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Adsorption behaviors of surfactants for chemical flooding in enhanced oil recovery
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JIEC 2067 1–7 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Adsorption behaviors of surfactants for chemical flooding in enhanced oil recovery Q1 Sangkwon a b
Park a,*, Euy Soo Lee a,1, Wan Rosli Wan Sulaiman b
Department of Chemical and Biochemical Engineering, Dongguk University, 3-26, Pil-dong, Chung-gu, Seoul 100-715, Republic of Korea Department of Petroleum Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 March 2014 Received in revised form 2 May 2014 Accepted 6 May 2014 Available online xxx
In this work, we investigated fundamental interfacial tension properties and adsorption behaviors of four surfactant candidates for the chemical flooding. For interfacial tension properties, we measured the surfactants’ critical micelle concentration (CMC) in brine solution and interfacial tension (IFT) values with model oil phases. For adsorption behaviors, we determined an adsorption equilibrium time and adsorption isotherms, which were analyzed by the Langmuir and the Freundlich models. Through integrating the results of interfacial tension properties and adsorption behaviors, dodecylbenzenesulfonate was found to be the most appropriate candidate because of less adsorption amount (mg/g adsorbent) and minimum interfacial tension among the tested. ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Enhanced oil recovery Chemical flooding Surfactants Interfacial tension properties Adsorption behaviors
7 8
Introduction
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Crude oil is recovered by three major processes: primary, secondary and enhanced oil recovery. In the primary recovery, crude oil is spurted out of petroleum reservoir by natural pressure of trapped liquid. As the primary process proceeds, reservoir pressure becomes lower than a certain value so that it cannot push the trapped oil toward producing wells any more. Then water or gas is injected to increase and maintain the existing pressure in the reservoir, which is usually known as the secondary oil recovery process. Possible amount of oil recovered in both the primary and secondary processes typically corresponds to about 20–50% of deposit depending on the characteristics of oil and reservoir [1,2]. The rest of oil remaining in the reservoir can be harvested by an enhanced oil recovery (EOR) process [1,2]. In the past, a variety of EOR methods have been used to recover light and heavy oils, and they can be roughly subdivided into thermal and non-thermal methods. Thermal methods, which are primarily used for heavy oils and tar sands, employ thermal energy supplied with hot water, steam, or electrical heating to collect the
* Corresponding author. Tel.: +82 2 2260 3362; fax: +82 2 2268 0188. E-mail address: [email protected] (S. Park). 1 Co-corresponding author.
remaining oil. Non-thermal methods, which are normally used for light oils, utilize other means such as chemical flooding, miscible displacement, or immiscible gas drives than thermal energy [2,3]. Chemical flooding is a process of injecting chemical solutions of surfactant, polymer, alkaline, or micelles into reservoir to squeeze the residual oil out by improving mobility and sweep efficiency [3,4]. The surfactant flooding of the chemical flooding methods has been considered most promising but uneconomical mainly because of significant loss of surfactants due to adsorption on rock surface and rock wettability change in an unproductive way [5,6]. Such an economical unfeasibility of the chemical flooding has limited its application to a minimal level. Nowadays the contribution of chemical flooding to worldwide production of crude oil is only a few percentage of worldwide production even if it is one of the major EOR methods in China [3,5]. When the chemical flooding is economically improved by minimizing chemical loss during the process, it would remain a promising process with better recovery efficiency. In this work, we study fundamental adsorption behaviors of four surfactant candidates for surfactant flooding and their interfacial tension properties for the purpose of identifying the most desirable surfactant. Surfactants’ adsorption behaviors and interfacial tension properties are investigated by measuring equilibrium adsorption isotherms onto kaolinite adsorbent and interfacial tension with model oil phases. The results are discussed in terms of adsorption amount, the consequent financial loss,
http://dx.doi.org/10.1016/j.jiec.2014.05.040 1226-086X/ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S. Park, et al., J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.05.040
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JIEC 2067 1–7 S. Park et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
2
54 55
and interfacial tension decrease, and the most suitable one is identified.
56
Experimental
57
Materials
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Four different types of surfactants were employed as candidates for the surfactant flooding. Two of them were nonionic surfactants; polysorbate20 (Tween20, PS20) and nonylphenyl polyoxyethylene glycol (Igepal CO-720, PONP), and the other two were anionic surfactants; dodecylbenzenesulfonate (sodium salt, DBS) and lignosulfonate (sodium salt, average Mw 8000, LS). These surfactants were selected because sulfonate and polyoxyethylene types of anionic and nonionic surfactants have been widely used during the last two decades [4]. Their molecular structures are shown in Fig. 1 and their molecular weights and hydrophilic– lipophilic balance (HLB) values are listed in Table 1. All the surfactants were purchased from Aldrich and used without further
Table 1 Molecular weight and HLB value of surfactant. Surfactant
Molecular weight
HLB
DBS PONP PS20 LS
348.5 749.0 1228 8000
11.0a 14.0a 16.7a 15b
a b
Referred to internet. Provided by the provider.
purification. As model oil phases, n-octane (Aldrich, reagent grade, 98%), toluene (Aldrich, anhydrous, 99.8%) and cyclohexane (Aldrich, anhydrous, 99.5%) was used on the basis of the fact that such paraffinic, aromatic and naphthenic hydrocarbons often can be representative components of crude oil in the aspects of interfacial and phase behaviors [7,8]. Kaolinite (Aldrich, analytical grade) was used as a model absorbent because it is the most common type of clays found in oil reservoirs and surfactant adsorption occurring on sandstone is known to be attributed to kaolinite component [9]. Sodium chloride (Aldrich, reagent grade, >99%) and double distilled water were used for the preparation of 1 wt% synthetic brine, which has been practically used as a basic medium of surfactant solution for surfactant flooding [2–4].
70 71 72 73 74 75 76 77 78 79 80 81 82
Critical micelle concentration
83
Critical micelle concentration (CMC) values of four surfactants were determined by a force tensiometer (Wilhelmy plate method, ˝ SS, K100). Aqueous surfactant solutions with different KRU surfactant concentrations from 0.001 to 5 wt% were prepared by dissolving the corresponding surfactants in 1 wt% synthetic brine. Their surface tension values were then measured at room temperature (23 1 8C). A surface tension profile was plotted as a function of surfactant concentration and the concentration at inflection point was decided as the CMC [10].
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Interfacial tension measurement
93
Interfacial tension (IFT) value between water phase (brine solution of 1 wt% surfactant) and oil phase (n-octane, toluene, or cyclohexane) was measured with a spinning drop tensiometer ˝ SS SITE100). A small droplet of n-octane was injected into a (KRU tubular vessel containing aqueous surfactant solution and the oil droplet suspended in the vessel became elongated to a cylindrical shape when the tubular vessel was spun at sufficiently large rotational speed (v). The IFT (gint) value was calculated from the v, the observed radius (r) of cylindrical oil droplet and the density difference (Dr) between two phases according to the following equation [11]:
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1 4
g int ¼ r3 Drv2
Fig. 1. Molecular structures of used surfactants.
(1)
Adsorption equilibrium time
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Prior to main adsorption isotherm experiments, an adsorption equilibrium time was determined by measuring adsorbed amount of surfactant as a function of time. An aqueous solution containing 1 wt% surfactant was prepared by dissolving proper amount of surfactant into 200 mL synthetic brine in a 500 mL stoppered flask. Ten grams of kaolinite adsorbent was prewashed by distilled water and dried in the air, and it was immersed into the surfactant solution. The whole stoppered flask was shaken using an orbital incubator shaker at 100 rpm and room temperature (23 1 8C). At a predetermined time, a small amount of supernatant of the solution was taken, filtered, and subject to UV–vis spectrophotometry
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measurement. The amount of surfactant adsorption, q (in mol/g adsorbent) was calculated by the following relationship: q¼
ðC Ai C Ae ÞW b 100MA W a
(2)
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where CAi, CAe, and MA denote the initial concentration and the equilibrium concentration of surfactant (wt%), and the molecular weight of surfactant, respectively. In the equation, Wb and Wa are the weight of synthetic brine and the weight of adsorbent, respectively.
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Adsorption behaviors
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For adsorption isotherm experiments, several surfactant solutions of different concentrations from 0.001 to 1.0 wt% were prepared and proper amount of adsorbent was added. After the mixtures were shaken at the same conditions as before for the predetermined equilibrium time, the solution concentration was determined by the same procedure as above. Based on the obtained data, adsorption isotherms were plotted and fitted to the Langmuir and the Freundlich adsorption models for further analysis. These two general models are known to describe adsorption behaviors from a liquid solution onto solid adsorbent properly. The Langmuir adsorption isotherm is based on the basic assumption that adsorption takes place at specific homogeneous sites and no adsorption can take place at the site which a solute already occupies. The fundamental feature for the model is that rate of adsorption is proportional to concentration gradient of solute and amount of bare surface [12]. The Langmuir adsorption model can be described by the following equation: q¼
147 146 148 149 150
qm bC Ae bC Ae þ 1
(3)
where CAe, qm, and b are the equilibrium surfactant concentration (mol/L), the maximum amount of solute adsorbed (mol/g adsorbent) and the Langmuir equilibrium constant, respectively. This equation is rearranged into the following linearized form: C Ae C Ae 1 ¼ þ q qm bqm
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From the plot of CAe/q versus CAe, slope and the intercept correspond to 1/qm and 1/bqm, respectively. Therefore, qm and b are readily calculated from the slope and the intercept [13]. The basic assumption of the Freundlich model is that adsorbent surface is composed of heterogeneous sites with different energy and amount of solute adsorbed per unit mass of adsorbent is a function of solute concentration. This model does not confine the adsorption layer to a monolayer but allows multilayers [14]. The Freundlich model quantitatively states that the adsorption amount is proportional to the equilibrium concentration of the solute as follows: 1=n
q ¼ aCAe 165 164 166 167 168 169
(5)
where a, CAe and n denote the Freundlich equilibrium constant, the surfactant concentration (mol/L) and the Freundlich constant, respectively. The constants a and n imply adsorption capacity and intensity, respectively [13,14]. By taking logarithm, this equation is linearized into the equation: ln q ¼ ln a þ
170 172 171 173 174
(4)
1 ln C Ae n
(6)
When ln q is plotted against ln CAe, the slope and the intercept are 1/n and ln a, respectively and thus a and n are readily determined.
Model parameters of qm, b, a, and 1/n were calculated by applying the two adsorption models to the isotherm data. Adsorption behaviors of four surfactants were then analyzed and discussed by comparing the parameters of one another.
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Results and discussion
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Critical micelle concentration
180
The measured surface tension profiles of brine solutions of surfactants (1 wt%) are shown in Fig. 2, and the CMC results are shown in Table 2. The CMC values of nonionic surfactants PONP and PS20 are about one third of those for aqueous solution without extra electrolytes (like NaCl) reported in the literature (3 104 mol/L and 9 105 mol/L, respectively) [15–17]. Meanwhile those of anionic surfactants DBS and LS were about one order of magnitude lower than the values for aqueous solution in the literature (1.59 103 mol/L for DBS and about 3 103 mol/L for LS) [18,19]. Such a large CMC decrease with extra electrolytes is well explained by the change of electrical atmosphere in the aqueous surfactant solution. In other words, electrolytes from the dissolved NaCl neutralize the effective head group charge of the surfactant, which results in reducing the electrostatic repulsion between the polar head groups of the surfactant. Consequently, the micelles are formed at lower concentration than in pure water [20]. The area per surfactant molecule (in A˚2) at the air–water interface at the CMC was estimated using the surface tension (g) data at temperature T and the Gibbs adsorption isotherm equation: 1 dg Gs ¼ (7) nRT d ln C A
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where Gs, R and CA denote the surface excess (mol/m2), the gas constant and the surfactant concentration (mol/L), respectively. Here, n is taken as 2 for DBS and LS because those surfactants dissociate into negatively charged surfactants and sodium cation in a solution. As seen in Table 2, PS20 yielded the largest molecular area of 87.3 A˚2 at the CMC, which is somewhat larger than the reported value of 60.3 A˚2 in the literature [21], while DBS showed the smallest one of 52.9 A˚2 about the same as the reported value of 51.8 A˚2 [22].
200 201 202 203 204 205 206 207 208 209
Adsorption equilibrium time
210
As a prerequisite step for adsorption isotherm experiments, an adsorption equilibrium time was determined by monitoring the
211 212
80 DBS PONP PS20 LS
70
γ (mN/m)
119 120
3
60 50 40 30 20 -16
-14
-12
-10
-8
-6
-4
ln CA Fig. 2. Surface tension profiles as functions of logarithmic concentration of surfactant.
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Table 2 Measured CMC, surface tension at CMC, and area per molecule of surfactant.
g at CMC (mN/m)
Area per molecule (A˚2)
DBS PONP PS20 LS
0.229 0.103 0.032 3.10
27.4 33.7 39.7 50.3
52.9 77.2 87.3 85.6
8 DBS PONP PS20 LS
6
-6
CMC (mmol/L)
q (x10 mol/g)
Surfactant
213 214 215 216 217 218 219
amount of adsorption as a function of time. Fig. 3 shows the adsorption equilibrium profiles of the four surfactants as functions of time. The adsorption equilibrium for all surfactants was completely attained after 1200 min (20 h). When the adsorption rate of surfactant was defined as the amount of adsorption per unit time (mol/g min), it was in the order of DBS > PONP > PS20 > LS from beginning to equilibrium.
220
Adsorption behaviors
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Adsorption isotherms were obtained by plotting the amount of adsorption (mol/g adsorbent) versus the equilibrium concentration of surfactant, and the results are shown in Fig. 4. The Langmuir model was applied to the adsorption isotherms and all the data showed quite good fits to the Langmuir adsorption equation (Eq. (3)) with r2 >0.98. The adsorption isotherm data were then fitted to the linearized equation (Eq. (4)) to estimate the maximum amount of surfactant adsorption, qm, and the Langmuir equilibrium constant, b with excellent linear fit of r2 >0.99. The obtained parameters are given in Table 3. The maximum amount of adsorption was in the order of DBS > PONP > PS20 > LS. These maximum adsorption amounts (for example, 9.36 106 mol/g for DBS and 0.66 106 mol/g for LS) are quite consistent with those reported in the literature [23,24]. Kaolinite is a 1:1 alumina silicate comprising a tetrahedral silica sheet bonded to an octahedral alumina sheet through the sharing of oxygen atoms between silicon and aluminum atoms in adjacent sheets [25]. Although both sheets are theoretically electrically neutral, kaolinite surfaces usually take charges in aqueous solutions because of Al3+ substitution for Si4+ in the tetrahedral sheet, and protonation and deprotonation of surface hydroxyl groups [26]. Therefore, kaolinite surfaces as well as the crystal edges have variable zeta-potential depending on solution pH and ionic strength [27,28]. The highest maximum adsorption amount of anionic surfactant DBS was somewhat unexpected because the zeta-potential of kaolinite has been reported to be significantly negative, approximately 10 to 40 mV with high concentration of electrolytes at natural pH (about 5.9) [27–29]. In other words,
4 2 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
CAe (wt%) Fig. 4. Adsorption isotherm data.
negatively charged DBS molecules were not expected to adsorb easily onto kaolinite surface with the significant amount of negative charge because of electrical repulsion. However, the DBS molecules with less hydrophilic characteristics (the lowest HLB) than other three surfactants are presumed to interact with kaolinite surface through hydrophobic interaction and resulted in such a relatively higher level of monolayer amount. This presumption is supported by a recent AFM study stating that Q2 kaolinite silica face has a modest level of hydrophobicity [30] (Fig. 5). The adsorption of anionic surfactants onto kaolinite can be explained as follows: in the initial stage of adsorption (at the low surfactant concentration), surfactant adsorption is derived mainly by hydrophobic attraction between surfactant’ hydrocarbon chains and kaolinite surface in neutral pH condition. As surfactant concentration increases, additional adsorption occurs through alkyl–alkyl hydrophobic interactions between bulk surfactant molecules and adsorbed surfactant molecules, and surfactant aggregates (called ‘solloids’ or ‘hemimicelles’ [9]) of small size are formed at the kaolinite surface. At the CMC, the adsorption level approaches to a plateau for a moment, and above the CMC surfactant molecules then adsorb to form bilayer and multilayers [9,23,31]. On the other hand, the adsorption of nonionic surfactants onto kaolinite can be described as follows: when the surfactant concentration is low, a weak adsorption of molecule occurs in the form of monomer. The polyoxyethylene part forms a hydrogen bonding with surface hydroxyl groups, and the lateral hydrophobic interactions between hydrophobic groups are not significant. At a higher concentration hemimicelles form, and the molecules tend to lie flat on the surface. Here, cooperative interactions between adsorbates are dominant, and amount of adsorption significantly increases. As the adsorption proceeds further, the adsorbed molecules’ orientation changes and surface aggregates starts to appear due to lateral alkyl–alkyl interactions [9,31,32]. When Eq. (3) is expressed with the surfactant concentration (CAe) in the unit of mol/L, the Langmuir equilibrium constant is Table 3 Estimated parameters of qm and b in the Langmuir adsorption model. Surfactant
Fig. 3. Adsorption equilibrium profiles as functions of time.
DBS PONP PS20 LS
Langmuir adsorption model parameter qm (106 mol/g)
b (103 L/mol)
DGo (kJ/mol)
9.36 4.09 2.81 0.66
0.412 1.04 1.47 5.46
14.7 16.9 17.8 21.0
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Fig. 5. Application of Langmuir model to adsorption isotherm data by (a) Eq. (3) and (b) Eq. (4).
286 287
related to the standard free energy of adsorption, DGo by the following equation:
DGo ¼ RTln K 289 288 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309
(8)
where R and T denote the gas constant and the adsorption temperature, respectively [33]. As given in Table 3, the D Go values for surfactants were in the order of LS > PS20 > PONP > DBS. Since a larger negative free energy change implies a more spontaneous adsorption, LS is thought to show the largest spontaneous adsorption while DBS the smallest. This highly spontaneous adsorption of LS can be explained by high hydrophobic interaction between bulky hydrocarbon chains of LS and hydrophobic kaolinite surface [34]. The Freundlich isotherm model, which is known to be widely used in heterogeneous systems [13], was applied to the isotherm data on electrically heterogeneous kaolinite [35]. As shown in Fig. 6, the model fits by Eqs. (5) and (6) were partially good, i.e. good (r2 = 0.96) at lower concentration but poor (r2 < 0.89) at higher concentration. Therefore, the Freundlich model was applied up to 0.25 wt% concentration and yielded excellent fits overall with r2 >0.99 as shown in Fig. 7. At a fixed temperature, the Freundlich constants, a and 1/n are related to adsorption capacity and adsorption intensity. The constant 1/n is related to the heat of adsorption, DHa by the equation: 1 RTln u ¼ n DH a
(9)
5
Fig. 6. Application of Freundlich model to adsorption isotherm data by (a) Eq. (5) and (b) Eq. (6).
where u is the surface coverage. Thus, at a constant surface coverage the smaller 1/n value indicates the larger heat of adsorption, i.e., higher adsorption intensity. As listed in Table 4, PONP showed the lowest 1/n value and all other three surfactants yielded somewhat larger values than PONP. These results are not consistent with the Langmuir adsorption model results stating that DBS had the largest maximum adsorption amount. This discrepancy is attributed to the assumption that the Freundlich model is not matched to our suggested adsorption mechanism as previously addressed. According to our suggestion, at low surfactant concentration the adsorption is initiated by hydrophobic interactions for anionic surfactant or hydrogen bonding for nonionic surfactant. Then at higher concentration surfactant molecules form aggregate, and finally multilayers at very high concentration. Apparently, the Freundlich model well explains the initial adsorption process on the heterogeneous kaolinite surface but fails to describe the later stages.
Table 4 Estimated parameters of a and 1/n in Freundlich adsorption model (up to CAe = 0.25 wt%). Surfactant
DBS PONP PS20 LS
Freundlich adsorption model parameter 1/n
a (104)
0.607 0.422 0.649 0.655
1.58 0.37 1.35 0.89
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Table 5 Measured interfacial tension, maximum adsorption amount (mg/g adsorbent), and calculated financial loss due to adsorption. Surfactant
DBS PONP PS20 LS a
Interfacial tension, gint (mN/m) model oil phase n-Octane
Cyclohexane
Toluene
2.79 11.8 23.5 49.9
4.53 12.3 16.2 25.8
2.84 8.60 12.9 17.2
Maximum adsorption amount (mg/g adsorbent)
Current price per unit weight ($/kg)a
Financial loss due to adsorption (million $)
3.26 3.06 3.45 5.28
47 210 38 270
3.3 13.7 2.8 30.3
Approximated price quoted from Aldrich in March, 2014.
328
Evaluation of surfactants for chemical flooding
329 330 331 332 333 334 335 336 337
Proper selection of surfactant is one of the key factors for a successful surfactant flooding, lowering oil recovery cost and enhancing its efficiency. Surfactants play a critical role of reducing the interfacial tension (IFT) between oil trapped in capillary pores and water surrounding the pores, thus allowing the oil to be mobilized [36,37]. In a typical surfactant flooding process, the ratio of viscous forces to interfacial tension forces is a critical parameter for oil recovery, and it is related to the capillary number, NVC as follows: mw (10) N VC ¼ V
g int
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where V, mw and gint are the Darcy velocity, the viscosity of displacing fluid, and the IFT between displaced and displacing fluid, respectively [1]. For the mobilization of unconnected oil
Fig. 7. Application of Freundlich model to adsorption isotherm data up to CAe = 0.25 wt% by (a) Eq. (5) and (b) Eq. (6).
droplets, the capillary number needs to be greater than 105 [1]. With a constant viscosity of displacing fluid, the interfacial tension needs to be reduced to maintain the capillary number larger than 105. The measured IFT values between the brine solutions of 1 wt% surfactants and the model oil phases are shown in Table 5. For all the model oil phases, the IFT value was in the order of LS > PS20 > PONP > DBS, and importantly DBS yielded much lower IFT than the other surfactants. For LS, the concentration of 1 wt% is less than a half of the CMC so that it was not enough to lower the interface tension. For an economical success of surfactant flooding, one needs to minimize the depletion of injected surfactant due to adsorption onto clays in the reservoir [36]. We calculated the maximum adsorption amounts obtained by the applying the Langmuir model in the unit of mg/g adsorbent, which were in the order of LS > PS20 > DBS > PONP as shown in Table 5. We roughly estimated financial loss due to the surfactant adsorption according to the following method: if a reservoir to be swept with a surfactant solution has the size of one acre (4047 m3) by 3 m deep, and the rock substrate is about 70% solid material having a density of 2.5 g/cm3, approximately 2.12 1010 g of adsorbent are then totally available for surfactant adsorption [37]. When DBS is used, a financial loss due to adsorption is evaluated to be about $3,300,000 by multiplying the total adsorbent weight by the maximum adsorption amount per unit adsorbent weight (3.26 g/kg) and the current price of DBS ($47 per kg). By combining the financial loss and the interfacial tension reduction results, DBS is identified as the most recommendable surfactant for the surfactant flooding among the tested. According to our surfactant flooding simulation, the oil recovery (in percentage of recovered oil to remaining oil) with DBS at optimum conditions was more than 38%, which is higher than that with alkoxy glycidyl sulfonate, one of the most widely used surfactants [38].
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Conclusions
375
This work provided fundamental information on interfacial tension properties and adsorption behaviors of several surfactant candidates for surfactant flooding based on interfacial tension and adsorption isotherm data. The experimental results and consequent analysis revealed some important features of adsorption behaviors and interfacial tension properties of the four surfactants and the following conclusions are drawn:
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1. Adsorption isotherm data of four surfactants were successfully analyzed by the Langmuir and the Freundlich models, and they were better fitted by the former rather than the latter. 2. The maximum adsorption amount of a surfactant on kaolinite surface was in the order of LS > PS20 > DBS > PONP when it was estimated in the unit of mg/g adsorbent, while it was DBS > PONP > PS20 > LS in mol/g adsorbent. 3. The interfacial tension between brine solution of 1 wt% surfactant and model oil phases was in the order of DBS > PONOP > PS20 > LS.
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4. By considering the financial loss caused by surfactant adsorption and the interfacial tension reduction, DBS was found to be the most appropriate candidate for the surfactant flooding among the tested.
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Acknowledgement
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Q3 Q4
This work was supported by the Dongguk University Research Fund of 2014.
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[14] A.W. Adamson, A.P. Gast, Physical Chemistry of Surfaces, 6th ed., Wiley-Interscience, New York, 1997. [15] S.K. Ghosh, P.K. Khatua, S.C. Bhattacharyya, Int. J. Mol. Sci. 4 (2003) 562. [16] M. Graca, J.H.H. Bongaerts, J.R. Stokes, S. Granick, J. Colloid Interface Sci. 315 (2007) 662. [17] T.S. Singh, S. Mitra, Eur. J. Chem. 1 (4) (2010) 341. [18] A. Fachini, I. Joekes, Colloids Surf. A: Physicochem. Eng. Asp. 201 (103) (2002) 151. [19] K.M. Askvik, S. Are Gundersen, J. Sjo¨blom, J. Merta, P. Stenius, Colloids Surf. A: Physicochem. Eng. Asp. 159 (1) (1999) 89. [20] D. Varade, T. Joshi, V.K. Aswai, P.S. Goyal, P.A. Hassan, P. Bahadur, Colloids Surf. A: Physicochem. Eng. Asp. 259 (2005) 95. [21] C. Kim, Y.L. Hsieh, Colloids Surf. A: Physicochem. Eng. Asp. 187–188 (2001) 385. [22] S.G. Oh, D.O. Shah, J. Phys. Chem. 97 (1993) 284. [23] N.V. Sastry, P.N. Dave, J. Surfactants Deterg. 2 (1999) 459. [24] F. Perche, Y.F. Houst, P. Bowen, P.H. Hofmann, Adsorption of lignosulfonates and polycarboxylates-depletion and electroacoustic methods, in: V.M. Malhotra (Ed.), Proceedings 7th CANMET/ACI International Conference on Superplasticizers and Other Chemical Admixtures in Concrete. Supplementary Papers, 2003, p. 1. [25] Y. Yukselen, A. Kaya, Water Air Soil Pollut. 145 (2003) 155. [26] B.K. Schroth, G. Sposito, Clays Clay Miner. 45 (1997) 85. [27] E.A. Stephan, C.G. Chase, Sep. Purif. Technol. 21 (2001) 219. [28] M. Alkan, O. Demirbas, M. Dogan, Microporous Mesoporous Mater. 83 (2005) 51. [29] A. Kaya, Y. Yukselen, J. Hazard. Mater. B120 (2005) 119. [30] X. Yin, J. Miller, Miner. Metall. Proc. 29 (2012) 13. [31] S. Paria, K.C. Khilar, Adv. Colloid Interface Sci. 111 (2004) 75. [32] Y. Deng, J.B. Dixon, G.N. White, Colloid Polym. Sci. 284 (2006) 347. [33] L. Yu, Colloids Surf. A: Physicochem. Eng. Asp. 274 (2006) 34. [34] X. Ouyang, Y. Deng, Y. Qian, P. Zhang, X. Qiu, Biomacromolecules 12 (9) (2011) 3313. [35] E. Tomba´cz, M. Szekeres, Appl. Clay Sci. 34 (1) (2006) 105. [36] Y. Wu, S. Iglauer, P. Shuler, Y. Tang, W.A. Goddard, Tenside Surfactant Deterg. 47 (2010) 152. [37] L.L. Wesson, J.H. Harwell, Surfactant adsorption in porous media, in: L.L. Schramm (Ed.), Surfactants: Fundamentals and Applications in the Petroleum Industry, Cambridge University Press, London, 2009. [38] W.R.W. Sulaiman, A Study on Application of Surfactants to Enhanced Oil Recovery, (Ph.D. dissertation), Dongguk University, 2012.
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JIEC 2067 1–7 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Adsorption behaviors of surfactants for chemical flooding in enhanced oil recovery Q1 Sangkwon a b
Park a,*, Euy Soo Lee a,1, Wan Rosli Wan Sulaiman b
Department of Chemical and Biochemical Engineering, Dongguk University, 3-26, Pil-dong, Chung-gu, Seoul 100-715, Republic of Korea Department of Petroleum Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 March 2014 Received in revised form 2 May 2014 Accepted 6 May 2014 Available online xxx
In this work, we investigated fundamental interfacial tension properties and adsorption behaviors of four surfactant candidates for the chemical flooding. For interfacial tension properties, we measured the surfactants’ critical micelle concentration (CMC) in brine solution and interfacial tension (IFT) values with model oil phases. For adsorption behaviors, we determined an adsorption equilibrium time and adsorption isotherms, which were analyzed by the Langmuir and the Freundlich models. Through integrating the results of interfacial tension properties and adsorption behaviors, dodecylbenzenesulfonate was found to be the most appropriate candidate because of less adsorption amount (mg/g adsorbent) and minimum interfacial tension among the tested. ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Enhanced oil recovery Chemical flooding Surfactants Interfacial tension properties Adsorption behaviors
7 8
Introduction
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
Crude oil is recovered by three major processes: primary, secondary and enhanced oil recovery. In the primary recovery, crude oil is spurted out of petroleum reservoir by natural pressure of trapped liquid. As the primary process proceeds, reservoir pressure becomes lower than a certain value so that it cannot push the trapped oil toward producing wells any more. Then water or gas is injected to increase and maintain the existing pressure in the reservoir, which is usually known as the secondary oil recovery process. Possible amount of oil recovered in both the primary and secondary processes typically corresponds to about 20–50% of deposit depending on the characteristics of oil and reservoir [1,2]. The rest of oil remaining in the reservoir can be harvested by an enhanced oil recovery (EOR) process [1,2]. In the past, a variety of EOR methods have been used to recover light and heavy oils, and they can be roughly subdivided into thermal and non-thermal methods. Thermal methods, which are primarily used for heavy oils and tar sands, employ thermal energy supplied with hot water, steam, or electrical heating to collect the
* Corresponding author. Tel.: +82 2 2260 3362; fax: +82 2 2268 0188. E-mail address: [email protected] (S. Park). 1 Co-corresponding author.
remaining oil. Non-thermal methods, which are normally used for light oils, utilize other means such as chemical flooding, miscible displacement, or immiscible gas drives than thermal energy [2,3]. Chemical flooding is a process of injecting chemical solutions of surfactant, polymer, alkaline, or micelles into reservoir to squeeze the residual oil out by improving mobility and sweep efficiency [3,4]. The surfactant flooding of the chemical flooding methods has been considered most promising but uneconomical mainly because of significant loss of surfactants due to adsorption on rock surface and rock wettability change in an unproductive way [5,6]. Such an economical unfeasibility of the chemical flooding has limited its application to a minimal level. Nowadays the contribution of chemical flooding to worldwide production of crude oil is only a few percentage of worldwide production even if it is one of the major EOR methods in China [3,5]. When the chemical flooding is economically improved by minimizing chemical loss during the process, it would remain a promising process with better recovery efficiency. In this work, we study fundamental adsorption behaviors of four surfactant candidates for surfactant flooding and their interfacial tension properties for the purpose of identifying the most desirable surfactant. Surfactants’ adsorption behaviors and interfacial tension properties are investigated by measuring equilibrium adsorption isotherms onto kaolinite adsorbent and interfacial tension with model oil phases. The results are discussed in terms of adsorption amount, the consequent financial loss,
http://dx.doi.org/10.1016/j.jiec.2014.05.040 1226-086X/ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: S. Park, et al., J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.05.040
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54 55
and interfacial tension decrease, and the most suitable one is identified.
56
Experimental
57
Materials
58 59 60 61 62 63 64 65 66 67 68 69
Four different types of surfactants were employed as candidates for the surfactant flooding. Two of them were nonionic surfactants; polysorbate20 (Tween20, PS20) and nonylphenyl polyoxyethylene glycol (Igepal CO-720, PONP), and the other two were anionic surfactants; dodecylbenzenesulfonate (sodium salt, DBS) and lignosulfonate (sodium salt, average Mw 8000, LS). These surfactants were selected because sulfonate and polyoxyethylene types of anionic and nonionic surfactants have been widely used during the last two decades [4]. Their molecular structures are shown in Fig. 1 and their molecular weights and hydrophilic– lipophilic balance (HLB) values are listed in Table 1. All the surfactants were purchased from Aldrich and used without further
Table 1 Molecular weight and HLB value of surfactant. Surfactant
Molecular weight
HLB
DBS PONP PS20 LS
348.5 749.0 1228 8000
11.0a 14.0a 16.7a 15b
a b
Referred to internet. Provided by the provider.
purification. As model oil phases, n-octane (Aldrich, reagent grade, 98%), toluene (Aldrich, anhydrous, 99.8%) and cyclohexane (Aldrich, anhydrous, 99.5%) was used on the basis of the fact that such paraffinic, aromatic and naphthenic hydrocarbons often can be representative components of crude oil in the aspects of interfacial and phase behaviors [7,8]. Kaolinite (Aldrich, analytical grade) was used as a model absorbent because it is the most common type of clays found in oil reservoirs and surfactant adsorption occurring on sandstone is known to be attributed to kaolinite component [9]. Sodium chloride (Aldrich, reagent grade, >99%) and double distilled water were used for the preparation of 1 wt% synthetic brine, which has been practically used as a basic medium of surfactant solution for surfactant flooding [2–4].
70 71 72 73 74 75 76 77 78 79 80 81 82
Critical micelle concentration
83
Critical micelle concentration (CMC) values of four surfactants were determined by a force tensiometer (Wilhelmy plate method, ˝ SS, K100). Aqueous surfactant solutions with different KRU surfactant concentrations from 0.001 to 5 wt% were prepared by dissolving the corresponding surfactants in 1 wt% synthetic brine. Their surface tension values were then measured at room temperature (23 1 8C). A surface tension profile was plotted as a function of surfactant concentration and the concentration at inflection point was decided as the CMC [10].
84 85 86 87 88 89 90 91 92
Interfacial tension measurement
93
Interfacial tension (IFT) value between water phase (brine solution of 1 wt% surfactant) and oil phase (n-octane, toluene, or cyclohexane) was measured with a spinning drop tensiometer ˝ SS SITE100). A small droplet of n-octane was injected into a (KRU tubular vessel containing aqueous surfactant solution and the oil droplet suspended in the vessel became elongated to a cylindrical shape when the tubular vessel was spun at sufficiently large rotational speed (v). The IFT (gint) value was calculated from the v, the observed radius (r) of cylindrical oil droplet and the density difference (Dr) between two phases according to the following equation [11]:
94 95 96 97 98 99 100 101 102 103 104
1 4
g int ¼ r3 Drv2
Fig. 1. Molecular structures of used surfactants.
(1)
Adsorption equilibrium time
105 106 107
Prior to main adsorption isotherm experiments, an adsorption equilibrium time was determined by measuring adsorbed amount of surfactant as a function of time. An aqueous solution containing 1 wt% surfactant was prepared by dissolving proper amount of surfactant into 200 mL synthetic brine in a 500 mL stoppered flask. Ten grams of kaolinite adsorbent was prewashed by distilled water and dried in the air, and it was immersed into the surfactant solution. The whole stoppered flask was shaken using an orbital incubator shaker at 100 rpm and room temperature (23 1 8C). At a predetermined time, a small amount of supernatant of the solution was taken, filtered, and subject to UV–vis spectrophotometry
108 109 110 111 112 113 114 115 116 117 118
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measurement. The amount of surfactant adsorption, q (in mol/g adsorbent) was calculated by the following relationship: q¼
ðC Ai C Ae ÞW b 100MA W a
(2)
122 121 123 124 125 126
where CAi, CAe, and MA denote the initial concentration and the equilibrium concentration of surfactant (wt%), and the molecular weight of surfactant, respectively. In the equation, Wb and Wa are the weight of synthetic brine and the weight of adsorbent, respectively.
127
Adsorption behaviors
128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145
For adsorption isotherm experiments, several surfactant solutions of different concentrations from 0.001 to 1.0 wt% were prepared and proper amount of adsorbent was added. After the mixtures were shaken at the same conditions as before for the predetermined equilibrium time, the solution concentration was determined by the same procedure as above. Based on the obtained data, adsorption isotherms were plotted and fitted to the Langmuir and the Freundlich adsorption models for further analysis. These two general models are known to describe adsorption behaviors from a liquid solution onto solid adsorbent properly. The Langmuir adsorption isotherm is based on the basic assumption that adsorption takes place at specific homogeneous sites and no adsorption can take place at the site which a solute already occupies. The fundamental feature for the model is that rate of adsorption is proportional to concentration gradient of solute and amount of bare surface [12]. The Langmuir adsorption model can be described by the following equation: q¼
147 146 148 149 150
qm bC Ae bC Ae þ 1
(3)
where CAe, qm, and b are the equilibrium surfactant concentration (mol/L), the maximum amount of solute adsorbed (mol/g adsorbent) and the Langmuir equilibrium constant, respectively. This equation is rearranged into the following linearized form: C Ae C Ae 1 ¼ þ q qm bqm
153 152 151 154 155 156 157 158 159 160 161 162 163
From the plot of CAe/q versus CAe, slope and the intercept correspond to 1/qm and 1/bqm, respectively. Therefore, qm and b are readily calculated from the slope and the intercept [13]. The basic assumption of the Freundlich model is that adsorbent surface is composed of heterogeneous sites with different energy and amount of solute adsorbed per unit mass of adsorbent is a function of solute concentration. This model does not confine the adsorption layer to a monolayer but allows multilayers [14]. The Freundlich model quantitatively states that the adsorption amount is proportional to the equilibrium concentration of the solute as follows: 1=n
q ¼ aCAe 165 164 166 167 168 169
(5)
where a, CAe and n denote the Freundlich equilibrium constant, the surfactant concentration (mol/L) and the Freundlich constant, respectively. The constants a and n imply adsorption capacity and intensity, respectively [13,14]. By taking logarithm, this equation is linearized into the equation: ln q ¼ ln a þ
170 172 171 173 174
(4)
1 ln C Ae n
(6)
When ln q is plotted against ln CAe, the slope and the intercept are 1/n and ln a, respectively and thus a and n are readily determined.
Model parameters of qm, b, a, and 1/n were calculated by applying the two adsorption models to the isotherm data. Adsorption behaviors of four surfactants were then analyzed and discussed by comparing the parameters of one another.
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Results and discussion
179
Critical micelle concentration
180
The measured surface tension profiles of brine solutions of surfactants (1 wt%) are shown in Fig. 2, and the CMC results are shown in Table 2. The CMC values of nonionic surfactants PONP and PS20 are about one third of those for aqueous solution without extra electrolytes (like NaCl) reported in the literature (3 104 mol/L and 9 105 mol/L, respectively) [15–17]. Meanwhile those of anionic surfactants DBS and LS were about one order of magnitude lower than the values for aqueous solution in the literature (1.59 103 mol/L for DBS and about 3 103 mol/L for LS) [18,19]. Such a large CMC decrease with extra electrolytes is well explained by the change of electrical atmosphere in the aqueous surfactant solution. In other words, electrolytes from the dissolved NaCl neutralize the effective head group charge of the surfactant, which results in reducing the electrostatic repulsion between the polar head groups of the surfactant. Consequently, the micelles are formed at lower concentration than in pure water [20]. The area per surfactant molecule (in A˚2) at the air–water interface at the CMC was estimated using the surface tension (g) data at temperature T and the Gibbs adsorption isotherm equation: 1 dg Gs ¼ (7) nRT d ln C A
181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199
where Gs, R and CA denote the surface excess (mol/m2), the gas constant and the surfactant concentration (mol/L), respectively. Here, n is taken as 2 for DBS and LS because those surfactants dissociate into negatively charged surfactants and sodium cation in a solution. As seen in Table 2, PS20 yielded the largest molecular area of 87.3 A˚2 at the CMC, which is somewhat larger than the reported value of 60.3 A˚2 in the literature [21], while DBS showed the smallest one of 52.9 A˚2 about the same as the reported value of 51.8 A˚2 [22].
200 201 202 203 204 205 206 207 208 209
Adsorption equilibrium time
210
As a prerequisite step for adsorption isotherm experiments, an adsorption equilibrium time was determined by monitoring the
211 212
80 DBS PONP PS20 LS
70
γ (mN/m)
119 120
3
60 50 40 30 20 -16
-14
-12
-10
-8
-6
-4
ln CA Fig. 2. Surface tension profiles as functions of logarithmic concentration of surfactant.
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Table 2 Measured CMC, surface tension at CMC, and area per molecule of surfactant.
g at CMC (mN/m)
Area per molecule (A˚2)
DBS PONP PS20 LS
0.229 0.103 0.032 3.10
27.4 33.7 39.7 50.3
52.9 77.2 87.3 85.6
8 DBS PONP PS20 LS
6
-6
CMC (mmol/L)
q (x10 mol/g)
Surfactant
213 214 215 216 217 218 219
amount of adsorption as a function of time. Fig. 3 shows the adsorption equilibrium profiles of the four surfactants as functions of time. The adsorption equilibrium for all surfactants was completely attained after 1200 min (20 h). When the adsorption rate of surfactant was defined as the amount of adsorption per unit time (mol/g min), it was in the order of DBS > PONP > PS20 > LS from beginning to equilibrium.
220
Adsorption behaviors
221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248
Adsorption isotherms were obtained by plotting the amount of adsorption (mol/g adsorbent) versus the equilibrium concentration of surfactant, and the results are shown in Fig. 4. The Langmuir model was applied to the adsorption isotherms and all the data showed quite good fits to the Langmuir adsorption equation (Eq. (3)) with r2 >0.98. The adsorption isotherm data were then fitted to the linearized equation (Eq. (4)) to estimate the maximum amount of surfactant adsorption, qm, and the Langmuir equilibrium constant, b with excellent linear fit of r2 >0.99. The obtained parameters are given in Table 3. The maximum amount of adsorption was in the order of DBS > PONP > PS20 > LS. These maximum adsorption amounts (for example, 9.36 106 mol/g for DBS and 0.66 106 mol/g for LS) are quite consistent with those reported in the literature [23,24]. Kaolinite is a 1:1 alumina silicate comprising a tetrahedral silica sheet bonded to an octahedral alumina sheet through the sharing of oxygen atoms between silicon and aluminum atoms in adjacent sheets [25]. Although both sheets are theoretically electrically neutral, kaolinite surfaces usually take charges in aqueous solutions because of Al3+ substitution for Si4+ in the tetrahedral sheet, and protonation and deprotonation of surface hydroxyl groups [26]. Therefore, kaolinite surfaces as well as the crystal edges have variable zeta-potential depending on solution pH and ionic strength [27,28]. The highest maximum adsorption amount of anionic surfactant DBS was somewhat unexpected because the zeta-potential of kaolinite has been reported to be significantly negative, approximately 10 to 40 mV with high concentration of electrolytes at natural pH (about 5.9) [27–29]. In other words,
4 2 0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
CAe (wt%) Fig. 4. Adsorption isotherm data.
negatively charged DBS molecules were not expected to adsorb easily onto kaolinite surface with the significant amount of negative charge because of electrical repulsion. However, the DBS molecules with less hydrophilic characteristics (the lowest HLB) than other three surfactants are presumed to interact with kaolinite surface through hydrophobic interaction and resulted in such a relatively higher level of monolayer amount. This presumption is supported by a recent AFM study stating that Q2 kaolinite silica face has a modest level of hydrophobicity [30] (Fig. 5). The adsorption of anionic surfactants onto kaolinite can be explained as follows: in the initial stage of adsorption (at the low surfactant concentration), surfactant adsorption is derived mainly by hydrophobic attraction between surfactant’ hydrocarbon chains and kaolinite surface in neutral pH condition. As surfactant concentration increases, additional adsorption occurs through alkyl–alkyl hydrophobic interactions between bulk surfactant molecules and adsorbed surfactant molecules, and surfactant aggregates (called ‘solloids’ or ‘hemimicelles’ [9]) of small size are formed at the kaolinite surface. At the CMC, the adsorption level approaches to a plateau for a moment, and above the CMC surfactant molecules then adsorb to form bilayer and multilayers [9,23,31]. On the other hand, the adsorption of nonionic surfactants onto kaolinite can be described as follows: when the surfactant concentration is low, a weak adsorption of molecule occurs in the form of monomer. The polyoxyethylene part forms a hydrogen bonding with surface hydroxyl groups, and the lateral hydrophobic interactions between hydrophobic groups are not significant. At a higher concentration hemimicelles form, and the molecules tend to lie flat on the surface. Here, cooperative interactions between adsorbates are dominant, and amount of adsorption significantly increases. As the adsorption proceeds further, the adsorbed molecules’ orientation changes and surface aggregates starts to appear due to lateral alkyl–alkyl interactions [9,31,32]. When Eq. (3) is expressed with the surfactant concentration (CAe) in the unit of mol/L, the Langmuir equilibrium constant is Table 3 Estimated parameters of qm and b in the Langmuir adsorption model. Surfactant
Fig. 3. Adsorption equilibrium profiles as functions of time.
DBS PONP PS20 LS
Langmuir adsorption model parameter qm (106 mol/g)
b (103 L/mol)
DGo (kJ/mol)
9.36 4.09 2.81 0.66
0.412 1.04 1.47 5.46
14.7 16.9 17.8 21.0
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Fig. 5. Application of Langmuir model to adsorption isotherm data by (a) Eq. (3) and (b) Eq. (4).
286 287
related to the standard free energy of adsorption, DGo by the following equation:
DGo ¼ RTln K 289 288 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309
(8)
where R and T denote the gas constant and the adsorption temperature, respectively [33]. As given in Table 3, the D Go values for surfactants were in the order of LS > PS20 > PONP > DBS. Since a larger negative free energy change implies a more spontaneous adsorption, LS is thought to show the largest spontaneous adsorption while DBS the smallest. This highly spontaneous adsorption of LS can be explained by high hydrophobic interaction between bulky hydrocarbon chains of LS and hydrophobic kaolinite surface [34]. The Freundlich isotherm model, which is known to be widely used in heterogeneous systems [13], was applied to the isotherm data on electrically heterogeneous kaolinite [35]. As shown in Fig. 6, the model fits by Eqs. (5) and (6) were partially good, i.e. good (r2 = 0.96) at lower concentration but poor (r2 < 0.89) at higher concentration. Therefore, the Freundlich model was applied up to 0.25 wt% concentration and yielded excellent fits overall with r2 >0.99 as shown in Fig. 7. At a fixed temperature, the Freundlich constants, a and 1/n are related to adsorption capacity and adsorption intensity. The constant 1/n is related to the heat of adsorption, DHa by the equation: 1 RTln u ¼ n DH a
(9)
5
Fig. 6. Application of Freundlich model to adsorption isotherm data by (a) Eq. (5) and (b) Eq. (6).
where u is the surface coverage. Thus, at a constant surface coverage the smaller 1/n value indicates the larger heat of adsorption, i.e., higher adsorption intensity. As listed in Table 4, PONP showed the lowest 1/n value and all other three surfactants yielded somewhat larger values than PONP. These results are not consistent with the Langmuir adsorption model results stating that DBS had the largest maximum adsorption amount. This discrepancy is attributed to the assumption that the Freundlich model is not matched to our suggested adsorption mechanism as previously addressed. According to our suggestion, at low surfactant concentration the adsorption is initiated by hydrophobic interactions for anionic surfactant or hydrogen bonding for nonionic surfactant. Then at higher concentration surfactant molecules form aggregate, and finally multilayers at very high concentration. Apparently, the Freundlich model well explains the initial adsorption process on the heterogeneous kaolinite surface but fails to describe the later stages.
Table 4 Estimated parameters of a and 1/n in Freundlich adsorption model (up to CAe = 0.25 wt%). Surfactant
DBS PONP PS20 LS
Freundlich adsorption model parameter 1/n
a (104)
0.607 0.422 0.649 0.655
1.58 0.37 1.35 0.89
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Table 5 Measured interfacial tension, maximum adsorption amount (mg/g adsorbent), and calculated financial loss due to adsorption. Surfactant
DBS PONP PS20 LS a
Interfacial tension, gint (mN/m) model oil phase n-Octane
Cyclohexane
Toluene
2.79 11.8 23.5 49.9
4.53 12.3 16.2 25.8
2.84 8.60 12.9 17.2
Maximum adsorption amount (mg/g adsorbent)
Current price per unit weight ($/kg)a
Financial loss due to adsorption (million $)
3.26 3.06 3.45 5.28
47 210 38 270
3.3 13.7 2.8 30.3
Approximated price quoted from Aldrich in March, 2014.
328
Evaluation of surfactants for chemical flooding
329 330 331 332 333 334 335 336 337
Proper selection of surfactant is one of the key factors for a successful surfactant flooding, lowering oil recovery cost and enhancing its efficiency. Surfactants play a critical role of reducing the interfacial tension (IFT) between oil trapped in capillary pores and water surrounding the pores, thus allowing the oil to be mobilized [36,37]. In a typical surfactant flooding process, the ratio of viscous forces to interfacial tension forces is a critical parameter for oil recovery, and it is related to the capillary number, NVC as follows: mw (10) N VC ¼ V
g int
339 338 340 341
where V, mw and gint are the Darcy velocity, the viscosity of displacing fluid, and the IFT between displaced and displacing fluid, respectively [1]. For the mobilization of unconnected oil
Fig. 7. Application of Freundlich model to adsorption isotherm data up to CAe = 0.25 wt% by (a) Eq. (5) and (b) Eq. (6).
droplets, the capillary number needs to be greater than 105 [1]. With a constant viscosity of displacing fluid, the interfacial tension needs to be reduced to maintain the capillary number larger than 105. The measured IFT values between the brine solutions of 1 wt% surfactants and the model oil phases are shown in Table 5. For all the model oil phases, the IFT value was in the order of LS > PS20 > PONP > DBS, and importantly DBS yielded much lower IFT than the other surfactants. For LS, the concentration of 1 wt% is less than a half of the CMC so that it was not enough to lower the interface tension. For an economical success of surfactant flooding, one needs to minimize the depletion of injected surfactant due to adsorption onto clays in the reservoir [36]. We calculated the maximum adsorption amounts obtained by the applying the Langmuir model in the unit of mg/g adsorbent, which were in the order of LS > PS20 > DBS > PONP as shown in Table 5. We roughly estimated financial loss due to the surfactant adsorption according to the following method: if a reservoir to be swept with a surfactant solution has the size of one acre (4047 m3) by 3 m deep, and the rock substrate is about 70% solid material having a density of 2.5 g/cm3, approximately 2.12 1010 g of adsorbent are then totally available for surfactant adsorption [37]. When DBS is used, a financial loss due to adsorption is evaluated to be about $3,300,000 by multiplying the total adsorbent weight by the maximum adsorption amount per unit adsorbent weight (3.26 g/kg) and the current price of DBS ($47 per kg). By combining the financial loss and the interfacial tension reduction results, DBS is identified as the most recommendable surfactant for the surfactant flooding among the tested. According to our surfactant flooding simulation, the oil recovery (in percentage of recovered oil to remaining oil) with DBS at optimum conditions was more than 38%, which is higher than that with alkoxy glycidyl sulfonate, one of the most widely used surfactants [38].
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Conclusions
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This work provided fundamental information on interfacial tension properties and adsorption behaviors of several surfactant candidates for surfactant flooding based on interfacial tension and adsorption isotherm data. The experimental results and consequent analysis revealed some important features of adsorption behaviors and interfacial tension properties of the four surfactants and the following conclusions are drawn:
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1. Adsorption isotherm data of four surfactants were successfully analyzed by the Langmuir and the Freundlich models, and they were better fitted by the former rather than the latter. 2. The maximum adsorption amount of a surfactant on kaolinite surface was in the order of LS > PS20 > DBS > PONP when it was estimated in the unit of mg/g adsorbent, while it was DBS > PONP > PS20 > LS in mol/g adsorbent. 3. The interfacial tension between brine solution of 1 wt% surfactant and model oil phases was in the order of DBS > PONOP > PS20 > LS.
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4. By considering the financial loss caused by surfactant adsorption and the interfacial tension reduction, DBS was found to be the most appropriate candidate for the surfactant flooding among the tested.
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Acknowledgement
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This work was supported by the Dongguk University Research Fund of 2014.
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