Article pubs.acs.org/EF
Adsorption of Novel Nonionic Surfactant and Particles Mixture in Carbonates: Enhanced Oil Recovery Implication Mohammad Ali Ahmadi† and Seyed Reza Shadizadeh*,‡ †
Department of Petroleum Engineering, Ahwaz Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran Department of Petroleum Engineering, Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran
‡
ABSTRACT: Over 40% of the current world conventional oil production comes from carbonate reservoirs, dominantly mature and declining giant oilfields. After primary and secondary oil production stages using tertiary oil production methods as part of an enhanced oil recovery (EOR) scheme is inevitable. Surfactant flooding aims at reducing the mobility ratio through lowering the interfacial tension between oil and water and mobilizing the residual oil. This article highlights the adsorption equilibrium of the combination of different types of nanosilica and Zyziphus Spina Christi, a novel surfactant, in aqueous solutions for EOR and reservoir stimulation purposes. A conductivity technique was used to assess the adsorption of the surfactant and nanosilica in the aqueous phase. Batch experiments were used to understand the effect of adsorbent dose on sorption efficiency as well. The adsorption data were examined using four different adsorption isotherm models (Langmuir, Freundlich, Temkin, and Linear), and the adsorption parameters were determined for each model. This study suggests that a Freundlich isotherm model can satisfactorily estimate the adsorption behavior of combination nanosilica and surfactant adsorption on carbonates. Results from this study can help in appropriate selection of surfactants in the design of EOR schemes and reservoir stimulation plans in carbonate reservoirs.
1. INTRODUCTION
Surfactants can be classified into different groups based on ionic nature of the headgroup, namely, anionic, cationic, nonionic, and zwitterionic.40,41 Nonionic surfactants are nonvolatile and benign compounds for the environment and are considered as suitable alternative for traditional solvents due to their ability to separate organic compounds from solid samples. Nonionic surfactants have effective solubilization toward water-insoluble or sparingly soluble organic compounds.42 This property makes nonionic surfactants a proper candidate for separation of polar and nonpolar compounds from different solid materials such as separation of aromatic hydrocarbon from solid environmental phases.43−49 Other factors affecting the rate of adsorption of surfactants are including the rock surface charge, fluid interface, and surfactant structure.50,51 Surfactant adsorption during flow of the surfactant solutions in porous systems is typically convoyed through a range of complex phenomena. Some research works related to the adsorption of commercial surfactants have been published but reasonable comparison of reported data is pretty difficult since surfactants with various purities have been employed.51−57 Surfactant adsorption is a phenomenon on solid−liquid interfaces where transport of surfactant molecules from the bulk phase to the interface occurs. This process can be explained as the interface is energetically favored by the surfactant compared to the bulk phase.55,56 The adsorption of surfactant may happen in the form of monomers at low concentrations of aqueous surfactant, if soil or sediment is
Carbonate rocks cover around 23% of the earth crust and contain as much as 50% of the world’s proven conventional oil reserves and over 20% of the world’s endowment of heavy oil, extra heavy oil, and bitumen. More than 40% of current world oil production comes from naturally fractured carbonate reservoirs (NFCRs), dominantly mature and rapidly declining giant oilfields in the Middle East. Primary and secondary oil production stages result in recovery factors (RF) of commonly not greater than 0.45. Over 50% of the oil originally in place (OOIP) is trapped in the reservoir rock as residual oil due to mobility issues and capillary barrier. Hence, to unlock this immense oil resource, implementation of tertiary oil production techniques as part of an enhanced oil recovery (EOR) scheme is inevitable. However, chemical EOR methods were never responsible for a significant EOR oil production worldwide. Nevertheless, surfactants are increasingly used as a well stimulation or wettability alteration agents in EOR projects in carbonates and this is an active research area for the scientists around the world.1−25 Among the chemical EOR techniques, surfactant flooding aims at reducing the mobility ratio through lowering the interfacial tension between oil and water and mobilizing the residual oil into the production well. This is feasible via adsorption of the surfactants by the reservoir rock.26−31 Surfactant flooding has been tried for some conventional oil reservoirs around the world with some success stories (i.e., Yates field in Texas9,32,33 and The Cottonwood Creek in Wyoming)5,9,32−36 but largely proved inefficient due to huge surfactant loss into the porous medium and also issues with adsorption and reactions with the reservoir rock.37−39 © 2012 American Chemical Society
Received: January 26, 2012 Revised: May 18, 2012 Published: May 18, 2012 4655
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synthetic surfactants having lipophilic and hydrophilic molecular parts have the same structure.62 Three cyclopeptide alkaloids as well as four saponin glycosides and several flavonoids can be extracted from the leaves of Zyziphus Spina Christi. Saponin, which is a biosurfactant, is produced from the leaves of Zyziphus Spina Christi. For the purpose of this study, the novel surfactant was extracted from the leaves by the spray dryer method. The leaves of Zyziphus Spina Christi were collected from the south of Iran, and the saponin was extracted by the spray dryer method. The total extracted powder contains saponin and flavonoids. The powder has a light brown color and is soluble in water and alcohol. The density of the powder is 0.09 g/cm3, and 1% of this powder in water has a pH of 5.7−5.9. Saponin is a natural and biodegradable nonionic surfactant. Properties of the novel surfactant are summarized in Table 1.62−64
present in the porous medium, while if the surfactant concentration increases, the adsorbed surfactant monomers tend to aggregate and form micelle-like structures. Depending on the number of layers for aggregated surfactant, the micelles are called admicelles for one layer and hemimicelles for two layers.53−56 When the micelle structures make a solid surface, the rate of adsorption might increase fast until the solid interface is covered completely by bilayers of surfactant. The aggregation of the surface occurs at concentrations lower than the critical micelle concentration (CMC) but beyond a specific critical concentration that is called hemimicellar concentration (HMC); Gaudin and Fuerstenau introduced this critical concentration for first time.53 Kinetics and equilibrium adsorption of surfactants are dependent on the nature or properties of surfactants and also the solid−liquid surface.54−57 Wayman58 and Scamehorn59 studied the adsorption of a dilute solution of sodium alkyl benzene sulfonate (SDDBS) as a surfactant on various clay minerals (e.g., kaolinite). They concluded that the adsorption isotherms match the Langmuir equation and they can be explained by using the Truber diagram. Moreover, a number of researchers (e.g., Meader52 and Trogus60) investigated the adsorption of sodium alkyl benzene sulfonate on bioglass, kaolin, and Berea sand core and indicated that the Langmuir equation is not appropriate to describe the adsorption isotherms of this particular materials. On the basis of their experimental results, a maximum adsorption magnitude was observed, occasionally followed by a minimum extent. Gogoi reported the adsorption equilibrium of Na-lignosulphonate onto the porous media of Oil India Limited (OIL) petroleum reservoir rocks. He demonstrated that adsorption increases with an increase in NaCl concentration but decreases with an increase in pH.61 The adsorption mechanism of the combination of different types of nanosilica and Zyziphus Spina Christi on a carbonate rock surface is not yet reported in the literature. This article highlights the adsorption behavior of combination nanosilica and Zyziphus Spina Christi in aqueous solutions as a novel nonionic surfactant to be implemented for EOR schemes in carbonates. Adsorption of the studied surfactant was assessed using a conductivity technique for the aqueous phase. Batch experiments were carried out to investigate the effect of the adsorbent dose on sorption efficiency. The isotherm adsorption data were also examined with four different isotherm adsorption models (Langmuir, Freundlich, Temkin, and Linear equations), and the adsorption parameters were determined in each case. Results from this study are presented and discussed in detail throughout this article and can be used for appropriate selection of surfactants in the design of EOR schemes and reservoir stimulation plans in carbonate reservoirs.
Table 1. Properties of the Saponin, a Novel Surfactant product
total extract powder of Zyziphus Spina Christi
used part preparation description color solubility in cold water solubility in alcohol pH value (10% solution) density LOD at 110 °C after 6 h total ash at 550 °C after 4 h applications
leaves spray drier fine powder brown soluble soluble 5.7−5.9 0.09 g/cm3 1.6−2% 11.7−12% medicine
2.2. Nanoparticles. Nanosilica, which has been successfully utilized as a thickening and rheology control agent in many polymeric systems, has been chosen for this study. Nanosilica is commercially available in the form of untreated (hydrophilic) silica, treated (hydrophobic or partially hydrophobic) silica, or as a mixture of silica and alumina (SiO2 and Al2O3 mixture). Hydrophobic types also have dispersibility problem in aqueous media as they cannot be easily wetted by water. Therefore only the hydrophilic and partially hydrophobic forms were used. As the surface area of nanosilica increases, its potential as a rheology control agent increases; hence, grades with surface areas below 200 m2/g are not usually recommended for thickening effects. Two types of commercially available hydrophilic fumed silica were used in this study, both having approximate specific surface areas of 200 m2/g. One was AEROSIL 200 provided by Evonik Degussa GmbH, which is known as the most frequently used type of AEROSIL fumed silica for thickening, thixotropizing, and reinforcement. The partially hydrophobic type used was AEROSIL R 816 provided by Evonik Degussa GmbH. AEROSIL R 816 is a fumed silica after-treated with hexadecylsilane based on AEROSIL 200. AEROSIL R 816 is commonly used as a rheology control additive in water-based coating systems. The Nano silica in this study is a kind of modified ultrafine powder (see Figure 1), which is made from SiO2 and an additive. As shown in Figure 1, the shape of a nanoparticle looks like an approximate sphere when observed under a TEM. Physical properties of AEROSIL R816 and AEROSIL 200 are shown in Table 2. 2.3. Adsorbent. Carbonate rock cores for the purpose of this study were taken from the Azadegan oilfield located in the Northern Persian Gulf. The samples under study are from Sarvak reservoir in this oil field. The porosity of the Sarvak reservoir is around 12.39%, while permeability for the reservoir varies between 1 md and 10 md depending on the position, pore structure, and rock lithology. Also, the average water saturation is 10.33%. Samples were disintegrated into small pieces by the jaw crusher and then grounded. Samples were then air-dried for 24 h followed by oven drying at 105 °C for 24 h. Dried rock samples were sieved by sieves no. 50 and no. 70 to obtain particles less than 297 μm and larger than 210 μm. The semiquantitative mineral composition of the crushed
2. MATERIALS AND METHODS 2.1. Surfactant. Zyziphus Spina Christi is a tree with spiny branches and commonly found in Jordan, Iran, Iraq, and Egypt. The concentration of saponins in Zyziphus Spina Christi is high.62 Saponins are natural surface-active substances (surfactants) present in more than 500 plant species.63,64 Their molecules include hydrophobic and hydrophilic parts. The hydrophobic part is composed of a triterpenoid or steroid backbone, and the hydrophilic consists of several saccharide residues, attached to the hydrophobic scaffold via glycoside bonds.65 The combination of the nonpolar sapogenin and water-soluble side chain enables saponin to change to foam. Most 4656
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Figure 1. Image of silica nanoparticles observed under TEM: (a) hydrophilic NanoSilica and (b) hydrophobic NanoSilica. carbonate rock determined by X-ray diffraction (XRD) is presented in Figure 2. Results are given in Table 3. The major phase of crushed carbonate is calcite and orthoclase with a minor of dolomite and clay.
Table 2. Physical Properties of Nanoparticles AEROSIL 200
AEROSIL R 816
behavior with respect to water
hydrophilic
appearance BET-surface area (m2/g) average primary particle size (nm) tapped density (g/L) SiO2 (wt %) Al2O3 (wt %) Fe2O3 (wt %) TiO2(wt %) HCl (wt %)
fluffy white powder 200 ± 25 12
partially hydrophobic fluffy white powder 190 ± 20 12
50 ≥99.8 ≤0.05 ≤0.01 ≤0.03 ≤0.025
40 ≥99.8 ≤0.05 ≤0.01 ≤0.03 ≤0.025
Table 3. Semi-Quantitative Mineral Composition (wt %) Determined by XRD for Reservoir Crushed Carbonate Rock calcite dolomite orthoclase clay
65.38 3.13 28.89 1.83
2.4. Preparation of Surfactant Solution. The stock solution of Zyziphus Spina Christi with concentrations of between 1000 and 80 000 mg/L were prepared by dissolving 0.10−8 g of Zyziphus Spina
Figure 2. XRD of crushed carbonate rock. 4657
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Christi in 1000 mL of deionized water in a volumetric flask. These solutions were diluted to obtain standard solutions containing 1000, 5000, 10 000, 15 000, 20 000, 40 000, 50 000, 60 000, 70 000, and 80 000 mg/L of the Zyziphus Spina Christi. 2.4.1. CMC Measurement. There are several methods such as UV− vis spectroscopy, voltametry, scattering techniques, calorimetry, surface tension, and conductivity to measure the CMC. In this study, the conductivity method was selected to conduct the CMC measurements. The concentration of the Zyziphus Spina Christi samples used was in the range of 1000 to 80 000 ppm. Conductivity of the solutions was determined from high concentration to low. A conductivity detector from the Crison Company (EC-Meter GLP 31+) was implemented in this research work. At first, the conductivity detector was calibrated by using a standard solution. In all of the experiments, the electrode was washed up with distilled water and after that with a peculiar solution. It is necessary to immerse the probe of the conductometer in solution to guarantee the accuracy of solution conductance. In the next step, the conductivity of terms of concentration of Zyziphus Spina Christi was measured as shown in Figure 3.
Figure 3. Conductivity vs surfactant concentration.
Figure 4. Adsorption density vs equilibrium surfactant concentration (a) with a different concentration of hydrophobic nanosilica and (b) with a different concentration of hydrophilic nanosilica.
2.5. Adsorption Experiment. Experimental tests were designed in two steps: the first one includes adsorption of Zyziphus Spina Christi on carbonate rock without nanosilica particles and the second step adsorption of surfactant were considered at different nanoparticle concentrations. Adsorption of surfactant on reservoir rock is determined by batch equilibrium tests on crushed core carbonate mineral tests. In batch equilibrium tests on crushed core, a known volume of surfactant solution at a known concentration is mixed with a specified mass of crushed rock in a centrifuge tube. Samples of surfactant solution were taken from the container in seven time steps; after 1, 2, 3, 6, 12, 24, and 48 h, we used a conductivity meter to determine the residual concentration at each time step. When the concentration remains constant with time, the system is at equilibrium and the test is completed. Batch adsorption experiments were carried out by allowing a precisely weighted amount of Zyziphus Spina Christi and different states of nanosilica to reach equilibrium with crushed carbonate solutions of various initial concentrations ranging from 1000 to 80 000 ppm and 500 to 2000 ppm for surfactant and nanosilica, respectively. The solid to liquid weight ratio was 1:15. Known weights of crushed carbonate samples (2 g) were added to a 30 mL solution of the nanosilica-Zyziphus Spina Christi at different concentrations. All the adsorption experiments on crushed rocks were performed at 28 °C and atmospheric pressure. Static adsorption isotherms were obtained by measuring surfactant concentration before and after equilibrating with crushed rock. A volume of 30 mL of each nanosurfactant solution was added to a centrifuge tube containing a 2 g mass of crushed rock. The samples were centrifuged for 15 min at 2500 rpm. Supernatants were separated and analyzed for residual surfactant concentrations. Adsorption density vs equilibrium surfactant concentration is presented in Figure 4. Changes in the surfactant concentration during the adsorption tests were determined using the conductometry method. The method of
measuring surfactant adsorption on the surface of solids such as carbonate rocks is based on determining the difference between the surfactant concentration in the aqueous phase before and after adsorption. Adsorption (q) measured by using this method can be calculated as the following:
q=
mtot.solution(C o − C) × 10−3 mcarbonate
(1)
where q = surfactant adsorption on rock surface, mg/g of rock; mtot.solution = total mass of solution in original bulk solution, g; Co = surfactant concentration in initial solution before equilibrated with carbonate rock, ppm; C = surfactant concentration in aqueous solution after equilibrated with carbonate rock, ppm; and mcarbonate = total mass of crushed carbonate, g. 2.6. Adsorption Models. An adsorption model is required to predict the loading on the adsorption matrix at a certain concentration of the component. The four well-known general adsorption isotherms that can be used to describe the equilibrium adsorption relation are described in this section, briefly. 2.6.1. Langmuir Isotherm. This model is obtained under the ideal assumption of a totally homogeneous adsorption surface and it may be represented as follows:66,67 qe =
qoK adCe 1 + K adCe
(2)
Thus, a plot of 1/qe vs 1/Ce should yield a straight line if the Langmuir isotherm is obeyed by the adsorption equilibrium. qo and Kad values 4658
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will be calculated from the slope and intercept of the graphed line, respectively. 2.6.2. Freundlich Isotherm. Freundlich developed an empirical equation to describe the adsorption process based on the assumption that the adsorbent has a heterogeneous surface composed of different classes of adsorption sites.68 Freundlich demonstrated that the ratio of the solute adsorbed onto a given mass of an adsorbent to the concentration of the solute in the solution was not constant at different solution concentrations. This isotherm does not predict any saturation of the adsorbent by the adsorbate. Hence, infinite surface coverage is predicted mathematically indicating multilayer sorption of the surface.67,68 This model is expressed by the following equation:68 qe = K f Ce1/ n
3.2. Adsorption Isotherm Study. As mentioned earlier in this text, adsorption isotherms can help in describing the adsorption mechanism of the surfactant on the surface of the adsorbent. Adsorption isotherms are characterized by specific constants that express the surface properties and affinity of adsorbent toward the adsorbed surfactant. It is important to evaluate the most appropriate correlations for equilibrium curves, to optimize the design of a sorption system. Langmuir, Freundlich, Temkin, and Linear isotherm models were employed to describe the adsorption equilibrium. Experimental isotherm tests were carried out at an equilibrium time for different dosages of adsorbate. 3.2.1. Langmuir Isotherm. The Langmuir constants qo and kad are related to the adsorption capacity and the energy of adsorption, respectively. When 1/qe was plotted against 1/Ce, a straight line with a slope of 1/qo was obtained, indicating that the adsorption of Zyziphus Spina Christi follows the Langmuir isotherm. The Langmuir constants Kad and qo were calculated from this isotherm, and their values are listed in Table 4.
(3)
n and Kf are the Freundlich constants. 2.6.3. Temkin Isotherm. The linear form of the Temkin isotherm is expressed as67,69,70 qe = B ln K t + B ln Ce
(4)
The adsorption data were analyzed according to above equation as shown in Table 6. 2.6.4. Linear Isotherm. The simplest type of adsorption isotherm is the linear increase described by the Henry equation as follows:67,71
Table 4. Parameters of Langmuir Adsorption Model Employed in This Study
q = KHC
NSHO concn (ppm)
where q is the amount adsorbed, C is the concentration, and KH is a constant in units of L/m2.
0
3. RESULTS AND DISCUSSION Adsorption of aqueous solutions of combination of nanosilica and Zyziphus Spina Christi as a nonionic was studied here. This surfactant has an ability to change the wettability of rock due to adsorption on the rock. Also, since saponin is a natural surfactant, it is considered as a environmental friendly surfactant. Important aspects of Zyziphus Spina Christi adsorption are discussed in detail as follows. 3.1. Important Parameters in Adsorption Process. In this section, the vital factors affecting of combination of nanosilica and saponin adsorption on carbonate samples are considered for more clarification about the interaction mechanism of a liquid phase and a solid phase in the particular case brought in this study. 3.1.1. CMC Measurement. As earlier mentioned in this text, CMC measurement aids in describing the adsorption mechanism of carbonate rock on the surface of the adsorbent under study. In order to determine the CMC, a plot of conductivity vs concentration should be constructed to determine the CMC value from the turning point in the curve (see Figure 3). When the concentration of the surfactant solution increases to a certain value, then its ions or molecules will come to the association reaction and will start to be micelles; a sharp change in the trend of the curve. Careful experimental measurements using highly purified systems revealed that somewhat gradual and continuous changes in the physicochemical properties occurs near the CMC. The micelles appeared to be polydisperse and that monomer activities changed above the CMC. 3.1.2. Surfactant Concentration. The influence of the initial saponin concentration on the amount of adsorption is shown in Figure 4. An increase in the concentration of surfactant leads to an increase in the adsorption capacity of the surfactant onto carbonate samples. This is due to the increase in the concentration gradient between the bulk and the surface of the carbonate rock with an increase in the initial concentration of the surfactant solution.
500 1000 1500 2000 NSHI concn (ppm) 500 1000 1500 2000
correlation 1/qe = 0.0962/Ce + 0.0637 1/qe = 0.228/Ce + 0.1398 1/qe = 0.3214/Ce + 0.212 1/qe = 0.5258/Ce + 0.3965 1/qe = 1.1795/Ce + 0.8122 correlation 1/qe = 0.0964/Ce + 0.0791 1/qe = 0.1134/Ce + 0.0909 1/qe = 0.1287/Ce + 0.1008 1/qe = 0.1475/Ce + 0.1105
R2
qo
Kad
0.9857
15.698
0.6621
0.9884
7.153
0.6131
0.9858
4.716
0.6596
0.9797
2.522
0.7540
0.9839
1.231
0.6885
R2
qo
Kad
0.9751
12.642
0.8205
0.9763
11.001
0.8015
0.9775
9.920
0.7832
0.9797
9.049
0.7491
3.2.2. Freundlich Isotherm. Unlike the Langmuir model, it describes reversible adsorption and is not restricted to the formation of the monolayer. The amount of adsorbed material is the summation of adsorption on all binding sites. The Freundlich equation predicts that the Zizyphus Spina Christi concentrations on the adsorbent will increase as long as there is an increase in the Zizyphus Spina Christi concentration in the solution. An exponent (n) value between 1 and 10 dramatically represents adsorption; the higher the n value, the stronger the adsorption intensity. KF has been used as an indicative parameter of the adsorption strength; a higher value of KF indicates a higher capacity of adsorption.68 The equilibrium adsorption data for Zizyphus Spina Christi was plotted in the linear form of the Freundlich isotherm (eq 3) from which the Freundlich constants KF and Freundlich exponent n were determined from the intercept and the slope of the plot, respectively (Figure 5). At 28 °C, the correlation coefficients for Freundlich model were >0.99. The n > 1 values with high KF values dramatically indicated adsorption of Zizyphus Spina Christi onto the carbonate adsorbent but fortunately the value 4659
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Freundlich equation with a correlation factor of 0.9913 and a Freundlich constant and heterogeneity factor of 5.1097 and 0.7217, respectively. 3.2.3. Temkin Isotherm. A plot of qe versus ln Ce enables the determination of the isotherm constants Kt and B. Kt is the equilibrium binding constant (L/mg) corresponding to the maximum binding energy and the constant B is related to the heat of adsorption. The values of the parameters are given in Table 6. Table 6. Parameters of Temkin Adsorption Model Employed in This Study NSHO concn (ppm) 0 500 1000 1500 2000 NSHI concn (ppm) 500 1000 1500 2000
Figure 5. Freundlich isotherm model (a) with different concentrations of hydrophobic nanosilica and (b) with different concentrations of hydrophilic nanosilica.
R2 0.7217
KF
n
0 500 1000 1500 2000 NSHI concn (ppm)
qe qe qe qe qe
= = = = =
5.1097(Ce) 2.2298(Ce)0.7285 1.5313(Ce)0.7218 0.8797(Ce)0.7094 0.4088(Ce)0.7175 correlation
0.9913 0.9914 0.9913 0.9908 0.9912 R2
5.1097 2.2298 1.5313 0.8797 0.4088 KF
0.7217 0.7285 0.7218 0.7094 0.7175 n
500 1000 1500 2000
qe qe qe qe
= = = =
4.6086(Ce)0.7017 3.9597(Ce)0.7036 3.5271(Ce)0.7055 3.1421(Ce)0.7094
0.9903 0.9904 0.9906 0.9908
4.6086 3.9597 3.5271 3.1421
0.7017 0.7036 0.7055 0.7094
Kt
B
0.8694
5.4663
4.7689
0.8702
5.4468
2.1009
0.8695
5.4604
1.4306
0.8678
5.4862
0.8087
0.8688
5.4418
0.3812
correlation
R2
Kt
B
qe = 4.18 ln(Ce) + 7.1489 qe = 3.6114 ln(Ce) + 6.1578 qe = 3.2326 ln(Ce) + 5.4984 qe = 2.9021 ln(Ce) + 4.9209
0.8667 0.867
5.5304 5.5019
4.18 3.6114
0.8672
5.4807
3.2326
0.8678
5.4501
2.902
Table 7. Parameters of Linear Adsorption Model Employed in This Study NSHO concn (ppm)
Table 5. Parameters of Freundlich Adsorption Model Employed in This Study correlation
qe = 4.7689 ln(Ce) + 8.1005 qe = 2.1009 ln(Ce) + 3.5611 qe = 1.4306 ln(Ce) + 2.4285 qe = 0.8087 ln(Ce) + 1.3766 qe =0.3812 ln(Ce) + 0.6458
R2
3.2.4. Linear Isotherm. A plot of the amount of adsorption q versus equilibrium concentration C have a slope of KH. The values of parameters are given in Table 7. The coefficient of
of n is lower than 1 and this value shows that adsorption of Zizyphus Spina Christi is low. The fit of data to the Freundlich isotherm indicates the heterogeneity of the sorbent surface. The magnitude of the exponent 1/n gives an indication of the adequacy and capacity of the adsorbent/adsorbate system. Therefore, the plot of log(qe) versus log(Ce) is a linear relationship with the y-intercept of log(KF) and a slope of 1/ n.67,68,71 Figure 5 shows the fitting of the adsorption isotherm using the Freundlich equation (eq 3). The values of parameters (n, kF) are given in Table 5. The Zizyphus Spina Christi adsorption isotherm for carbonate can be describe by the
NSHO concn (ppm)
correlation
correlation
R2
KH
C
0 500 1000 1500 2000 NSHI concn (ppm)
qe = 2.4621Ce + 3.0732 qe = 1.084Ce + 1.3485 qe = 0.7372Ce + 0.9234 qe = 0.4164Ce + 0.5257 qe = 0.1955Ce + 0.2463
0.9149 0.9147 0.9146 0.9151 0.915
2.4621 1.084 0.7372 0.4164 0.1955
3.0732 1.3485 0.9234 0.5257 0.2463
correlation
R2
KH
C
500 1000 1500 2000
qe = 2.1622Ce + 2.7284 qe = 1.8603Ce + 2.3543 qe = 1.6598Ce + 2.1046 qe = 1.4852Ce + 1.8847
0.9156 0.9155 0.9155 0.9153
2.1622 1.8603 1.6598 1.4852
2.7284 2.3543 2.1046 1.8847
determination obtained from the Temkin isotherm model is less than the R2 values obtained with the Langmuir, Freundlich, and Linear isotherm equations. This denotes that the experimental data better obey the Freundlich and Langmuir isotherms. 3.3. Economic Aspect. In addition to the technical feasibility of EOR surfactant flooding, economic feasibility should also be taken into account; however, the economic feasibility is basically dependent on the various parameters such as oil prices, international economies, and the cost of 4660
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surfactants. The EOR surfactant flooding process is mostly dependent on the cost of surface-active substance. The expenses consist of the preliminary investment to buy the surfactant and also the cost for replacement of the surfactant that has been declined through adsorption processes. Since Zyziphus Spina Christi taken in this study is a natural surface-active substance in the Middle East oil producing countries such as Iran, it is inexpensive and easily available; thus the cost of surfactant has not been considered to be an effective parameter. This adsorption experimental investigation provides a new tool for understanding the usefulness of this new surfactant in chemical EOR methods.
NOMENCLATURE
Acronyms
CMC = critical micelle concentration EOR = enhanced oil recovery ppm = part per million NSHI = hydrophilic nanosilica NSHO = hydrophobic nanosilica Variables
Γ = surfactant adsorption on rock surface, (mg/g-rock) Mtot.solution = total mass of solution in original bulk solution, (g) Co = surfactant concentration in initial solution before equilibrated with carbonate rock, (ppm) C = surfactant concentration in aqueous solution after equilibrated with carbonate rock, (ppm) Mcarbonate = total mass of crushed carbonate, (g) qo = adsorption capacity in Langmuir model (mg/g-rock) kad = energy of adsorption n = Freundlich constant kf = Freundlich constant Ce = equilibrium concentration (mg/L) Kt = equilibrium binding constant corresponding to the maximum binding energy (L/mg) B = Temkin constant is related to the heat of adsorption KH = constant in linear model (L/m2)
4. CONCLUSIONS The adsorption of the combination of different state of nanosilica and Zyziphus Spina Christi onto crushed carbonate rock from aqueous solutions was systematically investigated. Adsorption parameters for the Langmuir, Freundlich, Linear, and Temkin isotherms were determined and the equilibrium data were best described by the Freundlich isotherm model. The following main conclusions can be drawn based on results from this study: (1) The adsorption of surfactant onto carbonate results in the dose to achieve micellization being much greater than that for an aqueous system without carbonate. This was attributed to attraction forces between the negatively charged Zyziphus Spina Christi and the positively charged carbonate. (2) According to the results obtained in this study, as the nanosilica concentration increases, the magnitude of adsorption on the surface of carbonate rocks decreases. However, efficiency in the adsorption reduction of surfactant depends on the type of nanosilica. In the hydrophilic state, the efficiency in adsorption reduction is much lower than the hydrophobic state. This fact caused by the hydrophobic bond between the hydrophobic part of the surfactant and the hydrophobic part of nanosilica. (3) For the experimental data obtained, the Linear, Langmuir, and Temkin equilibrium adsorption models were not suitable for predicting the surfactant adsorption, but the Freundlich equilibrium adsorption was in good agreement between the experimental data and the model with R2 = 0.995 25 for combined hydrophilic nanosilica and surfactant and R2 = 0.991 17 for combined hydrophobic nanosilica and surfactant. (4) Since the Zyziphus Spina Christi, the raw material used in the investigation, is freely and locally available, the surfactant is expected to be economically viable for EOR methods, especially chemical flooding . Moreover, the newly introduced surfactant has no harmful effects on the environment.
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Subscript
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ad = adsorption o = maximum capacity of adsorption e = equilibrium f = Freundlich
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AUTHOR INFORMATION
Corresponding Author
*Address: Department of Petroleum Engineering, Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran, P.O. Box 619, Postal Code 6318714331. Phone: (+98)631-4420050-51. Fax: (+98)631-4423520. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Petroleum University of Technology for providing laboratory facilities. 4661
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