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Temperature and pressure effects on zeta potential values of reservoir minerals
Journal of Colloid and Interface Science 300 (2006) 788–794 www.elsevier.com/locate/jcis
Temperature and pressure effects on zeta potential values of reservoir minerals Karina Rodríguez a , Mariela Araujo b,∗ a Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela b Earth Science and Engineering Department, Imperial College, London, United Kingdom
Received 19 January 2006; accepted 6 April 2006 Available online 6 May 2006
Abstract An experimental study of the effect of temperature and pressure on zeta potential of typical reservoir minerals, including quartz, kaolinite, and calcite, is presented. Experiments included the design and construction of an electrophoretic cell for zeta potential measurements at variable pressure and temperature. Electrolyte concentration was varied in the range from 0.0001 to 0.1 M in the pH range from 2 to 9. For all the minerals it is found that the zeta potential decreases with temperature at a rate characteristic of each mineral; values are around −2.3 mV/◦ C for quartz, −0.96 mV/◦ C for kaolinite, and −2.1 mV/◦ C for calcite for pressure values less than 45 psi. The effect of pressure is found to depend on the mineral nature and pH of the electrolytic solution. In the case of quartz, a systematic increase in the value of the zeta potential with pressure is observed, whereas a decreasing trend is measured for the kaolinite. In the case of calcite, a decreasing trend is observed for pressures up to 45 psi, whereas the experimental data suggest an increasing trend for higher pressure values. © 2006 Elsevier Inc. All rights reserved. Keywords: Reservoir minerals; Zeta potential; Quartz; Kaolinite; Calcite; Pressure
1. Introduction Understanding the electrokinetic response of minerals present in natural porous structures is essential for the description of interfacial processes in such formations. In particular, rock–fluid interactions in reservoir rocks are of primary interest to the oil industry since they affect the fluid distribution, and therefore the macroscopic flow properties of reservoir fluids, information required for the prediction of future reservoir performance [1–3]. Minerals such as quartz, kaolinite, and calcite are present in many different forms of porous media, including reservoir rocks. In general, reservoir rock composition includes several types of clay minerals besides quartz and other metallic components [4]. In the literature, very little information is available on interfacial properties such as zeta potential at high temperatures, * Corresponding author.
E-mail address: [email protected] (M. Araujo). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.04.030
and even less is known about the effect of pressure on such properties. Interesting industrial processes such as flotation and enhanced oil recovery normally occur under pressure and temperature conditions different from ambient or lab conditions (20 ◦ C and 1 atm); thus any knowledge in this direction is of particular scientific, technical, and industrial interest. Electrokinetic response refers to the phenomena that take place at a solid/liquid interface as a result of an applied electrical potential gradient. They derive from interactions between macroscopic motion and diffuse electric charge. The electrokinetic response can be measured by several different methods according to the experimental conditions, such as electrophoresis, electroosmosis, streaming potential, or sedimentation potential [5]. Zeta potential measurements are commonly performed using electrophoresis and streaming potential techniques. Johnson used quartz samples to demonstrate that the zeta potential as determined using both methods is equivalent within experimental error [6]. The streaming potential method has been commonly used since the technique is most easily adaptable for measurements at nonambient conditions. In this work we use
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the electrophoresis technique in a new specially designed cell that allows exploring a limited temperature and pressure range, avoiding the presence of convectional currents and nonuniform cell expansion effects. Previous work reporting zeta potential measurements of quartz, kaolinite, and calcite was mostly performed under ambient conditions, i.e., at normal pressure (1 atm/14.7 psi) and temperature (20–25 ◦ C). High-temperature experiments were successfully performed by Kulkarni and Somasundaran [7] and Ramachandran and Somasundaran [8] using the streaming potential technique on quartz and kaolinite samples. Moulin and Roques studied the effect of calcite concentration under threephase (gas, liquid, and solid) conditions [9]. Hussain et al. studied the zeta potential on clay samples, including kaolinite, and the effect on coal flotation [10]. In this work, the zeta potential is studied as a function of the pH of the electrolytic solution for three different minerals, quartz, kaolinite, and calcite, in the temperature range from 20 to 45 ◦ C and pressures from 14.7 to 74.5 psi. As electrolytic solution, NaCl brine, at different concentrations ranging from 0.0001 to 0.1 M was used. It is found that for the three minerals, the zeta potential decreases with temperature at rates on the order of −2.3 mV/◦ C for the quartz, −0.96 mV/◦ C for the kaolinite, and −2.1 mV/ ◦ C for the calcite for pressure values less than 45 psi. The decreasing trend of the zeta potential with temperature was observed in previous reported experiments for the quartz and kaolinite in Refs. [7,8]. In terms of the pressure response, a systematic increase in zeta potential values with pressure is observed for the quartz at all pH values, whereas a monotonic decreasing trend is measured for the kaolinite. The calcite response is variable with pressure. For pressures less than 45 psi, a decreasing trend is observed, whereas the behavior seems to increase for higher pressure values. In this paper we first present in Section 2 the strategy followed in the experimental procedure, including the design and construction of the electrophoretic cell. Section 3 contains the experimental results of the zeta potential measurements with temperature and pressure and their discussion. 2. Experimental methods 2.1. Sample preparation and characterization Quartz, kaolinite, and calcite samples were obtained from commercial vendors. Solid characterization included elemental analysis by X-ray photoelectron spectroscopy (XPS) using a Leybold LH-11 with a monochromatic X-ray source, and surface area measurements performed through a BET isotherm. Elemental composition as derived from the XPS analysis is given in Table 1. Table 2 summarizes the corresponding specific surface areas of the chosen samples. Samples were submitted to a careful cleaning process previous to their preparation. The quartz samples were leached with concentrated nitric acid and repeated washing with distilled water. The kaolinite was subjected to repeated washing with NaCl using the procedure of Hollander et al. [11]. The
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Table 1 XPS elemental composition of the used samples (% atomic)
C–O C–H O Al Si–O Si–H Fe Ca
Quartz
Kaolinite
Calcite
– 26.58 40.65 – 32.77 – – –
–
15.99 39.14 32.15 – – – – 12.72
6.88 40.62 24.25 17.00 11.25 – –
Table 2 Specific surface area of selected reservoir minerals Mineral
SSA (m2 g−1 )
Quartz Kaolinite Calcite
1.25 ± 0.05 21.33 ± 0.06 1.40 ± 0.15
sample was washed several times with distilled water and then with triply distilled water until a constant pH of the supernatant was obtained. The samples were ground using conventional mechanical methods and sieved in a 400 mesh. As electrolytes, solutions with different NaCl concentrations (0.0001, 0.001, 0.01, and 0.1 M) were used. All glass material used was cleaned with H3 NO3 5 M, followed by a 50%–50% mixture of H2 SO4 and H2 O2 (3%), and finally washed with distilled water. Different solid/electrolyte suspensions were prepared using the selected set of minerals and the different electrolyte concentrations. Each suspension contained 100 mg of solid in 1 L of fluid. The solutions were let equilibrate for 24 h. Then experimental measurements including particle size distribution, solution conductance, and surface charge density were performed as part of the characterization process and are given in Appendix A. 2.2. Solid/electrolyte suspensions properties Solid/electrolyte suspensions were characterized by several measurements including density, viscosity, and surface tension. The typical expected behavior of these properties was observed as a function of temperature. Surface charge density was also determined as a function of the pH of the electrolytic solution in the range from pH 2 to 9 by potentiometric titrations at 20, 35, and 45 ◦ C. Potentiometric titrations were done with a Metron 683 unit with a Dosimat 665 dosification unit for a fast exchange of liquids. For the titrations, NaOH and HCl of known concentrations were used. Calibration buffers at pH 4 and 7 were used. 2.3. Zeta potential measurements The measurements were performed at room conditions using a conventional commercial electrophoretic cell (Zeta Meter 3.0). The cell setup contained three main elements: a highquality microscope for particle observation, an electrophoretic
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cell where the colloidal particles are introduced, and a power source that provides the electric field to the cell. A new cell was designed and built for measurements at variable pressure and temperature. The new electrophoretic cell was built from Teflon and Plexiglas. Material testing determined that the cell resist up to 130 ◦ C without bending. In terms of pressure the cell was able to support 110 psi at room temperature. Several resistant rubber rings were placed on the cell boundaries to avoid any leakage when it was operated under pressure. The cell was placed inside an external jacket for heating, which was performed by a continuous flow of water at the desired temperature. The jacket is made of Plexiglas and has two outlets, allowing its connection to a heat bath for temperature control of the system. Sensitivity to flow and electrical measurements were performed on the cell to verify the presence or absence of convection currents when the cell was operated under various temperature and pressure conditions. Fig. 1 presents different views of the cell. For the injection of the suspension samples, a syringe pump at the desired working pressure was used (constant-pressure injection mode). Due to the nonconductive nature of the cell jacket, temperature values were verified using two methods: (a) direct readings of temperature strips attached to the inner surface of the jacket close to the cell, and (b) a PT100 thermocouple connected to one of the ends of the glass capillary cell. Pressure was monitored through a gauge connected to the injection line. The sample was preheated to the same cell tem-
Fig. 1. Different views of the electrophoretic cell designed for measurements at variable pressure and temperature.
perature while being maintained under constant agitation prior its placement in the electrophoretic cell. From each suspension, five samples of 100 ml were taken. The samples were set to five different pH conditions (2, 4, 6, 8, and 9) using HCl and NaOH solutions at 0.1 M concentration at a constant agitation. This gave 20 suspension samples for each mineral except for the calcite, which suffers dilution at low pH values. The solutions were let equilibrate for 24 to 48 h. Once a solution was inside the electrophoretic cell, the two electrode terminals were connected, and the voltage was applied. The mobility of the particles was monitored through an oscilloscope. Between 30 and 50 measurements of zeta potential were taken for each suspension sample. Getting triplicate measurement sets for each mineral sample controlled experimental reproducibility. Handling of the large number of data from zeta potential measurements was possible by automatic capture and computer acquisition. The new electrophoretic cell was calibrated using a commercial Zeta Meter 3.0 as a reference for measurements under ambient conditions (20 ◦ C, 14.7 psi). Additional measurements were performed at 35 and 45 ◦ C and four more pressures, 30, 45, 60, and 74.7 psi. 3. Results and discussion 3.1. Quartz The results for quartz zeta potential measurements as a function of the pH of the electrolytic solution for 20, 35, and 45 ◦ C degrees are shown in Fig. 2 for 0.01 M concentration. It is observed that the zeta potential becomes more negative as the temperature is increased (i.e., the zeta potential absolute magnitude increases). Solid lines in the figure are just a guide to the observed data trend. This behavior is consistent with experimental results from previous authors [7,8]. At alkaline pH, elevation of the temperature caused significant changes in the final zeta potential values, partly due to the change in the pK of water and also due to the mineral solution equilibria. This is reflected as an almost linear dependence between the zeta potential value and the pH. Somasundaran noticed the same effect on streaming potential measurements up to 75 ◦ C [8]. Tewari and McLean [12] observed similar pH changes at elevated temperatures for the alumina–water system.
Fig. 2. Zeta potential as a function of pH for several temperatures. Diamonds correspond to data at 20 ◦ C, squares at 35 ◦ C, and triangles at 45 ◦ C for. Measurements are for electrolyte concentration of 0.01 M and 1 atm of pressure. Solid lines are just a visual guide for the observed trend.
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Fig. 3. Zeta potential as a function of pH for differential pressures. Symbols are diamonds (14.7 psi), squares (30 psi), triangles (45 psi), crosses (60 psi), and stars (74.7 psi). Solid line is just a visual guide to show trend of data. Measurements were performed at 20 ◦ C with a 0.01 M electrolyte solution.
Fig. 4. Zeta potential of kaolinite 0.01 M as a function of pH, at 20 ◦ C. Line is only a visual guide to the data trend.
The main cause for the quartz surface charge is the dissociation of the silanol groups at the interface. To understand the temperature dependence of the zeta potential of quartz it is necessary to analyze the mineral solution chemical equilibrium of the system at different temperatures. The temperature dependence of the solubility of crystalline quartz [13] can be described by the following equations:
Measurement of the zeta potential of kaolinite and other clay mineral particles is not so difficult if the particles are relatively large (>1 µm) and the electrolyte concentration is reasonably large (above about 20 µM) [5,16]. In the case considered here these conditions were fulfilled, giving us confidence in the results from the experiments, since the Smoluchowski equation can be used. Zeta potential values at room temperature are found to decrease with pH as shown in Fig. 4. This behavior is similar to those values reported by Hu and Liu for kaolinite samples [17]. A similar experimental trend was reported by Ramachandran and Somasundaran [8] in measurements at 25 ◦ C. In our experiments it was observed that as the temperature increases, the magnitude of the zeta potential increases for both acidic and alkaline pH values. The kaolinite temperature response can be understood in terms of the processes taking place in the system and the different species active in a given pH range. For alkaline pH values, zeta potential is negative and tends to stabilize for larger pHs. This result can be understood in terms of the activity of Al3+ , which is very high in the acidic region and decreases rapidly with increasing pH, a change of 16 orders of magnitude for the studied pH range (from 2 to 9). In the alkaline region the major − species are H3 SiO− 4 and Al(OH)4 and their adsorption causes the mineral to be highly negatively charged, resulting in a negative zeta potential. For acidic pH values the mineral surface is in fact more positively charged at higher temperatures. This could be attributed to dissolution and readsorption of Al3+ and Al(OH)2+ species, which have a high activity in this pH range. For pH values near the neutral range the relevant species are Al(OH)3 , H4 SiO4 , and H3 SiO− 4 . The net negative potential on the surface is attributed to the adsorption of H3 SiO− 4 , which is the only charged species that is active. In terms of the pressure response, data from Fig. 5 correspond to zeta potential values versus pressure for pH 4. It is observed that for pressure values up to 45 psi there is a clear linear decreasing trend with slope ∼ −0.96 mV/◦ C and for higher pressure values, the trend has a lower slope in the range of −(0.5/0.6) mV/◦ C. Note that the effect of temperature is to produce a shift of the zeta potential values and that with pressure there is also a decaying trend, almost linear, tending to stabilize for large pres-
SiO2 + 2H2 O = H4 SiO4 , log(H4 SiO4 ) = 0.151 − 1162/T . The reaction is independent of pH, and as can be seen, the H4 SiO4 formation is favored at higher temperatures. The number of ionizable ions per silicon atom is higher for a silicic acid surface than for a fresh quartz surface. Thus, the silicic acid surface is expected to have a larger surface charge density and hence a larger magnitude of the zeta potential. H3 SiO− 4 is the only major ionic species in solution. In alkaline solutions the equilibrium for quartz dissolution is governed by H4 SiO4 = H+ + H3 SiO− 4,
pK = 9.8,
−9.8 = − log(H4 SiO4 ) + log(H+ ) + log(H3 SiO− 4 ). Thus, as the pH increases while K is constant, log(H3 SiO− 4) must increase. Increases in both temperature and pH favor the formation of H3 SiO− 4 , leading to more negative zeta potential values. This approach is consistent with the analysis reported by Van Lier et al. [14]. Fig. 3 shows zeta potential values at 20 ◦ C for different pressures as indicated. A systematic decrease in the magnitude of the zeta potential value with pressure is observed. From the stability diagram of quartz [15], we see that the a-quartz phase in the temperature and pressure range studied here is stable, thus no phase transformations are occurring in the system, and that the increase in pressure can only change the suspension ordering at the solid–liquid interface in a monotonic variation, as seen in the experiments at a given temperature. The solubility of quartz is increasing with temperature and there is no pH dependence for the pH range between 2 and 9. For higher pH values (pH > 9), the solubility rises sharply, but this regime was not addressed in this study.
3.2. Kaolinite
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Fig. 5. Zeta potential versus pressure for kaolinite for three different pressures at pH 4 and 0.01 M of electrolyte concentration. Symbols correspond to 20 ◦ C (diamonds), 35 ◦ C (squares), and 45 ◦ C (triangles).
Fig. 6. Zeta potential versus pressure for calcite at pH 8 and three different temperatures. Data symbols are diamonds for 20 ◦ C, squares for 35 ◦ C, and triangles for 45 ◦ C. Solid lines are only visual guides to the data trend.
sures. The same behavior was observed for other pH values. A similar trend in temperature was reported by Ramachandran and Somasundaran for measurements of Na-kaolinite at 75 ◦ C at 1 atm (14.7 psi) [8]. However, these authors did not report any evidence of reaching a plateau region, since they did not have enough points, just reporting measurements for three points. The understanding of the detailed mechanism of the pressure response of the zeta potential of kaolinite is not simple, since several species are active in the studied pH range. For example, Al3+ and Al(OH)2+ have a decreasing activity as a function of pH, but they may reabsorb for pH ∼ 4, which is the case shown in Fig. 5, whereas the solubility of Al(OH)− 4 increases with pH, and the increase rate is higher when pH ∼ 4. Other species, such as H4 SiO4 , only display activity near pH ∼ 4 and a decreasing trend in solubility. The case of H3 SiO− 4 is interesting, since it shows increasing solubility for pH between 2 and 3, then decrease around pH ∼ 4, and linear increasing activity for pH values higher than 5 (see activity plot in Ref. [8]). The analysis of the phase diagram for kaolinite shows that no phase changes are expected in the pressure and temperature range studied [15]. The steadily decreasing trend observed in the zeta potential with pressure could be associated with local rearrangements of these active species near the inner and outer Helmholtz planes, and effect that tends to stabilize at high pressure values. Further detailed modeling is required to understand the detailed ordering mechanism.
Smani et al. [23], indicated that the surface charge is negative. More recent work suggests that the surface charge depends on the nature of the sample [24]. A similar situation also occurs with the zeta potential measurements, as reported by Moulin and Roques [9] and Amankonah and Somasundaran [25]. The dilution effect was also observed when the temperature and pressure were changed. An increase in the magnitude of the zeta potential was observed with the increase in temperature with characteristic curves for this mineral. In the studied pressure range, for pressure values less or equal to 45 psi, a decreasing trend of the zeta potential with temperature was observed with a rate factor on the order of −2.1 mV/◦ C. For higher pressure values, such a trend disappears, and data suggest that the zeta potential value tends to increase; however, only one set of experimental points display this last behavior for every measured temperature, as shown in Fig. 6. The pressure dependence of the zeta potential for the calcite is also characteristic of this mineral, as observed clearly in Fig. 6. At 20 and 35 ◦ C, zeta potential values are almost independent of pressure up to values of 45 psi. Then the behavior changes, giving rise to an apparent minimum around 60 psi for 20 ◦ C. An increasing trend was observed for higher pressure values. From the data at 45 ◦ C, there is a curvature in the functional form of the zeta potential however the characteristic apparent minimum at 60 psi observed for 20 ◦ C seems to be preserved with an increasing trend for higher pressures. The behavior of the zeta potential with temperature can be understood by analyzing the processes and species involved. Regarding the sign of the zeta potential, it is dependent on the number of excess Ca2+ or CO2− 3 ions available in solution, in agreement with the work of Berlin and Khabakov [26] and Douglas and Walker [22]. With an increase in temperature, the solubility of calcite decreases, as is also the case for CO2 . The Ca2+ ions have increased solubility as the temperature increases, allowing them to preferentially leave the calcite structure, leaving the surface with a negative charge. This is the simplest plausible mechanism to account for the observed response. In general, there are six dissolved species (H+ , OH− , 2− 2+ H2 CO3 , HCO− 3 , CO3 , and Ca ) that are active in this pH range, and the observed response is a combination of the behavior of all of them. In the experiment we did not monitor the species concentrations; thus we are unable to model their kinetic behavior.
3.3. Calcite For the calcite, as stated previously, only measurements at high pH were performed due to dilution of the mineral [18]. Sjoberg [19] noticed that calcite suspension in water is accompanied by a phenomenon of surface dissolution followed by a reversion to equilibrium through a mechanism of recrystallization. Thus, the equilibrium state at the surface depends on the kinetics of dissolution. The experiments show that calcite in aqueous media displays complex behavior mainly due to its solubility, which is governed by many chemical equilibria and the surface electrical charge. In the case of calcite, even the sign of the surface charge has been strongly disputed in the literature. Some authors, including Fuerstenau et al. [20] and Yarar and Kitchener [21], report a positive value, but others, such as Douglas and Walker [22] and
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More complicated processes associated with hydrolysis phenomena of surface carbonate ions could take place, as suggested by Ney [27], and Smani et al. [23]. They considered that the CaOH+ ions determine the intensity and sign of the zeta potential and that the charge is governed by hydrolysis phenomena of surface carbonate ions. Somasundaran and Agar [28] postulated the hydrolysis either of the surface Ca2+ and CO2− 3 ions or of the dissolved ions, the complexes formed being adsorbed. The electrolyte concentration has a strong effect on the zeta potential value. This can be ascribed to the change in the concentration of dehydrated calcium ions in the inner Helmholtz plane, due to variations in the dissolution rate. The pH dependency and the maximum value observed at pH 9 can be explained by the reactions of Ca2+ ions with hydroxyl ions on the one hand, and bicarbonate ions on the other. For the electrolyte concentrations used here the solubility limit was not reached during the performance of the experiments since no precipitation of solids was observed. However, such solubility effects may be enough to modify the structure of the electrical double layer, leading to the trend changes observed in the zeta potential measurements, as shown in Fig. 6 for high temperatures and pressures. In the hydrolysis interpretation of the surface behavior, the surface ions Ca2+ and CO2− 3 obey the reactions [28]
Table A.1 Particle size distribution of prepared suspensions
− − CO2− 3 + H2 O ↔ HCO3 + OH ,
A.2. Viscosity
− HCO− 3 + H2 O ↔ CO2 + H2 O + OH ,
Mineral/ electrolyte Quartz Kaolinite Calcite
Particle size (µm) 1.0 × 10−4 M
1.0 × 10−3 M
1.0 × 10−2 M
1.0 × 10−1 M
33.4 23.1 32.5
26.4 25.9 31.3
28.4 29.6 30.2
30.6 27.8 33.2
Acknowledgments The authors thank Yani C. Araujo and Hector Franco for their helpful contributions and encouraging discussions during the realization of this work. Appendix A. Summary of general electrolyte properties A.1. Particle size distribution Particle size distribution was determined using a Mastersizer unit from Malvern Instruments. Each measurement was taken three times and average values are reported in Table A.1. Particle size under normal conditions is in the range between 21 and 32 µm.
and
The viscosity of the suspensions was measured with a capillary viscosimeter. It displayed a monotonic increase with electrolyte concentration and pressure, and it tends to decrease with temperature.
CaOH+ + OH− ↔ Ca(OH)2aq .
A.3. Surface tension
Ca2+ + OH− ↔ CaOH+ ,
For acidic pH, the first two reactions proceed to the right and the two others to the left, whereas at high pH the opposite is true. This reflects the change in sign of the surface charge with pH due to hydrolysis and the corresponding behavior of the zeta potential [22,23]. The pressure response of the calcite can be understood from the fact that the calcite solubility slightly increases with an increase in pressure [15] that is accompanied by an increase in concentration of Ca2+ at the surface, thus leading to an overall decrease in the surface charge (becoming less negative) and a corresponding decrease in the zeta potential, as seen in Fig. 6. However, as the concentration of Ca2+ ions increases, they may + − combine with hydroxylic ions (Ca2+ surf + OH ↔ CaOHsurf ) and − − 2+ HCO3 (through the reaction Ca + HCO3 ↔ CaHCO+ 3 ) both resulting in a decrease of the zeta potential. The detailed local arrangements of ions and cations at the surface cannot be inferred from the measurements performed here and require detailed modeling, which is beyond the scope of this experimental work. The analysis given here is somewhat similar to that of Siffert and Fimbel [29] for the calcite zeta potential at 25 ◦ C, where results were associated with the change of ionic species superimposed on problems of hydration and positions of the various cations present in the inner and outer Helmholtz planes.
Surface tension was determined under room conditions by the Guingeli plate method. A Robal scale for better precision supports the experimental unit. It was found that surface tension increases with brine concentration as observed for the density and suspension viscosities. A.4. Conductance The conductance of the suspensions was measured with a Methrom conductimeter (686 Metrohm). Reproducibility of conductance values was ensured by repeated measurements. It is found that in all cases the conductance increases with temperature. References [1] R.P. Monicard, Properties of Reservoir Rocks: Core Analysis, Gulf Publishing Company, Paris, France, 1980. [2] N.R. Morrow, J. Pet. Technol. 42 (1987) 1476. [3] M. Araujo, Y.C. Araujo, Visión Tecnológica 8 (1) (2000) 19. [4] D. Tiab, E.C. Donaldson, Petrophysics, Gulf Publishing Company, Houston, TX, 1993. [5] R.J. Hunter, Introduction to Modern Colloid Science, Oxford Science Publication, New York, 1993. [6] P.R. Johnson, J. Colloid Interface Sci. 209 (1999) 264.
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[18] R.G. Compton, K.L. Pritchard, Philos. Trans. R. Soc. London Ser. A 330 (1990) 47. [19] E.L. Sjoberg, Geochim. Cosmochim. Acta 40 (1976) 441. [20] M.C. Fuerstenau, G. Gutierrez, D.A. Elgillani, Trans. AIME 241 (1968) 319. [21] B. Yarar, J.A. Kitchener, Inst. Min. Metall. Trans. Sect. C 79 (1970) 23. [22] H.W. Douglas, R.A. Walker, Trans. Faraday Soc. 46 (1950) 559. [23] M.S. Smani, P. Blazy, J.M. Cases, Trans. SME 258 (1957) 168. [24] A. Pierre, J.M. Lamarche, R. Mercier, A. Foissy, J. Persello, J. Dispers. Sci. Technol. 11 (1990) 611. [25] J.O. Amankonah, P. Somasundaran, Colloids Surf. 15 (1985) 335. [26] T.S. Berlin, A.V. Khabakov, Geochemistry 3 (1961) 217. [27] P. Ney, Zeta Potentiale und Flotierbarkeif von Mineralen, SpringerVerlarg, New York, 1973, pp. 96–101. [28] P. Somasundaran, G.E. Agar, J. Colloid Interface Sci. 24 (1967) 433. [29] B. Siffert, P. Fimbel, Colloids Surf. 330 (1984) 377.
Temperature and pressure effects on zeta potential values of reservoir minerals Karina Rodríguez a , Mariela Araujo b,∗ a Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuela b Earth Science and Engineering Department, Imperial College, London, United Kingdom
Received 19 January 2006; accepted 6 April 2006 Available online 6 May 2006
Abstract An experimental study of the effect of temperature and pressure on zeta potential of typical reservoir minerals, including quartz, kaolinite, and calcite, is presented. Experiments included the design and construction of an electrophoretic cell for zeta potential measurements at variable pressure and temperature. Electrolyte concentration was varied in the range from 0.0001 to 0.1 M in the pH range from 2 to 9. For all the minerals it is found that the zeta potential decreases with temperature at a rate characteristic of each mineral; values are around −2.3 mV/◦ C for quartz, −0.96 mV/◦ C for kaolinite, and −2.1 mV/◦ C for calcite for pressure values less than 45 psi. The effect of pressure is found to depend on the mineral nature and pH of the electrolytic solution. In the case of quartz, a systematic increase in the value of the zeta potential with pressure is observed, whereas a decreasing trend is measured for the kaolinite. In the case of calcite, a decreasing trend is observed for pressures up to 45 psi, whereas the experimental data suggest an increasing trend for higher pressure values. © 2006 Elsevier Inc. All rights reserved. Keywords: Reservoir minerals; Zeta potential; Quartz; Kaolinite; Calcite; Pressure
1. Introduction Understanding the electrokinetic response of minerals present in natural porous structures is essential for the description of interfacial processes in such formations. In particular, rock–fluid interactions in reservoir rocks are of primary interest to the oil industry since they affect the fluid distribution, and therefore the macroscopic flow properties of reservoir fluids, information required for the prediction of future reservoir performance [1–3]. Minerals such as quartz, kaolinite, and calcite are present in many different forms of porous media, including reservoir rocks. In general, reservoir rock composition includes several types of clay minerals besides quartz and other metallic components [4]. In the literature, very little information is available on interfacial properties such as zeta potential at high temperatures, * Corresponding author.
E-mail address: [email protected] (M. Araujo). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.04.030
and even less is known about the effect of pressure on such properties. Interesting industrial processes such as flotation and enhanced oil recovery normally occur under pressure and temperature conditions different from ambient or lab conditions (20 ◦ C and 1 atm); thus any knowledge in this direction is of particular scientific, technical, and industrial interest. Electrokinetic response refers to the phenomena that take place at a solid/liquid interface as a result of an applied electrical potential gradient. They derive from interactions between macroscopic motion and diffuse electric charge. The electrokinetic response can be measured by several different methods according to the experimental conditions, such as electrophoresis, electroosmosis, streaming potential, or sedimentation potential [5]. Zeta potential measurements are commonly performed using electrophoresis and streaming potential techniques. Johnson used quartz samples to demonstrate that the zeta potential as determined using both methods is equivalent within experimental error [6]. The streaming potential method has been commonly used since the technique is most easily adaptable for measurements at nonambient conditions. In this work we use
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the electrophoresis technique in a new specially designed cell that allows exploring a limited temperature and pressure range, avoiding the presence of convectional currents and nonuniform cell expansion effects. Previous work reporting zeta potential measurements of quartz, kaolinite, and calcite was mostly performed under ambient conditions, i.e., at normal pressure (1 atm/14.7 psi) and temperature (20–25 ◦ C). High-temperature experiments were successfully performed by Kulkarni and Somasundaran [7] and Ramachandran and Somasundaran [8] using the streaming potential technique on quartz and kaolinite samples. Moulin and Roques studied the effect of calcite concentration under threephase (gas, liquid, and solid) conditions [9]. Hussain et al. studied the zeta potential on clay samples, including kaolinite, and the effect on coal flotation [10]. In this work, the zeta potential is studied as a function of the pH of the electrolytic solution for three different minerals, quartz, kaolinite, and calcite, in the temperature range from 20 to 45 ◦ C and pressures from 14.7 to 74.5 psi. As electrolytic solution, NaCl brine, at different concentrations ranging from 0.0001 to 0.1 M was used. It is found that for the three minerals, the zeta potential decreases with temperature at rates on the order of −2.3 mV/◦ C for the quartz, −0.96 mV/◦ C for the kaolinite, and −2.1 mV/ ◦ C for the calcite for pressure values less than 45 psi. The decreasing trend of the zeta potential with temperature was observed in previous reported experiments for the quartz and kaolinite in Refs. [7,8]. In terms of the pressure response, a systematic increase in zeta potential values with pressure is observed for the quartz at all pH values, whereas a monotonic decreasing trend is measured for the kaolinite. The calcite response is variable with pressure. For pressures less than 45 psi, a decreasing trend is observed, whereas the behavior seems to increase for higher pressure values. In this paper we first present in Section 2 the strategy followed in the experimental procedure, including the design and construction of the electrophoretic cell. Section 3 contains the experimental results of the zeta potential measurements with temperature and pressure and their discussion. 2. Experimental methods 2.1. Sample preparation and characterization Quartz, kaolinite, and calcite samples were obtained from commercial vendors. Solid characterization included elemental analysis by X-ray photoelectron spectroscopy (XPS) using a Leybold LH-11 with a monochromatic X-ray source, and surface area measurements performed through a BET isotherm. Elemental composition as derived from the XPS analysis is given in Table 1. Table 2 summarizes the corresponding specific surface areas of the chosen samples. Samples were submitted to a careful cleaning process previous to their preparation. The quartz samples were leached with concentrated nitric acid and repeated washing with distilled water. The kaolinite was subjected to repeated washing with NaCl using the procedure of Hollander et al. [11]. The
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Table 1 XPS elemental composition of the used samples (% atomic)
C–O C–H O Al Si–O Si–H Fe Ca
Quartz
Kaolinite
Calcite
– 26.58 40.65 – 32.77 – – –
–
15.99 39.14 32.15 – – – – 12.72
6.88 40.62 24.25 17.00 11.25 – –
Table 2 Specific surface area of selected reservoir minerals Mineral
SSA (m2 g−1 )
Quartz Kaolinite Calcite
1.25 ± 0.05 21.33 ± 0.06 1.40 ± 0.15
sample was washed several times with distilled water and then with triply distilled water until a constant pH of the supernatant was obtained. The samples were ground using conventional mechanical methods and sieved in a 400 mesh. As electrolytes, solutions with different NaCl concentrations (0.0001, 0.001, 0.01, and 0.1 M) were used. All glass material used was cleaned with H3 NO3 5 M, followed by a 50%–50% mixture of H2 SO4 and H2 O2 (3%), and finally washed with distilled water. Different solid/electrolyte suspensions were prepared using the selected set of minerals and the different electrolyte concentrations. Each suspension contained 100 mg of solid in 1 L of fluid. The solutions were let equilibrate for 24 h. Then experimental measurements including particle size distribution, solution conductance, and surface charge density were performed as part of the characterization process and are given in Appendix A. 2.2. Solid/electrolyte suspensions properties Solid/electrolyte suspensions were characterized by several measurements including density, viscosity, and surface tension. The typical expected behavior of these properties was observed as a function of temperature. Surface charge density was also determined as a function of the pH of the electrolytic solution in the range from pH 2 to 9 by potentiometric titrations at 20, 35, and 45 ◦ C. Potentiometric titrations were done with a Metron 683 unit with a Dosimat 665 dosification unit for a fast exchange of liquids. For the titrations, NaOH and HCl of known concentrations were used. Calibration buffers at pH 4 and 7 were used. 2.3. Zeta potential measurements The measurements were performed at room conditions using a conventional commercial electrophoretic cell (Zeta Meter 3.0). The cell setup contained three main elements: a highquality microscope for particle observation, an electrophoretic
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cell where the colloidal particles are introduced, and a power source that provides the electric field to the cell. A new cell was designed and built for measurements at variable pressure and temperature. The new electrophoretic cell was built from Teflon and Plexiglas. Material testing determined that the cell resist up to 130 ◦ C without bending. In terms of pressure the cell was able to support 110 psi at room temperature. Several resistant rubber rings were placed on the cell boundaries to avoid any leakage when it was operated under pressure. The cell was placed inside an external jacket for heating, which was performed by a continuous flow of water at the desired temperature. The jacket is made of Plexiglas and has two outlets, allowing its connection to a heat bath for temperature control of the system. Sensitivity to flow and electrical measurements were performed on the cell to verify the presence or absence of convection currents when the cell was operated under various temperature and pressure conditions. Fig. 1 presents different views of the cell. For the injection of the suspension samples, a syringe pump at the desired working pressure was used (constant-pressure injection mode). Due to the nonconductive nature of the cell jacket, temperature values were verified using two methods: (a) direct readings of temperature strips attached to the inner surface of the jacket close to the cell, and (b) a PT100 thermocouple connected to one of the ends of the glass capillary cell. Pressure was monitored through a gauge connected to the injection line. The sample was preheated to the same cell tem-
Fig. 1. Different views of the electrophoretic cell designed for measurements at variable pressure and temperature.
perature while being maintained under constant agitation prior its placement in the electrophoretic cell. From each suspension, five samples of 100 ml were taken. The samples were set to five different pH conditions (2, 4, 6, 8, and 9) using HCl and NaOH solutions at 0.1 M concentration at a constant agitation. This gave 20 suspension samples for each mineral except for the calcite, which suffers dilution at low pH values. The solutions were let equilibrate for 24 to 48 h. Once a solution was inside the electrophoretic cell, the two electrode terminals were connected, and the voltage was applied. The mobility of the particles was monitored through an oscilloscope. Between 30 and 50 measurements of zeta potential were taken for each suspension sample. Getting triplicate measurement sets for each mineral sample controlled experimental reproducibility. Handling of the large number of data from zeta potential measurements was possible by automatic capture and computer acquisition. The new electrophoretic cell was calibrated using a commercial Zeta Meter 3.0 as a reference for measurements under ambient conditions (20 ◦ C, 14.7 psi). Additional measurements were performed at 35 and 45 ◦ C and four more pressures, 30, 45, 60, and 74.7 psi. 3. Results and discussion 3.1. Quartz The results for quartz zeta potential measurements as a function of the pH of the electrolytic solution for 20, 35, and 45 ◦ C degrees are shown in Fig. 2 for 0.01 M concentration. It is observed that the zeta potential becomes more negative as the temperature is increased (i.e., the zeta potential absolute magnitude increases). Solid lines in the figure are just a guide to the observed data trend. This behavior is consistent with experimental results from previous authors [7,8]. At alkaline pH, elevation of the temperature caused significant changes in the final zeta potential values, partly due to the change in the pK of water and also due to the mineral solution equilibria. This is reflected as an almost linear dependence between the zeta potential value and the pH. Somasundaran noticed the same effect on streaming potential measurements up to 75 ◦ C [8]. Tewari and McLean [12] observed similar pH changes at elevated temperatures for the alumina–water system.
Fig. 2. Zeta potential as a function of pH for several temperatures. Diamonds correspond to data at 20 ◦ C, squares at 35 ◦ C, and triangles at 45 ◦ C for. Measurements are for electrolyte concentration of 0.01 M and 1 atm of pressure. Solid lines are just a visual guide for the observed trend.
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Fig. 3. Zeta potential as a function of pH for differential pressures. Symbols are diamonds (14.7 psi), squares (30 psi), triangles (45 psi), crosses (60 psi), and stars (74.7 psi). Solid line is just a visual guide to show trend of data. Measurements were performed at 20 ◦ C with a 0.01 M electrolyte solution.
Fig. 4. Zeta potential of kaolinite 0.01 M as a function of pH, at 20 ◦ C. Line is only a visual guide to the data trend.
The main cause for the quartz surface charge is the dissociation of the silanol groups at the interface. To understand the temperature dependence of the zeta potential of quartz it is necessary to analyze the mineral solution chemical equilibrium of the system at different temperatures. The temperature dependence of the solubility of crystalline quartz [13] can be described by the following equations:
Measurement of the zeta potential of kaolinite and other clay mineral particles is not so difficult if the particles are relatively large (>1 µm) and the electrolyte concentration is reasonably large (above about 20 µM) [5,16]. In the case considered here these conditions were fulfilled, giving us confidence in the results from the experiments, since the Smoluchowski equation can be used. Zeta potential values at room temperature are found to decrease with pH as shown in Fig. 4. This behavior is similar to those values reported by Hu and Liu for kaolinite samples [17]. A similar experimental trend was reported by Ramachandran and Somasundaran [8] in measurements at 25 ◦ C. In our experiments it was observed that as the temperature increases, the magnitude of the zeta potential increases for both acidic and alkaline pH values. The kaolinite temperature response can be understood in terms of the processes taking place in the system and the different species active in a given pH range. For alkaline pH values, zeta potential is negative and tends to stabilize for larger pHs. This result can be understood in terms of the activity of Al3+ , which is very high in the acidic region and decreases rapidly with increasing pH, a change of 16 orders of magnitude for the studied pH range (from 2 to 9). In the alkaline region the major − species are H3 SiO− 4 and Al(OH)4 and their adsorption causes the mineral to be highly negatively charged, resulting in a negative zeta potential. For acidic pH values the mineral surface is in fact more positively charged at higher temperatures. This could be attributed to dissolution and readsorption of Al3+ and Al(OH)2+ species, which have a high activity in this pH range. For pH values near the neutral range the relevant species are Al(OH)3 , H4 SiO4 , and H3 SiO− 4 . The net negative potential on the surface is attributed to the adsorption of H3 SiO− 4 , which is the only charged species that is active. In terms of the pressure response, data from Fig. 5 correspond to zeta potential values versus pressure for pH 4. It is observed that for pressure values up to 45 psi there is a clear linear decreasing trend with slope ∼ −0.96 mV/◦ C and for higher pressure values, the trend has a lower slope in the range of −(0.5/0.6) mV/◦ C. Note that the effect of temperature is to produce a shift of the zeta potential values and that with pressure there is also a decaying trend, almost linear, tending to stabilize for large pres-
SiO2 + 2H2 O = H4 SiO4 , log(H4 SiO4 ) = 0.151 − 1162/T . The reaction is independent of pH, and as can be seen, the H4 SiO4 formation is favored at higher temperatures. The number of ionizable ions per silicon atom is higher for a silicic acid surface than for a fresh quartz surface. Thus, the silicic acid surface is expected to have a larger surface charge density and hence a larger magnitude of the zeta potential. H3 SiO− 4 is the only major ionic species in solution. In alkaline solutions the equilibrium for quartz dissolution is governed by H4 SiO4 = H+ + H3 SiO− 4,
pK = 9.8,
−9.8 = − log(H4 SiO4 ) + log(H+ ) + log(H3 SiO− 4 ). Thus, as the pH increases while K is constant, log(H3 SiO− 4) must increase. Increases in both temperature and pH favor the formation of H3 SiO− 4 , leading to more negative zeta potential values. This approach is consistent with the analysis reported by Van Lier et al. [14]. Fig. 3 shows zeta potential values at 20 ◦ C for different pressures as indicated. A systematic decrease in the magnitude of the zeta potential value with pressure is observed. From the stability diagram of quartz [15], we see that the a-quartz phase in the temperature and pressure range studied here is stable, thus no phase transformations are occurring in the system, and that the increase in pressure can only change the suspension ordering at the solid–liquid interface in a monotonic variation, as seen in the experiments at a given temperature. The solubility of quartz is increasing with temperature and there is no pH dependence for the pH range between 2 and 9. For higher pH values (pH > 9), the solubility rises sharply, but this regime was not addressed in this study.
3.2. Kaolinite
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Fig. 5. Zeta potential versus pressure for kaolinite for three different pressures at pH 4 and 0.01 M of electrolyte concentration. Symbols correspond to 20 ◦ C (diamonds), 35 ◦ C (squares), and 45 ◦ C (triangles).
Fig. 6. Zeta potential versus pressure for calcite at pH 8 and three different temperatures. Data symbols are diamonds for 20 ◦ C, squares for 35 ◦ C, and triangles for 45 ◦ C. Solid lines are only visual guides to the data trend.
sures. The same behavior was observed for other pH values. A similar trend in temperature was reported by Ramachandran and Somasundaran for measurements of Na-kaolinite at 75 ◦ C at 1 atm (14.7 psi) [8]. However, these authors did not report any evidence of reaching a plateau region, since they did not have enough points, just reporting measurements for three points. The understanding of the detailed mechanism of the pressure response of the zeta potential of kaolinite is not simple, since several species are active in the studied pH range. For example, Al3+ and Al(OH)2+ have a decreasing activity as a function of pH, but they may reabsorb for pH ∼ 4, which is the case shown in Fig. 5, whereas the solubility of Al(OH)− 4 increases with pH, and the increase rate is higher when pH ∼ 4. Other species, such as H4 SiO4 , only display activity near pH ∼ 4 and a decreasing trend in solubility. The case of H3 SiO− 4 is interesting, since it shows increasing solubility for pH between 2 and 3, then decrease around pH ∼ 4, and linear increasing activity for pH values higher than 5 (see activity plot in Ref. [8]). The analysis of the phase diagram for kaolinite shows that no phase changes are expected in the pressure and temperature range studied [15]. The steadily decreasing trend observed in the zeta potential with pressure could be associated with local rearrangements of these active species near the inner and outer Helmholtz planes, and effect that tends to stabilize at high pressure values. Further detailed modeling is required to understand the detailed ordering mechanism.
Smani et al. [23], indicated that the surface charge is negative. More recent work suggests that the surface charge depends on the nature of the sample [24]. A similar situation also occurs with the zeta potential measurements, as reported by Moulin and Roques [9] and Amankonah and Somasundaran [25]. The dilution effect was also observed when the temperature and pressure were changed. An increase in the magnitude of the zeta potential was observed with the increase in temperature with characteristic curves for this mineral. In the studied pressure range, for pressure values less or equal to 45 psi, a decreasing trend of the zeta potential with temperature was observed with a rate factor on the order of −2.1 mV/◦ C. For higher pressure values, such a trend disappears, and data suggest that the zeta potential value tends to increase; however, only one set of experimental points display this last behavior for every measured temperature, as shown in Fig. 6. The pressure dependence of the zeta potential for the calcite is also characteristic of this mineral, as observed clearly in Fig. 6. At 20 and 35 ◦ C, zeta potential values are almost independent of pressure up to values of 45 psi. Then the behavior changes, giving rise to an apparent minimum around 60 psi for 20 ◦ C. An increasing trend was observed for higher pressure values. From the data at 45 ◦ C, there is a curvature in the functional form of the zeta potential however the characteristic apparent minimum at 60 psi observed for 20 ◦ C seems to be preserved with an increasing trend for higher pressures. The behavior of the zeta potential with temperature can be understood by analyzing the processes and species involved. Regarding the sign of the zeta potential, it is dependent on the number of excess Ca2+ or CO2− 3 ions available in solution, in agreement with the work of Berlin and Khabakov [26] and Douglas and Walker [22]. With an increase in temperature, the solubility of calcite decreases, as is also the case for CO2 . The Ca2+ ions have increased solubility as the temperature increases, allowing them to preferentially leave the calcite structure, leaving the surface with a negative charge. This is the simplest plausible mechanism to account for the observed response. In general, there are six dissolved species (H+ , OH− , 2− 2+ H2 CO3 , HCO− 3 , CO3 , and Ca ) that are active in this pH range, and the observed response is a combination of the behavior of all of them. In the experiment we did not monitor the species concentrations; thus we are unable to model their kinetic behavior.
3.3. Calcite For the calcite, as stated previously, only measurements at high pH were performed due to dilution of the mineral [18]. Sjoberg [19] noticed that calcite suspension in water is accompanied by a phenomenon of surface dissolution followed by a reversion to equilibrium through a mechanism of recrystallization. Thus, the equilibrium state at the surface depends on the kinetics of dissolution. The experiments show that calcite in aqueous media displays complex behavior mainly due to its solubility, which is governed by many chemical equilibria and the surface electrical charge. In the case of calcite, even the sign of the surface charge has been strongly disputed in the literature. Some authors, including Fuerstenau et al. [20] and Yarar and Kitchener [21], report a positive value, but others, such as Douglas and Walker [22] and
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More complicated processes associated with hydrolysis phenomena of surface carbonate ions could take place, as suggested by Ney [27], and Smani et al. [23]. They considered that the CaOH+ ions determine the intensity and sign of the zeta potential and that the charge is governed by hydrolysis phenomena of surface carbonate ions. Somasundaran and Agar [28] postulated the hydrolysis either of the surface Ca2+ and CO2− 3 ions or of the dissolved ions, the complexes formed being adsorbed. The electrolyte concentration has a strong effect on the zeta potential value. This can be ascribed to the change in the concentration of dehydrated calcium ions in the inner Helmholtz plane, due to variations in the dissolution rate. The pH dependency and the maximum value observed at pH 9 can be explained by the reactions of Ca2+ ions with hydroxyl ions on the one hand, and bicarbonate ions on the other. For the electrolyte concentrations used here the solubility limit was not reached during the performance of the experiments since no precipitation of solids was observed. However, such solubility effects may be enough to modify the structure of the electrical double layer, leading to the trend changes observed in the zeta potential measurements, as shown in Fig. 6 for high temperatures and pressures. In the hydrolysis interpretation of the surface behavior, the surface ions Ca2+ and CO2− 3 obey the reactions [28]
Table A.1 Particle size distribution of prepared suspensions
− − CO2− 3 + H2 O ↔ HCO3 + OH ,
A.2. Viscosity
− HCO− 3 + H2 O ↔ CO2 + H2 O + OH ,
Mineral/ electrolyte Quartz Kaolinite Calcite
Particle size (µm) 1.0 × 10−4 M
1.0 × 10−3 M
1.0 × 10−2 M
1.0 × 10−1 M
33.4 23.1 32.5
26.4 25.9 31.3
28.4 29.6 30.2
30.6 27.8 33.2
Acknowledgments The authors thank Yani C. Araujo and Hector Franco for their helpful contributions and encouraging discussions during the realization of this work. Appendix A. Summary of general electrolyte properties A.1. Particle size distribution Particle size distribution was determined using a Mastersizer unit from Malvern Instruments. Each measurement was taken three times and average values are reported in Table A.1. Particle size under normal conditions is in the range between 21 and 32 µm.
and
The viscosity of the suspensions was measured with a capillary viscosimeter. It displayed a monotonic increase with electrolyte concentration and pressure, and it tends to decrease with temperature.
CaOH+ + OH− ↔ Ca(OH)2aq .
A.3. Surface tension
Ca2+ + OH− ↔ CaOH+ ,
For acidic pH, the first two reactions proceed to the right and the two others to the left, whereas at high pH the opposite is true. This reflects the change in sign of the surface charge with pH due to hydrolysis and the corresponding behavior of the zeta potential [22,23]. The pressure response of the calcite can be understood from the fact that the calcite solubility slightly increases with an increase in pressure [15] that is accompanied by an increase in concentration of Ca2+ at the surface, thus leading to an overall decrease in the surface charge (becoming less negative) and a corresponding decrease in the zeta potential, as seen in Fig. 6. However, as the concentration of Ca2+ ions increases, they may + − combine with hydroxylic ions (Ca2+ surf + OH ↔ CaOHsurf ) and − − 2+ HCO3 (through the reaction Ca + HCO3 ↔ CaHCO+ 3 ) both resulting in a decrease of the zeta potential. The detailed local arrangements of ions and cations at the surface cannot be inferred from the measurements performed here and require detailed modeling, which is beyond the scope of this experimental work. The analysis given here is somewhat similar to that of Siffert and Fimbel [29] for the calcite zeta potential at 25 ◦ C, where results were associated with the change of ionic species superimposed on problems of hydration and positions of the various cations present in the inner and outer Helmholtz planes.
Surface tension was determined under room conditions by the Guingeli plate method. A Robal scale for better precision supports the experimental unit. It was found that surface tension increases with brine concentration as observed for the density and suspension viscosities. A.4. Conductance The conductance of the suspensions was measured with a Methrom conductimeter (686 Metrohm). Reproducibility of conductance values was ensured by repeated measurements. It is found that in all cases the conductance increases with temperature. References [1] R.P. Monicard, Properties of Reservoir Rocks: Core Analysis, Gulf Publishing Company, Paris, France, 1980. [2] N.R. Morrow, J. Pet. Technol. 42 (1987) 1476. [3] M. Araujo, Y.C. Araujo, Visión Tecnológica 8 (1) (2000) 19. [4] D. Tiab, E.C. Donaldson, Petrophysics, Gulf Publishing Company, Houston, TX, 1993. [5] R.J. Hunter, Introduction to Modern Colloid Science, Oxford Science Publication, New York, 1993. [6] P.R. Johnson, J. Colloid Interface Sci. 209 (1999) 264.
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