0 ~m for Leduc and ST-86 and 1.6 ~m for Moutray crude. Electrophoretic mobilities were measured for crude oil emulsions as a function of pH in 10-3, 10-2 and i0-z M NaCI solutions at 25°C using a laser doppler apparatus,z7 For electrophoric mobility measurements, a 100-BI sample of each emulsion was diluted in 20 ml of water of the desired pH and sodium ion concentration. Buffer solutions of approximately i0-~ M Na+ concentration were used to control the pH at the desired level and to prevent pH drift at the electrode surface.
Although the theory adequately explains the measured ~ vs. pH relationships for all crude oils investigated in this study, the material constants listed in Table 3 are not necessarily a unique set of values for each crude oil. Additional experimental work would be needed to identify the chemical nature of charge groups at the interface. However, from the results so far, the following can be said about the chemical properties of surface active groups at the oil/water interface:
Suspensions of Ground Glass Microscope slides (Corning) were ground using a mortar and pestle until the resulting particles passed though a 120 mesh sieve (< 125 ~m). Approximately 50 mg of this sample were dispersed in 20 ml of an aqueous solution of the desired electrolyte concentration and pH. The electrophoretic mobility ~as measured within one hour after the preparation of the dispersion and again two weeks after preparation.
l)
sufficient surfactants are present in the crude oil to saturate the oil interface even after a large extension of the surface as in the formation of oil-in-water dispersions,
2)
carboxylic acids are the most dominant acid groups for Moutray and Leduc crude oils,
3)
a significant proportion of acids for ST-86 may be sulfate or sulfonate groups,
4)
the total number of surface active groups at the Moutray/water interface is approximately a quarter of those at the Leduc/water and ST-86/water interfaces, and
s)
steric hindrance may cause the lower coverage by surface active groups for the Moutray crude oil.
Zeta Potential Calculation Zeta potentials were calculated from the measured electrophoretic mobility. The OhshimaHealy-White equationz8 was used to correct for the electrophoretic relaxation effect of measurements in 10-3 M NaCl at pH > 6. Micromodel Displacements A glass micromodel with a regular twodimensional network of interconnecting-pores and throatswas used to demonstrate the wetting phenomena and displacement mechanisms due to the presence of crude oil. The model pattern has been described previously,s
More detailed discussion of properties of crude oil samples, ionbinding of charge groups with counterions, is beyond the scope of will be treated separately,s°
All waterfloods were conducted with an established water saturation in place. Thus the model was flooded with the specified brine, followed by crude oil, and finally the same brine was used in the waterflood. Fluids were not pre-equilibrated, but both brine and oil were in place in the model for at least several hours before the waterflood began. The model was cleaned with a series of solvents and dried between floods. Displacements were recorded on videotape.
the electric including the high valency this paper and
~ vs pH for Glass Surface Measured ~ vs. pH relationships in 10-2 and -~ I0 M NaCI solutions are shown in Fig. 8 using open symbols for a fresh dispersion and solid ones for the dispersion stored for two weeks in distilled water at room temperature. Significant reduction of I~I by storage (for example, -66 mV to -38 mV in 10-2 M at pH 7.0) can be seen. Similar aging phenomena of solid/water interfaces have been reported by several authors,s~-ss
RESULTS ~ vs pH for Crude Oils
Healy, Chan and Whites~ used pK_ = 0, pK+ = 6.0 and Ns = 5.0x10zs m-2 for the silica/water interface. These pK’s result in an IEP of 3.0. They also used 2.0 nm for the location of the shear plane, x, to estimate the zeta potential at KCI concentrations
Zeta potentials of three crude oils as a function of pH in 10-3, 10-2 and I0-~ M NaCI solutions are shown in Figs. 5, 6a and 7 for Moutray, ST-86 and Leduc crude oils, respectively. In these
320
SPE 16964
J.S. Buckley, K. Takamura, and N.R. Morrow
below 10-2 M. The value of IEP for the ground glass in our experiments appeared to be much lower, and a small value of x has to be used under our experimental conditions. Matching the measured ~ vs pH relationships for both fresh and aged dispersions required adjusting the location of the shear plane. The physical significance of this apparent change in x, if any, cannot be determined from our experiments, however.
Adhesion Experiments Results of adhesion tests for threecrude oils on the glass surface are summarized in Figs. 9-11. Here, the symbol + designates the condition where the oil is left on the glass surface, and o shows conditions where oil comes cleanly away. The experimentally determined adhesion diagrams clearly demonstrate the presence of two critical values, pH and electrolyte concentration. The adhesion of the crude oil only occurs below the critical value of pH and above the critical Na+ concentration. Temporary adhesion of oil to glass was observed for Leduc at intermediate values of pH. The oil drop was observed to adhere, but the contact angle through the water phase decreased with time, and eventually the oil drop was released from the surface. This process took from half an hour to two days, depending on brine composition and the initial contact time of oil on glass (before breaking off the oil drop). On Fig. ii, temporary adhesion is denoted by a 8 symbol.
5
solution because of the negative charg~ of the surface. This results in reduction of the dissociation of aqidic groups and increased protonation of basic groups on the crude oil surface as the crude oil and glass surfaces approach each other. Reversal of the sign of the crude oil surface may occur even when the bulk pH is greater than the pH at the IEP of the oil, as discussed by Healy, Chan and White3~ and Chan, Healy and White~s for other interfaces. A similar discrepancy between the predicted and measured critical pH has been reported for the coagulation of a bitumen emulsion in NaCI and CaCI2 solutions,s6
It is difficult to conclude, at this stage, whether or not the observed difference between measured and predicted critical Na+ concentration is due to the hydration force resulting from hydration of the glass surface. Coagulation of bitumen emulsions could be fully explained by the double layer interaction, indicating the absence of the hydration force at the oil/water interface. Near Na~ at pH > 6, most of the acidic groups on the crude oil are fully dissociated. In this condition, the double layer repulsion under the constant charge density assumption36
FD(h) = nkT ~2 [i+ (Yz+Y2)2 csch2(Kh/2)] ~ 4
(15) _
Results of the theoretically predicted boundary between the adhesion and non-adhesion are also superimposed in Figs. 9-11. For the ST-86 crude oil, the ~ vs pH relationships for the fresh emulsion were used to calculate the adhesion behavior. If the aging phenomena observed are indeed due to the leaching of basic groups, this will not occur during the adhesion measurement, because i) the adhesion test usually takes far less than one hour, and ii) the surface area to volume ratio of an oil droplet used for the adhesion test is very small compared to that of the oil in water emulsion.
(yz-y2)= exp(-Kh) ~-2 1 + ¼(yz+y2)2csch2(~h/2)
may be used. Therefore, the Na~ for each crude oil was estimated using Eqs. 12 and 15 and shown with dashed lines in Figs. 9 and 10. Those new Na~ values are closer to the measured values, but still smaller than actual observations.
As seen in Figs. 9-11, the theoretically calculated adhesion diagrams successfully reproduce the general features of the adhesion behavior of the crude oil on the glass surface. The main discrepancies between the experimental and theoretical results are:
Reduction of the zeta potential at high pH has often been reported~° and explained by the discreteness effect of dissociated groups at the interface.22 The lowering of the ~ may be contributing to adhesion at pH > 12. If the aged glass surface indeed has a porous gel layer, the dissolution of such a layer at high pH may also enhance the adhesion of the crude oil. Micromodel Displacements The high aspect ratio model exhibited trapping of mineral oil in pore bodies under strongly waterwet conditions,s With Moutray crude oil, the displacement mechanisms exhibited a greater variety of behavior than might be expected from the adhesion mapping (see Fig. 9) developed for the flat microscope glass. With brine composition and pH as variables, strongly oil-wet, intermediate-wet, and several examples of mixed wettability were observed. Video recordings were edited to obtain a series of displacements which illustrated the dominant effect of brine composition on displacement behavior.
i) (PHc)th < (PHc)obs, ii) [Na~]th < [Na~]obs and iii) [Na~]obs ~ I0-z M at pH 12, where (PHc)th, (PHc)obs, [Na~]th, and [Na~]obs are the theoretically predicted and experimentally measured critical adhesion pH and Na+ concentration, respectively. The observation that (PHc)th < (PHc)obs may be related to the assumption used for the calculation of the double layer interaction, that the ~ determined at infinite separation of the particles remains constant as separation (h) decreases during the adhesion experiment. At a bulk pH above the IEP for the glass, the H+ concentration in the vicinity of the gl-ass surface is much higher than in the bulk
CONCLUSIONS Adhesion of specific crude oils on flat glass surfaces occurs reproducibly under controlled conditions of pH and brine composition. Changes in the solid surface with time necessitate preequilibrating the glass with the brine of 321
Influence of Electrical Surface Charges
6
FA
interest at each set of conditions. Patterns of adhesion for two of the oils studied were very similar. Time dependent adhesion has also been observed. Stable emulsions of crude oil in brine can be formed, and the electrophoretic mobilities measured as a function of pH and brine composiElectrical properties of the crude tion. oil/brine interface changed with age of the emulsion for some oils suggesting chemical changes at the interface.
FH
repulsive (hydration) force
K_
e
charge on electron
ED
hydrogen ion at surface
equilibrium constant - 2nd proton amphoteric molecules
KaI, Ka2
equilibrium constants for acidic and basic components respectively, for deprotonation reactions
+ [Nac]obs
critical Na+ concentration below which adsorption is observed
+ [Nac]th
critical Na+ concentration below which adhesion is predicted numbers of acidic, basic and amphoteric species per m2
Naz, Na~, Ns nb
ion density in bulk critical pH below which adsorption is observed
(PHc)obs
critical pH below which adhesion is predicted
PHx
- log [H+], x = s, b
pKx
- log K,
PV
Alphanumeric Symbols
basic species in crude oil (protonated and unprotonated forms)
distance
equlibrium constant - ist proton amphoteric molecules
(PHc)th
B
force
Boltzman constant
Predictions of adhesion between oil and glass underestimated the critical pH above which no adhesion is observed. An additional force, e.g. hydration of the glass surface, is required to explain the non-adhesion at high ionic strength and high pH. Nevertheless, the general outline of the adhesional mapping can be predicted from the electrical charges at the interfaces. This provides a systematic way to predict wetting and adhesion behavior at a wide variety of surfaces of electrophoretic from a few measurements mobilities.
acidic or amphoteric species in crude oil (protonated and unprotonated forms)
force
hydrogen ion in bulk
Changes in isoelectric point with time for ST-86 were best modeled by a decrease in the number of basic groups at the interface while the acid density remained constant. This suggests that more of the basic groups are part of relatively short organic molecules which may diffuse into the water phase with time.
A
(van der Waals)
repulsive (double layer)
+ Hs
The parameters from curve fitting suggest that some oils (e.g. ST-86) have a larger contribution to the acidic fraction from strongly acidic components like sulfonates than others such as Moutray which have primarily carboxylic acids.
Hamaker constant (Eq. 12 only)
attractive
FD
h
Electrophoretic mobility as a function of pH and concentration of positive ions in the brine phase at constant temperature can be explained using the Ionizable Surface Group model with reasonable values of PKa’s for organic acids and bases. Both positively and negatively charged groups from the oil phase can be present at the interface simultaneously. Curve fitting to mobility data provides estimates of the surface charge densities and the PKa’s for average acidic and basic groups.
A
SPE 1696~
on the Wetting Properties of Crude Oils
x = az, a=, +, -
volume (normalized to total volume of pore space
T
temperature °K
x
location of
shear plane
Yi
reduced potential
Yo
reduced potential
Greek Characters
~ ~o
oil recovered (% of original oil in place)
f fractional coverage Na2/Naz
322 ,
potential in double layer surface potential
oo
surface charge density
°d
charge density in electrical double layer
~
electrical permittivity
~
zeta potential
7
J.S. Buckley, K. Takamura, and N.R. Morrow
SPE 16964
Wang, F.H.L.: "Effect of Wettability Alteration on Water/Oil Relative Permeability, Dispersion, and Flowable Saturation in Porous Media," paper SPE 15019 presented at the 1986 SPE Permian Basin Oil & Gas Recovery Conference, Midland, March 13-14.
inverse Debye-Huckel do~ble layer length, {(2e2nb)/(~KT)}T defined by Eq. 7b disjoining pressure
Alba, P.: "Discussion of Fatt, I. and Klikoff, W.A., Jr., ’Effect of Fractional Wettability on Multiphase Flow Through Porous Media’," Trans, AIME (1959) 216, 432.
maximum value of ~ as a function of h London wavelength Subscripts el, a2
Cuiec, L.: "Wettability and Rock/Crude-Oil Component Interactions," paper presented at the 21st Intersociety Energy Conversion Engineering Conference, San Diego, August 25-29, 1986.
distinguish more and less acidic species in crude oil, respectively distinguish equilibria involving positively and negatively charged forms of amphoteric species
S
surface
b
bulk
Anderson, B.: "Wettability Literature Survey-Part i: Rock-Oil-Brine Interactions and the Effects of Core Handling on Wettability~" paper SPE 13932, unsolicited. i0.
Sharma, M.M. and Wunderlich, R.W.: "The Alteration of Rock Properties Due to Interactions with Drilling Fluid Components," paper SPE 14302 presented at the 1986 SPE Technical Conf. & Exhib., Las Vegas, Sept. 22-25.
Ii.
Takamura, K. and Chow, R.S.: "A Mechanism for Initiation of Bitumen Displacement from Oil Sand," J. Can. Pet. Tech. (1983) 22, 22-30.
12.
Hall, A.C., Collins, S.H., and Melrose, J.C.: "Stability of Aqueous WettingFilms in Athabasca Tar Sands," Soc. Pet. Eng. J. (April 1983) 249258.
13.
Support for this work was provided by Arco Oil and Gas Company, Chevron Oil Field Research Company, Mobil Foundation, Shell Companies Foundation, and support is gratefully Statoil (Norway). That acknowledged.
Brown, C.E. and Neustadter, E.L.: "The Wettability of Oil/Water/Silica Systems With Reference to Oil Recovery," J. Can Pet. Tech. (1980) 19, 100-110.
14.
Morrow, N.R. and Harris, C.C.: "Capillary Equilibrium in Porous Materials," Soc. Pet. En~. J. (March 1965) 15-24.
REFACES
15.
Derjaguin, B.V. and Kussakov, Physicochim URSS (1939) i0, 153.
16.
Mohanty, K.K., Davis, H.T., and Scriven, L.E.: "Physics of Oil Entrapment in Water-Wet Rock," SPE Reservoir En~. (Feb. 1987) 113-128.
17.
Chow, R.S. and Takamura, K.: "Electrophoretic Mobilities of Bitumen and Conventional Crude-inWater Emulsions Using the Laser Doppler Apparatus in the Presence of Multivalent Cations," submitted to J. Colloid Interface Sci.
i, 2, i
distinguish between different surfaces
Other
[]
molar concentration of species within brackets
AC~OW-LEDGMENTS The authors wish to thank Ross Chow of the Alberta Research Council for the electrophoretic mobility measurements, Mary Graham of the PRRC for the fabrication of micromodels and performance of the videotaped displacements, and to Dr. Chris Lien of the PRRC for crude oil molecular weight measurements.
Morrow, N.R.: "A Review of the Effects of Initial Saturation, Pore Structure and Wettability on Oil Recovery by Waterflooding," PRRC Report 86-17, New Mexico Petroleum Recovery Research Center (Dec. 1986).
2. Anderson, W.G.: "Wettability Literature Survey-Part 6: The Effects of Wettability on Waterflooding," paper SPE 16471, unsolicited.
(1987).
3. Kyte, J.R. and Nauman, V.O.: "Effect of Reservoir Environment on Water-Oil Displacements," J. Pet. Tech. (June 1961) 579-582. 4.
M.: M. Acta
18.
Ohshima, H., Healy, T.W., and White, L.R.: "Approximate Analytic Expressions for the Electrophoretic Mobility of Spherical Colloidal Particles and the Conductivity of Their Dilute Suspensions," J.C.S. Faraday Trans. (1983) ~, 1613-1628.
19.
Takamura, K. and Chow, R.S.: "The Electric Properties of the Bitumen/ Water Interface. Part II: Application of the lonizable Surface-Group Model," Colloids & Surfaces (1985) 15, 35-48.
Salathiel, R.A.: "Oil Recovery by Surface Film Drainage in Mixed-Wettability Rocks," J. Pet. Tech. (Oct. 1973) 1216-1224. Morrow, N.R., Lim, H.T., and Ward, J.S.: "Effect of Crude-Oil-Induced Wettability Changes on Oil Recovery," SPE Formation Eval. (Feb. 1986) 89103. 323
8
Influence of Electrical Surface Charges
on the Wetting Properties of Crude Oils
SPE 1696~
20.
Harding, I.H. and Healy, T.W.: "Electrical Double Layer Properties of Amphoteric Polymer Latex Colloids," J. Colloid Interface Sci. (1985) 107, 382~397.
29.
Pashley, R.M. and Israelachvili, J.N.: "DLVO and Hydration Forces Between Mica Surfaces in Mg2+, Ca2+, Sr2+ and Ba2+ Chloride Solutions," J. Colloid Interface Sci. (1984) 97, 446-455.
21.
Healy, T.W. et el.: "Nernstian and Non-Nernstian Potential Differences at Aqueous Interfaces," J__. Electroanal. Chem. (1977) 80, 57-66.
30.
Takamura, K. and Chow, R.S., in prepmration.
31.
Kulkarni, R.D. and Sumasundaran, P.: "The Effect of Aging on the Electrokinetic Properties of Quartz in Aqueous Solution," Proc__., Symp. on Oxide-Electrochemical Soc. (1973), R.S. Alwitt (ed.), The Electrochemical Society, New York, 31-44.
32.
van Lier, J.A., de Bruyn, P.L., and Overbeek, J.Th.G.: "The Solubility of Quartz," J. Phys. Chem. (1960) 64, 1675-1682.
33.
Hunter, R.J.: Zeta Potential in Colloid Science~ Principles and Applications, Academic Press (1981) New York, 282.
34.
Healy, T.W., Chan, D., and White, L.R.: "Colloidal Behavior of Materials with lonizable Group Surfaces," Pure & Applied Chsm. (1980) 52, 1207-1219.
35.
Chan, D., Healy, T.W., and White, L.R.: "Electrical Double Layer Interactions Under Regulation by Surface Ionization EquilibriaDissimilar Amphoteric Surfaces," J.C.S. Farada’ Trans. (1976) ~, No. 72, 2844-2865.
36.
Takamura, K., Chow, R.S., and Tse, D.L.: "Prediction of Electrophoretic Mobilities and the Coagulation Behavior of Bitumen-in-Water Emulsions in Aqueous NaCI and CaCI2 Solutions Using the lonizable Surface Group Model," Proc____~., Symp. on Flocculation in Biotechnology and Separation Systems, San Francisco, CA (1986).
37.
Levine, S., Mingins, J., and Bell, G.M.: "The Discrete Effect in Ionic Double-Layer Theory," J. Electroanal. Chem. (1967) 13, 280-329.
22.
Healy, T.W. and White, L.R.: "lonizable Surface Group Models of Aqueous Interfaces," Adv. Colloid Interface Sci. (1978) ~, 303-345.
23.
Chow, R.S. and Takamura, K.: "Effects of Surface Roughness of Latex Particles on Their Electrokinetic Potentials," submitted to J. Colloid Interface Sci. (1987).
24.
Blake, T.D. and Kitchener, J.A.: "Stability of Aqueous Films on Hydrophobic Methylated Silica," J.C.S. Faraday Trans. (1972) ~, No. 68, 14351442.
25.
Gregory, J.: "Approximate Expressions for Retarded van der Waals Interaction," J. Colloid Interface Sci. (1981) 83, 138-145.
26.
Gregory, J.: "Interaction of Unequal Double Layers at Constant Charge," J. Colloid Interface Sci. (1975) 51, 44-51.
27.
Israelachvili, J.N. and Adams, G.E.: "Measurement of Forces Between Two Mica Surfaces in Aqueous Electrolyte Solutions in the Range 0-I00 nm," J.C.S. Faraday Trans. (1978) ~, No. 74, 975-1001.
28.
Pashley, R.M,: "DLVO and Hydration Forces Between Mica Surfaces in Li+, Na+, K+ and Cs+ Electrolyte Solutions: A Correlation of Double Layer and Hydration Forces With Cation Exchange Properties," J. Colloid Interface Sci. (1981) 83, 531-546.
324
Table i. Physical and Chemical Properties of Crude Oils Used for Adhesion and Electrophoresis Measurements.
Crude Oils
Viscosity at 25°C mPa’s
Density kg m-S
Molecular Weight
Acid No. mg KOH/g
Leduc
2.49
813
184
0.15
Moutray
5.23
838
226
0.26
2.52
13.01
875
246
0.15
8.14
St-86
Onset of Precipitation (ml pentane/g oil)
*No onset observed with up to 50 ml pentane
Table 2. Buffer Compositions Buffer
pH range
Constituents
Acetate
3.7 - 5.6
CH3COONa’nH20, CH3COOH
Phosphate
5.8 - 8.0
Na2HPO4"nH20, NaH2PO4"nH20
Phosphate/Citrate
2.2 - 8.0
Na2HPO4"nH20, C6HsOT"H20, KCI
Carbonate
9.7 - 10.9
Na2CO3"nH20, NaHCO3
Table 3. Material
Material constants used to predict zeta potentials. Na~ x lO-ZS
Na2 x lO-Z~
PKaz
PKa~
IEP (pH)
Liquids: Leduc
1.55
1.4
3.8
8.0
4.75
Moutray
0.40
0.i0
4.0
6.0
3.4
ST-86 (fresh)
2.00
2.08
1.7
5.8
4.3
4.0
1.5
Solid: Glass (aged)
2.5
-i.0
325
Ioo
8O
>-
~S T (ReOe fNi nGeLdY o i IV~ATER-WET
W
STRONGLY WATER-WET
6O
0
uJ
(,Reservoir temp. ond live crude oil)
uJ
~ABILITY
40 w
(East Texas film-refined oil) w 20
2O
r I, I ~,,, I I I I I I I I I I J I I I’ I ’ ~ ~
oo
(,0
1.5 2.0 2.5 1.0 WATER INJECTED (PV) (b) Mixed wettability vs. strongly water-wet. (After Salathiel, 197:5) 0
0.5
I.O 1.5 2.o 2.5 3.o WATER INJECTED (PV) Reservoir conditions vs. conventionol test. (After Kyte efo/, 1961)
~ 8O
0.5
~ 80 >W > 0 o w
40k ~ STRONGLY WATER-.WET
d
WEAKLY WATER-WET ( Moutray film- refined oil
/ 6O ~,/..[
STRONGLY WATER-WET (Refined oil )
4O
-
ESTIMATED o 2o
2O w
1.5 2.0 2.5 5.0 1.0 WATER INJECTED (PV) (c) Effect of time of contact with Loudon crude oil. (After Wang, 1986 ) 0
0.5
IO0
2.0 2.5 :5.0 1.0 1.5 (PV) WATER INJECTED (d) Film from Moutrey crude vs. strongly wet. (After Morroweto/,1986) 0.5
0
>z
o 2.c $ 80
w
w 60
> 0
~D
LOUOON CRUDE FRESH CORE ./}’ ~STRONGLY WATER-WET ...............
STRONGLY WAT’ER - WET (Cleaned core-refined oil)
w
._1
MOUTRAY FILM (d)
WEAKLY WATER-WET .___.___(Fresh core-refined oil’)
4O
-~5-~T~g~97~’~A L CORE
0.5 w
0
, , , , IIIII II1IIIIIIIIIII~rIII
0
0.5
1.0 WATER
1.5
2.0
2.5
5.0
INJECTED (PV)
(e) Cleaned vs. fresh core. (After Rathmelle;~/, 1975
o o
1.5 2.0 2_.5 3.0 1.0 INJECTED (PV) WATER Displacement efficiency (relative lostrongly water(f) wet conditions)for results shown in Figs. (a)-(e). 0.5
Fig. 1--Examples of effect of wettability on oil recovery. 326
5O ~. o’-~,..,
ST-86
~
0
~ _ I~Fr esh~Em_ulsion 1
"~..’.,,%"~ [,o"]= o., ~,
-50 -I00 -150
12
-200
2.
4
6
8 pH
I0
12.
14
+
tO
oo
+÷
÷
o
+
o
o
o
o 8
5O O. 0
o
o o =~
6
o
oo +o
+o
~~
+
~ -5o E o 0,01
MOUTRAY 0.1
1.0
[Ne+] (M) -200
2
0
4-
6
8 pH
I0 12 14
Fig, 9~Adheslon mapping for Moutray crude oil : + =adhesion obse~ed~ o=no adhesion obse~ed~ hatched region = predicted adhesion bounda~ dashed line = prediction revised using constant charge density assumption,
Fig, 6--Zeta potential as a function of pH and [Na +] for ST-86 crude oll emulsions: (a) stored 1 week (fresh) and (b) stored 2 weeks (aged).
5O
>
-50 -
-100 -150
- 200
0
2
4
6
8 pH
I0 12
Fig. 7--Zeta potential as a function of pH and [Na+] for Leduc crude oil emulsions,
14
[No+](M) Fig, 10--Adhesion mapping for ST-86 crude oil: + =adhesion observed, o=no adhesion observed, hatched region =predicted adhesion boundary, dashed line=prediction revised using constant charge density assumption, I
5O Crushed Gloss
IE
25
I0
0 -25 -5O
"r
8,-
.
4
LEDUC
-75
0
o O.Ol
0.1 [No+] (M)
1.0
Fig. 11--Adhesion mapping for Leduc crude oih +=adheslonobserved, o=noadhesionobserved,@=temporary Fig, 8--Zeta potential as a function of pH and [Na+] for suspensions of crushed microscope glass--solid adhesion observed, hatched reglon=predicted adhesion boundary. lines=aged suspensions, dashed lines=fresh suspensions, 328
PELLICULAR WATER
BULK ’WATER
-SOLID -SOLi D
PELLICULAR WATER
BULK WATER
b
BULK WATER
Fig, 2--Schematic of distribution of bulk and pellicular water in pores with (a) smooth and (b) rough walls,
75
75-
5O
5O
o 25
h(nm) -25
0
=, o.o~
pH 8 I I
-5O
-- 0,16 M --- 0.14 M ..... 0.10M
-50
b) Fig, 3--Comparison of ]3 vs, h relationships for (a) pH 4-6 at [Na +] = 0,01 M and (b) [Na +] = 0,1-0.16 M at pH 8,
IO
5O
i i i i i iiI
Non- edhesion
8
Na+-I = 0. I
Leduc~
-5O
"T"
-150
Adhesion 2 0.01
-200 0. I [No+] (M)
1.0
Fig. 4--Predicted adhesion boundaries for 3 crude oils, Lower limits correspond to ]3m~x =0; upper limits to ]3max =10 kPa, Adhesion is predicted to occur below and to the right of these curves.
327
III~IiIIIIII 0
2
4
6
pH
8
I0
12
Fig. 5--Zeta potential as a function of pH and [Na +] for Moutray crude oil emulsions.
14
