Evaluation of Low-Salinity Enhanced Oil Recovery Effects in

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Evaluation of Low-Salinity Enhanced Oil Recovery Effects in Sandstone: Effects of the Temperature and pH Gradient Hakan Aksulu, Dagny Håmsø, Skule Strand, Tina Puntervold,* and Tor Austad University of Stavanger, 4036 Stavanger, Norway ABSTRACT: Recently, we proposed a chemical mechanism for the low-salinity enhanced oil recovery (EOR) effect, where change in pH was a key parameter. In this paper, we demonstrate the importance of pH in the low-salinity water-flooding process in sandstones. We also try to relate the rate and size of the pH gradient to the low-salinity EOR potential at various temperatures. Static adsorption studies of basic material quinoline onto illite in the pH range of 3−8 showed that the adsorption was always higher in low-salinity water compared to high-salinity water, confirming that a decrease in salinity itself cannot be responsible for the wettability alteration in a low-salinity EOR process. In fact, previous work has shown that the adsorption and desorption processes of the organic material quinoline are mostly pH-controlled. Two reservoir and one outcrop sandstone cores were water-flooded successively with high-salinity−low-salinity−high-salinity brine at temperatures ranging from 40 to 130 °C, and pH and concentrations of Ca2+ and SO42− were recorded. In all cases, an increase in pH was observed as the injected fluid was switched from high-salinity brine to low-salinity brine and the pH decreased again to its initial value as the fluid was switched back to the high-salinity brine. Both the pH gradient and the rate of the pH gradient can be related to the desorption rate of Ca2+ from the clay surface, which is an exothermic process; i.e., the pH gradient and desorption rate decreased as the temperature increased. The presence of anhydrite in the rock material reduced the size of the pH gradient as well as the rate of desorption of Ca2+. The observed pH gradient in cleaned core material can be linked to the low-salinity EOR effect, and in that sense, the method can be used as a first screening test for possible low-salinity EOR potential.

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INTRODUCTION The chemical understanding of the mechanism for the lowsalinity (LS) enhanced oil recovery (EOR) effects in sandstone has improved significantly by recent publications. In laboratory studies, using both model compounds and crude oil, it has been verified that it is not the salinity gradient itself that increases the water wetness of clay minerals. In fact, the adsorption of organic material onto the clay surface increases as the salinity decreases at a given pH.1−4 It was very interesting to see that the model compound, quinoline, and an asphaltenic crude oil responded similarly regarding adsorption/desorption from clays at different salinities and pH values. It was pointed out by Fogden and Lebedeva2 that “in clay rich sandstones, low salinity flooding may perturb the brine pH from its initial rock buffered value, to counteract this direct effect of salinity on wettability”. This is of course a strong support of our chemical understanding of the LS EOR effect, which involves a two-step mechanism for wettability alteration at the clay surface:1,5 (1) H+ substitutes Ca2+ at negative sites of the clay surface as Ca2+ desorbs from the surface when the LS water displaces the highsalinity (HS) water. An alkaline environment close to the clay surface is created. (2) The acidic form of adsorbed organic molecules onto the clay is, by an ordinary acid−base reaction, transformed into the basic form, which has significantly lower affinity toward the clay surface. This proton-transfer reaction is known to be very fast. To observe LS EOR effects, clay minerals must be of low water wetness. Adsorption of active polar oil components onto clay minerals is at a maximum close to the pKa values of the active compounds (pH ∼ 5). At a given pH, the adsorption of organic components increases as the salinity decreases, because © 2012 American Chemical Society

of the competition between the different active species [cations such as Ca2+, H+, and basic (R3NH+) and acidic (−COOH) components in the crude oil] toward the negative sites of the clay. Thus, a negative salinity gradient, i.e., a reduction in salinity, will increase adsorption of organic material onto the clay. If, however, the pH is increased simultaneously to about 8−9, significant desorption of organic material takes place and the clay becomes more water-wet.1 At reservoir conditions, in the presence of acidic gases (CO2 and H2S) and other buffering components, the increase in pH is hardly observed in the produced water, but it does not mean that acid−base reactions are not taking place. Proton-transfer reactions are known to be very fast. If the initial formation brine is alkaline (pH > 7), the clay will remain rather water-wet and the potential for LS EOR effects is low, as was observed for the Snorre pilot because of the presence of large amounts of Plagioclase.6,7 In a tertiary LS EOR flood, it is important to maintain a sharp salinity gradient. Desorption of active cations from the clay surface will then impose a fast increase in pH, which leads to desorption of organic material from the surface, and a new oil bank will be created because of the mobilization of bypassed oil in the clay-rich areas. An increase in water wetness will increase the positive capillary forces, and the LS water can imbibe into new areas of water-wet pores and, in that way, improve the microscopic sweep efficiency. It has been experimentally verified that slightly water-wet conditions resulted in the lowest residual oil saturation, Sorw, in a waterReceived: January 27, 2012 Revised: April 24, 2012 Published: April 24, 2012 3497

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flood process.8 It may look like a paradox that, even after flooding at optimum wetting conditions, it is possible to improve the oil recovery in a tertiary LS process by enhancing the water wetness of clay-rich pores/areas of the reservoir. Usually, the rates of adsorption and desorption processes are quite different. A desorption process is normally much slower than an adsorption process. For divalent cations, the temperature could also play an important role, especially at high temperatures (Tres > 100 °C) because of high hydration energy of Ca2+ and Mg2+. In this paper, we will try to answer the following questions: (1) Can the observed pH gradient, ΔpH, during a HS−LS−HS flood sequence of a cleaned core give information about the potential for observing LS EOR effects? (2) Is the pH gradient sensitive to the reservoir temperature? (3) Can the slopes of the pH gradients give information about the rate of desorption and adsorption of cations? (4) Are the slopes of the pH gradients sensitive to temperature? Two sandstone cores from two different oil reservoirs and one outcrop core have been used in this study.

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Core Flooding. The cores were initially saturated with HS2 brine and left overnight at the test temperature. Next, the cores were flooded with HS2 brine at a rate of 4 pore volumes (PV)/day with a back pressure of 10 bar until a stable pH value was recorded in the effluent. The flooding fluid was then switched to LS2 brine, while the flooding rate was kept constant. When stable pH was obtained in the effluent fluid, the injected fluid was switched back to HS2. The flooding temperatures were 40, 90, and 130 °C, and all flooding tests were performed in that order, on the same core. Chemical Analysis. During the flooding period, samples of the effluent were collected and analyzed for pH and content of Ca2+ and sometimes SO42−, using an ion chromatograph Dionex ICS-3000. The quinoline was quantified using a Shimandzu UV-1700 spectrophotometer. Linear calibration curves for quinoline in both HS1 and LS1 were obtained by adjusting the pH to about 3.4 and measuring the absorbance at a wavelength of 312.5 nm.

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RESULTS AND DISCUSSION Quinoline is a basic polar component that exists in crude oil. The adsorption of quinoline onto illite at room temperature was studied using the high-saline brine HS1 and the low-saline brine LS1 at different pH values (Figure 1). As expected, on the

EXPERIMENTAL SECTION

Adsorption of Quinoline onto Illite. Milled outcrop formation illite was stirred in distilled water at pH 3 until stable pH, centrifuged, and further flushed with distilled water until stable pH around 5. The clay was dried at 90 °C to a constant weight. Adsorption of quinoline onto illite in aqueous solutions at different pH values was performed using a HS (25 000 ppm) and a LS (1000 ppm) brine. The

Table 1. Molar (mol/L) Brine Compositions Na+ Ca2+ Mg2+ Cl− ionic strength total dissolved solids (g/L) salinity (ppm)

SS

HS1

LS1

0.355 0.045 0.045 0.534 0.624 30.0

0.296 0.038 0.038 0.445 0.521 25.0

0.012 0.0015 0.0015 0.018 0.0208 1.0

1.54 0.090

HS2

0.0171

LS2

1.72 1.81 100.0

0.0171 0.0171 1.0

30000

25000

1000

100000

1000

Figure 1. Adsorption of quinoline versus pH at ambient temperature in LS brine, LS1 (1000 ppm), and in HS brine, HS1 (25 000 ppm). The dashed line represents the pKa value of quinoline (∼4.9).

compositions are given in Table 1. Each adsorption test was prepared with 10 wt % illite dissolved in the respective brines containing 0.01 M quinoline. The pH was adjusted by adding very small volumes of HCl and NaOH solutions. Core Material. Two reservoir sandstone cores (RC1 and RC2) and one outcrop sandstone core (OC1) were used in the tests. The reservoir cores were flooded with toluene and methanol to remove crude oil and then dried at 90 °C to a constant weight. The clay content and physical core data are listed in Table 2. The total clay content varied between 10 and 20 wt %. The relative amounts of illite, kaolinite, and chlorite were different. Brines. Static adsorption studies of quinoline onto illite: HS brine (HS1) and LS brine (LS1) were made by dilution of a 30 000 ppm stock solution (SS). Core flood experiments: The 100 000 ppm HS brine (HS2) and the 1000 ppm LS brine (LS2) was made by dissolving reagent-grade CaCl2 salt and/or NaCl salt in distilled water. All brine compositions are listed in Table 1.

basis of previous experience,1,5 the adsorption of basic material onto the clay was highest for the low-saline brine compared to the high-saline brine and the adsorption was at a maximum close to the pKa value of the basic material (∼4.9). The active species for adsorption is the protonated form of the base, (R3N−H)+. At pH > pKa, for instance when pH increased to alkaline conditions (pH > 8), the adsorption dropped drastically because the concentration of the protonated base decreased. Clay minerals are chemically unique in the sense that they are acting as cation exchangers because of their permanent negative charges, which must be charge-balanced. The different cations present will therefore compete for the negative sites on the clay, and the adsorption of polar components decreases as the concentration of active ions increases, i.e., as the salinity increases. The relative replacing power of the cations is

Table 2. Core Data core RC1 RC2 OC1

illite/mica (wt %) kaolinite (wt %) chlorite (wt %) 11.2 9.3 8.4

8.7 2.6 0.0

0.8 3.6 1.9

total clay (wt %) length, L (cm) diameter, D (cm) 20.7 15.5 10.3

6.90 5.18 7.03 3498

3.78 3.79 3.80

permeability, k (mD) porosity, Φ (%) ∼400 1000−2000

15.7 25.4 19.9

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eq 1 is moved to the left and a smaller pH gradient should be observed even in the absence of active buffers. Experimental work by Gamage and Thyne13 showed greater oil recoveries with LS brine injection at intermediate rather than high temperatures, and on the basis of this temperature effect, they suggested that an exothermic chemical reaction had to be a part of the LS mechanism. Equilibrium 1, desorption of active cations from the surface, will also be affected by the presence of dissolvable salts of Ca2+ in the formation, such as anhydrite CaSO4(s). The solubility of anhydrite will increase as the HS fluid is switched to the LS fluid, which will also move the equilibrium to the left and, thus, lower the pH gradient. Both the desorption rate and pH gradient will be important in desorbing organic material and, thus, creating a new oil bank when switching from HS to LS water. Usually, the desorption process is slower than the adsorption process, and in the case of clay materials, also the structure of the clay could play a role. The results obtained by flooding three different cores successively with HS2−LS2−HS2 are presented in the next sections. Reservoir Core RC1. The reservoir core RC1 is from a high-temperature reservoir (>100 °C) that contains HS formation water (>200 000 ppm). Figure 2 shows pH versus PV brine injected at temperatures ranging from 40 to 130 °C. The pH during the HS2 flood was close to 7 for the three cases.

dependent of course upon the relative concentrations of ions present, but assuming equal concentrations, it is generally believed to be of the following order:9 Li+ < Na + < K+ < Mg 2 + < Ca 2 + < H+

The proton, H+, which is the most reactive cation toward the clay surface, will also surely participate in the adsorption process. At pH < pKa, the concentration of H+ increases and the protons compete with other cationic material for the negative charges on the clay. Therefore, it has been suggested that nondissociated organic acids, RCOOH, adsorb onto the clay by hydrogen bonding, and it has been documented in the literature that adsorption increases as the pH decreases.10,11 It is very interesting to note that asphaltenic crude oil behaved in exactly the same way regarding pH and affinity to kaolinite.2 Sandengen et al.4 experienced that clay-containing cores became less water-wet as the brine salinity decreased, and they interpreted the observed increase in pH as the injected fluid was switched from HS to LS as an artifact resulting from doing laboratory experiments at low pressures. They proposed to avoid the pH increase by adding some CO2 to the system. We argue that the pH increase is no artifact and that the pH will increase if strong buffering agents, such as CO2 or H2S, are absent. With the acceptance that the LS EOR effect is related to increased water wetness, a decrease in salinity would have the opposite effect; i.e., a decrease in salinity should decrease the water wetness. Therefore, an increase in pH as active cations are desorbed from the surface is the key factor for the wettability alteration process.1,2,5 Using Ca2+ as an example for the active cation, the process could be described by the following equations: clay−Ca 2 + + H 2O ↔ clay−H+ + Ca 2 + + OH− + heat (1)

clay−R3NH+ + OH− ↔ clay + R3N + H 2O

(2)

clay−RCOOH + OH− ↔ clay + RCOO− + H 2O

(3)

Equation 1 describes the pH increase that results from the ion exchange that occurs when the LS brine enters the clay-rich zones of the reservoir. Equations 2 and 3 show the desorption of basic and acidic material, respectively, from the clay surface that is caused by the increased pH at the surface. A negligible increase in pH has been observed in the field and laboratory experiments containing buffers, such as CO2, H2S, and Mg2+.1,12 Equations 4−6 show how the pH can be buffered in such cases. CO2 (g) + OH− ↔ HCO3− −

−

Figure 2. Change in effluent pH versus PV-injected fluid for core RC1 at 40, 90, and 130 °C. The brine flooding sequence was HS2−LS2− HS2. The switches of fluids are indicated by dashed lines.

The first flood at 40 °C resulted in a rather small increase in pH when the flooding fluid was switched from HS2 to LS2 (ΔpH ∼ 0.6). The initial pH with HS2 was ∼7.2, and during the LS2 flooding, it stabilized at pH ∼ 7.8. When the injected fluid was switched back to HS2, the pH regained its initial value of ∼7.2. In the second flood at 90 °C, the pH increased smoothly from ∼7.1 to about ∼8.9 when switching from HS2 to LS2 water, resulting in ΔpH ∼ 1.8. When the injection fluid was switched back to HS2 again, pH decreased sharply to its initial value. In the third test at 130 °C, the shape of the pH curve was different from that observed at 90 °C. The initial pH when flooding with HS2 was ∼7.0. The following flood with LS2 rapidly increased pH, and it stabilized at ∼8.3. The pH gradient at 130 °C was ΔpH ∼ 1.3, and the decrease in pH back to its initial value was rapid when the injected fluid was switched back to HS2.

(4)

H 2S + OH ↔ HS + H 2O

(5)

Mg 2 + + 2OH− ↔ Mg(OH)2 (s)

(6)

Knowing that proton-transfer reactions (acid−base reactions) are very fast and, in fact, diffusion-controlled, the key reaction to create alkalinity, which initiates the wettability alteration, is desorption of active cations (e.g., Ca2+) from the clay surface, as shown in eq 1. Desorption of active cations is sensitive to the changes in the concentration, and it seems to be an exothermic process (i.e., ΔH < 0). Divalent cations are strongly hydrated in water, and the reactivity usually increases as the temperature increases because of dehydration. Thus, at high temperatures, 3499

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Figure 3. Concentrations of Ca2+ and SO42− in the effluent versus PV-injected fluid in core RC1 at 40, 90, and 130 °C. The flooding sequence was HS2−LS2−HS2.

The order of the ΔpH values 0.6, 1.8, and 1.3 at 40, 90, and 130 °C, respectively, was not in line with what should be expected according to the discussion related to eq 1 above; i.e., ΔpH should decrease as the temperature is increased. Therefore, the first test at 40 °C was repeated. This time, the pH gradient increased drastically, from ΔpH ∼ 0.6 in the first test to ΔpH ∼ 2.6 in the repeated test (Figure 2). Obviously, the chemical properties of the core had changed from the first to the second test at 40 °C. The concentrations of Ca2+ and SO42− in the effluents at the different temperatures were analyzed and plotted versus PVinjected fluid (Figure 3). Remember that the LS2 water only contained NaCl and that no Ca2+ was initially present. At 40 °C, the concentration of SO42− during the HS2 flood stabilized slightly above 20 mM, while during the LS2 flooding sequence, the concentration of Ca2+ and SO42− in the effluent stabilized at the same value, slightly below 20 mM (Figure 3a). The concentrations stayed at this value until the LS2 fluid was exchanged with the HS2 fluid. This observation can only be explained by dissolution of anhydrite, CaSO4(s). The core has been sampled from a high-temperature reservoir (Tres > 100 °C) with formation brine salinity > 200 000 ppm. It is therefore not unreasonable to presume that anhydrite is precipitated in the formation, because of the fact that CaSO4 solubility decreases with an increasing temperature and Ca2+ concentration. Thus, as the Ca2+-free LS2 water invades the pores, eq 1 is, because of the counterion effect, moved to the left because of the increased Ca2+ concentration caused by dissolution of CaSO4(s). Therefore, the resulting pH gradient is small, attributable to less desorption of Ca2+ from the clay. At 90 °C, the initial concentration of SO42− in the effluent of the HS2 water flood was close to 10 mM, which is half of the concentration observed at 40 °C (Figure 3b). This is normal

because the dissolution of anhydrite decreases as the temperature increases. The concentration of SO42− decreased all of the time during the HS2 and LS2 flood sequences to a very low concentration of about 0.02 mM. Even though the solubility of anhydrite decreases as the temperature increases, it appears that most of the dissolvable anhydrite in the core plug has been removed. There is no longer any correlation between the concentrations of Ca2+ and SO42−. At 130 °C, the concentration of SO42− in the effluent was quite stable and low in the range of 0.04 mM (Figure 3c), confirming that there was not much anhydrite left in the core. In the second repeated test at 40 °C, it was verified that the dissolution of anhydrite was very low during the LS2 flood, with a SO42− concentration of about 0.03 mM. The concentration of Ca2+ is about 0.6 mM, and its origin could be linked to desorption of Ca2+ from the clay surface or possibly to some dissolution of CaCO3; however, this is only speculation. With all anhydrite removed from the core, the resulting pH gradient from HS2−LS2−HS2 was larger than the first test at 40 °C (Figure 3d). The correlation between ΔpH and temperature, i.e., ∼2.6, ∼1.8, and ∼1.3 for 40, 90, and 130 °C, was then in harmony with the previous discussion. Thus, if dissolvable anhydrite is present in an oil reservoir, it could prevent LS EOR effects because desorption of active cations from the clay is decreased with the consequence that the pH gradient becomes smaller. In other cases, such as in carbonates14 or in sandstone containing dolomite,15 the presence of anhydrite may increase oil recovery in a LS EOR process. Reservoir Core RC2. The core RC2 was sampled from a reservoir with Tres ≈ 90 °C. The salinity of the formation brine (∼35 000 ppm) is comparable to that of seawater (∼33 000 ppm). The relationship between pH and PV-injected fluid of 3500

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90 and 130 °C, but the values were very low, not increasing beyond 0.1 mM, confirming negligible amounts of precipitated CaSO4 present in the core material. Further discussions take place in a later section. Outcrop Core OC1. For the outcrop core OC1, the initial pH during the HS2 flood varied between 6 and 7 in the temperature range of 40−130 °C (Figure 6). The lowest pH

HS2 and LS2 water is shown in Figure 4. In this case, the pH gradient is in the order of decreasing temperature, in line with

Figure 4. Change in effluent pH versus PV-injected fluid in core RC2 at 40, 90, and 130 °C. The brine flooding sequence was HS2−LS2− HS2. The switches of injection fluids are indicated by the dashed lines. The slopes of the pH change curves are indicated by the black lines.

the discussion linked to eq 1. A large gradient of about 3 pH units was observed for the flooding at 40 °C, and a ΔpH value slightly above 1 was obtained at 130 °C, confirming that the temperature is important regarding desorption of Ca2+ from the clay surface. Another interesting observation is the significant change in the slope of the pH gradient as the flooding fluid was switched from HS2 to LS2 for the different temperatures. When relating the slope of the pH gradient to the desorption rate of the active cations, it is expected that it is more difficult to achieve a sharp front of extra oil in a tertiary LS process at high temperatures (Tres >100 °C). At 130 °C, the value of ΔpH is small and it took a relatively long time to reach the pH plateau. The poor response of LS EOR effects at high temperatures has also been observed experimentally in the laboratory. A LS EOR effect was observed when a reservoir core was aged and flooded at 90 °C, but no LS EOR effect was observed when aging and flooding at 130 °C.16 The concentration of Ca2+ in the effluent is shown in Figure 5. Similar to the graph of pH versus PV, the concentration of Ca2+ stabilized at a certain value during the LS2 flood. The value varied between 0.5 and 1.0 mM for the different temperatures. SO42− effluent concentrations were measured at

Figure 6. Change in effluent pH versus PV-injected fluid in core OC1 at 40, 90, and 130 °C. The brine flooding sequence was HS2−LS2− HS2. The switches of injection fluids are indicated by the dashed lines.

was observed in the first test at 40 °C. The initial pH gives an indication of the interaction between the formation water and core minerals. This outcrop material contained about 30 wt % albite (NaAlSi3O8), which is a polysilicate of the plagioclase type. As an outcrop material, the sandstone has been exposed to fresh water for a very long time, and it is well-known that albite gives alkaline solution under low-saline conditions.17 The cation exchange upon first contact between albite and water giving alkaline solution can be represented by NaAlSi3O8 + H 2O ↔ HAlSi3O8 + OH− + Na +

(7)

Fresh water has passed through the sandstone for many years; thus, the equilibrium in eq 7 has moved to the right; i.e., NaAlSi3O8 has been transformed to HAlSi3O8. In the presence of HS brine, HS2 (100 000 ppm), acidic conditions are obtained. HAlSi3O8 + Na + ↔ NaAlSi3O8 + H+

(8)

The present test was performed by flooding the outcrop core successively with HS2−LS2−HS2 at the different temperatures (40, 90, and 130 °C). Because the core was not exposed to fresh water in between the tests at the different temperatures, it is quite reasonable that the pH was lowest in the first test at 40 °C. The presence of 0.3 wt % CaCO3 in the formation may complicate the interpretation of the variation in the initial pH at the different temperatures. Dissolution of CaCO3(s) is sensitive to the temperature, the pH, and the concentration of Ca2+ in the formation water. The observed pH value at the different temperatures could be a combination of the dissolution rate and thermodynamics related to CaCO3 and ion exchange on albite. It should be mentioned that no CaSO4 dissolution was detected during the floods at the various temperatures. The observed increase in pH as the flooding fluid was switched from HS2 to LS2 was significantly higher at 40 °C (ΔpH ≈ 3) compared to ΔpH ≈ 2 at 90 and 130 °C, which

Figure 5. Effluent concentration of Ca2+ versus PV-injected fluid in core RC2 at 40, 90, and 130 °C. The flooding sequence was HS2− LS2−HS2. 3501

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General Comments. From these experiments, it is not possible to relate the shape of the pH curves to the type of clay present in the cores, even though OC1 did not contain any kaolinite. All cores contained large amounts of illite (about 10 wt %; see Table 1). For this purpose, separate tests in prepared columns containing specific clay material and silica should be performed at various temperatures. If there was no dissolution of anhydrite or calcite, both the pH and concentration of Ca2+ remained constant during the LS2 flood, as shown by Figures 4 and 5. Because the LS2 fluid did not contain any Ca2+, the exchange of Ca2+ at the clay surface by H+ and Na+ must be in a steady state. On a molar basis, the concentration of Ca2+ was about 0.5 mM under the LS2 flood. The concentration of OH− varied between approximately 10−4 and 10−6 M at 40 and 130 °C. Thus, to compensate for the negative charges on the clay surface resulting from desorption of Ca2+, significant amounts of Na+ must also be adsorbed onto the clay surface. For all of the tested cores, the change in pH was completely reversible; i.e., the pH regained its initial value when the flooding fluid was switched from LS2 back to HS2. The slopes of the pH gradients indicated that the adsorption of Ca2+ onto the clay was faster than desorption of Ca2+ from the clay at the various temperatures. Gamage and Thyne13 performed a systematic study of the relationship between temperature (25−90 °C), pH gradient, and tertiary LS EOR effects using Berea core material. When the aging and flooding temperatures were the same, the LS EOR effect decreased as the temperature increased and the pH gradient, when switching the flooding fluid from the HS water to the LS water, decreased from about 1.5 at 25 and 40 °C to about 1.0 at 90 °C. Given the decreasing LS effect with the temperature, the authors suggested that an exothermic chemical reaction appeared to be an important component in the LS mechanism. This is completely in line with our observations that desorption of active cations, especially Ca2+, from the clay surface appeared to be an exothermic process, which could be indirectly determined by the increase in pH.

may be due to the fact that the starting pH was lower (Figure 6). The discontinuity of the pH versus PV curves observed for the different temperatures is very reproducible and may be related to the brine interaction with albite and CaCO3 as discussed above. In contradiction to the previous reservoir core RC2, the slopes of the pH gradients were quite similar at the different temperatures. The slope of the pH gradient when the flooding fluid was switched from LS2 to HS2 was very steep, which indicates a fast displacement of H+ by Ca2+ at the clay surface. The concentration of Ca2+ in the effluent is shown in Figure 7.

Figure 7. Effluent concentration of Ca2+ versus PV-injected fluid in core OC1 at 40, 90, and 130 °C. The flooding sequence was HS2− LS2−HS2.

In a previous study on the LS EOR effect performed on the same outcrop core material, an increase in pH was observed when LS brine was injected.18 The increase in pH was explained by ionic exchange between clay and invading brine and also calcite dissolution. The pH increase was temperaturedependent and decreased with an increasing temperature. The secondary oil recovery with HS brine at 35, 60, and 90 °C showed increased oil recovery with an increasing temperature. When switching to a LS brine, the core at the lowest temperature (35 °C) responded with the largest increase in oil recovery [∼10% original oil in place (OOIP)] and a pH gradient of ΔpH ∼ 2−2.5 was reported. At 90 °C, the LS effect was smaller than at 35 °C (∼5% OOIP), but unfortunately, no data on pH was reported in this case. These results are, however, in line with our observations, and they strongly support a link between the observed pH gradient in a system where strong buffering agents are absent and the potential for the LS EOR effect. It is also interesting to note that the presence of plagioclase in outcrop sandstone can have a positive impact on the LS EOR effect because of the acidic initial environment during core preparation/aging (eq 8). The adsorption of acidic and basic organic material onto clay minerals increases as the pH decreases, and this is a must for observing LS effects.1 On the other hand, plagioclase in reservoir core material at moderate brine salinities (35 000 ppm) can have a negative effect on the LS EOR effect, as observed for core material from the Snorre formation.7 In this case, the pH of the formation brine was above 7, eq 7 was moved to the right, thus preventing adsorption of organic material onto the clay, and the clay material remained quite water-wet. The potential for a LS EOR effect was therefore very low.

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CONCLUSION Adsorption and desorption to/from clays by organic material and cations have been studied under both static conditions (organic material) and dynamic conditions (cations), focusing on salinity and pH changes. The objective of the work was to confirm that pH is a key parameter in the LS EOR mechanism. The results are briefly summarized below: (1) In the pH range of 3−8, the adsorption of basic material quinoline is always higher in the LS water compared to the HS water, confirming that a decrease in salinity itself is not the only mechanism for the wettability alteration in a LS EOR process. (2) Successive core floods of two different reservoir cores and one outcrop core with HS−LS−HS water at different temperatures (40, 90, and 130 °C) confirmed an increase in pH as the injected fluid was switched from HS to LS, and the pH decreased to the original value as the fluid was switched back again from LS to HS. (3) In all cases, the pH gradient decreased as the temperature increased, confirming that desorption of Ca2+ from the clay surface is an exothermic process. (4) The rate of pH change, i.e., the slope of the pH gradient, can be related to the desorption rate of Ca2+ from the clay surface. As expected, the desorption rate appeared to decrease as the temperature increased. (5) The presence of anhydrite and/or calcite in the rock material affected the magnitude of the pH gradient as well 3502

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as the rate of desorption of Ca2+, which can both have an impact on the LS EOR potential. (6) Flooding a sandstone reservoir core successively with relevant HS−LS−HS brines at Tres and monitoring the pH and the composition of the produced water may be a valuable first approach to evaluate the LS EOR potential.

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(16) RezaeiDoust, A.; Puntervold, T.; Austad, T. A discussion of the low salinity EOR potential for a North Sea sandstone field. Proceedings of the Society of Petroleum Engineers (SPE) Annual Technical Conference and Exhibition; Florence, Italy, Sept 19−22, 2010; SPE Paper 134459. (17) Friedman, G. M.; Sanders, J. E.; Kopaska-Merkel, D. C. Principles of Sedimentary Deposits: Stratigraphy and Sedimentology; Macmillan Publishing Company: New York, 1992. (18) Cissokho, M.; Boussour, S.; Cordier, P.; Bertin, H.; Hamon, G. Low salinity oil recovery on clayey sanstone: Experimental study. Proceedings of the International Symposium of the Society of Core Analysts; Noordwijk, The Netherlands, Sept 27−30, 2009.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Talisman Energy Norge A.S. and Total Norge for financial support and permission to publish the results of this study.

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REFERENCES

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dx.doi.org/10.1021/ef300162n | Energy Fuels 2012, 26, 3497−3503