Synthesis and Physicochemical Investigation of Anionic-nonionic

Article Cite This: Energy Fuels 2019, 33, 6247−6257

pubs.acs.org/EF

Synthesis and Physicochemical Investigation of Anionic−Nonionic Surfactants Based on Lignin for Application in Enhanced Oil Recovery Shuyan Chen,†,‡ Yujie Zhou,‡ Hongjuan Liu,‡ Jingjing Yang,† Yingying Wei,† and Jianan Zhang*,‡ †

Department of Environment and Quality Test, Chongqing Chemical Industry Vocational College, Chongqing 401228, China Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China

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ABSTRACT: A series of eco-friendly anionic−nonionic surfactant lignin polyether sulfonates (LPES) based on alkali lignin from the paper-making industry was prepared through alkoxylation, allylation, and sulfonation reactions. The chemical structures of synthesized surfactants were determined by an infrared (IR) spectrogram and 1H nuclear magnetic resonance (1H NMR). In order to verify the adaptability of LPES surfactants applied in chemical enhanced oil recovery (EOR), the physicochemical properties including surface tension, salt tolerance, hydrophile lipophile balance (HLB) values, and interfacial tension for LPES surfactants were investigated through experiments. The critical micelle concentration (cmc) of LPES surfactants increased with the growth of ethylene oxide (EO) groups in the molecules, and the corresponding surface tension values were about 32−38 mN/m. Salt tolerance measurements prove that the LPES surfactants have a high tolerance to Na+ and Ca2+, and the HLB measurements demonstrate that the LPES surfactants have excellent emulsifying properties in oil/water (O/W) systems. Compared with Xinjiang crude oil, the LPES surfactants exhibit the desired ability of decreasing the interfacial tension (IFT) of Daqing/Huabei crude oil, which contains more colloid and asphaltene, and the LPES surfactants can reduce the IFT between Daqing (or Huabei) crude oil and brine to the 10−3 mN/m level in the presence of alkali. biodegradation, and high foaming capacity.10,11 The molecules of anionic−nonionic surfactants are composed of nonionic groups such as polyoxyethylene and anionic groups such as sulfonate, two different hydrophilic groups, so these surfactants have favorable interfacial properties.12 They can not only overcome the disadvantages of poor interfacial activity and high adsorption loss of nonionic surfactants but also make up the defects of poor salt tolerance of anionic surfactants.13,14 Therefore, the anionic−nonionic surfactants have a broad application prospect in the EOR field, especially in high temperature and high salinity reservoirs. Michels et al.15 designed and synthesized sulfonates and sulfates containing both oxyethylene and oxypropylene chains in the molecules, which were applied successfully for low concentration surfactant flooding in high salt reservoirs. Jin et al.16 synthesized Guerbet alcohol polyoxyethylene ether sulfate surfactant, and they believed that the ability of reducing the critical micelle concentration (cmc) and Krafft temperatures increased with increasing the number of oxyethylene groups in the molecules (from one to four). Chen et al.17 synthesized and evaluated the performance of alcohol ether carboxylate surfactants used in alkali−surfactant−polymer flooding; they found that the synthesized surfactants had the ability to decrease the IFT between Xinjiang crude oil and brine to the 10−3 mN/m level in a short period of time. Up to now, most of the research in this field mainly focuses on those anionic− nonionic surfactants with long fat alcohol chains as their hydrophobic groups. However, few studies have been

1. INTRODUCTION Petroleum is an important nonrenewable strategic resource in the world today. With the development of oil wells, it has been difficult for the oil production from conventional oil recovery methods in China to meet the needs of oil consumption. According to the research of the petroleum department in China, chemical enhanced oil recovery (EOR) techniques have been recognized to be suitable for developing the remaining oil of most oilfields after primary and secondary recoveries.1 Among various chemical EOR technologies, surfactant flooding has been given considerable attention in the past decades due to the unique functions of surfactants.2−4 The surfactants can dramatically improve the displacement efficiency by decreasing the interfacial tension (IFT) of oil/water and increasing the mobility ratio when they are injected into the formation.5−7 Meanwhile, the reservoirs’ conditions including oil characteristics, reservoir brine properties and temperature, surfactant adsorption loss, and IFT reduction will affect the displacement efficiency of surfactants throughout the surfactant flooding.8,9 The surfactant cost, as well as the above parameters, also plays an important role in the selection of appropriate surfactants. Therefore, it is highly important to lower the costs of recovering oil and explore new and highly efficient surfactants for EOR in the development of oilfields across China. The researchers also focus on the preparation of surfactants used in EOR through an environmentally friendly synthetic route using natural renewable biomass as raw materials. In recent years, anionic−nonionic surfactants have attracted more and more attention in the EOR field due to their outstanding properties, such as good solubility, high heat resistance, high salt tolerance, good compatibility, easy © 2019 American Chemical Society

Received: April 10, 2019 Revised: June 6, 2019 Published: June 10, 2019 6247

DOI: 10.1021/acs.energyfuels.9b01114 Energy Fuels 2019, 33, 6247−6257

Article

Energy & Fuels Scheme 1. Synthesis Routes of Lignin Polyether Sulfonates

conducted on anionic−nonionic surfactants containing phenyl as hydrophobic groups, particularly those with raw materials coming from natural renewable biomass.18,19 The surfactants containing phenyl groups in the alkyl chains may have superior properties compared to the surfactants containing fat alcohol chains.20 Therefore, it is attractive and significant to synthesize such anionic−nonionic surfactants that have phenyl groupcontaining carbon chains as hydrophobic groups. As the most abundant renewable lignocellulosic biomass containing various phenolic groups in nature, lignin is considered to make a great contribution to the production of bioproducts in the near future due to its huge availability.21 Lignin accounts for 20 to 30 wt % of the biomass by weight, and about 5.06 billion tons of industrial lignin are produced as the low value byproducts of delignification from the pulp and paper industry annually. According to statistics, only

approximately 20 wt % of industrial lignin is used in energy production and low-added value applications presently; much of the rest is either burned or poured into rivers.22 Lignin is a nonuniform, complex polymer with a hyperbranched structure containing many functional groups, such as alkyl group, carbonyl group, methoxyl group, and so on.23,24 It is reported that the total hydroxyl content of industrial sulfate lignin is 1.2−1.27 groups per C9 unit, wherein the phenolic hydroxyl content is 56 to 60 wt %.25 Therefore, it is attractive to strengthen the high value-added utilization of low-cost lignin waste as a biomass resource in industry. Lignin can be an ideal material for the development of surfactants based on biomass applied in the EOR field. Years ago, conventional lignosulfonates and their modified products were considered as potential surfactants for EOR. However, since the lignosulfonates have a complicated 6248


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Energy & Fuels Table 1. Composition of Simulated Brine from Daqing, Huabei, and Xinjiang Oilfields, China oilfield

Ca2+ (mg/L)

Mg2+ (mg/L)

Na+ (mg/L)

K+ (mg/L)

Cl− (mg/L)

SO42− (mg/L)

CO32− (mg/L)

HCO3− (mg/L)

Daqing Huabei Xinjiang

24.0 53.6 56.9

14.6 22.0 13.4

1738.8 3900.7 2085.0

0 32.2 0

1063.8 4737.5 2577.0

153.7 117.5 131.6

120.0 0 5.8

2501.8 1925.9 1156.9

(99.5 wt %), 3-chloropropene (98.0 wt %), sodium metabisulfite (99.0 wt %), tetrabutylammonium bromide (TBAB, 99.0 wt %), and benzethonium chloride (97.0 wt %) were used as obtained from Sigma Chemicals. All the chemical reagents employed in the experiments are of analytical pure grade. Three kinds of crude oil from Daqing, Huabei, and Xinjiang oilfields were supplied by Research Institute of Petroleum Exploration and Development, and the density of these crude oils is 0.85, 0.88, and 0.87 g/cm3, respectively. Table 1 shows the composition of simulated brine from Daqing, Huabei, and Xinjiang oilfields, China. 2.2. Synthesis and Structure Analysis of Lignin Polyether Sulfonates. 2.2.1. Synthesis of Lignin Polyoxypropylene Polyoxyethylene Ether. The lignin polyoxypropylene polyoxyethylene ethers were first synthesized through the synthetic pathway shown in Scheme 1 (Step 1). The purified alkali lignin (20 g) highly dispersed in 150 mL of alkali solution was added into the high-pressure reactor equipped with a stirrer and thermocouple temperature controller. Then, the temperature of the reactor was gradually increased to 70 °C with stirring; meanwhile, the air and water vapor of the reactor were replaced by nitrogen three times to ensure the reactor fully filled with nitrogen. Ten times the amount of PO (calculated by molar ratio) was added to the reactor, and the mixture began to react for 2 h at 70 °C. After that, a weighed amount of EO was introduced into the reactor with the nitrogen for the end-sealing reaction, and the reaction lasted at 70 °C until the pressure in the reactor dropped to −0.1 MPa. After the reaction, 20 wt % phosphoric acid was added to adjust the pH of the resulting mixture to 7, and the unreacted alkali lignin was removed by filtration. Then, the residual product was washed twice with cyclohexane, and excess solvent was evaporated using a rotary evaporator. Thus, the lignin polyoxypropylene polyoxyethylene ethers were obtained after drying in a vacuum oven. The short names for the obtained products were LPO 10EO, LPO10EO2 and LPO10EO4 according to different polymerization degrees. 2.2.2. Synthesis of Lignin Allyl Ether. As shown in Scheme 1 (Step 2), the mixture of lignin polyoxypropylene polyoxyethylene ether and 3-chloropropene in a molar ratio of 1.0:1.5 was added into the highpressure reactor. Then, twice the amount of potassium hydroxide (calculated by molar ratio) was added into the reactor, and the tetrabutyl ammonium bromide used as phase transfer catalyst was added into the reactor subsequently. The reaction mixture was stirred intensively, and the temperature gradually went up to 120 °C. In this process, the pressure in the reactor would automatically be generated and was maintained at 0.2−0.3 MPa. The reaction lasted for 5 h at 120 °C, and the oily products were attained. After that, the crude products, which were dissolved in 100 mL of hot distilled water, were shaken intensively and left for a while until layered. The lower layer is the unreacted reagents, which are dissolved in the aqueous phase, while the upper layer is the organic phase, and the organic phase was alternately washed with dilute hydrochloric acid and hot distilled water until the pH value of the aqueous phase was 7. Then, they were purified under reduced pressure to obtain the lignin allyl ethers. 2.2.3. Synthesis of Lignin Polyether Sulfonates. The mixture of lignin allyl ethers and sodium hydroxide with a molar ratio of 1.0:2.5, which was fully dissolved in 100 mL of distilled water, was first added into a 500 mL round-bottom flask and agitated at 80 °C for 30 min. Then, twice the amount of 30 wt % sodium metabisulfite solution (calculated by molar ratio) was added dropwise in 20 min, and the reaction lasted for 8 h at 80 °C (Step 3 in Scheme 1). After evaporating the water, the residual inorganic salts of the crude products were removed by hot ethanol, and the unreacted lignin allyl ethers were further extracted by petroleum ether. The lignin polyether sulfonates (LPES) could be obtained by distillation of the solvent under reduced pressure. The products LPO 10 EOSO 3 Na,

molecular structure, a wide molecular weight distribution, and a lack of structurally regular oleophilic groups, they are difficult to arrange orderly at the oil/water interfaces, and the lignosulfonates have relatively poor interfacial properties and high solubility when used solely. Therefore, the application of lignosulfonates is greatly limited in EOR, and it is necessary to combine them with other surfactants such as petroleum sulfonate as EOR displacement agents. Zhang et al.26 studied the combined systems of lignosulfonate and petroleum sulfonate, and the results indicated that the combined systems had good synergistic effects and could decrease the IFT of oil/ water after use; the higher the content of lignin, the higher was the surfactant concentration and the shorter was the time to reach the lowest IFT. Nevertheless, there are still two shortcomings in the combined systems: the combined method can not fundamentally change the hydrophilic and lipophilic groups of lignosulfonates as surfactants and their surface properties, and the price of combined systems is generally high at present, making them lack market competitiveness. To overcome these drawbacks, researchers have studied the chemical modifications of lignin surfactants based on alkali lignin; a variety of attempts such as alkylation, sulfonation, amination, oxidation, and carbonylation currently have been made to improve the surface and interfacial performance of lignin surfactants.27−29 However, in terms of the current modification methods, the adjustable range and flexibility of the modification were not large and the modified lignin surfactants could not achieve ultralow IFT of oil/water when used solely; the modification cost was too high for practical application.30,31 In this paper, a series of novel anionic−nonionic surfactants, lignin polyether sulfonates (LPES), was designed and synthesized with alkali lignin from the paper-making industry as raw materials, which could decrease the IFT of oil/water down to an ultralow value under a wide range. First, using alkali lignin as raw materials, the LPES surfactants with different polymerization degree were prepared by alkoxylation, allylation, and sulfonation reaction, and the synthesized pathway was depicted in Scheme 1. The structures of the synthesized surfactants were determined by a Fourier transform infrared (FT-IR) spectrogram and 1H nuclear magnetic resonance (1H NMR). Second, the physicochemical properties of LPES surfactants such as solubility, salt tolerance, hydrophile lipophile balance (HLB) values, surface tension, and IFT were studied experimentally. Meanwhile, the effects of surfactant concentration, alkali, salinity, and crude oil on the IFT were systematically conducted to check the mechanism of the synthesized surfactants at the oil/water interfaces.

2. EXPERIMENTAL SECTION 2.1. Materials. Alkali lignin is mainly derived from wheatgrass, which was supplied by Shandong Paper Mill, China. Potassium hydroxide (90.0 wt %), sodium hydroxide (99.0 wt %), phosphoric acid (85.0 wt %), hydrochloric acid (36.0 wt %), sodium chloride (99.5 wt %), and calcium chloride (96.0 wt %) were provided by Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Propylene oxide (PO, 98.0 wt %), ethylene oxide (EO, 98.0 wt %), cyclohexane 6249


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2.3.5. IFT Measurements. The variation of IFT between three kinds of crude oil and LPES solutions with time was measured by the SVT20N spinning drop IFT apparatus (Eastern-Dataphy, China) using the same procedures of the interfacial tension measurements as reported previously.33 The measurements were performed at the reservoirs’ temperatures of the corresponding crude oil (Daqing, 45 °C; Huabei, 54 °C; Xinjiang, 28.7 °C).

LPO10EO2SO3Na, and LPO10EO4SO3Na were synthesized by the above synthetic route. In addition, the sulfonate ion content of the LPES surfactants was determined by the two-phase titration method using benzethonium chloride as titrant, and the sulfonate ion contents of LPO10EOSO3Na, LPO10EO2SO3Na, and LPO10EO4SO3Na were 1.307, 1.348, and 1.351 mmol/g, respectively. 2.2.4. Structure Confirmation. The structures of the synthesized LPES surfactants were determined by Fourier transform infrared (FTIR), which was carried out in a PerkinElmer Spectrum (PerkinElmer Corporation, American) using a potassium bromide (KBr) pellet, and proton nuclear magnetic resonance spectroscopy (1H NMR), which was recorded by the AVANCE III spectrometer (Bruker, France). 2.3. Measurements. 2.3.1. Solubility Measurements. First, certain amounts of alkali lignin, lignin allyl ether, and lignin polyether sulfonate were prepared into two sets of solutions with a concentration of 1.00 g/L using distilled water, respectively. Then, 0.1 mol/L hydrochloric acid and 0.1 mol/L NaOH solutions were used to adjust the pH of each set of the above solutions to 1−2 or 10−12, respectively. The solubility of the above solutions was observed after 24 h at 25 °C. 2.3.2. Salt Tolerance Measurements. The salt tolerance of synthesized surfactants was investigated through the solubility of LPES surfactants in saline solutions containing different concentrations of Na+ or Ca2+. The LPES solution with a concentration of 1.00 g/L was first prepared with distilled water, and then, a series of NaCl and CaCl2 solutions with different concentrations was prepared. After that, the LPES solution was added to the saline solutions of different concentrations, and the concentration of salts at which the precipitation observed was considered to be the salt tolerance of LPES surfactants. 2.3.3. Surface Tension Measurements. Surface tension measurements of LPES solution prepared by distilled water with different concentrations were carried out with the Wilhelmy plate method using a dynamic QBZY-2 surface tensiometer (CANY, China) at 25 °C. To reduce the errors caused by adsorption kinetics, it was essential that the sample solutions be aged in the measuring tank for 30 min before being measured. The surface tension of the samples at each concentration was measured at least three times and averaged. The γ−c curve of the samples is plotted from the measured values of the surface tension and the surfactant concentrations, and the cmc of LPES surfactants can be obtained according to the inflection point of the curve. In addition, using the slope of the γ−c curve, other surface parameters such as the maximum surface excess concentration (Γmax) and area per molecule (Amin) can be calculated by the Gibbs adsorption isotherm equation. The corresponding calculation formulas are shown below: Γmax = −

1 ij d γ yz 1 jij d γ zyz jj zz = − jj zz 2RT k d ln C {T 4.606RT jk d log C z{

T

A min = (ΓmaxNA)−1

3. RESULTS AND DISCUSSION 3.1. Structure Confirmation. The FT-IR spectra of LPO10EO2SO3Na as an example of the synthesized lignin polyether sulfonates were obtained as illustrated in Figure 1.

Figure 1. FT-IR spectra of lignin polyether sulfonate.

The absorption peaks in the IR spectrum are helpful to confirm the synthesis of the reaction products since these absorption peaks correspond to the functional groups of the samples. The absorption band of O−H in LPES surfactants was shifted to 3450−3550 cm−1 due to the association between the hydroxyl group and the hydrogen bond in the molecule. The absorption peaks centered at 2980 and 2800 cm−1 demonstrate the asymmetric and symmetric stretching C−H band in −CH2− and −CH3 groups, and the relatively wide absorption band indicates that methyl and methylene groups are attached to the alkali lignin molecules after the alkoxylation reaction. The strong and wide absorption peaks appeared at 1620 and 1420 cm−1, which are characteristic of the benzene ring skeletal vibration. The peaks centered at 1210 cm−1 correspond to the C−O stretching vibration absorption, while the characteristic stretching vibration absorption of SO was observed at 1360 and 1150 cm−1, which confirms the presence of the sulfonate group in the LPES surfactants. The 1H NMR spectra of synthesized LPES surfactants were shown in Figure 2. The intense peaks appeared from δ 1.2 to 1.35 ppm and δ 1.40 ppm, corresponding to the methylene proton (−CCH2C−). The peaks seen at δ 1.55 and 1.80 ppm can be attributed to the methine proton (−CCHR2), which indicate that the long carbon chain containing plenty of methylene groups and methine groups is introduced into the alkali lignin molecules after the alkoxylation reaction. The peaks appearing in the range of δ from 3.30 to 3.45 ppm are attributed to the methine proton (−RCHO−) and methylene proton (−RCH2O−). The methylene proton (−CH2−SO3−) was observed with δ ranging from 3.60 to 3.70 ppm, which confirms the presence of the sulfonate group in the LPES

(1) (2)

wherein Γmax is the maximum surface excess concentration; Amin is the area per molecule; γ is the surface tension; R is the gas constant; T is the temperature in Kelvin; C is the surfactant concentration; NA is the Avogadro constant. 2.3.4. HLB Measurements. The HLB values of LPES surfactants were investigated by means of an emulsification method.32 The specific measured method was as follows: First, the standard oil samples with HLB values ranging from 7 to 16 were prepared with turpentine (HLB = 16) and cottonseed oil (HLB = 6) in a certain proportion. Then, a series of mixtures prepared by 30 mL of standard oil samples with different HLB values and 30 mL of LPES solution (0.4 wt %) was added into the cylinder with a stopper, and the cylinder was oscillated vigorously. The cylinder was left for a while until the aqueous phase and oil phase were completely separated, and the separation time of water/oil was recorded. The required HLB for emulsifying the standard oil corresponding to the emulsion with the longest water/oil separation time is the same as that of the tested sample. 6250


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Figure 2. 1H NMR spectra of lignin polyether sulfonate.

surfactants. The peak observed at δ 7.25 ppm is due to the aromatic protons in the LPES molecule. 3.2. Solubility Measurements. The molecular structure of alkali lignin is complex, and it can only be dissolved in alkaline solution. However, the structure of alkali lignin will be changed during the modification process, and the solubility will also be changed accordingly. In this experiment, the solubility changes of alkali lignin and the synthesized products of each step under different pH conditions were investigated by the solubility analysis method. The solubility of alkali lignin and products synthesized in each step is shown in Table 2.

allylation reaction, the polyether chain and allyl group are introduced into alkali lignin molecules, and its solubility is slightly improved. Lignin polyether and lignin allyl ether could not only be dissolved in 0.1 mol/L NaOH solution but also be slightly dissolved in 0.1 mol/L HCl solution, and the lignin polyether sulfonate obtained after the sulfonation reaction could be dissolved not only in 0.1 mol/L NaOH solution but also in 0.1 mol/L HCl solution. After the alkoxylation reaction, the introduction of multiple PO groups and EO groups greatly increases the relative molecular weight of alkali lignin, and the hydrophilicity and solubility of lignin polyether will be further improved due to the introduction of EO groups in the molecules; thus, the lignin polyether can be slightly dissolved in acidic solution, whereas the lignin polyether can be sodium modified under alkaline conditions and it has better solubility in alkaline solution. The reason for the solubility of lignin allyl ether in aqueous phase is similar to that of lignin polyether. After the sulfonation reaction, the strong hydrophilic sulfonate group is introduced into the lignin allyl ether molecule; therefore, the lignin polyether sulfonate has good solubility in both alkaline and acidic solutions. 3.3. Salt Tolerance. The salt tolerance of surfactants is a crucial indicator for evaluating the feasibility of surfactants in high salinity reservoirs. Lower salt-tolerant surfactants will cause precipitation in such reservoirs, resulting in a decrease of oil recovery. The solubility of synthesized LPES surfactants in

Table 2. Solubility of Alkali Lignin and the Products Synthesized in Each Step solubility sample

0.1 mol/L NaOH

0.1 mol/L HCl

alkali lignin lignin polyether lignin allyl ether lignin polyether sulfonate

soluble soluble soluble soluble

unsoluble slightly soluble slightly soluble soluble

As shown in Table 2, alkali lignin was only dissolved in alkaline solution (0.1 mol/L NaOH) and not dissolved in acidic solution (0.1 mol/L HCl). After the alkoxylation and

Table 3. Effects of NaCl Concentration on the Solubility of LPES Surfactantsa concentration of Na+ (mg/L) surfactant

5000

10 000

20 000

50 000

100 000

150 000

200 000

LPO10EOSO3Na LPO10EO2SO3Na LPO10EO4SO3Na

− − −

− − −

− − −

− − −

− − −

− − −

+ − −

Notes: −, the solution was clear and transparent, and no new phase was produced; +, a new phase was produced in the solution.

a

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values of surface tension continually reduced to a plateau region as the surfactant concentrations increased. The decrease in surface tension at the beginning indicates the adsorption of surfactant molecules at the gas/liquid interface,35 and the number of surfactant molecules adsorbed at the gas/liquid interface increases with an increase in the surfactant concentration. When surfactant molecules aggregate in bulk solution to form micelles, the surfactant molecules reach the saturated adsorption at the gas/liquid interface and the break point can be clearly seen on the γ−c curves.36 The surface activity parameters including Ccmc, γcmc, Γmax, and Amin were calculated according to the γ−c curves and the eqs 1 and 2, and the obtained calculated values are listed in Table 5. As shown in Table 5, the lowest surface tension values of 32.87, 35.79, and 38.02 mN/m were obtained for LPO10EOSO3Na, LPO10EO2SO3Na, and LPO10EO4SO3Na at the concentrations of 0.04807, 0.08312, and 0.09712 g/L, respectively. It indicates that the LPO10EOSO3Na is easier to form micelles in solution compared with LPO10EO2SO3Na and LPO10EO4SO3Na. The hydrophilicity and solubility for LPES surfactants increase as the number of EO groups in the LPES molecules increase, resulting in the aggregation of surfactant molecules to form micelles in solution at higher concentration and an increase of cmc value. Furthermore, the hydrophilic chain segments of the LPES molecules increase with the growth of the EO chain length in the molecules, making the area occupied by surfactant molecules at the gas/liquid interface increase. Meanwhile, the surfactant molecules are loosely arranged at the gas/liquid interface due to the electrostatic repulsion between molecules and the resistance of the hydration layer, making surfactant molecules difficult to access. As a result, the adsorption of surfactant molecules at the gas/liquid interface is reduced and the adsorption efficiency is lowered. Therefore, the ability to lower the surface tension of the solution is deteriorated, and the value of γcmc increases. It could also be seen that the LPO10EOSO3Na had a lower value of area per molecule Amin and a higher value of surface excess concentration Γ max as compared to that of LPO10EO2SO3Na and LPO10EO4SO3Na. As the number of EO groups in molecules increases, the proportion of the hydrophilic headgroup in the LPES molecules decreases and the packing of surfactant molecules at the gas/liquid interface is relatively loose, resulting in a decrease of surface excess concentration Γmax and an increase in area per molecule Amin.37 In addition, the area per molecule Amin of surfactant molecules at the gas/liquid interface and the value of γcmc are parallelrelated: the smaller the γcmc value, the smaller is the Amin. 3.5. HLB Measurements. Surfactants are the molecules in which the lipophilic groups and hydrophilic groups are combined, and the HLB value is usually used to represent the balance between hydrophilic groups and lipophilic groups. The stronger the lipophilicity of the surfactant, the smaller is the HLB value and vice versa. It is well-known that the HLB value as a semiempirical scale for the selection of surfactants has been widely applied in the EOR field.38 The surfactants

different concentrations of NaCl and CaCl2 solution is shown in Tables 3 and 4, respectively. The concentration of NaCl was Table 4. Effects of CaCl2 Concentration on the Solubility of LPES Surfactantsa concentration of Ca2+ (mg/L) surfactant

200

400

600

800

1000

2000

3000


− − −

− − −

− − −

− − −

− − −

− − −

− − −

Notes: −, the solution was clear and transparent, and no new phase was produced; +, a new phase was produced in the solution. a

in the range of 5000 to 200 000 mg/L, and the concentration of CaCl2 varied from 200 to 3000 mg/L. As shown in Table 3, the salt tolerance of LPES surfactants increased along with an increase in the number of EO groups in the LPES molecules; no precipitation was observed in LPO10EO2SO3Na and LPO10EO4SO3Na solutions up to a NaCl concentration of 200 000 mg/L while a new phase was observed in the LPO10EOSO3Na solution with 200 000 mg/L NaCl. For the LPES surfactants with a fixed hydrophobic carbon chain length, the hydrophilicity of surfactants generally increases as the EO chain length in the surfactant molecules increases, and more hydrogen bonds can be formed between the water molecules and the hydrophilic head groups, resulting in difficulty for the precipitation to form in salinity solutions. From Table 4, it can also be seen that no precipitation was observed in the samples up to a CaCl2 concentration of 3000 mg/L. The salt content in high salinity reservoirs is between 10 and 15 wt %;34 thus, the results indicate that the synthesized surfactants have good salt tolerance and can be widely used in high salinity reservoirs. 3.4. Surface Tension Measurements. The plots of surface tension γ (mN/m) versus c (g/L) for LPES surfactants at 25 °C were shown in Figure 3. As depicted in the curves, the

Figure 3. Variation of surface tension of LPES surfactants as a function of surfactant concentration at 25 °C.

Table 5. Calculated Surface Activity Parameters of LPES Surfactants at 25 °C surfactant

Ccmc (g/L)

γcmc (mN/m)

Γmax (mol/m2)

Amin (m2)


0.04807 0.08312 0.09712

32.87 35.79 38.02

1.30 × 10−6 9.83 × 10−7 8.84 × 10−7

1.28 × 10−18 1.69 × 10−18 1.88 × 10−18

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Figure 4. HLB value of the synthesized surfactants LPO10EOSO3Na (a), LPO10EO2SO3Na (b), and LPO10EO4SO3Na (c).

affected by various factors such as the characteristics of various reservoirs (salinity, temperature, viscosity, and crude oil with different alkane carbon numbers) and the type and concentration of surfactant, etc.41 In this experiment, the effects of surfactant concentration, alkali, salinity, and crude oil on the IFT of oil/water were investigated systematically. 3.6.1. Effects of Surfactant Concentration. The equilibrium IFT between Daqing crude oil and brine containing different concentrations of LPES surfactants at 45 °C was first measured without any additions, and the results are shown in Figure 5. The concentrations of LPES surfactants ranged from 0.1 to 0.6 wt %. As depicted in the figure, the equilibrium IFT between LPO10EOSO3Na solution and Daqing crude oil decreased first and then increased with the surfactant

with high lipophilicity are not easily soluble in the aqueous solution, while the hydrophilicity of the surfactant is too strong to facilitate its adsorption at the gas/liquid interface, and the surfactants owning appropriate HLB values can be practically employed in EOR.39 Figure 4 shows the HLB values of LPES surfactants with different degrees of polymerization. The results indicate that the HLB values of LPO10EOSO3Na, LPO10EO2SO3Na, and LPO10EO4SO3Na are 9, 10, and 10, respectively. The HLB values of LPES surfactants are between 8 and 13, signifying that they have better emulsifying properties in oil/water (O/W) systems on the basis of the relationship between the physicochemical properties of surfactant and the HLB value.40 When the surfactant molecules contain a longer hydrophobic carbon chain, where there is an increase in the hydrophilicity character upon the increase of EO group number in the prepared surfactant molecules, the HLB values increase. As for the fewer numbers of EO groups in LPES molecules, the introduction of sulfonate groups leads to a significant increase in the hydration of hydrophilic groups of surfactant molecules, thereby increasing the hydrophilicity of surfactant molecules and increasing the HLB value. As for the larger number of EO groups in LPES molecules, the introduction of sulfonate groups has no obvious effects on the hydration of the hydrophilic groups of surfactant molecules, and the hydrophobicity of long carbon chain leads to a trade-off simultaneously; thus, the increase in HLB value is not significant. 3.6. IFT Measurements. The IFT is an important parameter for studying the interface phenomenon of liquid− liquid systems. In the surfactant flooding field, it is well-known that the IFT between oil and water is the basis for researching the interfacial performance of surfactants, and it is also the standard for screening surfactant flooding formulations. During the displacement process, the IFT between oil and water is

Figure 5. Effects of LPES concentrations on the IFT between Daqing crude oil and brine at 45 °C. 6253


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Energy & Fuels concentration, and the lowest equilibrium IFT of 0.1724 mN/ m was obtained at the surfactant concentration of 0.4 wt %. The interfacial activity between LPO10EO 2 SO 3Na (or LPO10EO4SO3Na) solution and Daqing crude oil was analogous to that of LPO10EOSO3Na. It is worth noting that the further increase of surfactant concentration has little effect on the IFT of oil/brine when the lowest IFT value is obtained. The results indicate that surfactant concentration is one of the important factors affecting the IFT of oil/water. At lower surfactant concentrations, the decrease of IFT with time is slower. At higher surfactant concentrations, the decrease of IFT with time is rapid and the equilibrium IFT is reached within 10 to 20 min. Furthermore, the time required to reach the lowest IFT decreases along with the increase of surfactant concentration. The dynamic change of adsorption and desorption of surfactant molecules at the oil/water interface occur when the surfactant is in contact with crude oil, and the adsorption rate of surfactant molecules at the oil/water interface is determined by the diffusion rate of surfactant molecules from bulk solution to the oil/water interface. The larger the surfactant concentration, the greater is the concentration gradient of bulk solution and the interface layer and the faster is the diffusion rate of surfactant molecules. As a result, the faster the adsorption rate of surfactant molecules at the oil/water interface, the shorter is the time of lowering the IFT of oil/water. The lowest value of IFT is attained when the surfactant molecules reach saturated adsorption at the oil/water interface; any further increase of the surfactant concentration can not reduce the IFT of oil/ water. In general, the surfactant concentrations used in chemical flooding range from 0.05 to 0.60 wt %, and the minimal IFT between brine and Daqing crude oil can be attained with the optimum surfactant concentration of 0.4 wt %. 3.6.2. Effects of Alkali Concentration. A minimum IFT can be obtained for surfactants at which the adsorption rate and desorption rate of surfactant molecules at the oil/water interface are balanced. Usually, the conventional single surfactants hardly satisfy the requirements of lowering the IFT of oil/water to ultralow values, and more and more attention has been paid to the synergistic effects of reducing the IFT in mixed systems of surfactants and alkali.42 The effects of NaOH concentrations on the IFT between Daqing crude oil and brine containing LPES surfactants at the concentration of 0.4 wt % were investigated at 45 °C, and the results are illustrated in Figure 6. In the chemical solution, the NaOH concentrations varied from 0 to 1.0 wt %. From the curves, it could be seen that the equilibrium IFT of LPO10EOSO3Na/crude oil reduced sharply after the addition of NaOH into the solution in contrast to the LPO10EOSO3Na used solely. Furthermore, a decrease in IFT was found for the LPO10EOSO3Na along with the increase in the NaOH concentration. The values of equilibrium IFT for LPO10EOSO3Na had decreased by about 2 orders of magnitude with 0.4 wt % NaOH relative to the use of only LPO10EOSO3Na. Analogous interfacial properties were observed with LPO10EO2SO3Na and LPO10EO4SO3Na. Additionally, the equilibrium IFT between Daqing crude oil and brine containing the LPES surfactants could reach the ultralow values (

Synthesis and Physicochemical Investigation of Anionic-nonionic

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