Characterizations of surfactant synthesized from palm oil and its application in enhanced oil recovery

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Characterizations of surfactant synthesized from palm oil and its application in enhanced oil recovery Neha Saxena a, Nilanjan Pal a, Swapan Dey b, Ajay Mandal a,∗ a b

Enhanced Oil Recovery Laboratory, Department of Petroleum Engineering, Indian Institute of Technology (ISM), Dhanbad 826 004, India Department of Applied Chemistry, Indian Institute of Technology (ISM), Dhanbad 826 004, India

a r t i c l e

i n f o

Article history: Received 19 January 2017 Revised 12 September 2017 Accepted 12 September 2017 Available online xxx Keywords: Palm oil Surfactant synthesis Surface tension Interfacial tension Contact angle Enhanced oil recovery

a b s t r a c t The present work deals with synthesis of an anionic biodegradable surfactant for application in chemical enhanced oil recovery (EOR) process. Alpha sulfonated ethyl ester (α -SEE) was synthesized from palm oil via trans-esterification process. The surfactant was characterized by FTIR, GC, TGA, FE-SEM and EDX analyses. Critical micelle concentration (CMC) was determined by surface tension measurement at air– aqueous interface. The efficiency of α -SEE was studied by calculating the interfacial tension (IFT) between crude oil and surfactant solution, and by investigating the ability of the surfactant to alter the wettability nature of carbonate and quartz surfaces. Salt effect was studied at CMC of surfactant to obtain an ultra-low IFT value of the order of 10−3 mN/m at optimal salinity. Addition of organic alkali also showed synergistic effect on IFT between crude oil and surfactant solution. The surfactant favorably altered the wettability of oil-wet carbonate and quartz surfaces to water-wet, which is desirable for oil recovery. The surfactant showed potential application in EOR owing to its enhanced interfacial properties and rockwetting characteristics. Flooding experiments were conducted with surfactant slugs at different α -SEE concentrations to achieve about 25%–27% additional oil recoveries after conventional flooding. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction The global energy demand is highly dependent on oil and gas extraction in the face of current energy needs. Since 2013, it became impertinent for petroleum industry to demand and develop better techniques for improving oil production. In the present scenario of low crude oil price, there are a few limitations of enhanced oil recovery techniques as a consequence of high project cost and longer lead time periods. Enhanced oil recovery is a strained and costly affair in less accessible areas, thereby paving the necessity to extract oil by employing more economical and effective methods. Traditionally oil recovery techniques follow primary, secondary and tertiary (EOR) processes [1,2]. Among these recovery processes, chemical flooding with polymers and surfactants is gaining importance because of its high potential to extract oil that is otherwise not recoverable by primary and secondary recovery [3]. However, as the most of the oil in existing reservoirs are now matured with lower production rates, it is the right time to develop cost effective EOR techniques which can be applied efficiently in the oil field with existing facilities [4]. Surfactants are

∗

Corresponding author. E-mail address: [email protected] (A. Mandal).

chemicals that are known for their use in detergents, cosmetics and textile sectors [5,6]. Surfactants reduce the interfacial tension between the injected water and the crude oil trapped in the reservoir to an ultra-low value. Recently, much attention is given on use of biodegradable and environmental-friendly surfactants in oil recovery application [7,8]. Several efforts are being made to develop low cost chemicals from natural resources for successful application in chemical enhanced oil recovery [9]. Bio-enzyme catalyzed esterification is employed to synthesize fatty acid esters from vegetable oils like castor oil, Jatropha oil [10,11]. The efficacy of the chemicals in oil recovery processes is dependent on capillary pressure, permeability and wettability of rocks and interfacial tension. Though substantial amounts of oil are extracted by the injection of surfactant solutions in chemical enhanced oil recovery, some oil still remain in the rock pores [12]. This is because of high interfacial tension between the oil water interface which traps the oil droplets by capillary forces. Surfactants help in lowering the interfacial tension between the oil and water phases, and alter the wettability of the oilwet reservoir rock to water-wet [13,14]. Surfactant selection plays a pivotal role in achieving cost-effectiveness and favorable interfacial properties in EOR processes. Polyethylene glycol based esters are generally produced by chemically catalyzed esterification processes. However the yield of product is not satisfactory in such

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Please cite this article as: N. Saxena et al., Characterizations of surfactant synthesized from palm oil and its application in enhanced oil recovery, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.014

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Table 1 Properties and composition of Palm oil. Palm oil properties

Fatty acid constituents of palm oil

Properties @ 27 °C

Value

Fatty acid

Constituents (%)

Acid value (mg KOH/g) Density (g/cc) Viscosity (cap) Saponification value (mg KOH/g)

11.34 0.912 42.08 202.7

Oleic Linoleic Palmitic Stearic Myristic Free fatty acid

37.0 10.0 42.0 6.6 1.7 2.7

reactions and also proceeds toward undesirable side products [15,16]. In recent decades, esterification reactions carried out in the presence of biocatalysts viz. enzymes which include lipases, hydrolases, etc. attracted the attention of scientists and researchers for synthesizing fatty acid esters of selective fatty acid and polyol systems. The catalytic action of lipases to catalyze fatty acids was investigated by Hayes in different studies [17–20]. According to Hayes (1992) the hydroxyl group present on the fatty acid chain is subject to the catalytic action of lipases, and the length of acid chain has a minor effect on the reaction mechanism [17]. Ziemann and Wagner [21] synthesized wax esters of trihydroxy palmitic acid by Rhizomucor miehei and obtained moderate yields. Lang et al. [22] studied the laboratory synthesis of wax esters of 17-hydroxy and 3-hydroxy stearic acids using lipase as catalyst and found that the reaction rates were higher using Mucor miehei as the biocatalyst. Steffen and co-workers [23] also applied the same method to synthesize monoglycerides of 17-hydroxy and 12-hydroxy stearic acids to achieve higher yields with R. miehei. The objective of this study is to synthesize an anionic surfactant from palm oil which can be used in the field of enhanced oil recovery. The chemical and morphological structure of α -SEE was confirmed by FTIR, GC, TGA and FE-SEM and EDX. Surface tension studies were conducted in order to determine the CMC value of the synthesized surfactant. The interfacial tension at surfactant solution–crude oil interface and contact angle variation of surfactant solution on quartz and carbonate rock with time were also measured to study the efficacy of the α -SEE surfactant. The α -SEE surfactant is expected to be green and biodegradable nature. The enhanced oil recovery by injection of small pore volume of surfactant slug after conventional water flooding were also investigated by performing flooding experiments.

Lipase was added subsequently to the reaction mixture 10.0% (w/w) of the starting material [6]. The reaction mixture was constantly stirred for 48 h and, in between, the progress of the reaction was monitored by checking the TLC (Thin Layer Chromatography) in a mixture of petroleum ether-ethyl acetate (70:30) as mobile phase. Excess lipase was filtered off. Fatty acid residue was removed by washing with a mixture of diethyl ether and a small amount of sodium bicarbonate solution. The lipase-catalyzed esterification of palm oil in the presence of polyethylene glycol (PEG 600) aids in conversion of free fatty acids present in oil into ester. The remaining solution was evaporated under vacuum and a light yellow semi-viscous liquid (compound 1) was obtained. Sulfonation reaction: The product (compound 1) obtained in esterification reaction was used to synthesize the desired product (α -SEE) by sulfonation of palm oil ethyl ester. In this reaction, chlorosulfonic acid (2.89 g) was added to pyridine (10.0 mL) in a round bottom flask. The reaction was carried out in ice bath under constant stirring for 30 min. Then, the reaction mixture was heated to 60 °C till a clear solution was obtained. The solution was then quenched with a saturated solution of sodium carbonate and sodium bicarbonate in a separating funnel. Normal butanol (30 mL) was poured into the separating funnel to dissolve the components which remained unreacted in the organic layer to obtain the desired product in aqueous layer and heated to temperature of 120 °C to remove water and pyridine from desired sulfonated product. Organic impurities from the final sulfonated product were removed by washing it with petroleum ether twice. The final white solid product (compound 2) was obtained by vacuum drying the product at 60 °C for 24 h which was characterized with FTIR and GC. The palm oil used in the synthesis chiefly contains oleic acid, palmitic acid along with small amount of linoleic acid, myristic and stearic acid, hence, the resultant surfactant formed in the process is a mixture of the sulfonated ester. 2.3. Reaction scheme of α -SEE

2. Materials and experimental methods

Fig. 1 shows the reaction mechanism for the synthesis of α -SEE. The fatty acid was first esterified in presence of lipase by cleavage of ethyl group from PEG which gets attached to the oxygen atom of fatty acid to form ethyl ester. In sulfonation step, the pyridine was added as it abstracts the α -hydrogen of the ester group, being the most reactive site for the reaction to take place efficiently as compared to other available reactive sites like unsaturation present in fatty acid. The sulfonate group is attached to α -position of ester group.

2.1. Materials required

2.4. Characterization of α -SEE by FTIR, GC, TGA, FESEM analyses

Palm oil used in the synthesis of the sulfonated ester was procured from the local market. Its properties and composition are shown in Table 1. The enzyme lipase from R. miehei was obtained from Sigma Aldrich. Poly ethylene glycol 600 was obtained from Loba chemicals. Chlorosulfonic acid, sodium carbonate anhydrous, sodium bicarbonate, diethyl ether, petroleum ether, ethyl acetate and pyridine were procured from Merck (India). The crude oil used in the experiments was procured from Oil India Limited, Assam. The obtained crude oil has a total acid number 0.84 mg KOH/g, gravity of 18.9° API, density of 0.9244 g/cc and viscosity of 4.26 cp at 27 °C.

The FTIR analysis was done by Perkin Elmer-Spectrum 2 spectrometer to identify the functional groups present in the newly synthesized sulfonated ester surfactant from palm oil. A pellet of synthesized surfactant in combination with KBr was used in FTIR analysis. The surface structure was analyzed by FE-SEM analysis using SUPRA-55 ZEISS Germany. The synthesized surfactant was kept in desiccator for 24 h for removal of moisture and was coated with gold to obtain better accuracy. Gas Chromatography (Jeol GCMate II) was used to determine the constituents of surfactant synthesized from palm oil. The TGA analysis of α -SEE was done to determine its thermal degradability by thermo gravimeter analyzer (Netzsch-STA 449 Jupiter).

2.2. Synthesis method Esterification reaction: Palm oil and poly ethylene glycol (PEG) 600 were weighted in stoichiometric amounts in a round bottom flask, mixed with constant stirring and the reaction mixture was heated at a temperature of around 60 °C for about 30 min.

2.5. Interfacial and flooding properties of α -SEE surfactant 2.5.1. Surface tension measurements The surface tension and critical micelle concentration (CMC) of synthesized surfactant was calculated by using Du Nouy ring

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Fig. 1. Reaction scheme for the synthesis of α -SEE.

method (KRUSS Easy Dyne, Germany) at 27 °C. Surface tension was determined for aqueous solutions with different surfactant concentrations. Surface tension values for individual samples were measured three times to ensure the accuracy of results. The measured value of surface tension of distilled water was 72.3 mN/m at 27 °C. After each test the platinum ring used in the experiment was cleaned by acetone and was dried with flame for the next set of experiments. 2.5.2. Salt tolerance studies The concentration of salt in surfactant solution at which surfactant starts precipitating is known as salt tolerance of the surfactant. Salt tolerance of α -SEE surfactant solution at CMC value was measured at 27 °C. Aqueous solutions were prepared by adding salt in the range of 1% to 12% NaCl to surfactant solution at CMC. Initially, 1 wt.% NaCl was added to the solution. The mixture was

stirred at 20 0 0 rpm for 15 min. Similar procedure was repeated for successive salt additions in surfactant solution until surfactant precipitation occurs. 2.5.3. Interfacial tension measurements Oil–aqueous interfacial tension values between the surfactant solutions of different concentrations and crude oil was calculated by spinning drop tensiometer (SVT 20, Dataphysics) at 27 °C. The surfactant solutions at various concentrations ranging from 20 0 0 ppm to 12,0 0 0 ppm were used. The α -SEE solution was injected into a capillary tube through a syringe. Progressively, a drop of crude oil was injected to the tube through rubber septum. The IFT between the two liquids of different densities was determined using SVT 20 software. The experiment was conducted at room temperature at a speed of 30 0 0 rpm for individual sample. The equilibrium interface tension for surfactant solution is obtained by

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Fig. 2. Schematic representation of chemical flooding experimental setup α -SEE solution in sandpack system.

Eq. (1)

σ = ω R ρ / 4 2 3

(1)

where ω represents the angular velocity of the system, R represents the radius of crude oil droplet, σ is the interfacial tension between two fluids under consideration and ࢞ρ is the density difference between surfactant solution and crude oil. The interfacial tension between crude oil and distilled water interfaces was found to be 18.0 mN/m at 27 °C. 2.5.4. Contact angle determination To carry out a study on alteration of wettability of reservoir rock from oil-wet rock to water-wet rock surface, the contact angle between the newly synthesized surfactant solution and oil-wet rock surface had been studied using Drop shape Analyzer (KRUSS DSA 25, Germany) at 27 °C. Precisely controlled tempering and humidity chambers were used to provide a realistic model of the process conditions to avoid vibration, air effect and other mechanical effects. To calculate the contact angle, 5–10 μL of surfactant solution was taken in the syringe and carefully dropped onto the surface of oil wet rock (carbonate and quartz). The test was repeated three times to maintain the consistency in the results for individual samples. Initially, the rock was suitably prepared for wettability study by washing it with deionized water, and was then put into the crude oil for ageing in oven at 65 °C for 15 days. After being aged for 15 days the oil-wet rock was then vacuum dried overnight.

at 240 psig to irreducible water saturation. The initial water saturation was determined on the basis of mass balance. The crude oil used in the flooding experiments was collected from Oil India Limited, Assam (India). The oil has a total acid number of 0.840 mg KOH/g, gravity of 18.90°API and viscosity of 4.6 cP at 27 °C. Darcy’s law was used to calculate the effective permeability to oil (ko ) and effective permeability to water (kw ) at irreducible water saturation (Swi ) and residual oil saturation (Sor ), respectively. For a horizontal linear system, flow rate is related with permeability as shown in Eq. (2)

q=

kA dp μ dx

(2)

where, q is the volumetric flow rate (cm3 /s), A is the total crosssectional area of the sandpack system (cm2 ), μ is the fluid viscosity (cp), ddxp is the pressure gradient (atm/cm) and k is the permeability in Darcy. Water-flooding was initially conducted by subjecting the coreholder placed horizontally to a constant injection pressure at 35 psig. After water flooding, when water-cut reached above 95%, around 0.5 pore volume (PV) of surfactant slug was injected. This was followed by chase water flooding (2.0 wt.% brine solution). The flooding tests were repeated for all surfactant systems with varying concentrations. 3. Results and discussion 3.1. Characterization of synthesized anionic surfactant

2.5.5. Flooding experiments Sandpack flood tests were employed for the evaluation of the effectiveness of the synthesized surfactant (α -SEE) to enhance the oil recovery after conventional water flooding. It consists of four components viz. a core holder, a displacement pump (Teledyne Isco), chemical solution cylinder for crude oil, brine and surfactant solution and a fraction collector. A schematic of the experimental setup is depicted in the Fig. 2. As per the homogeneous sand packing model, the geometry was chosen as L = 45 cm and r = 3.5 cm. The core holder was tightly packed with uniform sands (60 mesh) and saturated with 2.0 wt.% brine. It was flooded with the brine at 35 psig and the absolute permeability was calculated from the flow rate and pressure drop through sand pack using the Darcy’s law equation. The sand pack was then flooded with the crude oil

3.1.1. FTIR study of palm oil ethyl ester The FTIR spectrum of synthesized palm oil ethyl ester derived from palm oil is shown in Fig. 3. The FTIR spectrum of palm oil ethyl ester shows an absorption band at 3471 cm−1 , arising due to the stretching vibration of –OH bond. Peak at 3008 cm−1 corresponds to cisolefinic=C–H double bond. The symmetrical and asymmetrical stretching vibrations of methylene group (–CH2 ) is indicated by peaks at 2854 cm−1 and 2924 cm−1 . A strong peak at 1746 cm−1 is attributed to C=O bond from ester as the main functional group. The presence of a peak at 1464 cm−1 shows the bending vibration of aliphatic groups –CH2 and –CH3 . The asymmetrical axial stretching of the C–O ester group is identified by the absorption peaks at 1237 cm−1 , 1163 cm−1 and 1116 cm−1 . The

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Fig. 3. FTIR spectrum of Palm oil ethyl ester.

Fig. 4. FTIR spectrum of α -SEE.

peak near 722 cm−1 is due to the out-of-plane vibration of cisdi-substituted olefins. FTIR spectrum of the synthesized surfactant proved that palm oil is an ethyl ester of saturated and unsaturated fatty acids. 3.1.2. FTIR study of α -SEE Fig. 4 shows the FTIR spectrum of synthesized surfactant derived from the palm oil. The spectra which appear in palm oil ethyl ester are also present in synthesized surfactant. The absorbance for different functional groups and bonds are shown in Fig. 4. Additional peaks at 1116 cm−1 and at 850 cm−1 , 617 cm−1 indicate the stretching vibration of sulfonate groups (S=O) and (S–O) respectively. Thus, the FTIR data confirms that the sulfonation of palm oil ethyl ester produced the corresponding ethyl ester sulfonate. The various peaks obtained in the FTIR spectra are discussed in Table 2.

3.1.3. GC analysis of α -SEE Fig. 5 shows the gas chromatogram of the surfactant synthesized from palm oil. The purity of the final product is obtained at ≥76%. The surfactant synthesized from palm oil is a mixture of various fatty acid ethyl esters, which is confirmed by GC analysis. A strong peak at retention time (RT) 17.22 with 43.09% peak area shows that palmitic acid ethyl ester (C16:0 ) is constituted in the synthesized product. The presence of oleic acid ethyl ester (C18:1 ) is attributed to a peak at RT 19.43 and peak area percentage of 21.65%. A peak at RT 19.65 with 11.80% area in the GC chromatogram corresponds to stearic acid ethyl ester (C18:0 ). 3.1.4. FESEM analysis of α -SEE FESEM analysis of α -SEE was performed to study the surface morphology. Fig. 6(a) at 10 KX zoom shows that the surfactant molecules appear to be in cluster form and are compact in nature.

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N. Saxena et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–13 Table 2 Characteristic adsorption bands corresponding to different functional groups in palm-oil based surfactant. Characteristic adsorption bands (cm−1 )

Description of functional group

310 0–370 0 280 0–30 0 0 1735–1750 1620–1680 1350–1480 1120–1160 850 & 617 10 0 0–130 0 1350–1480

Stretching vibration of O–H bonds Stretching vibration of alkyl C–H bonds (asymmetric CH3 and CH2 stretching and symmetric CH2 ) Stretching vibration of ester groups C = O bonds Stretching vibration of C = C bonds of alkene Bending vibration of alkyl C–H bonds Stretching vibration of the O = S = O bonds of sulfonate Stretching vibration of S-O bonds Stretching of C–O of ester group CH3 asymmetric bending

Fig. 5. Gas chromatogram showing the retention time for different components of the α -SEE.

Fig. 6. FESEM image of α -SEE at different magnification: (a) 10 KX (b) 100 KX.

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Fig. 7. EDX analysis of the α -SEE.

Image shown in Fig. 6(b) at 100 KX zoom illustrates that particles are rod-like and irregular in shape, and are well dispersed within the area under analysis. 3.1.4. EDX spectrum of α -SEE In EDX analysis of synthesized surfactant, five elements are observed viz. carbon (C), oxygen (O), sodium (Na), sulfur (S) and gold (Au) as shown in Fig. 7. The gold (Au) peak characterized in the spectra was due to the presence of coating materials used along with the surfactant during the analysis. All the remaining elements present correspond to different components of the α -SEE. EDX data clearly shows the presence of sulfur as chlorosulfonic acid was used in the sulfonation reaction. The presence of sodium shows that the anionic surfactant exists in the form of sodium salt. No extra peak of any other element is depicted in the EDX spectra, which indicates that the α -SEE surfactant is pure, without any other elemental impurities. 3.1.5. Thermal gravimetric analysis of α -SEE The thermal stability of α -SEE was studied by thermogravimetric analysis (TGA). The conventional reservoir temperature lies in the range between 80 °C 120 °C [24]. Hence, it becomes important to investigate the thermal degradation characteristics of the surfactant for effective application in EOR processes. The weight loss percentage is depicted with increase in temperature in Fig. 8. TGA plot shows that the initial thermal loss occurs from 96 °C to 150 °C, wherein about 20% of loss in weight is observed. No further loss in weight was observed above this temperature, which shows that the α -SEE surfactant is thermally stable under reservoir conditions which are pre-requisite for its application in field of enhanced oil recovery. The presence of both saturated and unsaturated fatty acids in palm oil supports the good thermal stability of α -SEE as these fatty acids require high temperature for its degradation. The anti-oxidants present in palm oil aid in good thermal stability.

Fig. 8. Thermal stability curve for α -SEE surfactant.

3.2. Interfacial properties of α -SEE surfactant

Fig. 9. Variation of surface tension with concentration of synthesized surfactant at 27 °C.

3.2.1. Surface tension of α -SEE solutions The surface tension versus concentration curve of α -SEE solutions with concentration ranging from 0 ppm to 16,0 0 0 ppm is depicted in Fig. 9. The CMC of this surfactant was obtained as the interception between the decreasing slope and horizontal constant line of the surface tension curve plotted against concentration.

With the increase in concentration of synthesized surfactant solution the surface tension values lowers as there is increase in adsorption of surfactant molecules at the interface of air and water [25] and this trend was observed until the concentration reached CMC value of 80 0 0 ppm with surface tension of 32.5 mN/m. After

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Fig. 11. Interfacial tension versus concentration of α -SEE at 27 °C. Fig. 10. Effect of time elapse on interfacial tension of α -SEE at different concentrations at 27 °C.

the CMC, the surface tension curve attains a constant value as the surface gets saturated with the surfactant molecules. 3.2.2. Salt tolerance analysis of α -SEE solutions Generally salt tolerance of surfactants vary between 5 × 104 mg/L (5 wt.%) to 8 × 104 mg/L (8 wt.%) [26,27]. The salt tolerance level of the surfactant solution is found to fall in this range. Salt tolerance studies revealed that no surfactant precipitation occurred in the range between 1% NaCl and 7% NaCl for α -SEE surfactant solutions at CMC (80 0 0 ppm). As the salt content was increased to 8% NaCl, surfactant begins to precipitate in solution. The surfactant is resistant to salt effects up to 7% NaCl (7 × 104 mg/L) concentration. Therefore, the synthesized surfactant shows good tolerance to reservoir salinity and can be effectively used in both low salinity and high salinity reservoirs for oil recovery applications. 3.2.3. Interfacial tension (IFT) of crude oil and α -SEE Studies on dynamic interfacial tension (DIFT) between crude oil and α -SEE surfactant solutions at 27 °C are depicted in Fig. 10. The interfacial tension initially decreases to a minimum value, due to the existence of dynamic equilibrium between adsorption and desorption of α -SEE molecules at oil–water interface. As aqueous phase interacts with oil phase, surfactant molecules show diffusion phenomenon from the bulk to the interface, and at interface of oil and water phase adsorption occurs. At first, the adsorption rate of molecules was higher than desorption rate which lead to reduction in DIFT. Finally, when an equilibrium state was achieved and adsorption and desorption forces were balanced, the DIFT approached toward a constant value [28]. Fig. 11 shows the variation of equilibrium IFT with surfactant concentration at 27 °C. With increase in α -SEE (alpha-sulfonated ethyl ester) concentration in aqueous solution, more surfactant molecules are adsorbed at the interface and IFT is reduced [29]. When surfactant concentration reaches critical micelle concentration (CMC) value, the oil–aqueous interface is saturated with surfactant molecules and micelles begin to form in the aqueous bulk phase. At 80 0 0 ppm (CMC), the value of IFT was found to be 1.17 × 10−2 mN/m. As the surfactant concentration is further increased, the micelle formation takes place resulting in slight increase in IFT. This is attributed to change in the distribution of surfactant molecules in the oil and water phase, caused as a result of increase in surfactant concentration [30]. 3.2.4. Effect of salt concentration on IFT Addition of salt plays a synergistic effect on the interfacial activity of surfactant molecules at oil–aqueous interface. The effect

Fig. 12. Effect of salinity on IFT at CMC (80 0 0 ppm) of α -SEE at 27 °C.

of salt concentration on IFT value of surfactant solutions at CMC is depicted in Fig. 12. The IFT shows a bell shaped curve with an optimal salinity of 2% NaCl content in surfactant solution at CMC. Initially, it decreases to a minimum value of 8.30 × 10−3 mN/m at 2% salt content followed by an increase in the IFT value. Initially for palm oil-based anionic surfactant, the oil–aqueous interface is negatively charged. On addition of NaCl salt, some Na+ ions are attracted toward the adsorption layer as counterions, whereas other cations form the electrical double layer [31]. With further increase in salt concentration, more counterions are attracted toward the interface, reducing the effective charge in the interface layer [32,33]. This reduces the electrostatic repulsive forces between anions carried by polar head group of α -SEE molecule, leading to thinning of the interface layer. As a result, surfactant molecules are more closely packed at the oil–aqueous interface and IFT is drastically reduced [30,34]. After salt concentration in solution exceeds optimal salinity value, the ionic interactions between the hydrophilic group (negatively charged group of surfactant) and the cationic part of salt (Na+ ) are subsequently decreased in surfactant solution [35]. 3.2.5. Effect of pH on IFT Alkalis play a synergistic effect on the oil–aqueous interfacial tension. Addition of alkali improves pH of the solution and makes the solution more alkaline in nature. On alkali addition, the reactivity with acidic component of crude oil is increased due to the formation of in-situ surfactants by the deprotonation of acids present in crude oil [36–38]. Fig. 13 shows the influence of pH due to addition of mono-ethanolamine (MEA) on IFT between crude oil and surfactant solution at CMC. IFT was found to decrease with increase in pH value. Initially, this decrease was sharp at

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Fig. 13. Effect of pH on IFT at CMC (80 0 0 ppm) of α -SEE at 27 °C. Fig. 14. FTIR spectrum of dry quartz rock and crude oil aged quartz rock.

low pH values. IFT decrease by alkali addition is likely a consequence of the synergism among surfactant molecules, ionized acid and unionized acid to form a mixed adsorbed interfacial layer [28,39]. Consequently, adsorption of surfactant molecules at oil– aqueous interface is increased, resulting decrease of IFT. However, IFT trend showed gradual and slower reduction with increase in organic alkali concentrations (pH value). Minimum IFT value of 6.81 × 10−3 mN/m was achieved at solutions with 12 pH. Reduction in IFT with pH may be attributed to the increased ionic strength, which prevents the surfactant from adsorbing onto the surface of rocks. Furthermore, MEA causes lesser reservoir damage due to its organic nature and low alkali consumption in the reservoir [40,41]. 3.3. Wettability alteration studies 3.3.1. Characterization of quartz and carbonate rock surfaces A water-wet rock surface possesses greater oil recovery that can be achieved by water flooding. When the calculated amount of surfactant solution is injected to the oil-wet or intermediate wet rock the trapped oil is recovered efficiently due to the change in wettability of rock from oil-wet to water-wet. This change in wettability is governed by three factors: the electrostatic force between the ions, the forces of attraction between surfactant and polar components of crude oil, and surface characteristics of the rock. The FTIR analysis of quartz rock, dry and aged with crude oil is depicted in Fig. 14. The spectrum of dry quartz show bands at 458 cm−1 and 689 cm−1 corresponding to asymmetric and symmetric bending vibrations of Si–O bonds whereas peaks at 782 cm−1 and 1081 cm−1 , corresponds to stretching vibration for Si–O bond. The Si–O bond between the quartz rock surfaces clarifies the presence of silica as its main component, whereas the aged quartz rock shows bending vibrations of C–H of substituted benzene at 1076 cm−1 and C=C stretching vibration at 1630 cm−1 respectively. The absorption band at 2850 cm−1 and 2922 cm−1 depicts C–H symmetric vibration of the saturated hydrocarbons and O–H band due to stretching vibration at 3435 cm−1 . C=O stretching peaks appeared between 1627 cm−1 to 1870 cm−1 which confirms the presence of carbonyl components like acids, aliphatic esters and aldehydes in the crude oil. Many new peaks are observed which corresponds to the components of crude oil, thereby confirming the ageing of quartz with crude oil. Characterization of the functional groups present in, dry and oil-wet carbonate rock were done using the FTIR technique, and the results are shown in Fig. 15. The FTIR spectrum of dry carbonate rock surface shows absorption peaks at 868 cm−1 and

Fig. 15. FTIR spectrum of dry and aged carbonate rock with crude oil.

1794 cm−1 corresponding to stretching vibration for Ca–CO3 bonds for calcite. Carbonate rock aged in the crude oil show absorption bands at about 3436 cm−1 , depicting bending vibration of O–H bonds. A new peak at 2947 cm−1 corresponds to alkyl C–H bond, thereby confirming that the rock was aged with crude oil. 3.3.2. Effect of surfactant concentration on contact angle The contact angle values of surfactant solution at different concentrations with time were measured to study the wettability alteration of oil-wet rock surfaces. The sessile drop method was used to calculate the contact angle of oil wetted rock surfaces at different surfactant concentrations. Initially the test was conducted with distilled water and the contact angle calculated were found to be greater than 90° at the start, indicating that the rock surface under study was oil-wet. In the presence of surfactant, the contact angle gradually decreased with elapse of time and reached a constant value after 10 0 0 s, showing that the rock surface had changed its wettability to water-wet. This decrease in contact angle value is dependent on the type of rock surface, type of surfactant used and the acid components of crude oil adsorbed onto the surface of rock used [42]. The plot of contact angle versus time for carbonate and quartz rock surfaces at different concentrations is shown in Figs. 16 and 17 respectively.

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Fig. 16. The contact angle of oil wet carbonate surface through surfactant dosing at different concentration at 27 °C. Fig. 18. Effect of salinity on contact angle at CMC (80 0 0 ppm) of α -SEE solution at 27 °C on Quartz surface and Carbonate rock surface.

quently, α -SEE surfactant synthesized from palm oil proves to be highly efficient in altering the wettability of rock from oil-wet to water-wet,which ultimately aids in enhancing the oil recovery.

Fig. 17. The contact angle of oil wet quartz surface through surfactant dosing at different concentrations at 27 °C.

Surfactant concentration plays a pivotal role in controlling the dynamic wetting characteristics of both quartz and carbonate rocks. The interaction between SO4 2− ions in the palm oil-based surfactant and organic compounds on the rock surface displaces the adsorbed fatty acids (in crude oil) adsorbed on rock surface and, therefore, reduces oil-wetness [43,44]. This improves the oildisplacing ability of the surfactant at the interface between oil-wet rock and aqueous solution of surfactant [45]. The lowest drop in contact angle is observed at CMC value (80 0 0 ppm) of the synthesized surfactant. For carbonate rock surface, surfactant solution at CMC showed contact angle decrease from 82.5° to 20.7° in 10 0 0 s. Contact angle is found to decrease from 80.3° to 17.3° in 10 0 0 s for quartz surface at CMC (80 0 0 ppm). This shows that ability to alter nature of the rock surface from oil-wet to water-wet surface is most favorable at CMC. At surfactant concentrations above CMC, rate of decrease of contact angle with time is reduced, showing that adhesion tension between rock and surfactant solution does not increase with concentrations beyond CMC [46]. Variation in contact angle shows that the alteration of wettability of rock is due to the effect of two mechanisms predicted: first is ion-pair attraction between the anionic polar head groups of the surfactant units and cationic charged components present in the crude oil adsorbed on the rock surface [47,48]. Another possible reason for this variation can be the adsorption of negatively charged group of anionic surfactant onto the positively charged surface of rock [49]. Conse-

3.3.3. Effect of salinity on contact angle Salinity plays a crucial role in wettability alteration behavior of surfactant solutions. Hence, there is a need to study the effect of salt on alteration of wettability of oil wet rock surfaces at optimal surfactant concentration of 80 0 0 ppm at 27 °C. Fig. 18 shows the influence of salinity on contact angle behavior of quartz and carbonate rocks respectively for 600 s. Addition of salt (NaCl) improves the wettability behavior of oil-wet rock due to increased rate of accumulation of surface active components at oil–aqueous interface. The contact angle value decreases with the increase in salt percentage in surfactant solution at CMC. This may be due to the salting-out of surfactant molecules in aqueous phase in the presence of NaCl [50]. As salt concentration in CMC surfactant solution increases, some water molecules are attracted by salt ions. This reduces the availability of water molecules to interact with surfactant molecules in bulk solution. As a result, faster diffusion rate of surfactant molecules from bulk to interface is achieved and additional surfactant molecules precipitate to the rock–aqueous interface to form stronger hydrophobic surfactant-surfactant interactions [13,51]. This phenomenon is observed till optimal salinity value is reached. At optimal salinity (2% NaCl), wetting characteristics are more favorable than other solutions for application in EOR. Contact angle values are found to decrease from oil-wet conditions to 7.7° for quartz and 12.9° for carbonate rock surfaces respectively in 600 s. The decrease in value of contact angle at optimal salinity is due to the reduced electrostatic repulsion force as more sodium ions are bound to oil wet surface which results in alteration of wettability [52]. Beyond optimal salinity, the repulsive electrostatic and hydration forces at the rock–solution interface increase in order to avoid droplet spreading at the surface [53]. This reduces the adsorption of the surfactant onto the rock surface and thus, increases the value of contact angle with increase in salinity above the optimal salinity [54]. It is, therefore, concluded that salt– surfactant mixtures show better wettability characteristics as compared to surfactant solutions on both quartz and carbonate rock surfaces [55].

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11

Fig. 19. Flooding performance of α -SEE slugs in sandpack system at different surfactant concentrations: (a) 80 0 0 ppm, (b) 10,0 0 0 ppm, (c) 12,0 0 0 ppm, and (d) additional oil recoveries. Table 3 Flooding results of the different systems. Sandpack sample No.

Porosity (%)

Permeability, k (Darcy) kw (Sw = 1)

ko (Swi )

I

31.24

4.47

0.58

II

31.64

4.75

0.65

III

31.97

5.10

0.70

Design of chemical slug for flooding

Oil Recovery (%OOIP) at 95% water cut

Additional recovery (% OOIP)

Swi

Soi

Sor

0.5 PV 80 0 0 ppm SMES + Chase water 0.5 PV 10,0 0 0 ppm SMES + Chase water 0.5 PV 12,0 0 0 ppm SMES + Chase water

51.10

25.80

21.14

78.86

19.20

51.70

26.10

20.35

79.65

18.10

51.50

26.30

20.10

79.90

17.00

3.4. Oil recovery by flooding of synthesized surfactant solutions While planning for surfactant flooding in chemical enhanced oil recovery, it is important to design the surfactant slug with optimum concentration of surfactant. Optimum surfactant concentration basically depends on the CMC of surfactant, adsorption of surfactant onto the reservoir rock, salt tolerance and interfacial behavior at oil–aqueous interface. In this study, flooding tests were conducted with the injection of α -SEE surfactant slugs at three different concentrations. During surfactant flooding, a significant amount of surfactant is adsorbed on the rock surface. Hence, flooding studies are carried out with surfactant slugs at concentrations higher than CMC value (80 0 0 ppm) to account for these adsorption losses. This ensures that under reservoir conditions, at least CMC of surfactant is maintained for efficient recovery of oil. Sandpack saturated with crude oil (obtained from Oil India, Assam) was initially flooded with brine solution (2% NaCl). Water

% Saturation

breakthrough was found to occur after injection of small pore volume of displacing fluid during flooding test due to lower viscosity of water as compared to that of crude oil. Almost 52% of original oil in place (OOIP) is recovered after conventional water flooding due to high porosity of the sandpack system. When water-cut reached up to ∼95%, injection of surfactant slug improved the oil recovery rate with drastic reduction in water-cut percentages. However, with further increase in pore volume of slug injected, the oil recovery increased with simultaneous increase in water-cut. The flooding performance of different surfactant slugs (80 0 0 ppm, 10,0 0 0 ppm, 12,0 0 0 ppm) injected into the sandpack systems are depicted in Fig. 19(a), (b) and (c) respectively. After injection of surfactant slug, the oil droplets trapped in pores are mobilized due to reduction of interfacial tension between oil and displacing fluid [56,57]. With increase in surfactant concentration, oil saturation increases due to increase in the coalescence of mobilized oil droplets to form oil bank [58–60]. This allows better oil

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displacement by injected fluid with increase in surfactant concentration [60,61]. Behind the oil bank, the surfactant prevents backflow and subsequent retrapment of mobilized oil in the pores. After injection of surfactant slug, the chase water flooding is continued till no further recovery of oil. A comparative study of additional oil recoveries after water flooding by surfactant solution at different concentrations is illustrated in Fig.19(d). It has been found that additional recoveries of 24.70%, 25.80% and 27.60% of OOIP after water injection were observed for three different concentrations of surfactant. Surfactant slugs with higher concentration always contain sufficient concentration to achieve low IFT and low contact angle even after adsorption. Thus additional recovery increases with increase in surfactant concentration. A comparative picture of oil recovery at different surfactant concentrations along with different petro-physical properties are shown in Table 3. 4. Conclusions The effectiveness of an anionic surfactant synthesized from a natural vegetable resource, palm oil has been investigated for application in enhanced oil recovery. The surfactant, alphasodium ethyl ester (α -SEE) has been suitably characterized by FTIR, GC, EDX and FE-SEM techniques for the determination of functional groups, presence of various esters, elemental composition and molecular surface morphology, respectively. Thermogravimetric analysis shows good thermal stability of α -SEE surfactant up to reservoir temperature. Critical micelle concentration (CMC) of the synthesized surfactant is found to be 80 0 0 ppm at 27 °C. An ultralow IFT of 1.17 × 10−2 mN/m is observed at the crude oil– aqueous solution interface at CMC. The value of IFT is further reduced to 8.30 × 10−3 mN/m under optimal salinity conditions (2% NaCl) at CMC. By addition of organic alkali (MEA) IFT reduced to very low values, and a minimum value of 6.81 × 10−3 mN/m was obtained at pH 12 for alkali-surfactant system at CMC. The surfactant shows good wetting characteristics by altering the nature of the both quartz and carbonate rock surfaces from oil-wet to waterwet. Contact angle values are found to initially decrease with surfactant concentration up to CMC, after which it starts increasing. Effect of addition of salt shows much lower contact angle reduction as compared to that of pure surfactant solutions. Flooding experiments show additional recoveries of 25.80%, 26.10% and 26.30% OOIP by the injection of different surfactant slugs with 80 0 0 ppm, 10,0 0 0 ppm and 12,0 0 0 ppm concentrations respectively after conventional water flooding for sandpack systems with 2% brine salinity. Acknowledgments The authors gratefully acknowledge the financial assistance provided by Oil India Limited [Contract No. 62O6917], Duliajan, Assam, India to the Department of Petroleum Engineering, Indian Institute of Technology (ISM), Dhanbad, India. Authors would also like to thank Central Research Facility, IIT (ISM), Dhanbad, India for technical assistance and support. References [1] Green DW, Willhite GP. Enhanced oil recovery. SPE textbook series. 1st ed. Texas: Richardson; 1998. [2] Majidaie S, Muhammad M, Tan IM, Demiral B. Green surfactant for enhanced oil recovery. In: 2011 Natl. Postgrad. Conf. - Energy Sustain. Explor. Innov. Minds; 2011. p. 1–5. [3] Babu K, Pal N, Saxena VK, Mandal A. Synthesis and characterization of a new polymeric surfactant for chemical enhanced oil recovery. Korean J Chem Eng 2016;33(2):711–19. [4] Ahmadi AM, Shadizadeh RS. Experimental investigation of adsorption of a new nonionic surfactant on carbonate minerals. Fuel 2013;104:462–7.

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Please cite this article as: N. Saxena et al., Characterizations of surfactant synthesized from palm oil and its application in enhanced oil recovery, Journal of the Taiwan Institute of Chemical Engineers (2017), https://doi.org/10.1016/j.jtice.2017.09.014