Performance Evaluation of Novel Sunflower Oil-Based Gemini

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Performance Evaluation of Novel Sunflower Oil-based Gemini Surfactant(s) with different Spacer Lengths: Application in Enhanced Oil Recovery Nilanjan Pal, Narendra Kumar, Amit Verma, KEKA OJHA, and Ajay Mandal Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02744 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 30, 2018

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Sunflower oil-derived Gemini Surfactants for EOR application. 338x190mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Performance Evaluation of Novel Sunflower Oil-based Gemini Surfactant(s) with different Spacer Lengths: Application in Enhanced Oil Recovery Nilanjan Pal, Narendra Kumar, Amit Verma, Keka Ojha, Ajay Mandal* Department of Petroleum Engineering Indian Institute of Technology (Indian School of Mines), Dhanbad-826004, India *Corresponding Author, Email: [email protected] Abstract A series of novel, non-ionic gemini surfactants (GSs) with varying spacer lengths were synthesized from sunflower (Helianthus) oil for application in enhanced oil recovery (EOR). The surfactants were characterized by 1H-NMR and TGA analyses. Critical micelle concentration (CMC) values increased with temperature due to delay in micellization of GS molecules in bulk phase. Hydrolytic stability studies revealed that GS solutions possess the ability to displace acidic crude oil through reservoir pores. Crude oil miscibility studies showed the formation of stable emulsion systems. Ultra-low interfacial tension (IFT) was achieved at the oil-aqueous interface in the presence of salt. Surfactant solutions exhibited good tolerance to varying salinity and hardness conditions. GS solutions showed favourably low lime soap dispersion requirement (LSDR), indicating improved dispersing ability. GS-based foam systems showed enhanced kinetic stabilities with increasing concentration; and pseudoplastic flow character that are considered desirable for EOR operations. Half-life times decreased with temperature due to thinning and subsequent rupture of foam film boundary. Single-phase continuous emulsion(s) were observed during 15 days for n-heptane/GS/aqueous-based emulsions. Dynamic light scattering (DLS) and microscopic investigations showed that emulsion stability decreased with time due to gradual coalescence of oil droplets. Therefore, studies pertaining to characterization and performance evaluation of synthesized gemini surfactants confirm their potentiality as effective oil-displacing agents under reservoir conditions. Keywords Natural gemini surfactants; Critical micelle concentration; Performance evaluation; Interfacial tension; Emulsions; Foam stability Introduction


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The petroleum industry is one of the still-emerging, mature sectors capable of addressing the energy crisis issue on a global scale. Crude oil available within rock pores in existing reservoirs is generally extracted by primary, secondary and tertiary recovery techniques. Even after the incorporation of primary and secondary oil extraction methods, about two-thirds of the original oil in place (OOIP) cannot be recovered from the reservoir. At this stage, the application of tertiary or enhanced oil recovery (EOR) techniques by the introduction of tailor-made chemicals come into play [1-3]. Surfactants designed specifically for this purpose are employed to alter the properties of reservoir fluids [2-6]. Modifications or alterations in the surfactant structure at the synthesis stage have proven to be useful in improving its suitability in EOR operations [5, 6]. The surfactant under analysis are generally injected into the reservoir in solution form, as emulsion or foam. Surfactant molecules act by migrating from the bulk aqueous phase to the oilaqueous interface and adsorbing onto the available vacant sites [7]. This reduces the energy barrier required for oil displacement; and interfacial tension (IFT) is reduced. Emulsion stability is important in assessing the surfactant’s ability to effectively displace oil. Surfactant-stabilized emulsions plug the high permeability pores in reservoir, forcing the injected displacing fluid to move through the low permeable pores, resulting in better additional oil recovery [8]. Foaming systems, when introduced into reservoir, decrease the relative gas permeability in reservoirs and aid in improved crude oil mobility control [9, 10]. Foams also help in reducing gravity setting of displaced oil in reservoirs; and also prevent viscous fingering caused by low dispersion of gas in connate water [10]. Therefore, it is necessary to analyse the physicochemical attributes and performance evaluation of surfactant formulations involving different systems to gain a thorough perspective into the utility and functionality of a surfactant system in EOR applications. Gemini surfactants are unique “surface-active agents” that hold enormous potential in petroleum recovery applications [11, 12]. A gemini or dimeric surfactant molecule essentially consists of two hydrophilic heads and two hydrophobic tails with a flexible or rigid spacer chain. Gemini surfactants possess self-aggregation properties at low concentrations, leading to low critical micelle concentration (CMC) values in aqueous systems. Gemini surfactants are proved to exhibit enhanced thermal and surface properties in comparison to their monomeric counterparts [11-14]. Gemini surfactants show tunability due to their ability to self-assemble at low concentrations, in addition to forming stable emulsion systems with hydrophobic compounds [15, 16]. In fact, spacer length plays a pivotal role in understanding the physicochemical


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behavior of gemini surfactant system [15-17]. The growing need for indigeneity and costeffectiveness in commercial operations has generated opportunities in the field of research to synthesize “green” surfactants that are easily available and cost-profitable with low environmental footprint [18-20]. Castro et al. [18] studied the physicochemical properties of a family of non-ionic gemini surfactants from glycolipids, and investigated the effect of structural attributes on surfactant efficacy. Moran and others [19] found that the behavior of amino acidbased gemini surfactants showed better surface and aggregation properties in comparison to similar surfactants of monomeric nature. Performance evaluation of dimeric surfactants showed superiority in terms of surface tension, foaming, emulsification ability and lime-soap dispersibility as reported in earlier studies [20, 21]. Green synthesis and characterization properties of gemini surfactant from waste cooking oil was investigated with improved surface active properties, showing a major stride in recycling sector for the manufacture of surface-active agents [22]. Recently, Hussain et al. [23] reported the characterization properties of novel aminoamide gemini surfactants with low CMC values, surface tension reduction property and thermal stability, confirming their potentiality in oil recovery operations. Literature on the synthesis and characterizations of gemini surfactants synthesized from natural resources for favourable application in EOR is not very extensive. Despite their beneficial traits, gemini surfactants are relatively new in the field of surfactant science and not employed on a large scale. Therefore, strengthening the application of natural gemini surfactants has the ability to create long-term effects in chemical-induced EOR processes. In this paper, the suitability of a family of two bis(monoglyceride-1-hydroxymethyl-2fattyacidester)-α,ω-alkanediether surfactants of dimeric nature for effective application in EOR have been investigated. This work justified novelty as well as significance in two different aspects: synthesizing a novel cost-effective gemini surfactant family from a naturally occurring raw material, i.e. sunflower oil, and achieving favourable interfacial, rheological and stabilizing properties by the analysis of GS-based aqueous, emulsion and foam systems for suitable application in chemical oil recovery processes [24]. The effect of temperature and spacer length (s = number of aliphatic methyl atoms in spacer chain) on the aggregation and surface properties of gemini surfactant (GS) systems have been discussed. Surface tension experiments were performed to determine the CMC of the surfactants in aqueous media. The performance properties of GS solutions were evaluated in terms of hydrolytic stability, salt tolerance, hardness


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tolerance and lime soap dispersing ability to predict the efficacy of synthesized surfactant systems. The decay behavior of GS-stabilized foams were measured as a function of time elapsed,

surfactant

concentration

and

temperature.

Emulsification

abilities

of

n-

heptane/GS/aqueous systems were studied using stability studies at different time intervals. Material and Methods The starting raw material for GS syntheses, sunflower oil, was obtained from the local market. Glycerol and calcium oxide were used during the first step of synthesis, during which transesterification of vegetable oil in triglyceride form occurs to obtain a monoglyceride ester intermediate. Potassium hydroxide (KOH) and tetra butyl ammonium bromide (TBAB) were employed during the second stage of synthesis. The GS spacer length was controlled by the addition of 1,4-dibromobutane or 1,6-dibromohexane into the reaction mixture. The drying of the obtained solution was achieved using anhydrous sodium sulfate (purchased from TCI Chemicals). Vacuum filtration of the solution was conducted using a Buchner funnel apparatus fitted with a Tarsons Rocky Vac 300 Pump to extract the final surfactant in liquid form. For experimental studies, double distilled water (DDW) extracted from water distillation apparatus was used. The crude oil was obtained from Ahmedabad oil field. Crude oil analysis showed a total acid number (TAN) of 0.044 mg KOH/g, kinematic viscosity of 61.47 cSt and gravity of 23.55° API at 303 K. Sulfuric acid and sodium hydroxide employed during stability studies were collected from Sigma Aldrich Co. During salt tolerance, the effect of sodium chloride salt (NaCl) from Rankem Chemicals on stability was investigated. Hard water was prepared in the laboratory by addition of CaCO3 during hardness tolerance and lime soap dispersibility studies. Heptane used as oil phase during emulsification studies was obtained from Alfa Aesar. Laboratory synthesis of gemini surfactants (GSs) Sunflower oil (0.05 mol, 43.89 gm) and glycerol (0.15 mol, 13.80 gm) were homogenously stirred in the presence of calcium oxide (CaO) catalyst during transesterification. During this process, organic group of glycerol replaced the organic group of ester in presence of base catalyst. After the dispersion of CaO (1.0% of the total weight of oil) in the reaction mixture, the temperature was increased to 353 K for a period of 45 minutes. Thereafter, the reaction was allowed to continue at 493-513 K for four hours in nitrogen atmosphere. Excess glycerol collected at the bottom of reaction mixture was extracted by washing with water, and


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subsequently evaporated under vacuum to form intermediate as a yellow semi-viscous liquid. The ester was not formed in ionized state and therefore, not soluble in water. However, glycerol ester was soluble in organic solvents such as methanol. Hence, the formation of glycerol ester (intermediate product) was confirmed by solubility test in methanol. Glycerol ester intermediate (0.05 mol, 18.35 gm) was added to 20% potassium hydroxide (KOH) solution (45 ml) and tetrabutylammonium bromide TBAB (0.50 gm). The mixture was stirred vigorously for 30 minutes at 313 K, followed by dropwise addition of 1,4-dibromobutane (0.025 mol, 5.40 gm) or 1,6-dibromohexane (0.025 mol, 6.10 gm). At the end of addition, the resulting solution was continuously stirred at 373 K for 24 hours. The organic phase was dried and separated over sodium sulphate (anhydrous). Then, the product was filtered under reduced pressure using Buchner funnel apparatus. The final non-ionic gemini surfactant (GS) was obtained as yellowish viscous liquid. Reactions showing the two stages of surfactant synthesis from sunflower oil are depicted in Fig. 1.

Fig. 1. Reaction equations showing two stages of synthesis of non-ionic surfactant from sunflower oil.

Surfactant characterization by 1H-NMR and TGA methods


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Fourier transform-nuclear magnetic resonance (NMR) employs pulse waves of radio-frequency to excite the nuclei in magnetic field. 1H-NMR spectra was obtained in Bruker AVANCE III 500 MHz (AV 500) multi nuclei solution NMR Spectrometer. The specimen was dissolved in a deuterium (D2O) lock solvent in 5-mm NMR tube. The concentration of gemini sample used must be not be very low to avoid undesirable low signal-to-noise ratio and also not very high to minimize exchange effects. Hence, an optimal amount of surfactant was dissolved in the solvent for characterization. The same procedure was used for both synthesized surfactants. For TGA analysis, the GS samples (≥ 10 mg) were placed in platinum (Pt) sample holders and studied in argon

(Ar)

atmosphere

at

a

heating

rate

of

5

Kelvin/minute

in

Diamond

Thermogravimetric/Differential Analyzer (Perkin Elmer, USAA). Surface tensiometry CMC values of SF-4-SF and SF-6-SF surfactants at different temperatures were identified using surface tensiometry measurements in Easy Dyne K20 Tensiometer instrument. Using Du Noüy ring technique, the surface tension was measured as the amount of force needed to lift a platinum ring slowly from the surface of solution. The CMC values of each synthesized surfactant system were recorded at 303 K, 323 K and 343 K. Hydrolytic stability 15 ml of GS solutions (at corresponding CMC) were initially mixed with 15 ml of 0.05 M base (NaOH) in alkaline hydrolysis test. Similarly, acid hydrolytic stability test was performed by adding 15 ml of 1.00 M sulphuric acid (H2SO4) to surfactant solution. At 303 K, aqueous solution containing 0.05 M NaOH has a pH value of 13.34, whereas the pH of 1.00 M H2SO4 solution in distilled water is 0.5. Therefore, addition of non-ionic GS aids in understanding the influence of pH change from neutral to acidic/basic condition as well as in predicting the extent of surfactant’s suitability in EOR processes. The time required by the initially transparent sample solution(s) to change into a clouded phase was taken as measure of hydrolytic stability. Miscibility studies The crude oil miscibility was investigated by visual observation for synthesized GS solutions at varying temperatures. Crude oil and aqueous surfactant solution (at corresponding CMC values) were poured in the ratio 1:1 into a borosilicate glass tube, and mixed continuously for 12 hours in


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a Rivotek horizontal shaker. The miscibility time of crude oil in the prepared solution was noted to predict the surfactant’s ability to produce tertiary oil. Rheological investigations The viscosities of aqueous gemini surfactant solutions in the absence and presence of crude oil sample were studied in a cup-and-bob (coaxial cylinder) setup with the aid of Bohlin Gemini 2 Rheometer (Malvern Instruments Limited, UK). Temperature conditions were varied for each sample solution with shear rates ranging between 0.1 and 1000 s-1. The viscosity of pure GS solutions were compared with crude oil-aqueous solution mixtures in order to predict the rheological characteristics as a result of crude oil miscibility in the displacing fluid (injected surfactant solution). Oil-aqueous IFT measurements The spinning drop technique was employed to study the IFT behavior of surfactant solutions with varying concentrations in aqueous phase. IFT values were measured using SVT20 tensiometer (Dataphysics, Germany) at different temperatures. Crude oil was injected into a capillary tube filled with GS solution and allowed to rotate at 3500 rpm. When a stable oil drop was formed, profile-fitting of the crude oil drop was performed to determine the value of IFT. Before each study, the capillary tube was rinsed with toluene to remove oil traces, washed with acetone to remove surfactant/salt traces and finally dried. Salt tolerance Aqueous solutions of synthesized surfactants, SF-4-SF and SF-6-SF were tested to check their corresponding tolerance to the presence of salt ions. NaCl salt was added in increasing concentrations to GS solutions, and rotated at 2500 rpm for 15 minutes in Remi R-24 Research Centrifuge. Salt tolerance studies were conducted by heating the solution under analysis to desired temperatures and visually inspected for precipitate formation. Hardness tolerance The hardness tolerance of surfactant was measured as the amount of calcium carbonate required to cause precipitation of aqueous GS solution. As per the United States Geological Survey, hard water and very hard water can be classified as containing ≥120 ppm calcium carbonate (CaCO3) in water. The concentration of CaCO3 was gradually increased and the resulting solution was stirred to achieve homogenous mixing. The solution was then allowed to settle for a period of 20


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minutes to observe any coagulating or precipitating effect of the synthesized GSs. This experiment continued until bailing-out effect of GSs was observed and the solution tolerance limit was identified. Lime-soap dispersing ability Initially, lime soap solution was prepared as a precipitated solution in the laboratory by uniform mixing of 5 ml oleic acid (0.5 wt. %) solution and 10 ml hard water (prepared by addition of 200 ppm CaCO3 in water). Thereafter, aqueous solutions of GSs were prepared and gradually added in small doses to the earlier prepared lime solution. The resulting mixture was then rotated for a constant period of 10 minutes. This process was continued till precipitation or coagulation in the cylinder eventually disappear. The amount of surfactant solution, required for a continuous single phase solution with no visible precipitates, was noted and used in the determination of lime soap dispersion requirement (LSDR) as shown in Equation (1). 𝐿𝑆𝐷𝑅 =

𝑉𝐺𝑆 × 𝐶 × 100

(1)

0.5 × 10 × 𝑉𝑜

In the above relation, VGS is the volume of GS solution added to the initially prepared lime soap solution (10 ml) whereas VO is the volume of oleic acid solution added (5 ml). Smaller the value of LSDR %, greater is the dispersing ability or dispersibility of the surfactant under analysis. Foam generation and stability studies During foam behavior studies, synthesized GSs in predetermined concentrations were initially added in 100 ml of distilled water and allowed to mix at 400 rpm using magnetic stirrer till the solution formed a homogenous mixture. The solution was then agitated at 3000 rpm using a laboratory waring blender for a period of 180 seconds. The foam, thus formed, was poured into a 500 ml measuring cylinder to investigate its stability at different time intervals. The foam stability was measured as a time-dependent function of foam volume decay ratio for both GS systems at varying concentrations under analysis. Liquid drainage from the total foam volume was noted with elapse of time, and their corresponding decay ratios were calculated. Half-life time determination was another approach employed as a standard measure of foam stability. It was calculated as the time required by foam to reduce to one-half of its initial foam height (at time = 0). Atmospheric pressure conditions were employed for all experiments with temperature values at 303 K and 343 K. In order to ensure repeatability of results, all tests were conducted at least three times and the average values were reported. The microscopic images of freshly


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prepared foams were obtained with the aid of Olympus Microscope (BX51) equipped with Stream Image Analysis software. The viscosity of GS-stabilized foam systems were analysed at different temperatures using a coaxial cylinder assembly setup in Bohlin Gemini 2 Rheometer (Malvern Instruments Limited, UK). Emulsion formulation and characterization Emulsions were formed by mixing aqueous GS solution and oil (n-heptane) with the help of magnetic stirrer at 1200 rpm for period of 4 hours. 9 ml aqueous solution and 1 ml oleic phase were added during emulsification process. This allowed uniform mixing of oil and water phases in the presence of surfactant. Surfactant concentrations were varied to study their relative stabilities. The prepared emulsions were stored in cylindrical glass tubes at 303 K, 323 K and 343 K and their phase behavior images were taken at regular intervals over a course of 15 days. The size distribution of emulsion droplets were investigated at 303 K using Zetasizer Nano S90 (Malvern, Germany) light scattering apparatus equipped with He-Ne laser (633 nm, max. 4 mV). Results were obtained at 90° scattering angle using 21CFR part 11 software to determine the stability of prepared emulsions. Olympus Microscope (BX51) was employed to study the morphology of the emulsion droplets. Results and Discussion Characterization of synthesized surfactants 1H-NMR

studies

The chemical structures of the synthesized gemini surfactants, SF-4-SF and SF-6-SF, were confirmed using 1H-NMR spectroscopy results. The description of each peak signal obtained is assigned to eight local environments of the GS molecules as shown in Table 1. The respective 1H-NMR

plots showing signal intensity versus chemical shift values for SF-4-SF and SF-6-SF

surfactant is shown in Figs. 2(a) and 2(b). The presence of characteristic peaks at (δ) values of 0.961, 0.958 ppm (triplet, SF-4-SF) and 1.293, 1.290 ppm (singlet, SF-6-SF) correspond to the presence of terminal methyl (-CH3) and methylene (-CH2-) groups constituting long chain triglycerides. The presence of unsaturation for each GS is evident from a singlet at 1.907 and 1.946 ppm (δ) in the NMR spectra, showing the presence of double bonds (H2C=CH=CH2) in the fatty acid chains. The presence of a doublet at chemical shift (δ) values of 2.492-2.569 ppm; and 2.248-2.277 ppm is attributed to the existence of two ester groups (R’-COO) in the GS molecule.


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Peaks of varying intensities are obtained at 3.315-3.425 ppm for SF-4-SF and 3.324-3.481 ppm for SF-6-SF, exhibiting the effect of protons situated in the polymethylene spacer chain in GS molecule. Resonance signals in the range between 3.605-3.757 ppm and 3.539-3.916 ppm correspond to the ether functional groups present in the synthesized gemini compounds. Characteristic signals in the range 4.621-4.785 ppm for SF-4-SF and 4.496-4.628 ppm for SF-6SF is indicative of the proton bonded to oxygen atom, i.e. -OH groups present in the synthesized GSs. The olefinic protons of fatty acids is identified due to a multiplet in the chemical shift region in the range 5.045-5.289 ppm and 5.016-5.279 ppm for SF-4-SF and SF-6-SF respectively. The above results show that the non-ionic GSs synthesized from sunflower oil, is a mixture of mono/di-glyceride esters of unsaturated and saturated fatty acids such as linoleic acid, oleic acid, palmitic acid and stearic acid. The synthesized surfactants, hence, consist of a dimeric structure with fatty acid ester bonds located on either side of polymethylene aliphatic spacer and ether head (polar) groups. Table 1. 1H-NMR data showing description of proton type for synthesized non-ionic GS compounds. Peak number 1

Chemical shift (ppm) of peak SF-4-SF SF-6-SF 0.961 0.958 14-4-14 14-6-14

Structural assignment Terminal –CH3 and –CH2 groups present in the glyceride

2

1.293

1.290

alkyl tail groups

3

1.907

1.946

H2C=CH unsaturation(s) present in fatty acid chain

4

2.492-2.569

2.248-2.277

Two ester groups in triglyceride alkyl groups

5

3.315-3.425

3.324-3.481

-CH2 protons situated in gemini spacer chain

6

3.605-3.757

3.539-3.916

Ether polar functional head groups in dimer structure

7

4.621-4.785

4.496-4.628

Intramolecular –OH groups

8

5.045-5.289

5.016-5.279

Olefin groups (double-bonded carbon atoms) present in fatty acid chain


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Fig. 2. 1H-NMR spectra showing resonance signals at varying chemical shift (ppm) values for gemini surfactants: (a) SF-4-SF and (b) SF-6-SF.

Thermal gravimetric analyses (TGA) The thermal stabilities of the synthesized GSs aid in predicting gemini surfactant characteristics at elevated temperatures. Fig. 3(a) and Fig. 3(b) show the thermal loss curves of SF-4-SF and SF-6-SF surfactants in the temperature range between 298 K and 573 K. Two stages of thermal degradation are identified by thermogravimetric studies. A sharp slope change in the first-order derivative plot is concurrent with the sample mass loss in the weight loss versus temperature curve. Initial degradation starts at 373 K showing the loss of intramolecular hydroxyl (–OH) groups with an average mass loss of 3.47% and 3.37% for GSs with four and six methylene atoms in the spacer respectively. This is identified by the presence of a negative peak in the derivative curve. Thereafter, a second loss in surfactant weight occurs at a temperature of 448 K due to the decomposition of ester groups present in GS structure. 14.19% and 13.38% degradation losses are observed at this stage for SF-4-SF and SF-6-SF surfactants respectively. A


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downward peak is obtained in the weight derivative versus temperature plot at this region, indicating the degrading influence of temperature on surfactant structure. This thermal loss escalates sharply due to subsequent decomposition of the carbon atoms in the sunflower oil fatty acid chain, in addition to the previously degraded triglyceride ester bonds. The influence of temperature rise on the synthesized surfactants ceased at about 518 K for both analysed specimens. Therefore, it is concluded that the synthesized GSs are capable of retaining their respective structures under varying reservoir conditions. 100 3.47 wt.% loss (SF-6-SF) 3.37 (SF-4-SF)

95

Effect of

(a)

temperature rise on GS ceases

3.47 wt.% loss (SF-4-SF) 14.19 wt.% loss (SF-4-SF)

90

13.38 wt.% loss (SF-6-SF)

s-4-s s-6-s

85

First degradation stage

Weight percent (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

75

Second Degradation Stage

70 298

323

348

373

398

423

448

473

Temperature (K)


498

523

548

573

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0.05 0.00 -0.05

(b)

-0.10 -0.15

s-4-s s-6-s

-0.20

First degradation stage

Surfactant weight derivative (% / K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-0.25 Loss of

-0.30

-OH groups

-0.35

Decomposition of ester groups

Second Degradation Stage

-0.40 298

323

348

373

398

423

448

473

498

523

548

573

Temperature (K)

Fig. 3. Results of (a) thermogravimetric analyses (TGA); and (b) differential thermogravimetric (DTG) analysis of synthesized non-ionic GSs with different spacer lengths (s = 4, 6) in the temperature range of 298 K to 573 K.

CMC determination by surface tensiometry Surface tension data of aqueous GS solutions with varying concentrations are analysed in order to determine their CMC values at different temperatures. When surfactant CMC is reached, the molecules begin to form aggregates and surfactant adsorption process ceases at the air-aqueous interface. Therefore, surface tension value is observed to be minimum at CMC and does not decrease further at higher concentrations. The CMCs of SF-4-SF as well as SF-6-SF surfactants along with their corresponding surface tension values are tabulated in Table 2. With increase in spacer chain length, the CMC value is observed to increase as a result of improved hydrophobic nature of SF-6-SF in comparison to SF-4-SF. Greater the surfactant hydrophobicity, greater is its relative tendency to remain in un-aggregated state in solution. The micellization of synthesized GSs is delayed with temperature increase. This behavior is attributed to the increased migration of GS molecules to the ‘vacant’ sites onto the interface, whereas the iceberg structures formed by intermolecular GS-water bonding are gradually destroyed with temperature in the bulk phase.


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Table 2. Surface tension and CMC data of synthesized gemini surfactant solutions at 303 K, 323 K and 343 K. Non-ionic

Temperature (K)

CMC (mM)

GS solution

SF-4-SF

SF-6-SF

Surface tension at the airaqueous interface (mN/m)

303

0.1496

27.6

323

0.1995

22.6

343

0.2244

19.4

303

0.2409

25.1

323

0.3373

19.4

343

0.3855

18.0

Influence of temperature on CMC of synthesized GSs In aqueous systems containing GSs, water molecules form iceberg structures due to hydro-bond arrangement with surfactant dimer molecules. The relative tendencies of GS molecules to adhere to or segregate from these structures is the primary driving phenomenon for micelle formation in the bulk phase [25]. The correlation between CMC of gemini and temperature (T) is described by an empirical equation of second order as shown in Equation (2). ln(𝐶𝑀𝐶) = 𝐴 + 𝐵𝑇 + 𝐶𝑇2

(2)

The experimental CMC values are fitted using the above equation. The fitting parameters along with polynomial model plots are shown in Fig. 4. The CMC values are found to increase with temperature due to enhanced molecular velocities of GS molecules as a consequence of improved thermal activity of the aqueous solution [25]. This leads to shortened interactions among the GS alkyl tail chains. Therefore, temperature significantly influences the micellization processes in GS aqueous solution. Also, CMC is observed to increase with spacer length due to enhanced hydrophobic character of the dimeric surfactant molecules. This behavior is attributed to the strengthening influence of motion of ordered GS molecules from free solvated state to micellar form, which decreases solution entropy.


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-5.50 -5.75 -6.00 -6.25

Natural logarithm of CMC

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-6.50

-7.00 -7.25

y = Intercept + B1*x^1 + B2*x^2

Equation Model Residual Sum of Squares

2nd order polynomial 2.41104E-4

-3.27085E-4

0.99547

0.99542

Adj. R-Square

Value

-6.75 SF-4-SF

SF-6-SF

-7.50

Page 16 of 40

Intercept B1 B2 Intercept B1 B2

SF-4-SF SF-6-SF

--

Standard Error

-31.66548 0.1332 -1.90637E-4 -35.66374 0.15962 -2.29088E-4

3.05765 0.01896 2.93443E-5 3.56136 0.02208 3.41784E-5

-7.75 -8.00

2 ln (CMC) = -35.66374 + 0.15962 T - 0.00022909 T 2 R = 0.99542

-8.25 -8.50

2 ln (CMC) = -31.66548 + 0.13320 T - 0.00019064 T 2 R = 0.99547

-8.75 -9.00 -9.25 293

303

313

323

333

343

353

Temperature (K)

Fig. 4. Plots showing the fitted models for synthesized GS solutions between natural logarithm of CMC and temperature. Solid lines represent fitting data obtained using second order polynomial equation.

Hydrolytic stability The hydrolytic stabilities of non-ionic GS systems at corresponding CMC values in acidic and alkaline media at varying temperatures are shown in Table 3. During NaOH addition, pH value of pure surfactant solution is observed to increase from 7.44 to 11.82 (SF-4-SF) and 7.69 to 12.31 (SF-6-SF) at 303 K. On the contrary, addition of H2SO4 acid shows decrease in pH to acidic region to 2.54 and 2.93 for SF-4-SF and SF-6-SF surfactants respectively. Hydrolytic stability times are measured as ~26 min (SF-4-SF) and ~32 min (SF-4-SF) under alkaline condition. GS/acid solutions show much improved stabilities, to values as high as ~10-11 hrs for SF-4-SF and ~12-13 hrs for SF-6-SF systems. The synthesized surfactants show lower stability in the presence of sodium hydroxide (NaOH). This is because of the presence of ester groups in non-ionic surfactant molecules, which are more easily hydrolysed in basic solution. On the other hand, acid-catalysed hydrolysis of ester-based GSs is very slow, thereby leading to very long stabilities. Time required for base-catalysed hydrolysis of GS solutions does not exceed 60 min, whereas acid-hydrolysis of GS solution is observed after a considerable time-period. Thus, SF-4SF and SF-6-SF surfactants are more susceptible to attack from hydroxide ions (-OH) and effectively retain their ability to remain stable in dilute acid phase. Hydrolytic stability increases


Page 17 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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with increasing spacer chain length due to the decreased hydrophilicity of the GSs. The nonionic head groups do not face repulsive electrostatic interactions and remain farther apart for SF6-SF system, thereby exhibiting better stability in different media. Temperature increase is found to have an inverse effect on the hydrolytic stability of aqueous GS solutions. With rise in temperature, thermal activity of the molecules are enhanced, resulting in increased rate of hydrolytic reaction. At elevated temperature (343 K), basic solutions show shorter stabilization times in comparison to acidic systems with pH value increasing to as high as 11.13 and 11.65 for SF-4-SF and SF-6-SF surfactants respectively, and lasting for a few minutes (< 5 min). The pH value is observed to decrease to 2.01 for SF-4-SF/H2SO4 system (~ 80 min) and 2.42 for SF-6-SF /H2SO4 system (~115 min) in acidic medium, showing better hydrolytic stabilities. This proves that non-ionic GS solutions possess the capability to retain their tolerance to hydrolysis in the presence of acidic crude oil and other hydrophobic components trapped within rock-pores. Table 3. Time stabilities of GS solutions (at CMC) in acidic and basic media at different temperatures GS

T (K)

CMC (mM)

Surface tension (mN/m)

pH of GS solution

pH of GS/H2SO 4

pH of GS/NaOH solution

Hydrolytic Stability to acid (hh:mm:ss)

Hydrolytic Stability to base (hh:mm:ss)

solution

SF-4-SF

SF-6-SF

303

0.1496

27.6

7.44

2.54

11.82

10:45:00

00:26:28

323

0.1995

23.3

7.29

2.28

11.49

04:10:00

00:09:16

343

0.2244

19.4

7.07

2.01

11.13

01:20:00

00:02:40

303

0.2409

24.6

7.69

2.93

12.31

12:50:00

00:31:50

323

0.3373

19.1

7.51

2.51

11.96

06:35:00

00:11:02

343

0.3855

15.4

7.36

2.42

11.65

01:55:00

00:04:50

Crude oil miscibility In the presence of synthesized surfactants, the crude oil is observed to mix completely with aqueous phase. Fig. 5 shows the time-dependent miscibility of crude oil in SF-4-SF and SF-6-SF surfactant solutions (at corresponding CMC) at 343 K. When surfactant slug is injected, formation of in-situ emulsions within the reservoir is a desirable mechanism for recovery processes. This in-situ emulsion offers an ultra-low IFT compared to simple surfactant solution and loss of surfactant by adsorption is lesser [26-28]. Surfactants are effectively adsorbed onto


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the oil droplet-aqueous interfaces, which provides desirable oil-attracting properties [29, 30]. This leads to dispersion of oil trapped within rock pores, causing the alteration of nature of oilsaturated rock grains to a favorable water-saturated state [27-29]. As a result, crude oil trapped in reservoir pore-throats are displaced and produced by emulsification process. Initially, after mixing, a single continuous emulsion phase is observed, showing complete miscibility of crude oil in surfactant solutions. However, the surface interaction energy gradually increase with time, resulting in decrease in oil saturation. Phase separation first begins at the end of 6 hours for SF4-SF system; and after 12 hours for SF-6-SF system. This is indicative of the accumulation of crude oil (lower density) at the top of the solution as a result of decreasing miscibility. The higher density aqueous GS solution accumulates at the bottom of the tube. At the end of 48 hours, the oil and aqueous phases exhibit very low miscibility. Complete immiscibility is not observed, due to partitioning of GS molecules onto the oil-aqueous interface. Crude oil remains miscible in SF-6-SF solution as compared to SF-4-SF solution. Therefore, better oil recovery is expected in case of SF-6-SF slug injection. Miscibility is a favorable trait for chemical injecting fluids since a dynamic shear always exists in a reservoir and interfacial energy required to flow the crude oil trapped within rock pore-throats must decrease.


Page 18 of 40

Page 19 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fig. 5. Images showing crude oil miscibility at 343 K in aqueous solution containing (a) SF-4SF; (b) SF-6-SF surfactants at different time intervals. Viscosity improvement in the presence of crude oil The flow behavior of crude oil emulsions is important in assessing the ability of injected surfactant solution to solubilize crude oil remaining trapped within the pores of reservoir rocks. Fig. 6 shows the effect of transition of surfactant solutions into crude oil/GS/aqueous emulsions at 343 K. The viscosity of surfactant solutions generally follow pseudoplastic behavior up to a critical shear rate value. The pseudoplastic or shear thinning character of surfactant solutions is beneficial for chemical oil recovery operations due to better injectivity of surfactant solution into reservoir formation. As GS spacer length increases, the stability of crude oil-aqueous emulsion systems increase due to delayed micellization of GS molecules in aqueous phase and subsequent increase in CMC value. It is observed that the viscosity values of GS aqueous phase decrease from 30-45 mPa.s at very low shear rates to 3.99 mPa.s for SF-4-SF (0.2244 mM; CMC) and 4.91 mPa.s for SF-4-SF (0.3855 mM; CMC). In the presence of crude oil, a single-phase continuous emulsion is formed. The GS molecules adsorb onto the interface and interact with crude oil components to form ordered structures in the crude oil phase [31, 32]. As a result, the oil displacement ability is found to improve drastically in oil/GS/aqueous systems. From analysis of the viscosity versus shear rate plots, it is evident that viscosities of SF-4-SF and SF-6-SF


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based crude oil emulsions staggeringly increase to values as high as 133.68 mPa.s and 173.71 mPa.s respectively due to crude oil addition. The formation of crude oil-based emulsions increases the viscosity of the displacing phase, resulting in plugging high-permeability pores in mature reservoirs [33, 34]. This forces the displacing fluids to enter into and pass through the rock pores with low permeabilities, leading to improved mobility control and oil productivity performance [32-35]. SF-4-SF (CMC) solution

SF-4-SF / crude oil emulsion

SF-6-SF (CMC) solution

SF-6-SF / crude oil emulsion

1000

Viscosity (mPa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

100

10

1 1

10

100

1000

-1

Shear rate (s )

Fig. 6. Viscosity versus shear rate for SF-4-SF and SF-6-SF surfactant systems (at corresponding CMC) in the absence and presence of crude oil at 343 K.

Interfacial tension (IFT) at the crude oil-aqueous interface A desirable mechanism for oil recovery is the reduction of IFT due to the adsorption of surfactant molecules at the oil-aqueous interfaces [36, 37]. Fig. 7 shows the variation of IFT with salinity (NaCl, wt. %) at 303 K, 323 K and 343 K. Lower IFT values are achieved in case of SF6-SF than SF-4-SF solutions, indicating better oil displacing ability. For pure SF-4-SF and SF-6SF solutions at corresponding values of CMC, the respective values of IFT at the oil-aqueous interface are measured as 4.546×10-1 mN/m and 3.195×10-1 mN/m at 303 K; 1.719×10-1 mN/m and 1.237×10-1 mN/m at 323 K; and 9.482×10-2 mN/m and 8.998×10-2 mN/m at 343 K. In the presence of salts, IFT is observed to reduce to ultra-low magnitudes of the order of 10-2 mN/m. This is attributed to the better partitioning of GS molecules and subsequent thinning of the crude


Page 21 of 40

oil-aqueous interface [38]. As evident from the figure, IFT initially decreases with salinity up to a specified point (optimal salinity). At 343 K, crude oil/aqueous GS systems showed IFT reduction to as low as 1.038×10-2 mN/m and 8.624×10-3 mN/m for SF-4-SF and SF-6-SF systems respectively. Beyond optimal salt concentrations, repulsive interactions among salt ionsalt ion groups become dominant, resulting in bulk aggregate formation and slight increase in IFT. 1

303 K

323 K

343 K

(a)

Interfacial tension (mN/m)

Optimal salinity

0.1

0.01 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Salinity (wt.% NaCl) 1

303 K

323 K

343 K

(b)

Optimal salinity

Interfacial tension (mN/m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1

0.01

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Salinity (wt.% NaCl)

Fig. 7. Oil-aqueous IFT versus salinity plots at different temperatures for (a) SF-4-SF; and (b) SF-6-SF surfactant systems.

Salt tolerance studies Fig. 8 shows the salt tolerance values of the synthesized GS solutions at CMC at 303 K, 323 K and 343 K. The tolerance levels of non-ionic GSs at corresponding CMC values are investigated


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by gradually increasing the percentage of salt (NaCl) in aqueous surfactant solution, till precipitation or coagulation is observed. The synthesized GSs does not precipitate up to 24% NaCl for SF-4-SF and 25% NaCl for SF-6-SF at a temperature of 303 K. This behavior may be attributed to the bailing-out effect of GS molecules in aqueous solution [39, 40]. At salt concentrations higher than tolerance levels, self-association among water molecules via hydrogen bonding are promoted. This tightens the structure of water and weakens its solubilizing power. As a result, the number of water molecules available for interaction with the polar head of GSs are reduced and causing bailing-out effect due to salt-induced precipitation. The salt tolerances for SF-4-SF and SF-6-SF surfactants are identified as 28% NaCl and 30% NaCl respectively at 323 K. At 343 K, salt tolerance limit improved to respective values of 32% NaCl and 34% NaCl for SF-4-SF and SF-6-SF. With rise in temperature, the solubilities of salt ions as well as GS molecules increases, resulting in higher salt tolerance levels. 35

SF-4-SF SF-6-SF

30

Salt tolerance (% NaCl)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

25 20 15 10 5 0 303

323

343

Temperature (K)

Fig. 8. Salt tolerance of synthesized SF-4-SF and SF-6-SF surfactants at CMC.

Hardness tolerance Hardness of water is determined by the concentration of multivalent cations, caused due to the presence of high mineral content [41, 42]. The hardness tolerance of surfactant-hard water mixtures in terms of ppm CaCO3 versus temperature is shown in a 2D bar diagram in Fig. 9. The composition of calcium carbonate (CaCO3) in aqueous phase is increased gradually until the solution mixture shows precipitation of GSs under experimented conditions. It is observed that hardness tolerance decreases with increase in CaCO3 concentration. This is because surfactant precipitates are formed more easily when divalent ion concentration in water is higher. The


Page 23 of 40

calcium ions (Ca2+) ions destroy the surfactant properties of soap by forming soap-scum. This property contributes to the soap-consuming capacity of the GS solution that prevents the lathering or foaming of soaps. Increasing temperature promotes the formation of calcium carbonate instead of bicarbonate, resulting in decreased solubility. Therefore, the hardness tolerance of GS solution shows the reverse effect with temperature rise. Respective hardness limits for SF-4-SF and SF-6-SF aqueous surfactant solutions are found to be 3000 ppm (CaCO3) and 4400 ppm (CaCO3) at 303 K; 2800 ppm and 4000 ppm at 323 K; and 2500 ppm and 3800 ppm at 343 K. When the concentration of CaCO3 in aqueous GS solution exceeds tolerance limit, scaling phenomenon is observed through CaCO3 precipitation. This reduces the concentration of Ca2+ ions present in the solution, thereby reducing the calcium stability of the aqueous sample. SF-6-SF shows better hardness tolerance than SF-4-SF surfactant systems due to its improved hydrophobic character, which allows the GS molecules to interact more effectively with water molecules and delays the scaling process. SF-6-SF

353

SF-4-SF 3800

343

Temperature (K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2500

333

4000

323

2800

313

4400

303

3000

293 0

1000

2000

3000

4000

5000

Hardness Tolerance (ppm CaCO3)

Fig. 9. Hardness tolerance expressed in terms of ppm CaCO3 at different temperatures for GS aqueous solutions at CMC.

Lime-soap dispersing ability Lime-soap dispersing ability is a relative measure of the ability the surfactant to disperse uniformly throughout the continuous phase [43]. The lime-soap dispersing ability results for both synthesized GS systems at different concentrations are shown in Fig. 10. Proper selection of optimal formulations showing good LSDR rates as well as favourable shelf-life on the basis of


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application is extremely critical [44]. SF-6-SF GS system shows lower percentage values of lime soap dispersion requirement (LSDR), showing better dispersibility over SF-4-SF systems. This may be attributed to the larger stearic hindrance generated by SF-4-SF molecules at the interface. The number of surfactant molecules arranged at the interface is more in case of GS systems with shorter spacer chain length. This leads to greater stearic hindrance effect in which the polar group is seemingly weakened by the neighbouring polar groups of other GS molecules. As temperature increases, their lime-soap dispersing abilities increase due to increased solubilization of the added GS molecules. This causes the disappearance of formed coagulants in solution and, consequently, increases surfactant dispersibility. The LSDR values of SF-4-SF and SF-6-SF surfactants at corresponding CMC values are found to decrease with increasing GS concentration due to the transition of aggregates existent at low concentrations into micellar structures in aqueous phase. This reduces the size of the surfactant area available for ion-surfactant interactions and reduces the LSDR percentage values. Moreover, the surfactant systems shows kinetic stabilizations over a considerable time period. This is measured as the time required for the mixture to return to its previous turbid state at a specified temperature. This time span is referred to as the shelf-life of surfactant solution. Though temperature rise reduces the LSDR% of GS solutions, it is also found to reduce the shelf-life of the solution. It is noted that the destabilization process is irreversible. 200

20.0

303 K

323 K

(a)

343 K

175

17.5 150

LSDR (%)

15.0

125 100

12.5

75 10.0 50 7.5

25

5.0

0 40

80

120

160

200

240

280

320

Surfactant concentration (ppm)


360

400

Destabilization times (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

Page 25 of 40

20.0

LSDR (%)

303 K

323 K

(b)

343 K

200

17.5

175

15.0

150

12.5

125

10.0

100

7.5

75

5.0

50

2.5

25

0.0

Destabilization times (min)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 40

80

120

160

200

240

280

320

360

400

Surfactant concentration (ppm)

Fig. 10. Lime soap dispersion requirement (LSDR) percentages and destabilization times for aqueous GS solutions with varying concentrations of (a) SF-4-SF; (b) SF-6-SF at 303 K, 323 K and 343 K. Solid lines represent LSDR% and dotted lines show destabilization time values.

Foam stability and rheological measurements The foaming behavior of surfactant solutions is of enormous significance due to its wide range of applications. In this study, the synthesized non-ionic GSs are analysed to investigate their foaming stabilities as a function of concentration and temperature. When the prepared foam shows complete stability to drainage and coalescence, the decay ratio is found to be unity [45]. On the contrary, phenomenon of complete foam breakdown corresponds to a value of zero foam decay ratio. In order to compare the relative foam stabilities of aqueous GS solutions at varying temperatures, the foam volume decay rate (Rt) is calculated from the equation (3). 𝑅𝑡 =

𝑉𝑡𝑜𝑡𝑎𝑙 ― 𝑉𝑑𝑟𝑎𝑖𝑛𝑎𝑔𝑒

(3)

𝑉𝑡𝑜𝑡𝑎𝑙

where, Vtotal and Vdrainage respectively correspond to the values of total foam volume and free drainage volume at a specified time (t). Fig. 11 shows the foam decay rate versus time plots for SF-4-SF and SF-6-SF solutions in air-aqueous systems. As time elapses, the height of foam gradually decreases, resulting in reduction in decay ratio. Aqueous foam is a metastable system that shows continuous drainage of aqueous phase from plateau border or lamella of foam bubbles. Slower the rate of drainage, greater is the stability of foam [46, 47]. This results in the formation of larger foam bubbles and a marked increase in the number of liquid drainage


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channels. The microscopic images showing the variation of foam bubbles formed in the presence of SF-6-SF surfactant (at CMC) at the end of 120, 600, 900 and 1500 seconds are depicted in Figs. 12(a), 12(b), 12(c) and 12(d) respectively. Initially, smaller size bubbles are uniformly distributed throughout the area under analysis, which begin to gradually increase with time. A state of non-uniformity in foam bubble distribution with both small and large size bubbles is observed at intermediate times. Eventually, the small bubbles coalesce and give way to the appearance of large bubbles only with reduced foam stability. Therefore, it is evident that the probability of film rupture increases and coalescence of foam bubbles occurs with passage of time. This causes foam instability and increase in the average size of foam bubbles. Analyses of foam stability curves show that the decay ratio, Rt increases with increasing GS concentration in aqueous solution. At low surfactant concentrations, effective tendency of liquid phase to withdraw through the foam border is observed, leading to thinning of the foam and generation of a surface tension gradient inside the bubble. With increasing GS concentrations above CMC value, the Van Der Waals forces of attraction between the bubble surfaces are weakened and the bubble movement reduces significantly, resulting in slower film rupture. Therefore, a critical thickness of foam boundary must increase with GS content in aqueous solution, thereby lowering the rate of foam volume decay [48, 49]. The synthesized non-ionic GSs are observed to exhibit maximum foam stability with low decay rates at four times the CMC of surfactant. 1.0

(a)

CMC 2*CMC 4*CMC

0.9 0.8

Foam decay ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 40

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

200

400

600

800

1000

1200

1400

1600

Time (s)


1800

2000

2200

Page 27 of 40

1.0

(b)

CMC 2*CMC 4*CMC

0.9 0.8

Foam decay ratio

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

Time (s)

Fig. 11. Effect of GS concentration on variation of foam volume decay ratio at 303 K with elapse of time for (a) SF-4-SF; (b) SF-6-SF air-aqueous GS systems.

Fig. 12. Microscopic images of foam stabilized by SF-6-SF solution (at CMC) under 100 µm magnification at varying time intervals: (a) time = 120 s; (b) time = 600 s; (c) time = 900 s; (d) time = 1500 s at 303 K.


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Fig. 13(a) and Fig. 13(b) summarize the influence of temperature on the respective foam stabilities of SF-4-SF and SF-6-SF systems at varying concentrations. It is clear from the analysis of half-life time versus concentration plots that foam stabilities are effectively improved by increasing the concentration of surfactant in aqueous systems. The half-life times of SF-4-SF stabilized foam systems are 240 seconds, 330 seconds and 360 seconds when surfactant concentrations correspond to 1, 2 and 4 times the CMC value(s) respectively. Foam stabilities improved for CMC, twice CMC (2×CMC) and four times CMC (4×CMC) solutions in the presence of SF-6-SF surfactant where respective half-life times increased to 300 seconds, 375 seconds and 420 seconds. This is attributed to the increased hydrophobicity of SF-6-SF gemini molecules, which reduces strength of attractive forces among foam surfaces, delays rupture and retains better foamability. Half-life times are found to decrease significantly with increased thermal activity due to thinning of the foam film surface and subsequent rupturing [50]. Half-life times of GS-stabilized foams are significantly reduced at 343 K for surfactant solutions. At CMC, (2×CMC) and (4×CMC), the half-life time values are respectively measured as 75 seconds, 100 seconds and 150 seconds for SF-4-SF; and 90 seconds, 135 seconds and 180 seconds for SF-6-SF systems. As temperature increases, the hydrogen bonds between water molecules and non-ionic polar heads of GSs are destroyed. This causes faster desorption of GS molecules from foam films and the liquid drainage rates are subsequently increased.

SF-4-SF 420

Half-life time (s)

SF-6-SF

(a)

360 300 240 180 120 60 0 180

Half-life time (s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 40

(b)

150 120 90 60 30 0 0.1

0.2

0.5

1

2

Number of times of CMC of gemini surfactant (n*CMC)


4

Page 29 of 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fig. 13. Foam stabilities of aqueous GS systems depicted as half-life time versus surfactant concentration plots at (a) 303 K; (b) 343 K surfactants.

To further corroborate favourability of foam systems in oil recovery operations, the rheological properties of the GS-based foams as a function of increasing shear rates are studied for EOR application. Fig. 14 depicts the viscosity versus shear rate curves for foaming systems stabilized by non-ionic GSs at a temperature of 343 K. The apparent viscosity of analysed foaming compositions shows a shear rate-dependent pseudoplastic behavior due to the orientation of entangled micelles with disturbances caused due to varying shear rates. During porous media flow, coalescing effect of lamella (liquid films) is an important factor influencing the viscosity of foam systems. With increase in shear rate, foam bubble film undergoes tensile deformation resulting in easy rupture and subsequent decrease in foam viscosity. At 343 K, foams stabilized in the presence of GSs (at CMC) are observed to maintain a minimum viscosity of 21.12 mPa.s and 34.96 mPa.s for SF-4-SF and SF-6-SF systems respectively. When surfactant concentration is increased beyond CMC values, foam stabilities are observed to increase due to slower rate of bubble rupture, which also has the effect of increasing foam viscosities. It is found that SF-4-SF and SF-6-SF show improved viscosities at four times CMC with obtained values 52.56 mPa.s and 61.37 mPa.s respectively at high shear rate (~995 s-1). These data values are much higher than those obtained in case of surfactant solutions. This is attributed due to the increased turbulence of bubbles existent abundantly in foams in comparison to those dispersed in the base surfactant solution [51, 52]. This property of viscosity enhancement of surfactant-stabilized foams is useful in oil recovery operations for oil mobility control by surfactant-induced displacement of trapped crude oil present in rock formations.


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10000

SF-4-SF at CMC SF-6-SF at CMC SF-4-SF at 4*CMC SF-6-SF at 4*CMC

Viscosity (mPa.s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 40

1000

100

10 0.1

1

10

100

1000

-1

Shear rate (s )

Fig. 14. Effect of shear rate on the viscosity of surfactant-stabilized foams at 343 K.

Emulsion stability studies A desirable property of surfactants is emulsification, in which oil and water molecules come into contact and sufficiently mix with each other [53, 54]. The synthesized GSs act as emulsifying agent responsible for long-term stability of the emulsion phase. A single continuous emulsion is observed in all experimented systems, which retains its physical appearance over a period of 15 days. No free organic (upper) and/or aqueous (lower) layers are seen during this period. Furthermore, increase in temperature causes no variations in physical appearance. Therefore, it can be concluded that both SF-4-SF as well as SF-6-SF formed emulsions that exhibited kinetic stability over a long time period and remained visibly unaffected by changes in temperature. It is pertinent to note that though no significant physical changes are observed with increasing time, stability cannot remain unaffected on a microscopic scale and prepared emulsions must exhibit decreased time-dependent stabilities [55]. The phase behavior of n-heptane/SF-6-SF (CMC)/water systems at different time intervals at 303 K, 323 K and 343 K are depicted in Fig. 15.


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Fig. 15. Phase behavior of n-heptane/SF-6-SF (CMC)/aqueous emulsion systems after 1 hour, 1 day, 7 days and 15 days at (a) 303 K; (b) 323 K; (c) 343 K.

Stabilities of the prepared emulsions containing GSs at corresponding concentrations are investigated using dynamic light scattering (DLS) results at different time intervals. The average hydrodynamic diameter of different GS solutions at different time intervals is depicted in Fig. 16. SF-6-SF shows lesser droplet sizes in comparison to SF-4-SF systems due to more effective accumulation of GS molecules at the oil-water interface that lends better stability to oil droplets dispersed in aqueous media. The average hydrodynamic diameter (dh) values at corresponding surfactant CMC values are initially found to be 1.79 µm for SF-4-SF emulsion; and 1.38 µm for SF-4-SF emulsion. With elapse of time, the rate of droplet coalescence increases, whereas that of droplet breakdown decreases. At the end of 15 days, the respective average sizes for SF-4-SF and SF-6-SF are found to be 3.89 µm and 3.50 µm. This results in increasing dh values with passage of time. With increase in concentration, initial droplet size is observed to decrease upto a particular concentration, and thereafter increases due to agglomeration of micelles in the emulsion phase [56].


Energy & Fuels

6.0

1 hour 7 days

5.5

12 hours 15 days

(a)

1 day

Average droplet size, dh (m)

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 200

400

600

800

1000

SF-4-SF concentration (ppm) 5.5

1 hour 7 days

5.0

Average droplet size, dh (m)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12 hours 15 days

(b)

1 day

4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 200

400

600

800

1000

SF-6-SF concentration (ppm)

Fig. 16. Plots showing average hydrodynamic diameter of emulsion droplets versus surfactant concentration for (a) SF-4-SF and (b) SF-6-SF dimeric surfactants at different time intervals.

Fig. 17 shows the particle size distribution of dispersed oil droplets in the emulsion phase at the end of 1 hour and 15 days. It is found that the average droplet size distribution profile shifts to the right as the age of the emulsion increases from 1 hour to 15 days. Therefore, droplet sizes in oil-in-water coarse emulsions gradually increase with time. This is attributed to the gradual thickening and subsequent rupture of oil droplets, which result in the coalescence of oil [57, 58]. As a result, larger size emulsion droplets are formed with increasing time. Initially, the size of nheptane/SF-4-SF/water and n-heptane/SF-6-SF/water emulsions are found in the range 0.2493.106 µm and 0.214-2.861 µm respectively. At the end of 15 days, the respective droplet sizes are observed in the range 0.639-6.234 µm and 0.475-5.315 µm for SF-4-SF and SF-6-SF


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stabilized emulsions. In order to confirm the results of DLS, microscopic image analysis of emulsion drops are also performed. The micrographs of n-heptane/SF-6-SF (CMC)/water based coarse emulsion at 303 K after 1 hour and 15 days are depicted in Fig. 18. A few emulsion droplets with sizes close to the average hydrodynamic diameter range are also labelled in the images. The oleic phase (n-heptane) molecules are dispersed in water phase, resulting in formation of a single continuous oil-in-water emulsion. The GS molecules adsorb at the oilaqueous interface to allow sufficient mixing of oil and water phases. It is evident that the size of the oil droplets increase and emulsion stability decreases with passage of time. During emulsification, two different processes: droplet breakdown and re-coalescence occur in the presence of emulsifier (surfactant). Initially, the emulsion remains completely stable when the process of breakdown of oil droplets dominates over the coalescence (or aggregation) process. At initial times, the newly formed oil droplets are coated with GS molecules that prevent them from coalescing [57, 59]. However, with further elapse of time, the re-coalescence begins to occur and the droplet aggregation rate increases; and larger oil droplets are formed over previously formed small droplets. This is due to the thinning of the oil droplet boundary, which leads to their rupture and subsequent coalescence [57, 59]. It may be concluded that with ageing of prepared emulsions, surface area of oil globules increase and their stabilities decrease. 40

30

Intensity (%)

(a) 1 hour

SF-4-SF SF-6-SF

35

25 20 15 10 5 0 40 35

0.1

1

(b) 15 days

10

30

Intensity (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

25 20 15 10 5 0 0.1

1

Hydrodynamic diameter - dh (m)


10

Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 17. Droplet size distribution profile for SF-4-SF and SF-6-SF stabilized emulsions at the end of (a) 1 hour; (b) 15 days with varying concentrations at 303 K.

Fig. 18. Micrographs of n-heptane/GS/water emulsion stabilized by SF-6-SF surfactant (at CMC) under 10 µm magnification at different time intervals: (a) 1 hour; (b) 15 days.

Conclusions EOR processes using “green” gemini surfactant systems is a relatively new field in the petroleum production sector. Studies pertaining to the synthesis and evaluation of natural resource-derived gemini surfactants are very limited, and more so, their application in oil recovery processes is nearly non-existential. Studies show that structure of non-ionic gemini surfactants have an effect on surfactant efficacy [18]. In addition, non-ionic gemini systems, especially those synthesized using natural resources, show cost-effectiveness and environment-friendly nature, in addition to achieving improved interfacial and bulk aggregation properties [19, 21-23]. Other known advantages of employing surfactants of this nature are identified as adaptability in a wide range of systems such as foams, emulsions and aqueous formulations [20, 21]. These properties have contributed toward a demand create novel gemini surfactants with potentiality in crude oil displacement studies. In this paper, a class of bis(monoglyceride-1-hydroxymethyl-2-fattyacidester)-α,ω-alkanediether gemini surfactants (GSs) with varying spacer lengths were synthesized from sunflower oil and characterized for application in EOR. Thermal gravimetric analyses (TGA) showed that the nonionic GSs retain their structural integrity even under reservoir conditions. CMC increased with temperature due to the destruction of “iceberg” structures formed by intermolecular GS-hydro bonding, which delays the micellization process. The surfactants exhibited good stability in


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acidic solutions, indicating favourability in crude oil displacement. According to crude oil miscibility studies, SF-6-SF emulsion (at CMC) showed better stability in comparison to SF-4SF emulsion at 343 K. Addition of salt showed synergistic effects in oil-aqueous interfacial behavior. Ultra-low IFT of the order of 10-2 to 10-3 mN/m was achieved under optimal salinity conditions. The GS systems also showed favorable salt and hardness tolerance levels. Surfactants also showed favourable lime soap dispersibility, thereby showing suitability in long-term reservoir projects. Foaming studies revealed that the rate of foam film rupture decreases at GS concentrations greater than CMC. Half-life times were calculated as a measure of foam stabilization ability of air-aqueous systems. The synthesized surfactants formed single-phase emulsions at different temperatures, which remained stable over a significant time period. DLS studies confirmed that the prepared oil/surfactant/aqueous formulations encountered slow recoalescence of the oil droplets, resulting in gradually decreasing stability of emulsions. In summary, the synthesized non-ionic GSs exhibited beneficial performance properties that may contribute towards their functionality as well as potentiality as chemical injecting fluids in pure and mixed systems for residual oil extraction in mature reservoirs. Acknowledgements The authors would like to acknowledge the Department of Petroleum Engineering, IIT (ISM) Dhanbad, India for supporting this study. Declaration of Interest The authors declare no competing financial and personal interests. References 1. Ahmadi, M. A.; Zendehboudi, S.; Shafiei, A.; James, L. Nonionic Surfactant for Enhanced Oil Recovery from Carbonates: Adsorption Kinetics and Equilibrium. Ind. Eng. Chem. Res. 2012, 51, 9894-9905. DOI: 10.1021/ie300269c. 2. Negin, C.; Ali, S.; Xie, Q. Most common surfactants employed in chemical enhanced oil recovery. Petroleum 2017, 3 (2), 197-211. https://doi.org/10.1016/j. petlm.2016.11.007. 3. Lu, J.; Liyanage, P. J.; Solairaj, S.; Adkins, S.; Arachchilage, G. P.; Kim, D. H.; Britton, C.; Weerasooriya, U.; Pope, G. A. New surfactant developments for chemical enhanced oil recovery. J. Pet. Sci. Eng. 2014, 120, 94-101. https://doi.org/10.1016/j.petrol.2014.05 .021. 4. Khanamiri, H. H.; Nourani, M.; Tichelkamp, T.; Stensen, J. Å.; Øye, G.; Torsæter, O. LowSalinity-Surfactant Enhanced Oil Recovery (EOR) with a New Surfactant Blend: Effect of


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Performance Evaluation of Novel Sunflower Oil-Based Gemini

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