Effect of pH and Layer Charge on Formation Damage in Porous Media

The effect of pH and layer charge on the swelling, migration of clay particles, and resulting permeability reduction due to changes in salt concentrat...
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Effect of pH and Layer Charge on Formation Damage in Porous Media Containing Swelling Clays K. Krishna Mohan† and H. Scott Fogler* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48019 Received September 5, 1996. In Final Form: February 20, 1997X The effect of pH and layer charge on the swelling, migration of clay particles, and resulting permeability reduction due to changes in salt concentration has been studied. Porous media containing swelling clays exhibit a drastic decrease in permeability when subjected to salinity changes under acidic conditions. Experiments suggest that this reduction in permeability is caused by macroscopic swelling and release of clay aggregates. The critical salt concentration of sodium chloride at which the transition from crystalline to osmotic swelling regimes occurs is shown to be independent of pH for Wyoming montmorillonite. The layer charge of the clays is also shown to influence the critical salt concentration at which the transition occurs. A 30% reduction in layer charge of Wyoming montmorillonite is sufficient to collapse the clays irreversibly. These effects of pH and layer charge on the critical salt concentration are explained using a surface complex model that predicts the diffuse layer charge as a function of the surface charge, ionic strength, pH, and the affinity of the counterions to the surface. This model can be extended to predict the diffuse layer charge and the critical salt concentration in multicomponent solutions to provide a predictive tool for preventing permeability reduction in porous media containing swelling clays.

Introduction The permeability or hydraulic conductivity of natural porous formations is of primary importance in several areas such as oil production, landfill remediation, groundwater contamination, and soil infiltration. Most oil- and gas-producing formations contain clay minerals that were originally deposited during sedimentation (detrital clay) or precipitated from fluids flowing through the matrix (authigenic clay). Changes in the aqueous composition of the permeating fluids can cause these clays to block pores by swelling and migration thus reducing the permeability of the formation. This reduction of permeability, also called formation damage, is undesirable for oil/gas production but is beneficial for minimizing migration of contaminants in landfill sites. The extent of swelling and migration of the clay minerals depends on the surface properties of the clays and porous medium and the solution conditions of the permeating fluids including ionic composition, ionic strength, and pH. This paper focuses on the effect of pH and layer charge of the swelling clays on the permeability reduction caused by swelling and migration. The results of this study are of interest in phenomena which involve changes of pH of the flowing fluid including petroleum production operations such as alkaline flooding, acidizing, and steamflooding.1-4 Background The phenomena of the dependence of permeability of clay-bearing oil reservoirs on the aqueous composition including pH, salinity, and ionic composition of the permeating fluids is generally referred to as water * Author to whom all correspondence should be addressed. † Currently at International Paper, Longmeadow Rd, Tuxedo Park, NY 10987. X Abstract published in Advance ACS Abstracts, April 1, 1997. (1) Khilar, K. C. Water Sensitivity of Berea Sandstone. Ph.D Dissertation, The University of Michigan, Ann Arbor, MI, 1981. (2) Vaidya, R. N. Fines Migration and Formation Damage. Ph.D Dissertation, The University of Michigan, Ann Arbor, MI, 1991. (3) Leone, J. A.; Scott, M. E. SPE Reservoir Eng. 1988, 3, 1279. (4) Scheurman, R. F.; Bergersen, B. M. SPE Reprint Series on Formation Damage 1990, 154.

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sensitivity. The reduction in permeability is a manifestation of the changes in the colloidal interaction between clay particles themselves and between clay particles and larger grains of the porous medium such as quartz. In a previous study on Berea sandstone, it was shown that reducing the salinity of flowing fluid below a threshold value known as critical salt concentration causes permeability reduction to the sandstone.1,2 The critical salt concentration of sodium chloride for preventing permeability reduction in this sandstone was also shown to increase with pH of the flowing solution.2 Furthermore, under sufficiently acidic conditions such as pH 2.0, Berea sandstone does not exhibit water sensitivity.2 These results can be explained by the effect of pH and salinity on the migration of kaolinite clays present in the sandstone. High pH and low salinity values cause the kaolinite particles to migrate and be captured in narrow constrictions of the sandstone thus resulting in permeability reduction. The migration is dependent on the colloidal interaction between the clay particles and the quartz grains in the porous media. The dependence of kaolinite migration on pH is due to the fact that a substantial portion of the charge on these minerals is generated by protonation and deprotonation of surface hydroxyl groups. One of the faces of the particle is an alumina surface with a point of zero charge around 7.2 which becomes positively charged in acidic solutions and negatively charged in basic media.5 Thus the total negative charge of the kaolinite particle varies with pH and alters the balance of electrostatic repulsion and van der Waals attraction between the clay particle and quartz grains. Consequently, the fines migration and the attendant permeability reduction to the porous media containing kaolinites is inhibited at low pH and is significant at high pH. In contrast to kaolinite, smectites in natural porous formations can cause permeability reduction by swelling as well as migration. Swelling of clay particles reduces the pore throat size and increases the resistance to flow. The swelling of clay minerals is known to be a function (5) van Olphen, H. An Introduction to Clay Colloid Chemistry; John Wiley & Sons: New York, 1977.

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of the type of cation and the clay layer charge.6 The critical salt concentration or the threshold concentration at which the permeability reduction is first observed is related to the swelling of the montmorillonites.7-9 Swelling of montmorillonites occurs in two regimes, namely, crystalline and osmotic, and can be measured by X-ray diffraction.6 A majority of the natural smectite particles are composed of platelets that assume d spacing values of 15 Å in high concentrations sodium chloride solutions (>1 M NaCl). This d spacing corresponds to the presence of two water layers between the individual platelets of the clay particle. As the concentration of sodium chloride is reduced, the d spacing is not affected until a specific concentration is reached at which an additional layer of water is admitted and the d spacing increases to 19 Å. Further reduction in the concentration of sodium chloride does not alter the d spacing until at a specific concentration between 0.6 and 0.25 M, the d spacing increases by 20 Å. Below this concentration, the d spacing increases continuously with decreasing concentration of sodium chloride. Crystalline swelling refers to the incremental increase of d spacing from 15 to 19 Å while osmotic swelling refers to the continuous increase in d spacing above 40 Å with decreasing concentration of sodium chloride. As mentioned previously, the critical salt concentration of sodium chloride required to prevent permeability reduction in porous media containing montmorillonite has been shown to be related to the transition of the swelling from crystalline to osmotic regimes.6,8-9 We have shown that this transition can be modeled as a case where the platelets which constitute the montmorillonite particle move from a primary minimum to secondary minimum due to increased electrostatic repulsion resulting from the overlap of the diffuse double layers of the platelets. The concentration at which this transition occurs is a function of the type of cation with weakly hydrated ions such as potassium not exhibiting the transition. Reduction of the salinity of the flowing fluid below the critical salt concentration causes eventual migration of montmorillonite particles and a resultant decrease in permeability of the porous media. In contrast to the effect of cations and salinity, the influence of pH on the formation damage of smectitic media has not received much attention previously. Packed bed studies with montmorillonitic soils have been reported to exhibit pH-dependent permeability reduction in the range of pH 6-9.10 However, the mechanisms underlying this dependence on pH have not been delineated. Electrophoretic mobilities of montmorillonite particles and the rheological properties of montmorillonite suspensions have been shown to be dependent on the pH of solution suggesting the presence of pH-dependent groups on the particle surface.11,12 According to the widely accepted Hoffmann structure for montmorillonite, the charge on the faces of the clay platelets is taken to be independent of pH and all the pH-dependent charge is assigned to the edges which have broken bonds in the alumina laye.11 An alternative structure attributed to Edelman suggests that as much as 20% of the surface charge on the basal surfaces can be attributed to hydroxyl groups that are capable of (6) Norrish, K. Faraday Discuss. Chem. Soc. 1954, 18, 120. (7) Mohan, K. K.; Vaidya, R. N.; Reed, M. G.; Fogler, H. S. Colloids Surf. A 1993, 73, 237. (8) Mohan, K. K. Water Sensitivity of Porous Media Containing Swelling Clays. Ph.D Dissertation, The University of Michigan, Ann Arbor, MI, 1996. (9) Mohan, K. K.; Fogler, H. S. AIChE J., In press. (10) Suarez, D. L.; Rhoades, J. D.; Lavado, R.; Grieve, C. M. Soil Sci. Am. J. 1984, 48, 50. (11) Lee, Swartzen-Allen; Matijevic, E. J. Colloid Interface Sci. 1975, 50, 143. (12) Sohm, R.; Tadros, Th. F. J. Colloid Interface Sci. 1989, 132, 62.

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dissociating.13,14 Recent studies on clay pillaring provide some support to this theory.15 The location of the pHdependent groups on the montmorillonite particles is expected to influence any pH-dependent formation damage in smectitic porous media. In addition to the variation with pH, smectites exhibit differences in surface charge due to different levels of isomorphic substitution in the lattice. Previous experiments in our laboratory with sandstones from a typical reservoir containing swelling clays indicate the charge heterogeneity of the samples from this reservoir.7-9 The swelling of clays has been shown to be a function of the layer charge which is an indication of the charge deficiency of the mineral.16 Slade and co-workers have reported that smectites with high layer charge such as Drayton montmorillonite do not disperse even in deionized water while the smectites with a low charge such as found in Wyoming montmorillonite swell indefinitely under similar conditions.16 In other studies, packed bed experiments have demonstrated that the exchangeable sodium and potassium content influences the permeability reduction at low ionic strengths only for low-charged smectites.17 In view of the above, a systematic study was undertaken to delineate the effect of pH and layer charge both of which determine the surface charge of the smectite on the formation damage to smectitic porous media. Materials and Methods The experiments were conducted on reservoir sandstones from Stevens formation of the Elk Hills Reservoir and packed beds using acid-washed glass beads (Polysciences) and standard clays Wyoming montmorillonite (SWy-2) and Cheto montmorillonite (SAz-1). The sandstones from the Stevens formation contain 7-8% expandable clays by weight of which a significant portion are the smectites. These sandstone samples were supplied by Chevron Petroleum Technology Co. The standard montmorillonites were supplied by the Clay Minerals Repository and were selected based on their differences in layer charge. Wyoming montmorillonite is typically available in the sodium form while Cheto montmorillonite was supplied in the calcium form. The Cheto montmorillonite was converted to sodium form by repeated washings with 1 M NaCl and then deionized water to remove excess salt. The reservoir sandstone samples were supplied in cylindrical form(1 in. diameter, 2 in. length) with a Teflon tubing heat shrunk around them to ensure the structural integrity. Packed beds of glass beads and clay were prepared using a heat shrinkable Teflon tubing as an enclosure. Two steel disks (1 in. diameter) with several holes drilled in them served as fluid distributors at the inlet and outlet of the packed beds. One end of Teflon tubing (4 in. long) was heat-shrunk on to a distributor and a steel screen (nominal opening 100 µm) that constituted one end of the packed bed. This tubing was placed vertically by resting the closed end on a flat surface. The sand-clay mixture was added from the top and the remaining length of Teflon tubing was heat shrunk after another set of distributor and steel screen was placed on top of the sand-clay mixture. The heat shrinking method of packed bed preparation eliminates the edge effects that are observed in typical packed beds with hard enclosures (glass, steel). The apparatus and the procedures used for conducting coreflood experiments using reservoir cores and packed beds have been described in detail elsewhere.8 Briefly, the coreflood experiments involve placing a cylindrical sample of sandstone (1 in. diameter) known as the core in a Hassler cell and flowing fluids through the sample to measure the pressure drop across (13) Edelman, C. H. Verre Silicates Ind. 1947, 12, 3. (14) Grim, R. E . Clay Mineralogy, 2nd ed.; McGraw Hill: New York, 1968. (15) Bukka, K.; Miller, J. D.; Shabtai, J. Clays Clay Miner. 1992, 40, 92. (16) Slade, P. G.; Quirk, J. P.; Norrish, K. Clays Clay Miner 1991, 39, 234. (17) Alpertovich, N.; Shainberg, I.; Keren, R.; Singer, M. J. Clays Clay Miner 1988, 33, 443.

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Figure 1. Water shock of Stevens sandstone and XRD of smectites at low pH. the length of the sample. The Hassler cell is used to ensure that the flow is unidirectional in the core. The effluent from the core was flowed through a pH probe, spectrophotometer, and conductivity probe to monitor effluent pH, turbidity, and conductivity, respectively. In the following, water shock refers to a coreflood experiment in which the composition of the injected fluid is changed abruptly from a high salt concentration to deionized water. The X-ray diffraction (XRD) procedures were designed to measure the d spacing of clays in a wet state. Samples from the reservoir sandstone were obtained by disaggregation of the core, sonication, and size separation. Clay samples were filtered on to a 0.22 µm filter and then treated with the same sequence of fluids as the core sample or packed bed was exposed to. After equilibrating with each solution, the clay cake is covered with an X-ray transparent film (Kapton) and placed in the sample holder of a Rigaku goniometer. The effect of layer charge on the permeability reduction was also studied by conducting experiments on reduced-charge clays prepared using the Greene-Kelly method.18,19 Briefly, lithium montmorillonite and sodium montmorillonite suspensions were mixed in stoichiometric proportions and allowed to equilibrate overnight. The amounts were chosen in such a way that predetermined fractions of the cation exchange capacity were satisfied with the lithium ions. The clay was heated to 250 °C for 24 h in silica crucibles to allow the lithium to migrate into the octahedral layer. This migration reduces the octahedral charge of the clay.19 The clay was then suspended in 1 N MgCl2 at a concentration of 0.5-2.0 g/100 mL and then washed in 0.01 N MgCl2. The clay was then washed with 1 N NaCl to saturate with sodium and washed several times in deionized water to remove excess salt. The clay was freeze-dried and stored until further use.

Results Effect of pH. The water sensitivity of a typical reservoir sample, Stevens sandstone under acidic conditions, is demonstrated by the experiment shown in Figure 1. The abrupt reduction of sodium chloride concentration to zero, also referred to as water shock, causes significant reduction in permeability of the sandstone. This sandstone contains several clay minerals including smectites, mixed layer clays, and kaolinites. As explained previously, experiments on Berea sandstone show that the migration of kaolinites due to salinity reduction is inhibited under acidic conditions. Consequently, the permeability reduction in the Stevens sandstone is attributed to that caused by the response of expandable clays including smectites and mixed layer clays to the changes in aqueous composition. These changes can be monitored by the XRD scans of clay samples from this core exposed to the same fluids as shown in Figure 1. In this figure, the clay sample in (18) Brindley, G. W.; Ertem, G. Clays Clay Miner 1971, 19, 399. (19) Jaynes, W. F.; Bigham, J. M. Clays Clay Miner 1987, 35, 440.

Figure 2. Water shock of a packed bed with 3% montmorillonite in acidic conditions.

1 M NaCl concentration exhibits two peaks. The peak at 19 Å corresponds to a low-charged smectitic fraction with three water layers while the 15 Å peak represents a lowcharged vermiculite or a high-charged smectite with two water layers. As the pH is decreased to 2.0 at the same ionic strength, there is virtually no change in the peaks. However, reducing the salinity to deionized water at the same pH causes crystalline swelling where the clay platelets with a d spacing of 15 Å move apart to increase the platelet separation by another layer of water to assume d spacing of 19 Å. The above experiment shows that smectitic sandstones are water-sensitive under acidic conditions in contrast to reported observations on kaolinitic sandstones. The results of a similar experiment conducted in a packed bed with a standard clay Wyoming montmorillonite are shown in Figure 2. These results also show that reducing the pH of the injected 1 M NaCl solution from pH 7.0 to pH 2.0 does not alter the permeability of the medium. As the salt solution is replaced with deionized water at pH 2.0, the permeability decreases rapidly in a manner similar to a water shock experiment under neutral pH conditions.7-9 In addition, the permeability response of the packed bed is similar to that of Stevens sandstone for similar conditions and is substantially different than that of Berea sandstone. As mentioned previously, the water sensitivity of Berea sandstone is suppressed significantly in acidic conditions. The packed bed and coreflood results show that the smectites are water sensitive under low-pH conditions as well. From an operational standpoint, it would be useful to know if and how the critical salt concentration for permeability reduction varies as a function of pH. The results of coreflood experiments and XRD measurements to determine the effect of pH on the critical salt concen-

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Figure 3. Permeability reduction and particle release in packed beds containing Wyoming montmorillonite during step reduction in sodium chloride as a function of pH: (a) injection pH ) 2.0; (b) injection pH ) 7.0; (c) injection pH ) 10.5.

tration experiments of sodium chloride are shown in Figures 3 and 4. As described above, the critical salt concentration of sodium chloride required to prevent permeability reduction in smectitic sandstones corresponds to the concentration at which the clays change from crystalline to osmotic swelling regimes.8,9 The results of the coreflood experiments show that the critical salt concentration is between 1 M NaCl and 0.5 M NaCl at all the three values of injection pH. In all these coreflood experiments, the pH of the flowing solution was changed at 1 M NaCl and then salinity was reduced at the same injection pH value. It should be noted that while the injection pH is maintained at a constant value, the effluent pH cannot be maintained at a constant level during the salinity change because of the transient ionexchange that takes place between sodium ions and hydrogen ions in the bulk.2 Consequently, the permeability changes that occur are at higher pH values than the injection pH. For example, the effluent pH of the core injected with a solution at pH 2.0 was typically between 3.5 and 4.0. A majority of this change can be attributed to ion exchange as in the case of Berea sandstone, but higher pH values may also be caused by slow leaching of glass beads and clay particles. In all the experiments,

Mohan and Fogler

changing the pH at high salt concentration (1 M NaCl) results in no change in the permeability of the packed bed. From the results of the coreflood experiments, one can conclude that the injection pH does not appear to influence the critical salt concentration of sodium chloride significantly for these packed beds. This behavior is in contrast to the Berea sandstone where the critical salt concentration is reduced to zero in the acidic conditions. The diffractograms of Wyoming montmorillonite (SWy2) indicate that the critical salt concentration at which all the clay expands to the osmotic regime is very close to 0.4 M and is relatively insensitive to the injected pH. The diffractograms also indicate that the diffraction intensity decreases progressively with decreasing NaCl concentration suggesting that there is a heterogeneity in charge distribution in the montmorillonite samples. This observation is supported by variation in layer charge observed by alkylammonium methods.20 From the results described above, it can be concluded that the critical salt concentration of Wyoming montmorillonite is insensitive to pH. While the coreflood experiments with packed beds at different injection pH conditions do not show any differences in the CSC, the final permeability and the absorbance of the effluent in these experiments exhibit some variation. The salinity reduction experiments show that the final permeability reduction for injection of deionized water at pH 2.0 is similar to the water shock at low pH. The final permeabilities of the cores injected with deionized water at near neutral pH and alkaline conditions (pH 10.5) are similar with the values becoming higher than the initial permeability. In these cases, the permeability also becomes unsteady because the beds become unconsolidated due to removal of clay from the interstices. The absorbance data indicate that there is virtually no clay migration outside the bed for the core injected with pH 2.0 solution while significant migration is observed for the other corefloods at higher pH values. The loss of permeability and the lack of particulates in the effluent of the low-pH experiment suggest two possible explanations for the permeability reduction. First, the clay particles are not entrained in the flowing fluid and increase in size with the decreasing salinity and reduce the cross sectional area for flow in the pore constrictions. Second, the clays do migrate and are captured immediately and therefore cause a reduction in permeability by pore blockage. The migration of montmorillonite particles was studied in a set of experiments designed to minimize capture in the packed beds. These experiments were conducted in short glass tubes with large glass beads and a relatively small amount of clay. The large grain sizes increase the mean pore diameter and minimize plugging by any released particles. The concentration of released particles is low because of the small amounts of clay present in the packed tubes. These experiments were designed to examine the effect of sodium chloride concentration and pH on the release of montmorillonite clay particles, and the results are summarized in Figure 5. These experiments show that at low ionic strength (10-4 M), particles are released at all values of injection pH in the range 3.5-10.0. At a given concentration of sodium chloride, the amount of particles released increases with increasing pH. At a given pH, the amount of particles released increased with decreasing concentration of sodium chloride. In addition, particles are released even at a pH 3.5 for sufficiently low NaCl concentrations (10-4 M). The pH of the effluent from the packed bed during (20) Lagaly, G. Clays Clay Miner 1979, 27, 1.

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Figure 4. X-ray diffractograms of Wyoming montmorillonite (SWy-2) as a function of NaCl concentration and pH: (a) pH ) 2; (b) neutral pH; (c) pH ) 10.00.

Figure 5. Release of SWy-2 particles as function of pH and NaCl concentration: (a) 0.1 M NaCl; (b) 10-2 M NaCl; (c) 10-4 M NaCl.

the coreflood experiment at pH 2.0 was between 3.0 and 4.0. Therefore, the results of the release experiments at pH 3.5 apply to this set. It is evident from the coreflood experiments that most of the permeability reduction occurred above the NaCl concentration required to initiate particle release. Consequently, the permeability reduction has to be attributed to swelling of the clays in the initial locations. As described below, this voluminous swelling, which is referred to as macroscopic swelling, is a combination of edge-face interactions and one-dimensional swelling that has been described above. As mentioned previously, the edges of montmorillonite particles can acquire a pH-dependent charge by virtue of the aluminol groups that are a result of broken bonds. The aluminol groups Al(OH) can acquire a proton at pH values below the point of zero charge around 7.2 to become positively charged while the basal charge is predominantly negative. These differences in charge can result in different modes of particle arrangement as proposed by van Olphen.5 The observed permeability behavior in the packed bed experiments can be explained using the changes in the aggregate structure of montmorillonite particles with pH and ionic strength. Under high salt concentrations (1 M NaCl) and at neutral pH, the particles and the aggregates of particles are expected to be in a face-face configuration as shown in parts a and e of Figure 6 because of the high surface area of faces compared to the edges and lack of any charge on the edges. As the pH is reduced to 2.0 in the 1M NaCl concentration, the edge charge increases, which leads to electrostatic attraction between the edges and faces. However, the high ionic strength suppresses this interaction and the aggregate structure does not change as shown in Figure 6b. As the salt concentration is reduced from 1 M NaCl at pH 2.0 to a lower concentration at the same pH, the attraction between the edges and faces increases and the repulsion between the faces of platelets within a particle and between particles in an aggregate increase. These interactions lead to the structure shown in Figure 6c. As the ionic strength is reduced further, the face-face repulsion and the edge-face attraction increase resulting in the aggregates assuming the structure shown in Figure 6d.

Figure 6. Changes in aggregates of montmorillonite Particles with ionic strength and pH.

Under acidic conditions of pH 2.0, the glass beads of the packed bed are virtually uncharged, thus minimizing the repulsion between the glass beads and the aggregates. The large aggregates reduce the cross-sectional area for flow and hence the permeability. In neutral pH conditions, the changes in particle and aggregate morphology are shown in parts e and f of Figure 6. The absence of any charge on the edges in neutral pH conditions eliminates the edge-face attraction. As the ionic strength is decreased, the particles swell and migrate when the repulsion becomes high. Consequently, the permeability decreases initially and increases as particles migrate out of the core. The edge-face flocs that result in permeability reduction during a water shock under acidic conditions can be dispersed by altering the composition of injected fluid such that the edges of the clay particles becomes negatively charged. This change can be accomplished by increasing the pH of the injected fluid to neutral or alkaline values. The results of such an experiment are shown in Figure 7, where the absorbance of the effluent fluid from a packed bed along with the permeability is reported. The absorbance shows a slight increase during the change from salt water to freshwater at pH 2.0. This transient increase can be attributed to swelling induced migration, which is discussed elsewhere.7,8 As soon as the injected water is changed from deionized water at pH 2.0 to neutral

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Figure 7. Permeability restoration in a packed bed after water shock at low pH.

deionized water, the permeability starts increasing along with absorbance in the effluent. These results indicate that increasing the pH of the injected deionized water causes the edge-face flocs of swelling clays to become dispersed and be entrained in the flowing fluid. These results show that permeability reduction as well as fines migration can be observed in smectitic sandstones during injection of low ionic strength acidic solutions in contrast to kaolinitic sandstones. This difference between sandstones can be explained by the differences in the surface charge and how this surface charge varies with pH and ionic strength. The coreflood experiments and diffraction experiments show that the CSC of Wyoming montmorillonite is insensitive to the injection pH in the range of 2-10.5. These results suggest that the basal charge of Wyoming montmorillonite is independent of pH. However, other smectites with different levels of octahedral and tetrahedral substitution may indeed exhibit a pH dependent basal charge. Effect of Layer Charge. As shown in the previous section and reported elsewhere, the XRD of swelling clays from Stevens sandstone and Wyoming montmorillonite show that the 19 Å peak does not disappear completely at a threshold concentration of sodium chloride.8,9 Instead, the intensity of peak decreases progressively with concentration. This behavior is consistent with the fact that clay minerals by nature do not have a uniform surface charge and the variation in charge causes the critical salt concentration to vary. Naturally occurring smectites have a layer charge which can vary from -0.6 to -1.2 electrons per unit cell. The distribution of the layer charge between the tetrahedral and octahedral layers is also unique to a smectite. It should be noted that the variation in layer charge includes that contributed by the pH-dependent component of charge. The effect of the layer charge on the water sensitivity of clays was studied by conducting coreflood experiments and XRD measurements on clays prepared by modifying the layer charge of standard clays, Wyoming montmorillonite(SWy-2) and Cheto montmorillonite (SAz-1). The unmodified layer charges of these minerals are -0.68 and -1.14, respectively. The Greene-Kelly method described previously was used to reduce the layer charge of Wyoming montmorillonite by 30%-100% and that of Cheto montmorillonite by 20%. The results of a water shock experiment on packed beds containing the reduced charged clays are shown in Figure 8 along with results from a water shock experiment on an untreated packed bed. The water shock on a packed bed with untreated Wyoming montmorillonite causes a rapid decrease in permeability which then increases to its initial permeability. On the other hand, packed beds with the reduced charge clays show virtually no reduction in

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Figure 8. Water shock of packed beds with reduced-charge Wyoming montmorillonite.

Figure 9. Variation in critical salt concentration as a function of layer charge.

permeability due to the flow of deionized water. The results in Figure 8 also show that the charge reduction significantly limits the migration of clays and the corresponding permeability changes. XRD analysis on dry clays showed that the reduced charge Wyoming montmorillonites were completely collapsed while untreated clays showed two water layers. These results suggest that a reduction of only 30% of the layer charge is sufficient to collapse Wyoming montmorillonite irreversibly. It should be noted that reducing the layer charge does not eliminate swelling in all cases as demonstrated in results with Cheto montmorillonite. The CSC of Cheto montmorillonite identified by conducting XRD on a clay sample equilibrated with progressively decreasing concentrations of sodium chloride was shown to be approximately 0.01 M. However, reducing the octahedral charge by 20% causes the critical salt concentration of sodium chloride to increase to about 0.1 M. The results from the experiments on the reduced layer charge clays are summarized in Figure 9. At low values of layer charge, the critical salt concentration of sodium chloride is low due to a lack of sufficient charge in the diffuse double layer to develop sufficient repulsion for overcoming the van der Waals attraction. At higher values of layer charge, the critical salt concentration increases because of an increase in the charge of the diffuse double layer. The decrease in critical salt concentration of sodium chloride for clays at even higher layer charge may be attributed to an increased affinity of the counterions to the surface which leads to a reduction in the diffuse double layer charge. In the next section, the variation of the critical salt concentration of sodium chloride with layer charge is explained by using a surface complex model to calculate the diffuse layer charge. Surface Complex Model for Charge/Potential Behavior of Clay Particles. The experimental results

pH and Layer Charge on Clays

presented above show that the solution pH does not influence the critical salt concentration of sodium chloride for Wyoming montmorillonite as reported for kaolinite. In addition, the layer charge of smectites has a significant effect on the critical salt concentration. In previous work,8,9 we have shown that the critical salt concentration of sodium chloride for smectites is related to the transition between crystalline and osmotic swelling regimes. This transition can be described by modeling the energy of interaction between clay platelets as a combination of electrostatic repulsion, van der Waals attraction, and short range repulsion. These energy calculations show that the critical salt concentration corresponds to the concentration at which the platelets in a clay particle move from a primary minimum to a secondary minimum.8,9 The experimental results presented above can be rationalized by modeling the effect of the pH and the layer charge on this transition between the primary and secondary minima. The essential feature of the model that can quantitatively predict the critical salt concentration of sodium chloride for Wyoming montmorillonite is the assumption that the diffuse layer charge remains constant with ionic strength and particle separation distance.8,9 The diffuse layer charge needed for these predictions was estimated by analyzing swelling pressure data available in the literature.21 The assumption that diffuse layer charge is invariant with respect to salt concentration was verified using literature data for concentrations between 10-2 and 10-4 M NaCl.22 This model was also able to explain the dependence of swelling on the type of the cation based on the variation of the diffuse layer charge with the cation. A sensitivity analysis of the various parameters showed that the critical salt concentration is strongly dependent on the diffuse layer charge with the critical salt concentration being less than 10-5 M below a diffuse layer charge of 2.4 µC/cm2. Above this value of the diffuse layer charge, the critical salt concentration of sodium chloride is linearly related to the diffuse layer charge. This variation in the critical salt concentration with diffuse layer charge provides a method for explaining the observed effects of layer charge and pH on the critical salt concentration. It should be noted that the critical salt concentration is a useful property from an operational viewpoint for prevention of permeability reduction in several oil production operations including drilling, waterflooding, and steamflooding. In this paper, we use a surface complex model to describe the charging behavior of smectites and other clay minerals and determine the diffuse layer charge of these minerals as a function of ionic strength, pH, and layer charge. The predicted diffuse layer charge is used to rationalize the experimental observations described in the previous sections. Surface complex models have been used widely in describing the charge/potential behavior of inorganic materials such as oxides in an attempt to understand their electrochemical behavior and to predict speciation of metal ions in the environmental field.23,24 The application of surface complex modeling in aqueous geochemistry has been recently addressed in a comprehensive review.25 A significant number of surface complex modeling studies (21) Viani, B. E.; Low, P. F.; Roth, C. B. J. Colloid Interface Sci. 1983, 96, 229. (22) Zhang, F.; Low, P. F. J. Colloid Interface Sci. 1995, 173, 34. (23) James, R. O.; Parks, G. A. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum: New York, 1982; Vol. 12, p 119. (24) Hayes, K. F.; Leckie, J. O. J. Colloid Interface Sci. 1987, 115, 564. (25) Davis, J. A.; Kent, D. B. In Mineral-Water Interface Geochemistry; Reviews in Mineralogy; Hochella, M. F., White, A. F., Eds.; Mineralogical Society of America: Washington, DC, 1990; Vol. 23, Chapter 5, p 177.

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Figure 10. Distribution of counterions in the surface complex model.

have focused on oxide minerals such as rutile, alumina, and silica while a few studies have considered clay minerals as well.23,26,27 The primary difference between oxide minerals such as silica, alumina, and clay minerals is the origin of the surface charge. The charge on silica and alumina particles is primarily attributed to dissociation and protonation of surface hydroxyl groups. While the silica and alumina sheets in the clay minerals could generate surface charge by dissociation and protonation of surface hydroxyl groups, the surface charge in montmorillonites is usually attributed to isomorphic substitution in the tetrahedral and the octahedral layers. As mentioned previously, recent studies on clay pillaring provide support to the Edelman structure of montmorillonite which proposes that as much as 20% of the surface charge on the basal surface is generated by dissociation of hydroxyl groups.13,14 Our experimental data with swelling of Wyoming montmorillonite do not suggest any pH-dependent component to the charge on the basal surfaces. However, this observation does not rule out any pH-dependent groups on other natural smectites. In contrast to Wyoming montmorillonite, hydroxyl groups present on the basal plane of kaolinite particles can generate a pH-dependent charge on the basal surface. A surface complex model has been formulated for predicting the surface and diffuse layer charge of a uniformly charged surface containing two different types of sites and follows the general methodology outlined in the literature.23,26 A schematic of the model illustrating the various layers in the model is shown in Figure 10. The surface used in this model is representative of the face or basal plane of the clay minerals and is assumed to contain strongly acidic and weakly acidic sites. As described previously, strongly acidic sites may be representative of the isomorphic charge on the clay while weakly acidic sites could represent any hydroxyl groups present on the face or charges located in the tetrahedral region for montmorillonite. In this schematic, the particle surface is the 0 plane and the center of all the ions present in the Stern layer is at β while the diffuse double layer originates at the plane d. The ionization of the surface groups in an aqueous solution can be written as (26) Tombacz, E.; Abraham, I.; Gilde, M.; Szanto, F. Colloids Surf. 1990, 49, 71. (27) Braggs, B.; Fornasiero, D.; Ralston, J.; St. Smart, R. Clays Clay Miner. 1994, 42, 123.

2870 Langmuir, Vol. 13, No. 10, 1997

SOH S SO- + H+;

TOH S TO + H ; -

+

Mohan and Fogler

int Ka,SOH S

int Ka,TOH

S

[SO-][H+]0 [SOH]

[TO-][H+]0 [TOH]

(1)

NTOH )

106F {[TOH] + [TO-] + [TOM]} A

(13)

(2)

NSOH )

106F {[SOH] + [SO-] + [SOM]} A

(14)

Here, SO- denotes the strongly acidic sites while TOdenotes the weakly acidic sites. In the standard triplelayer model (TLM) used here, the hydrogen ions are assumed to be adsorbed by forming inner-sphere complexes.23 The hydrogen ion concentration at the surface [H+]0 can be calculated from the concentration of the hydrogen ion in the bulk, [H+]b, by using the Boltzmann distribution. The activity coefficients for all the species are assumed to be unity in the following analysis.

[H+]0 S [H+]be(-eψ0/kT)

(3)

where, ψ0 denotes the surface potential, e the electronic charge and k the Boltzmann constant, and T the absolute temperature. The binding of the cation with the two types of sites can be characterized by the relationships

SO- + M+ S [SOM];

int KM,SOM S

TO- + M+ S [TOM];

int KM,TOM S

[SOM] [SO-][M+]β [TOM] [TO-][M+]β

(4)

(5)

In contrast to hydrogen, the cation M+ is adsorbed to the surface by forming outer-sphere complexes.23 Consequently, the concentration of cation M+ (sodium) is calculated at the β plane by using the Boltzmann distribution.

[M+]β S [M+]be(-eψβ/kT)

(6)

The charge at various locations on the solid-liquid interface(O, β,d) is given by

σ0 ) -

106F {[SO-] + [TO-] + [SOM] + [TOM]} (7) A σβ )

106 F {[SOM] + [TOM]} A

σd ) -[8kT[X-]b]1/2 sinh

( ) eψd kT

(8)

(9)

Here, F is the Faraday constant, [X-]b is the concentration of the anion in the bulk, and A is the area of solid-liquid interface. The charge in the diffuse layer is related to the diffuse layer potential by the solution of the PoissonBoltzmann equation. The potentials at the surface, β plane, and diffuse layer plane are related to the charge on the surface and diffuse layer by the following relations.

ψ0 - ψβ ) σ0/C1

(10)

ψβ - ψd ) - σd/C2

(11)

Finally, electroneutrality and site balance provide the necessary constraints.

σ0 + σβ + σd ) 0

(12)

These simultaneous equations (1-14) can be solved to yield the distribution of potential and charge from the interface for known values of the total number of sites, surface area, capacitances, and concentrations of cation and hydrogen in the bulk along with the ionization and binding constants. The lattice charge and surface area can give estimates of these quantities. The site density was estimated from the total surface charge density as 14 µC/cm2 which is equivalent to 8.75 × 1013 sites/cm2. The surface charge density used in these calculations is within the range of surface charge densities observed for most natural smectites 8.7-20.8 µC/cm2.5,28 The capacitance values are estimated by using a dielectric constant of 10 for the inner water layers and the distance from the surface plane to β plane is taken to be the dehydrated radius of sodium ion while the outer plane starts at the hydrated radius of the sodium ion. The ionization constants and binding constants for oxide minerals and surfaces with dissociating groups have been estimated by using potentiometric titration and zeta potential data.23 It has been reported that the estimated binding constants estimated for oxide minerals tend to be usually high because the measured zeta potentials are typically low.23 As described in detail elsewhere, the interpretation of diffuse layer potential which is usually approximated as the zeta potential from measured electrophoretic mobility is typically complicated by several phenomena including surface conductance double layer polarization.8 Currently available theories cannot be used to accurately interpret zeta potentials of platy particles such as clays from measurements of electrophoretic mobility.7,8 This limitation makes it difficult to obtain accurate values for the binding and ionization constants. Consequently, in this work the surface complex model is used to develop an understanding of the variation in diffuse layer charge with concentration for different binding constants and to make qualitative predictions on the effect of pH and layer charge on CSC. The values of the pKM dictate the binding strength of the ions to the Stern layer and are affected by the surface properties including degree of surface hydration, location of charge, and the hydration of the ions. At high absolute values of pKM, which indicate a greater degree of binding in the Stern layer, the charge present in the diffuse double layer is low and decreases significantly at higher concentrations of sodium chloride. On the other hand as the binding constants decrease in magnitude, the diffuse layer charge increases and is less sensitive to salt concentration. It should be noted that a value of 2.9 µC/cm2 was used for the diffuse layer charge in the simulations which predicted the transition in swelling regimes quantitatively.8,9 This value of diffuse layer charge is predicted by the surface complex model for low binding constants and this charge is relatively invariant with the concentration of sodium chloride as verified elsewhere.8,9 This model can be used to explore the effect of pH on the critical salt concentration for surfaces with different types of acidity. The results of these simulations are shown in Figures 11 and 12 where the diffuse layer charge (28) Sposito, G. In Clay-Water Interface and its Rheological Implications; Guven, N., Pollastro. R. M., Eds.; CMS Workshop Lectures, 1992; Vol. 4.

pH and Layer Charge on Clays

Langmuir, Vol. 13, No. 10, 1997 2871

Figure 11. Variation in diffuse layer charge with concentration and pH for strongly acidic surfaces.

Figure 13. Effect of total number of sites on diffuse layer charge for low affinity Stern layer binding.

Figure 12. Variation in diffuse layer charge with concentration and pH for weakly acidic surfaces.

Figure 14. Effect of total number of sites on diffuse layer charge for high affinity Stern layer binding.

on these surfaces is plotted as a function of salt concentration for various pH values. The pH affects the dissociation of the groups and consequently the surface and diffuse layer charges. The figures also show the variation of the critical salt concentration as a function of the diffuse layer charge. The intersection of the CSC curve with the pH curves gives the critical salt concentration at that pH and the diffuse layer charge. If the salt concentration falls below the CSC at this pH, swelling or migration will occur. From Figure 11, we observe that for strongly acidic surfaces which are representative of montmorillonite particles, the pH has an effect on the CSC between pH 2 and 4. At higher pH values, all the curves intersect the CSC curve at the same point indicating that the CSC is insensitive to pH in this range. This behavior is expected because the sites are fully dissociated and the changes in hydrogen concentration are not significant enough to alter the surface charge and hence the electrostatic repulsion. Consequently, the critical salt concentration is not affected by pH for the surface with strongly acidic sites above pH 4. The absence of any dependence of CSC on pH above 4.0 is similar to the experimental observations on Wyoming montmorillonite. The discrepancy in the acidic range between experiment and prediction can be attributed to the dissociation constant used for the strongly acidic site. The pKa value of -1.0 makes the surface charge vary between pH 2 and pH 4. A lower value for this site would eliminate this variation and make the surface charge invariant with pH. In contrast to the results presented above, the critical salt concentration of weakly acidic surfaces is a strong function of pH as shown in Figure 12. At low pH values, the surface charge is low because of incomplete dissociation. Therefore, the electrostatic repulsion is not sufficient to overcome the van der Waals attraction even at low salt

concentrations. However, at high pH values (8-10) the weak acid sites become fully dissociated and consequently exhibit the same critical salt concentration. The curve for pH 6 shows an interesting result in that the CSC curve intersects the pH curve at two different points. As the concentration is dropped at pH 6, the critical salt concentration is reached at a certain diffuse layer charge. As the concentration is reduced further at the same pH, the critical salt concentration is exceeded at a certain point. This behavior is expected to yield different results based on the initial starting point. For the system described, under low electrolyte conditions the particles do not possess sufficient repulsion to overcome the van der Waals attraction. In the same manner, at high electrolyte concentrations, the repulsion is suppressed significantly to prevent overcoming the van der Waals attraction. The weakly acidic surface results are representative of charge potential relationships observed with kaolinite. As explained previously, experimental results obtained on Berea sandstone indicate that the CSC for the release of kaolinite particles is a strong function of pH.2 The surface complex model can also be used to examine the effect of layer charge on the diffuse layer charge. The variation in layer charge of the clay particle is equivalent to changing the number of dissociating groups of the surface. Experimentally, the reduction in layer charge of Wyoming montmorillonite was realized by heat treatment after saturation with lithium ions. As mentioned previously, the layer charge per unit cell can vary from -0.6 to -1.2 per unit cell for naturally occurring smectites. These smectites can have different affinities for sodium ions in the Stern layer. The diffuse layer charge of smectites at different values of surface charge density is shown for two types of surfaces in Figures 13 and 14, one

2872 Langmuir, Vol. 13, No. 10, 1997

with a high and one with a low affinity for binding of sodium ions in the Stern layer, respectively. The base case in these figures is a surface with 8.75 × 1013 sites/cm2. As mentioned previously, this surface with a low binding constant (pKM ) 1.0) yields a diffuse layer charge that can result in sufficient repulsion for the predicted CSC to match the experimental observation. If the total number of sites of this surface are reduced by 50%, the critical salt concentration changes to 0.2 m. However, our experimental observations indicate that reduction of layer charge by 20% is adequate to prevent swelling of this clay completely. This discrepancy can be explained by noting the possibility of a change in the affinity of the ions to the surface by the Greene-Kelly procedure. The experimental observations indicated that the clay particles became hydrophobic suggesting that the surface hydration has been altered by this treatment. This change can potentially alter the affinity of the sodium ions to the surface. Figure 14 shows the diffuse layer charge of a surface on which there is a strong affinity of the counterions to the Stern layer. The diffuse layer charge on these high affinity surfaces is lower than that required for expansion at all concentrations of salt. Thus, these clays cannot generate sufficient repulsion to expand. Similar arguments can be used to explain the changes in the swelling properties of smectites that have a higher total charge than Wyoming montmorillonite. Thus the maximum in CSC as a function of the layer charge can be rationalized by the understanding of the variation of diffuse layer charge with affinity of counterions to the Stern layer and the changes in the total number of sites. In order for these predictions to be quantitatively compared with experimental data, exact estimates of the ionization and binding constants are needed. Such estimates can only be obtained by reliable measurements of diffuse layer potential and surface charge as a function of the aqueous composition. As mentioned previously, currently available techniques are successful in predicting diffuse potential of monodisperse spherical particles and cannot be applied for polydisperse platy particles such a montmorillonites.8 Consequently, we have used the model to make qualitative predictions that can be compared with trends in experimental results. The surface complex model has been shown to be useful in predicting the diffuse layer charge and resulting critical salt concentration in monovalent salt solutions and pH conditions. The model can be used to identify conditions under which the diffuse layer charge is constant and also predict the critical salt concentration for systems whose diffuse layer charge varies with concentration. In addition, the model can explain the dependence of the critical salt concentration on the affinity of the counterions to the surface. This model can be extended to predict the critical total ionic strength in systems with multiple cations which have different affinities to the surface. The application

Mohan and Fogler

of this model to multicomponent solutions and to surfaces with different proportion of strong and weak acid sites is given elsewhere.8 Summary The effect of pH on swelling and release of smectite particles has been studied. Permeability experiments show that natural sandstones that contain expandable clay minerals (smectites, mixed-layer clays) exhibit water sensitivity in acidic solutions in contrast to sandstones that contain primarily nonswelling clays (kaolinite). Packed bed experiments with Wyoming montmorillonite confirm the water sensitivity of porous media containing swelling clays in acidic conditions. The concentration of sodium chloride at which swelling of Wyoming montmorillonite proceeds from crystalline to osmotic regimes appears to be relatively insensitive to pH. Particle release experiments show that the migration of montmorillonite due to a salinity reduction is reduced under acidic conditions. However, particle release is observed at extremely low ionic strengths even under low pH conditions. The reduction in permeability of packed beds containing swelling clays under low salinity, acidic conditions is attributed to macroscopic swelling of clay aggregates in place. The effect of layer charge of smectites on swelling and permeability damage has also been studied. It is shown that a reduction of 30% of the layer charge of Wyoming montmorillonite can cause complete collapse of clay layers and make the clays insensitive to water composition. On the other hand, the critical salt concentration of sodium chloride for Cheto montmorillonite increases with reduction in the layer charge. These results indicate the critical salt concentration of sodium chloride varies with the layer charge and exhibits a maximum. This maximum may be related to the affinities of the counterions to the surface and the nature of the surface of clay mineral. A surface complex model has been used to explain the invariance of the diffuse layer charge with ionic strength for Wyoming montmorillonite and the variation in diffuse layer charge with surface charge and affinity. This model can be used to explain the observed variation of CSC of sodium chloride with pH for weakly acidic surfaces such as kaolinite particles and the lack of such a dependence for strongly acidic surfaces such as montmorillonite particles. The model can be extended to predict the diffuse layer charge and the critical salt concentration in multicomponent solutions. Acknowledgment. The authors acknowledge the financial support received from the sponsors of The Industrial Affiliates Program for Flow and Reaction in Porous Media at The University of Michigan. LA960868W