Role of Kaolinite Clay Minerals in Enhanced Oil Recovery by Low

Jul 3, 2018 - Both laboratory studies and field observations have confirmed increased oil recovery by low salinity water injection beyond that obtaine...
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Role of Kaolinite Clay Minerals in Enhanced Oil Recovery by Low Salinity Water Injection Tina Puntervold,* Aleksandr Mamonov, Zahra Aghaeifar, Gunvor Oline Frafjord, Gyrid Marie Moldestad, Skule Strand, and Tor Austad

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University of Stavanger, 4036 Stavanger, Norway ABSTRACT: Both laboratory studies and field observations have confirmed increased oil recovery by low salinity water injection beyond that obtained by standard water injection. Whether extra oil is produced or not depends on certain reservoir conditions, and among them is the content of clay minerals. Kaolinite clay minerals have been reported to impact the low salinity enhanced oil recovery (EOR) potential, and they are believed to be involved in the initial wetting of sandstone reservoirs. In this work, the effect of temperature on the wetting of clay surfaces was studied. The adsorption of the polar organic base, quinoline, onto kaolinite clay minerals was investigated at ambient and high temperature (130 °C) versus pH. The experiments were performed using brines with different ionic compositions and salinities. A discussion of the effect of brine chemistry, pH, and temperature on quinoline adsorption was also included. Finally, wettability alteration processes by adsorption and desorption of polar organic molecules were used to explain the low salinity EOR effect observed in claycontaining sandstone reservoirs. The experimental results showed that the adsorption of quinoline onto kaolinite clay minerals was strongly dependent on pH at both ambient and at 130 °C. However, the adsorption at high temperature was reduced, which could affect the initial wetting of a sandstone reservoir system. The adsorption process was reversible by adjusting pH, and adsorption of quinoline was in general higher in a low salinity brine than in a high salinity brine. Thus, releasing basic polar crude oil components like quinoline from the kaolinite clay surface requires an increase in the brine pH, and not only a lowering of the salinity of the injection brine.

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INTRODUCTION Low salinity (LS) or Smart Water injection involves injection of a brine with ionic composition different from that of the formation water (FW). In that process, the initial chemical equilibrium established between crude oil, brine, and rock (COBR) is disturbed, and different chemical interactions may take place depending on the brine composition, the crude oil properties, and the surface mineralogy. Laboratory research and field pilot tests have shown that LS injection into sandstone rocks can enhance oil production beyond that observed for FW or a high salinity (HS) brine.1−4 There seems to be a general consensus that the enhanced oil recovery (EOR) is related to wettability alteration, preferably from slightly water-wet to more water-wet conditions, and that the clay content in the rock material is important. The LS EOR mechanism by which the oil is released from mineral surfaces creating more water-wet conditions has been interpreted in several ways, among them fines migration,5 pH-induced surfactant generation,6 multicomponent ion exchange,7 ionic double layer expansion,8,9 pH-induced wettability alteration,2 osmosis,10 and electrostatics.11 Experiments performed on fired sandstone cores, containing no clay minerals, did not show any LS EOR effects.5 However, a few studies have reported LS EOR effects with low or no amount of clays present. Pu et al. observed effects in sandstone cores low in clay but containing dolomite and anhydrite,12 while Farzaneh et al. worked with synthetic borosilicate cores of high permeability 4500 mD.13 Clay minerals, in general, have a very large surface area, much larger than that of quartz.14 Therefore, although it is © XXXX American Chemical Society

present in smaller amounts than quartz in sandstone material, it covers a large amount of the available surface area. The clay minerals also have cation exchange capacity (CEC) at the surfaces, and in some cases in between the layered sheets. The CEC is dependent on the type of clay, and according to Allard et al.14 kaolinite has a CEC of 9.0 mequiv/100 g at pH 5, while other clay minerals such as chlorite and montmorillonite have values of 50 and 700 mequiv/100 g, respectively. Mica (illite) has been reported with a CEC of 10−40 mequiv/100 g.15 Out of the mentioned clay minerals, kaolinite has the lowest CEC, but despite that, Lager et al.7 reported a correlation between kaolinite content and increased oil recovery. However, a similar correlation should also be attempted for the other types of clays. Experiments have shown that kaolinite, illite, and montmorillonite are able to adsorb polar organic base molecules (quinoline), which could influence the reservoir rock wettability, and thereby possibly correlate with oil recovery.16,17 Because of the CEC, the clay minerals have permanent negative surface charges that must be charge-balanced by oppositely charged ions. Such ions can be inorganic cations from the brine phase, or they can be polar organic acids or bases from the crude oil phase.18 The acidic material present in the crude oil is mainly represented by the carboxylic group, RCOOH, most of which are present in the asphaltene and resin fractions of the crude oil. The basic material in crude oil is Received: March 7, 2018 Revised: June 8, 2018

A

DOI: 10.1021/acs.energyfuels.8b00790 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels present as nitrogen-containing aromatic molecules, R3N:. Both the basic and acidic material are polar components, and therefore an excess concentration of these components is present at the oil−water interface and can undergo acid−base reactions, i.e., accept or release protons, H+, as pH changes; see eqs 1 and 2. R‐COOH + H 2O V RCOO− + H3O+

(1)

R3N:+H 2O V R3NH+ + OH−

(2)

The charge of the polar organic molecules is dependent on pH of the brine phase. Both acidic and basic materials can adsorb onto negatively charged clay minerals, and it is the protonated species, R-COOH and R3NH+, that have the highest affinity for the kaolinite clay surface.17,19 Note that the pKa values of the corresponding acids, R-COOH and R3NH+, are quite similar, close to 5, meaning that the concentrations of the two reactive species vary similarly regarding pH. Experimental results performed by Madsen and Lind19 showed that adsorption of benzoic acid decreased significantly when pH was raised from 5.3 to 8.1. According to Brady et al.,11 the kaolinite clay edges are positively charged at low pH, pH < 6, while the basal plane is negatively charged at pH > 2. As pH increases, both surfaces become increasingly negative, and thus CEC seems to be a more important property than the anion exchange capacity (AEC), which has been reported by Allard et al.14 to be very low, 0.8 and 0.6 at pH 5 and 8, respectively. A chemical model for the low salinity EOR effect has been proposed by Austad et al.2 and verified by Rezaeidoust et al.,20 where adsorption of polar organic molecules, both acids and bases, onto clay minerals plays a major role. It was proposed that at low pH, pH ≈ 6, protonated basic material adsorbed to the negatively charged clay sites by electrostatic interaction. It was also proposed that the acidic carboxylic material adsorbed onto the clay minerals by hydrogen bonds to the basal plane. Brady et al.11 linked oil adhesion by negatively charged carboxylic acids to the positively charged kaolinite edge sites in their model to predict LS EOR effects. However, because of the experimental work conducted by Madsen and Lind19 and the kaolinite CEC values versus AEC values,14 the basal planes of clays seem to be more involved in crude oil adsorption than the charged edges. LS EOR effects are not limited to the content of kaolinite clay minerals, as LS EOR effects have also been observed in core material containing only illite clay minerals.2,4,20 It has been verified that an optimum in adsorption of quinoline onto illite clay took place at pH values close to the pKa value, i.e., at pH ≈ 5, Figure 1.16 In another experimental study, it was concluded that adsorption of quinoline onto either kaolinite, illite, Na-montmorillonite, or Ca-montmorillonite was significantly higher at pH 5 than at pH 8−9.20 The polar components present in the crude oil serve as the anchor molecules for the crude oil to generate a mixed-wet clay surface. It has previously been suggested by Gamage and Thyne,21 and shown experimentally by Aksulu et al.,16 that increased reservoir temperature can have an effect on the potential for wettability alteration by LS water flooding. It was therefore of interest to investigate how increased temperature can affect adsorption of polar organic bases onto kaolinite clay minerals, and as such affect their wettability. Static adsorption tests of quinoline onto kaolinite clay minerals were performed versus pH at ambient and high temperature, 130 °C. A discussion of the effect of pH, brine salinity, and composition

Figure 1. Adsorption of quinoline at ambient temperature onto illite clay mineral using HS1 brine (25 000 ppm) and LS1 brine (1000 ppm) as a function of pH.16 The ionic compositions of the brines LS1 and HS1 are identical to LSQ and HSQ, respectively.

on adsorption was also performed, and finally adsorption and desorption processes were used to explain the LS EOR effect in clay-containing sandstone reservoirs.

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EXPERIMENTAL SECTION

Material. Kaolinite Clay Mineral. Powdered kaolinite clay mineral was provided by PROLABO. The white kaolinite powder was washed with distilled water before use. Water analysis by ion chromatography (IC) after contact with the clay powder revealed that the kaolinite clay powder was clean. Only 12 mM of Na+ cations were present in the water phase after cleaning. The BET surface area was determined to 12.00 ± 0.03m2/g for the kaolinite powder. Quartz Particles. The quartz material was provided by Sibelco and contained >98% pure silica. For the experiments, a 50:50 mixture of fine (average size 75 μm) and coarse (average size 200 μm) quartz mineral particles was used, having an average particle size of ∼100 μm. The BET surface area was determined to 0.1−0.2 m2/g. Brines. The brines used in this study were a high salinity brine (HS), a low salinity brine (LS) and a high salinity CaCl2-brine (HSCa). All brines were made by dissolving known amounts of salts in distilled water. All salts used were reagent grade and provided by Merck. The brine compositions and properties are given in Table 1.

Table 1. Brine Compositions and Properties (10−3 mol/L = mM) brine ion

HS [mM]

LS [mM]

Na Ca2+ Mg2+ Cl− ionic strength (M) TDS (mg/L)

355.0 45.0 45.0 535.0 0.624 30000

13.7 1.7 1.7 20.5 0.024 1150

+

HSCa [mM] 270.3 540.6 0.811 30000

Quinoline−A Polar Organic Model Compound. A known amount of quinoline, pKa = 4.9, delivered by Merck, was dissolved in acidified distilled water to a concentration of ∼0.07 M, hereafter called the quinoline stock solution. The quinoline stock solution was adjusted to pH 5 with HCl. From the stock solution, the test brines containing 0.01 M quinoline were made by dilution in HS, LS, or HSCa brine, resulting in a high salinity brine-quinoline solution (HSQ), a low salinity brine-quinoline solution (LSQ), and an HSCa-quinoline solution (CaQ). B

DOI: 10.1021/acs.energyfuels.8b00790 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels The degree of protonation of quinoline increases as the pH of the solution goes below the pKa value and reaches 100% around pH ≈ 3.5.17 Methods. Sample Preparation. In all adsorption experiments, the test samples contained 10 wt % solid material, kaolinite clay powder or fine quartz particles. A total of 1.0 g of solid was added to a test tube together with 9.0 g of a brine-quinoline solution. The pH was adjusted to the desired value by addition of very small volumes of strong acid, HCl, or strong base, NaOH. The test tubes were gas and pressure sealed and put on rotation (2−3 rpm) at the test temperature to equilibrate. If needed, more HCl or NaOH was added to tune the test pH, followed by a final equilibration for 24 h by rotation at the test temperature. Quinoline Adsorption Measurements. Adsorption of the model base quinoline onto kaolinite was investigated at different salinities and pH at ambient temperature and at 130 °C. Brines with 0.01 M dissolved quinoline were used, and 10 wt % kaolinite clay was added to the sample solutions. The pH of the samples was adjusted by adding very small volumes (μL) of concentrated HCl and NaOH solutions prior to equilibration. After equilibration, the samples were immediately centrifuged at ambient temperature to separate the solid and the aqueous phases. Centrifugation at ambient temperature will cool down the samples, but the contact area between clay particles and water will be quickly reduced due to sedimentation. Thus it is assumed that the chemical equilibrium between adsorbed and solubilized quinoline will be negligibly affected by the temperature decrease. A total of 50 μL of the supernatant was pipetted out and diluted 100 times with distilled water at pH 3 prior to performing the absorbance measurements. The pH values given in the figures were determined at ambient temperature after equilibration and just before the dilution and subsequent analysis of quinoline content in the supernatant. For each test series, at least three parallel samples were analyzed. The concentration of quinoline in the aqueous phase was analyzed by UV spectroscopy. Quinoline absorbance was measured at the peak absorbance wavelength for quinoline at 312.5 nm. Using a calibration curve of absorbance versus quinoline concentration at pH ≈ 3, the amount of adsorbed quinoline was calculated by mass balance. The absolute error in the measured adsorption values, determined from 7 identical measurements, was lower than 0.2 mg/g kaolinite clay. The average relative error was calculated to 2.2%. For the quartz measurements, the absolute error was slightly higher, due to the material being more heterogeneous in particle size.

polar organic base, quinoline, onto kaolinite clay minerals versus pH, at ambient and high temperature, 130 °C. Adsorption of Polar Organic Crude Oil Base Components onto Kaolinite Clay. Experimental results have previously shown that quinoline adsorbed onto illite clay mineral in the pH range 2−8, Figure 1, and that the adsorption was at the maximum when pH of the brine solution was close to the pKa value of quinoline.16 At the same time, it was noticed that the adsorption of quinoline onto the clay surface was higher when present in a low salinity brine, as opposed to that in a high salinity brine. In this study, the adsorption of quinoline onto kaolinite clay was studied as a function of pH, in the presence of brines with different compositions and salinities, and the results are presented in Figure 2.

Figure 2. Adsorption of quinoline at room temperature onto 10 wt % kaolinite clay in different brines; HSQ (25000 ppm), LSQ (1000 ppm), and CaQ (25000 ppm). Adsorption was measured as a function of pH.

As expected, and in agreement with similar results for illite clay,16 the adsorption of quinoline onto kaolinite clay was the highest in the LSQ brine. This observation contradicts the LS EOR theory that the clay surface becomes more water-wet in the presence of a low salinity compared to a high salinity brine. However, this is only true when there is a simultaneous increase in pH during LS injection. At constant pH, the adsorption is mostly higher in LSQ than in HSQ brine. The maximum adsorption of quinoline in high salinity brine HSQ was about 30% lower than that for LSQ. The reason for the difference in adsorption can be explained by the difference in brine composition, Table 1. The LSQ brine has an ionic strength far below that of the HSQ, and hence the content of cations is much lower in LSQ. As a consequence, there is less competition between the active species of the organic base and the inorganic cations in the brine phase for the negatively charged adsorption sites at the clay surface. Another important observation is that the adsorption is very much dependent on pH. The maximum adsorption was observed close to pH 5, which is consistent with the pKa value for quinoline. The results are in line with previously published similar work by Helmy et al. stating that “quinoline adsorption goes through a maximum around the pKa value, and it has been shown that increasing or decreasing pH above/below this value causes desorption”.22 At the pKa value, there are equal amounts of protonated and neutral quinoline molecules present in the brine solution. By increasing pH above pH 5, the relative amount of positively charged quinoline molecules decreases

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RESULTS AND DISCUSSION Sandstone rock material consists of varying amounts of different types of minerals such as quartz, clays, feldspars, anhydrite, calcite, and others. It is well-known that the clays have a large surface area due to their sheet-like structure, but they may be present in relatively low amounts, compared to the content of quartz. As an example, the BET surface area measured for the kaolinite clay powder used in this study was 60 times higher than finely milled quartz, also used in this study. Thus, the crude oil contacts a large surface area consisting of clay minerals. It is a general belief that the clays are the main wetting material of sandstone rock. In Smart Water or LS flooding, the main purpose is to alter the wettability of a rock from a mixed wetting to a more water-wet rock surface, thereby releasing some of the oil components sticking to the surface, and improving the microscopic sweep efficiency.18 To do so, the adsorption process must be reversed, so that desorption or release of oil components takes place. More water-wet mineral surfaces create increased positive capillary forces, which promote spontaneous imbibition of water into bypassed pores, and as such improves the microscopic sweep efficiency. In this study, the effect of temperature on the wetting of clay surfaces was studied by investigating the adsorption of the C

DOI: 10.1021/acs.energyfuels.8b00790 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels and reaches 0% close to pH 7.17 Neutrally charged molecules have lower affinity for the negatively charged kaolinite surface, and as seen in Figure 2, the adsorption of quinoline dropped for all brines, confirming that the wettability alteration and Smart Water EOR effect is not a salinity effect but a pH effect reducing the reactivity of polar organic components toward negatively charged mineral surfaces. By decreasing pH below pH 5, the relative amount of protonated base molecules increases, and the adsorption should in theory increase. However, at pH < 5 the concentration of protons, H+ or H3O+, becomes significant; 0.1 mM at pH 4 and 1 mM at pH 3. At these concentrations, the protons are efficiently competing with the protonated base molecules for the negative sites at the clay surface.22 Proton has a high affinity for clay surfaces, and its molecular size is small. Consequently, the adsorption of quinoline will drop even though the concentration of positively charged quinoline is increased. Adsorption of quinoline was not much affected by the ion composition at high salinities. As seen in Figure 2, there was no significant difference between the adsorption of quinoline in HSQ or CaQ brine. The total amount of ions in both brines was sufficient to dominate the adsorption process and suppress the adsorption of quinoline onto the kaolinite clay. Clearly, from the results in Figure 2, adsorption of quinoline is governed by pH rather than by the salinity of the brine; i.e., larger differences in adsorption are observed by pH changes than by changing the brine salinity. Whether it is correct to use a model compound to represent the behavior of a real crude oil is always questionable. The quinoline is a smaller molecule compared to the average base compounds present in crude oil and may thus be more reactive than real crude oil bases. However, a study performed by Fogden23 on two crude oil systems showed similar results as those of the model base. Adsorption of crude oil was dependent on pH, and one crude oil showed significantly higher adsorption in the LS brine. The same pH dependence as that observed for the quinoline experiments discussed above was also confirmed; in the typical reservoir pH range, i.e., pH 6−8, adsorption decreased with increasing pH. In the work by Fogden,23 the adsorption maximum was at pH 6, which is at a slightly higher pH level than for the quinoline results presented in Figure 2. However, in Fogden’s work there were no measurements performed at pH 5. Crude oil consists of a vast amount of different molecules containing carboxylic or basic functional groups that all have slightly different pKa values. On the basis of the similarity in adsorption behavior between crude oil and the model base quinoline, it was concluded that quinoline could be used as a good representative of polar organic crude oil bases in the study. Adsorption of Quinoline onto Quartz. Quartz is the main rock mineral present in sandstones, and it typically contributes to 60−90 wt % of the rock. The quartz grain sizes are much larger than the clay grain sizes, and thus quartz contributes with a lower surface area. The adsorption of quinoline in LSQ onto fine-grained quartz particles was measured versus pH and compared with quinoline adsorption onto kaolinite. Adsorption was expected to be the highest in the LSQ brine due to lower competition between quinoline and inorganic ions. The BET specific surface area of 0.2 m2/g for quartz is much lower than for that for kaolinite clay, 12 m2/ g. The comparison results are presented in Figure 3.

Figure 3. Comparison of quinoline adsorption onto 10 wt % kaolinite clay or quartz in low salinity brine (LSQ). The adsorption is measured at room temperature as a function of pH.

From the results in Figure 3, it can be seen that the adsorption of quinoline onto quartz is significantly lower than that onto kaolinite, and never reaches adsorption levels above 1 mg/g. In addition, the adsorption onto quartz seems to be less pH dependent than the adsorption onto kaolinite clay and illite clay in Figure 1. The observed low adsorption of quinoline at pH 5 is believed to be due to experimental error related to the use of a quartz mixture containing both coarse and fine particles, in combination with rather low adsorption values. Allard et al.14 have performed experimental work to determine ion exchange capacities and surface areas of several rock minerals, and among them quartz and kaolinite. They measured BET surface areas of 11 and 0.3 m2/g solid, which are comparable to the measured BET areas in this work of 12 and 0.2 m2/g for kaolinite and quartz particles, respectively. Regarding cation exchange capacity, CEC, they found that kaolinite had CEC values between 5 and 20 mequiv/kg, depending on pH, where the highest value was observed close to pH 6. For quartz, the CEC was generally low at pH 4−9, never exceeding 0.5 mequiv/kg. Compared to clay minerals, the CEC and also the adsorption of the polar organic component quinoline can be neglected. Effect of Temperature on Adsorption of Quinoline onto Kaolinite Clay. How the adsorption of quinoline onto kaolinite clay was affected by temperature was investigated by performing the same type of adsorption experiments on kaolinite clay at elevated temperature, 130 °C. The adsorption of quinoline onto kaolinite clay in the presence of LSQ, HSQ, and CaQ as a function of pH is presented in Figure 4. The quinoline adsorption at high temperature, Figure 4, shows the same adsorption behavior as that observed at ambient conditions, Figure 2. The adsorption of quinoline is about 25% higher in LSQ than that in HSQ or CaQ, the two latter showing similar results. Adsorption is not only dependent on salinity, but the pH dependence is also significant, as it was at ambient conditions. The adsorption behaviors in LSQ, HSQ, and CaQ at ambient temperature and 130 °C have been compared in Figure 5. Divalent cations like Ca2+ are strongly hydrated (high hydration enthalpy), and as the temperature increases, the ions become dehydrated and their reactivity increases.24 Therefore, their ability to compete for the adsorption sites at the clay surfaces increases, thereby lowering the adsorption of quinoline. The drop in adsorption maximum with an increase in D

DOI: 10.1021/acs.energyfuels.8b00790 Energy Fuels XXXX, XXX, XXX−XXX

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expected that the maximum quinoline adsorption will shift toward a lower pH. At high temperature and high pH, it cannot be ruled out that some precipitation or complexation may take place between divalent cations and hydroxide ions. These interactions reduce the activity of the divalent cations, thus promoting a higher adsorption of polar organic base components. However, in this case, as illustrated in Figure 4, the ability to compete for adsorption sites overrules the complexation and possible precipitation processes, thus reducing the amount of adsorbed quinoline at elevated temperature. As a result, at reservoir conditions, both inorganic cations from the FW and organic molecules from the crude oil can adsorb onto clay surfaces, wherein the amount of adsorbed polar organic base components and polar organic acid components19 strongly depends on initial reservoir pH and will determine the initial reservoir wetting state. Effect of pH on Adsorption of Crude Oil Components onto Kaolinite Clay Minerals. The reactivity of the polar organic components is dependent on pH. At low pH, both bases and acids are protonated, while at high pH they are deprotonated as shown in eqs 3 and 4.

Figure 4. Adsorption of quinoline at 130 °C onto 10 wt % kaolinite clay in LSQ, HSQ, or CaQ brine, measured as a function of pH.

temperature was the largest in LSQ, with a difference of about 1.1 mg/g, while in HSQ and CaQ the respective drops were 0.8 and 0.6 mg/g, taking the peak values at each temperature. The adsorption maximum was observed at pH ≈ 4, which is about one pH unit lower than that at ambient conditions. According to Le Châtelier’s principle, at elevated temperatures the equilibrium pH of water decreases as the process of water dissociation is endothermic. The pKa (−log(Ka)) value for quinoline is expected to decrease as acid dissociation constants (Ka) generally increase with increasing temperature.25 At ambient conditions, the adsorption maximum is observed at pH ≈ pKa, at which half of the quinoline molecules exist in the protonated form. Therefore, at elevated temperatures it is

organic bases carboxylic acids

R3NH+ V R3N:+ H+

(3)

R‐COOH V RCOO + H −

+

(4)

This means that both types of components will have different charges depending on the pH of the brine phase. The acidic components are neutrally charged at low pH, and negatively charged at high pH. The bases are positively charged at low pH, and neutrally charged at high pH. These differences

Figure 5. Adsorption of quinoline onto kaolinite clay at 23 and 130 °C, measured as a function of pH in (a) LSQ, (b) HSQ, and (c) CaQ brines. E

DOI: 10.1021/acs.energyfuels.8b00790 Energy Fuels XXXX, XXX, XXX−XXX

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observed extra oil recovery by low salinity injection. In Figure 7 the adsorption of quinoline onto kaolinite clay has been

in charges will impact the affinity of the polar basic organic components for the negatively charged kaolinite clay surfaces, as reported by Burgos et al.17 Adsorption of quinoline was dependent on pH, with generally higher adsorption measured at pH 4 than at pH 7. The positively charged quinoline molecules, present in excess at pH 4, have higher affinity for the negatively charged kaolinite clay surface, than the neutrally charged molecules, present in excess at pH 7. The results of Burgos et al.17 also confirmed higher adsorption in a low salinity brine compared to that in the higher salinity brine. That the ability of crude oil components to wet mineral surfaces is dependent on pH has also been concluded in other wettability studies. Buckley and Morrow26 concluded that pH was the dominant factor for the crude oil’s ability to wet the silica surface. Didier et al.27 also observed crude oil adhesion to sands in only a certain pH interval. Reversibility of the Adsorption of Quinoline onto Kaolinite. Because of the strong pH effect on adsorption or adhesion of crude oil to mineral surfaces, the question is if it is possible to reverse adsorption by changing pH. An experiment was designed to test just that. Six parallel samples were prepared by adding kaolinite to LSQ (1000 ppm) brine, and another six parallel samples were prepared by adding kaolinite to HSQ (25000 ppm) brine, Figure 6. The pH

Figure 7. Adsorption of quinoline onto 10 wt % kaolinite at pH 5 and pH 8 measured at ambient temperature as a function of brine salinity.20

measured in varying salinity brines from 0 to 25 000 ppm and at two different pH levels, 5 and 8. The various salinity brines were made by diluting the HS brine in Table 1 by distilled water. At pH 5 the adsorption was the highest in 0 ppm brine, i.e., distilled water, because of no competition from inorganic ions, except H+, which is present at 0.01 mM concentration. When the salinity increased, the amount of inorganic ions increased, and adsorption decreased slightly. By increasing salinity from 0 to 25 000 ppm at pH 5, the reduction in adsorption was about 20%. However, when the same adsorption study was performed at pH 8, the adsorption was drastically reduced at all salinities. Thus, the change in adsorption by salinity adjustment was minute compared to that by pH adjustment. Therefore, it cannot be the salinity itself which is responsible for the extra oil recovered by low salinity flooding. Actually, the low salinity brine promotes increased adsorption at constant pH. Effect of Formation Water Composition on Adsorption of Crude Oil Components onto Kaolinite Clay Minerals. Clay minerals are chemically unique, as they are acting as cation exchangers. According to Velde,28 the various cations have different affinities for the clay surfaces. In general, the affinities increase in the following order:

Figure 6. Reversibility in adsorption of quinoline onto 10 wt % kaolinite with varying pH at ambient temperature. The salinity of samples 1−6 is 1000 ppm while samples 7−12 is 25 000 ppm.20

was adjusted to pH 5 in all samples, and adsorption of quinoline to the kaolinite clay was determined at ambient temperature. As shown in Figure 6, the results were reproducible, and the highest adsorption was observed in the low salinity brine, verifying the results presented in Figure 2. Desorption was attempted by increasing the pH in the samples. At pH 8−9 the measured adsorption was negligible, confirming that most of the previously adsorbed quinoline molecules had desorbed from the kaolinite clay mineral surface. By adjusting the pH back to pH 5.5, the quinoline molecules readsorbed to the same level as that observed initially, confirming that the adsorption process is reversible and that quinoline molecules can be forced on and off the kaolinite clay minerals by merely adjusting pH. When pH was reduced to pH 2.5, quinoline adsorption dropped due to the competition with the protons, now present at increased concentration, as discussed previously. Effect of Salinity on Adsorption of Crude Oil Components onto Kaolinite Clay Minerals. It is a general belief that the salinity difference is the main reason behind the

Li+ < Na + < K+ < Mg 2 + < Ca 2 + <