Article pubs.acs.org/IECR
Effect of Salts, Alkali, and Temperature on the Properties of Sodium Oleate Hydrogel Neetha V Thampi,*,† Rohith P. John,† Keka Ojha,‡ and Udayabhanu G. Nair† †
Department of Applied Chemistry and ‡Dept. of Petroleum Engineering, Indian School of Mines, Dhanbad, Jharkhand 826004, India S Supporting Information *
ABSTRACT: A detailed study has been done on the rheological properties of sodium oleate based hydrogels that are developed in the presence of organic and inorganic salts. Shear stability of the NaOL gels were found to be a strong function of the salt, alkali, and the fabrication conditions. The addition of alkali to NaOL/ KCl hydrogel enhanced the viscoelastic properties, which were observed from static and dynamic rheological tests. The hydrogel is miscible with hydrocarbon and can act as a clean gel during a postfracturing job. The quantitative estimation of migration of NaOL to microemulsion medium during gel break test can be done by Sudan dye solubilization and Methylene blue complexation methods, and the results show that the addition of both KCl and NaOH favors NaOL micelle growth and its partitioning tendency toward the hydrocarbon phase.
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based fluids like Erucyl bis-(hydroxyl ethyl) methylammonium chloride (EHAC) are considered clean gels because they break easily by contacting reservoir fluids, which leads to the absence of insoluble residues after fracturing of tight and damaged reservoirs. In the presence of binding and nonbinding salts, EHAC forms a highly entangled network of the wormlike micelle with high viscosities (∼104 Pa·s) with shear thinning and viscoelastic nature is retained even at 90 °C.9 Anionic, nonionic, and zwitterionic surface-active molecules are currently receiving a strong attention due to environmental capabilities. Regarding this, the preferred surfactants were anionic, zwitterionic, and their synergistic combinations as they were water wet and also compatible with the mutual solvents used in the reservoir treatment. They were used in greater volume than any other surfactants, because of their high potent detergency and low cost of manufacture. Sodium oleate (NaOL) is preferred in many biocompatible formulations due to its nontoxic nature. Wormlike micellar growth with high viscoelastic properties has been reported in sodium oleate and its mixed surfactant combinations either with cationic or zwitter ionic surfactants.10,11 This surfactant self-assembles into wormlike micelles in the presence of either an inorganic salt, which screens the inter micellar electrostatic interactions, or a binding salt, which reduces the micellar surface charge and they received major emphasis in the oil industry, especially in enhanced oil recovery (EOR) as a rheology modifier.12 Viscoelastic fluids based on sodium oleate/nanoparticle claimes
INTRODUCTION Viscoelastic (VES) fluids are composed of low-molecularweight surfactants that can form long elongated micellar structures that contribute to the viscoelastic behavior in order to increase fluid viscosity.1 The molecular weight of surfactant molecules is usually less than 1000, and they consist of a hydrophilic head and a long hydrophobic tail.2 The physical nature of the surfactant depends on its tail length, branching of the tail, number of double bonds, and cis/trans configuration.3 The surfactant molecules are like monomers, and the volume ratio of the surfactant tail group to the headgroup is defined as the micelle packing ratio. When the packing ratio is below onethird (headgroup volume dominant), the packing arrangement of the surfactant molecules will produce spherical micelles. If the value is from one-third to one-half (headgroup volume moderately to slightly dominant), the surfactant molecules will arrange into long cylindrical micelles. Surfactants with higher packing ratio tend to form larger aggregates of wormlike micelles.4,5 The ability of the surface-active agents (surfactants) to form structured aggregates in the solution, especially in semidilute regime, leads to their practical importance and the scientific interest of the researchers. Depending on the chemical structure of the surfactant molecules, solvent, thermodynamic characteristics of solution in the system; direct and reverse micelles of different shape, lamellae, vesicles, and others were formed. The solutions of surfactants, where giant “wormlike” micelles are formed, reaching a length of few microns, are of particular interest. Such long micelles behave like polymer chains, and at high concentrations, they form a network of topological links; as a result, solutions acquire viscoelastic properties and can be used as rheology modifiers in drag reducing, enhanced oil recovery applications.6−8 Surfactant© XXXX American Chemical Society
Received: January 12, 2016 Revised: April 30, 2016 Accepted: May 4, 2016
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DOI: 10.1021/acs.iecr.6b00158 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research features of a successful fracturing fluid in terms of easy preparation, good viscosity, low frictional resistance, and good thermal stability even above 100 °C. These fluids are more sensitive to formation water than hydrocarbons.13−15 Similar to surfactant/salt or (cationic/zwitter ionic)/anionic surfactant combinations, hydrophobically modified watersoluble polymers (HMWSP) reported to have strong interactions or complex formations with anionic surfactants (for example, sodium oleate) in aqueous medium and give stable hydrogels. These hydrogels give better salt tolerance and viscoelastic properties and are reported to be suitable for high temperature fracturing operations.16,17 In the present work, efforts were made to develop a stable sodium oleate hydrogel using various inorganic and organic salts; this probably can act as a precursor medium where amphiphilic polymers (HMWSP) can be efficiently blended in small proportions. The static and dynamic rheology of hydrogel in different salt, alkali, fabrication, thermal environments are studied in detail. Sand settling tests in these hydrogels can give information about its static proppant suspension performance. Surfactant based gel can act as clean fracturing gel only when it undergoes a drastic viscosity reduction in the presence of reservoir hydrocarbons (gel break), this in effect provide better well clean up properties; hence zero formation damage and support the post fracturing treatments. Miscibility tests are model tests generally performed in the presence of water/hydrocarbons to examine the breaking characteristics of the fracturing fluid. The end results of a gel break are always an emulsion (E) or microemulsion (ME). To our knowledge, there are very few methods available to quantitative determination of the amount of surfactant that partitioned to E continuous phase relative to the surfactant concentration in the aqueous phase and these methods are often quite complicated.18 In present work, biphasic microemulsions/emulsions of Winsor I (W I) type are the end results of gel break in the presence of heptane. Here, two simple and direct methods are used for relative quantification of the surfactant that partitions to the ME/E phase after gel break test. This concentration in aqueous phase is considered to be the bulk surfactant concentration, and the remaining surfactant is assumed to be adsorbed at the microemulsion/emulsion phase; hence the partition coefficient can be determined using a general formula. The effect of salts and pH on distribution coefficient of NaOL was also studied in detail.
Methods. Proppant Suspension Studies. Static proppant settling tests were performed in 7.5 cm × 30 cm glass cylinders to investigate whether the static proppant settling is a function of viscosity of the suspension medium. These measurements were performed at a proppant concentration of 14 vol % 20/40 mesh sand. This proppant concentration was chosen because the maximum settling rate is observed at this value. At lower values, proppant clustering is prominent; at higher concentrations, hindered settling becomes important. The method of experimentation was based on Stoke’s law within the creep flow regime. We have not yet investigated the dynamic proppantsettling, although we expect that the applied shear influences proppant settling rates in the viscoelastic fluids. Break Test of Surfactant Gel Sample. Glass bottles of 250 mL bottles were filled with 50 g of NaOL gel of different compositions together with 50 g of Heptane (Fluka; as received; mass density 922 kg/m3). The closed bottles were rolled on a bottle roller at approximately 15 rpm for 12 h at 30 °C. After a rest period of typically 2 days, the oleate partitioning between the water and oil in each bottle was determined by comparing the oleate concentrations in the aqueous phase before and after the partitioning using UV spectroscopy [refer to Figure S1 in the Supporting Information]. The amount of sodium oleate that had disappeared from the water phase was supposed to reside in the interface or microemulsion phase. In the calculations involved it was assumed that after surfactant partitioning, all oil and water molecules reside in the oil and water phases, respectively, and that the volume of any surfactant−solvent mixture is equal to the summed volume of the pure components. The surfactant concentration in aqueous layer was determined by following methods. Method A: Preparation of SurfactantSudan III Dye Stock Solution. Aqueous fractions after gel break test were collected using separating funnel and transferred to 50 mL centrifuge tubes. Sudan III crystals (0.1 mg) were added to the tubes which were sealed and left 2 days with periodic stirring. Aqueous phases were separated and dyed with the Sudan III crystals in sealed tubes. All the samples were centrifuged in a benchtop IEC clinical centrifuge (Damon/IEC Division, Needham Hts., MA) and the supernatant absorbance were measured with a Shimadzu UV-2400 PC double beam spectrophotometer (Tokyo, Japan) operating under ambient conditions with surfactant solutions at equal concentrations without dyes serving as the sample background using 1 cm optical path cuvettes. Measuring the surfactant concentration in the oil phase was not well possible due to the high UV absorption by the Sudan in hydrocarbon medium itself. The amount of dye solubilized in each sample was measured by monitoring the absorbance wavelength (λmax = 523 nm) against surfactant concentration. For standard calibration plot, dye solubilization experiments were repeated with different concentrations of NaOL (range ∼0.01−8%). Method B: Preparation of Surfactant−Methylene Blue Indicator Stock Solution. Sodium oleate−MB dye complexation method is an example of extractive spectrophotometry. The sodium oleate exhibits basic character essentially due to the presence of carboxylate group. Anionic sodium oleate forms an association complex with cationic methylene blue dye in a basic medium, which can be extracted with chloroform. Aqueous fractions after gel break test are collected using separating funnel and transferred to 50 mL centrifuge tubes. Methylene blue (MB) in ethanol prepared by dissolving 10 mg of Methylene blue in 50 mL of dehydrated ethanol. This reagent
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EXPERIMENTAL SECTION Materials. Sodium oleate, NAOL (cis-9-octadecenoic acid sodium salt), anionic surfactant is a soap derived from naturally occurring fatty acid (main constituent of palm oil) of purity >95% was obtained from Loba Chemie Pvt Ltd., Mumbai. Sodium chloride (NaCl), ammonium chloride (NH4Cl), potassium chloride (KCl), sodium hydroxide (NaOH), tetra ethylammonium bromide (TEAB, NEt 4Br), tetra butyl ammonium bromide (TBAB, NBu4Br) all were >98% purity were obtained from Merck, India, were used without further purification. Alkaline concentration of aqueous medium kept at 0.025 M unless otherwise mentioned. This precaution is taken to prevent soap hydrolysis and subsequent formation of acid soap crystals. NaOL/salt samples were made by adding salt to NaOL solutions followed by mild heating (40 °C) with constant stirring. Sudan III crystals and Methylene Blue of purity >98% were obtained from CDH, Mumbai, and used without further purification. B
DOI: 10.1021/acs.iecr.6b00158 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. (a) Shear viscosity vs NaOL surfactant concentration. The change in slope indicates phase transition. (b) Shear viscosity vs shear rate plot for surfactant concentrations (regions II and III).
and plate sensor (40 mm diameter, 3° angle). The plate temperature was controlled by the Peltier unit, and the internal resolution of the temperature with the present equipment is 0.01 °C. The frequency spectra were recorded in the linear viscoelastic regime of the samples as determined from the dynamic strain sweep measurements. The frequency sweep measurements were carried out by varying the angular frequency from 0.01 to 130 rad/s. Sample for the measurement at each temperature left for at least 5 min before measurement such that it could attain stability in rheological parameters (η and G). Interfacial and Surface Tension Measurements. Interface surface tension measurements were performed using a spinning drop tensiometer (KRUSS SITE 100) and surface tension measurements were performed using a ring tensiometer (KRUSS K 11). This instrument is manually controlled, and it is possible to make several measurements of the same sample, by manually lowering and raising the vessel containing the solution, thereby passing the ring repeatedly through the weight maximum. All measurements were performed at 30 °C. All solutions were prepared at least 24 h before measurements. In order to avoid the formation of very surface-active acid/soap complexes, the oleate solutions were made slightly alkaline before measurements by adding sodium or potassium hydroxide. The ring and the measuring vessel were cleaned using deionized water and treated with a gas flame after measurements. The samples were allowed to stabilize for 30 min in the measuring vessel before measurement. In spite of these precautions, the reproducibility between measurements on the same sample was only of the order ±0.1 mN m−1.
was added dropwise to the 20 mL aqueous fraction taken in centrifuge vials and stirred to ensure complete mixing. MB oleate salt formed is insoluble in water and thus precipitates out. The solution was centrifuged repeatedly and supernatant solution was checked for further precipitation. Process repeated until precipitation is complete. The mixture was left at room temperature overnight to allow the ethanol to evaporate. The MB oleate precipitate was then extracted with chloroform and the amount of MB oleate complex solubilized in CHCl3 was measured by monitoring the absorbance wavelength (λmax = 640 nm) against surfactant concentration. With the help of standard calibration graph, oleate concentration in aqueous layer can be determined. For standard calibration graph, the test can be repeated with different concentrations of NaOL (range ∼0.1−8 wt %) in aqueous solutions. [The optical and regression characteristics, precision and accuracy, standard deviation (SD), relative standard deviation (RSD), and the range of error at the 96% confidence level of prescribed methods are summarized in Table S1]. Steady Shear Rheological Measurements. All the samples (2g each) for the rheological measurements were prepared by taking required amount of chemicals in clean and dry glass bottles with screw cap. The samples were homogenized and kept in a thermostat bath at 40 °C with the accuracy of the thermometer ±0.5 °C for at least 24 h before the measurements. The viscous samples were mixed using the magnetic stirrer. The flow characteristic of the NaOL samples were measured using the Physica Rheolab MC1 (stress controlled rheometer) with cone and plate sensor (50 mm diameter, 1° angle). The plate temperature is controlled by the TC 20 unit. Sample volume of 2 mL used for each measurement and the thickness in the middle of sensor was kept at 0.2 mm. Viscosity (η) of the samples obtained from the steady shear measurement with the shear rate ranging from 1 to 1000 s−1 at 30 °C. To study the effect of the temperature on the gel rheological behavior, steady shear measurements at 30−80 °C performed. The fluid was heated to 90 °C and shear rate was set to 11 s −1; thermal stability checked for 60 min at the given temperature. Dynamic Shear Rheology Measurements. The frequency sweep analyses of the samples were carried out in a controlled stress rheometer, Anton Paar Physica MCR 101, with the cone
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RESULTS AND DISCUSSION Steady Shear Rheological Studies. Effect of Surfactant Concentration on Rheology, Proppant Suspension Performance of NaOL in Dilute and Semidilute Regime. According to Asadi et al., a good fluid for proppant suspension is one that exhibits high zero shear viscosity without an undue increase in high shear viscosity.19 Solutions of surfactants capable of forming wormlike micelles are very attractive systems from a rheological standpoint since they exhibit high zero-shear viscosity (η0), a critical factor in evaluating proppant transport
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DOI: 10.1021/acs.iecr.6b00158 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. Effect of inorganic salts on shear viscosity of 13 wt % NaOL hydrogel: pH (a) 7.9 and (b) 9.8.
Figure 3. (a) Effect of alkyl ammonium salt on shear viscosity of NaOL hydrogel at 40 °C. (b) Various micelle solubilization sites: (I) micelle−water interface, (II) between the hydrophilic head groups, (111) palisade layer, (IV) deeply in the palisade layer, and (V) inner core of the micelle.
of fracturing fluids.20,21 Shear viscosity (η) changes with NaOL concentration are shown in Figure 1a and found to be minimal up to critical rod concentration (CRC) ∼ 1.5%. Below CRC (region I) is the dilute regime, where micelles are far apart from each other and the viscosity remains low, on the order of that of water, and is found to be insufficient for proppant suspension. Above the CRC, in the semidilute regime, spherical micelles grow further to rodlike micelles and an improvement in viscosity is observed. At high surfactant concentrations, above CRC the rodlike micelles grow rapidly to cylindrical (threadlike or wormlike micelles) ones and overlaping of entangled threadlike micelles results in a dynamic three-dimensional network, the reason for the high viscosity of the sample. The viscoelastic behavior of the fluids in region II can be seen easily by simple solution swirling and visually observing the recoil movement of air bubbles trapped in the solution after the swirling motion is stopped. The degree of shear thinning and viscoelastic behavior of NaOL solutions in the semidilute regime increase as surfactant concentration changes from region II to region III (Figure 1b). Viscous solutions in region III exhibit a typical rheogram characterized by a high viscosity region at low shear rate and
shear thinning region at high shear rate. The high viscosity observed in the Newtonian region can be interpreted as the result of entanglement of worm like micelles and shear thinning is due to the alignment of the aggregates as the shear rate is increases. The proppant settling measurements also split shear thinning concentration regime of NaOL samples into two sections: poor and unacceptable velocity (settling velocity, VS > 5 cm/min) observed in region II. Marginally acceptable proppant suspension (0.5 ≤ VS ≤ 5 cm/min) maintained that in region III even at low concentrations along with high viscosity make it a stable medium for proppant transport. Effect of Salts and Alkali on Rheology and Suspension Performance of NaOL Hydrogel. One of the most common shortcomings of anionic surfactants compared to the cationic and nonionic surfactants is the extensive salting out phenomena at high salinity conditions. The change in surfactant rheological properties as a function of salts (organic and inorganic) is also an important aspect to investigate seriously since this helps to assess the credibility and stability of a formulation as a proppant suspension medium for moderate temperature fracturing applications. Surfactant stock solutions (7−13 wt %) were prepared in water with salts (0.1−1 2 wt %) to study the effect D
DOI: 10.1021/acs.iecr.6b00158 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. (a) Effect of pH on shear viscosity (at 111 s −1): 7 and 13 wt %. (b) Viscosity vs shear rate for 13 wt % NaOL at 70 °C, KCl ∼ 1.1 wt %.
and organic counterions (Et4N+, Bu4N+) in NaOL micelle environment trigger micellar morphology to different extends. Compared to organic salts like TEAB and TBAB, polar inorganic salts (NaCl, KCl, NH4Cl) are believed to be solubilized in aqueous medium, mainly between the hydrophilic head groups in micelle/water interface (regions I and II in Figure 3b). A high degree of counterion binding in this case may overcome headgroup repulsion by holding the oppositely charged counterion between head groups of similar charge; headgroup repulsion is depressed and growth of more rod- or disk-shaped micelles to wormlike micelles (WLMs) becomes favored as in the presence of inorganic salts. The addition of inorganic salt at its optimum salt concentration (represented by peak regime) maximizes the growth of wormlike micelles to a point; this finally accentuates the 3D network overlapping of WLMs, hence the viscosity of the composition. Counterion binding increases with counterion size, colloidal stability, and viscosity of NaOL solution in the presence of salts follow the Hofmeister series (NH4+ ≈ K+ > Na+).23 Series reversal was observed with change in pH of surfactant solution in the present study. At lower pH (7.7−7.9), the degree of hydration by inorganic salts in sodium oleate salt mixture does follows a Hofmeister series, but at slightly increased pH; viscosity changes follow a new series: K+ > NH4+ > Na+. This order is clearly observed at a pH above 10. The addition of alkali here greatly screens the headgroup repulsive forces between surfactant molecules providing excess Na+ ions. In all cases experimented here, NaOL + KCl hydrogel give high gel viscosity. Addition of NH4Cl (0.6 wt % or above) in highly alkaline gel medium visually impart hazy (or milky tinge) appearance in the gel (may be the onset of phase separation). At high salt additions (0.6 wt % or above), the solution turns cloudy at a room temperature. This indicated a cloud point phenomenon or upper consolute phase behavior in NaOL/ NH4Cl solutions. This mixture exhibits an upper consolute solution temperature/upper cloud point (UCST/UCP) close to 36.5 °C. Clouding seems to be induced by attractive interactions between the micelles induced mostly by various inorganic and organic salts etc. and results in micelle cluster formation.24,25 The addition of KCl to NaOL solution at both pH conditions made the solutions viscous, but the solutions did not cloud. Addition of NH4Cl to highly alkaline NaOL solution,
of the presence of salts on solution viscosity. pH of the solutions was kept in the range 7.6−8. The change in shear viscosity of surfactant solutions with the addition of both types of salts is shown in Figures 2a and b and 3a. There is an increase in viscosity of in the semidilute regime compositions of sodium oleate with the addition of both strong (inorganic salts) and weak (organic salts) electrolytes. Salt addition is due to the growth of cylindrical or wormlike micelles through the screening of the electrostatic repulsions between the negatively charged surfactant head groups by salt counterion. The shear viscosity vs salt concentration plot depicts a peak region for viscous gels in the presence of inorganic salts (Figure 2a and b). The existence of a peak region in the diagram suggests that the cylindrical/wormlike micelle grow up to a point and further salt addition has no influence on the electrostatics of headgroup interactions and this represent the optimum salt concentration for viscous NaOL hydrogel. The gel−sol transition happens at further addition of salt and it clearly indicates the disruption of gel network structures. This may be either due to decrease in the mean size of the wormlike micelles, micellar branching, or to oleic acid salting out. Above optimum salt concentration, addition of salt induces turbidity in surfactant solution. Visual inspection of NaOL samples with quaternary ammonium salts reports that they are less viscous than those made with inorganic salts. In aqueous media, surfactants with bulky or loosely packed hydrophilic groups and long hydrophobic groups tend to form spherical micelles because area occupied by them is large, while those with short, bulky hydrophobic groups and small, closepacked hydrophilic groups tend to form lamellar or cylindrical micelles. The essential consideration pertaining to the area occupied by the head groups is the work necessary to overcome the electrical repulsion experienced by the like charges. A surfactant carrying a large charge on a relatively small charge bearing atom will inherently be more apt to form spherical micelles due to the high energy needed to overcome the prohibitive charge density of the headgroup. Unlike homogeneous solvents, micelles (spherical, disc or cylindrical or worm like micelles) possess different solubilization environments, ranging from the polar micelle−water interface to the relatively nonpolar hydrocarbon core of the micelle as shown in Figure 3b.22 The extent of solubilization of inorganic (Na+, K+, NH4+) E
DOI: 10.1021/acs.iecr.6b00158 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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when exposed to proposed reservoir treatment conditions, is essential here. We now report our preliminary findings with two quaternary ammonium salts, Et4NBr and Bu4NBr. Bu4NBr salt is known to cause clouding in solutions of anionic surfactants such as SDS. Surprisingly, however, we observe clouding phenomena in high alkaline NaOL medium (pH ≈ 9.5−9.8) in the presence of these organic salts; even though earlier studies reported the absence of clouding in the same system in neutral/mild alkaline conditions.26 Both systems (NaOL/Et4NBr and NaOL/ Bu4NBr) show upper consolute behavior with UCST of 38.5 and 43.3 °C respectively. Clouding is more prominent in Bu4NBr incorporated system and heating induce micellar growth in the order (Et)4 N+ > (Bu)4N+. Micellar solutions have a general tendency to solubilize a certain amount of organic salts (hydrophobic or partly hydrophobic).30 Longer chain, less polar compounds penetrate more deeply to micelle interiors than short-chain, more polar material and depth of the penetration in the palisade layer depends on the ratio of polar to nonpolar structures in the solubilizate molecule. The binding and solubilization of tetra alkyl ammonium counterions in anionic micelles is well documented in earlier studies.31,32 Longer chain non polar ammonium salt like Bu4N+ penetrate more deeply in the micellar interior than short chain, more polar Et4N+. The solubilization of organic additives in different micellar region can be correlated with the structural organization of micellar aggregates and their mutual interactions.33,34 In the semidilute regime, the wormlike micelles happen to be the possible targets. Rodlike/wormlike micelles solubilize a large amount of organic additives, the amount and the solubilization environment can play an important role in the final micellar shape. Et4N+ and Bu4N+can bind by inserting its alkyl tails into the hydrophobic interior of a micelle; however, not all four tails can be inserted due to geometric limitations.10 At higher salt additions, more and more TEAB/TBAB cram into the palisade layer of the micelle, organic salts become one of the major components in the micelle. Because one or two tails remain exposed to water, Et4N+ counterion will protrude into the headgroup area. The binding/screening effect of Et4N+ is relatively weak but still better than Bu4N+. Et4N+counterions mitigate the headgroup charge, they also occupy extra space at the headgroup. Suffering from the steric hindrance effect, Et4N+ no longer remains superior to NH4+ at high concentrations since the headgroup area remains high and the packing parameter changes results in shortening of worm like micelles. The probability of this type of binding and screening effects are rare in the case of Bu4N+ and it rather prefer to be solubilized in the shallow palisade layer. It has been proposed by Kabir-ud-Din et al. that interfacial partitioning of organic salts/additives causes micellar growth while interior solubilization produces swollen micelles.34,35 The volume factor of TBAB possessing a big head but a short tail becomes unfavorable for modifying the packing parameter to be suitable for growth of wormlike micelles. The interior solubilization of Bu4N+ provides swelling to the already grown micelle and releases the requirement of the surfactant chain to reach the center of the core.36 The volume factor of TBAB poses an obstacle to worm like micellar growth. Thus, weak counterion binding in the presence of organic salts along with the geometry of the resulting surfactant-counterion complex hinder the growth of wormlike micelles in semidilute regime. This will diminish the 3D network density, hence the viscosity.
on the other hand, gave rise to a less viscous medium and give a cloudy tinge to the solution which become more turbid at higher salt additions at room temperature. The cloudy tinge disappears on heating. Drastic increase in viscosity is noted with NaOL/NH4Cl system above UCP temperature. This suggest that the NaOL cylindrical/wormlike micelles in the presence of NH4+ ions undergo a progressive growth on heating to the UCP, but they form a micellar cluster at room temperature due to the onset of an intermicellar attractive interaction. Although such clusters are known to form in micellar solutions of nonionic surfactants on approaching the UCP, their formation at room temperature is the first of its kind for them. The observed unusual viscosity increase with temperature here has possibly been triggered by lesser dehydration and strong binding by NH4+ ions. Heating induces the relocation of NH4+ ions from micellar interiors (from regions III and IV since Na+ ions probably occupy regions I and II in a high pH NaOL/ NH4Cl medium kept at room temperature) to corona (micellar exterior layers) at a temperature close to UCP.24 NaOL/tri ethylammonium chloride (Et3NHCl) mixture is a perfect example of system that exhibit prominent clouding phenomena at high temperature. NaOL/Et3NHCl show a lower cloud point (LCP), and when heated, it turned cloudy and eventually separated into two liquid phases.26 Figure 4a represent the effect of the addition of alkali NaOH on the NaOL + KCl viscosity at a 111 s−1 shear rate. The figure depicts that, with the addition of NaOH to particularly the semidilute regime NaOL composition, the viscosity of the fluid increases for test temperature 70 °C. The addition of alkali here greatly screens the headgroup repulsive forces between surfactant molecules with excess Na+ ions.13,14 As a result, long, flexible anionic wormlike micelles are formed in the solution, which helps in the entanglement of micelles into a transient network and imparts high viscosity to the sample. So with the synergic binding of Na+ and K+ counterions, the resulting surfactant solution resembles a polymer solution with viscoelasticity.27−29 The shear viscosity changes in dilute regime compositions of NaOL with pH was found to be negligible. This is because of the low density of cylindrical micelle aggregates in region II. Addition of alkali has no effect on these loose networks as observed from its shear viscosity changes. Figure 4b shows variation of the viscosity with increasing shear rate for different pH for a 13 wt % NaOL fluid in the presence of KCl. Gel shear stability at high shear was also found to be a strong function of the alkalinity of the medium. At the optimum pH ∼ 9.8, NaOL hydrogels show a steady and consistent behavior at both low and high shear rate (Figure 4b). This high shear and thermal stability were attributed to the enhanced shear induced alignments of the dense worm like micelles contributing to dense 3D network in the presence of alkali (hence the increased pH value), all the micelles will be aligned in one particular direction to resist the shear stress more strongly than the ordinary sample. Alignment of the worm like micelles resembles the polymer chains, and can entangle to increase the shear viscosity. Beyond to the limit of the pH or NaOH concentration, the gel viscosity starts to decrease and exhibit a destabilizing tendency particularly at the higher shear rates. Surfactant precipitation occurs at high pH conditions. Bottom hole temperature (BHT) of reservoirs formations had a large influence on fluid rheology and viscoelastic response; thus, study on fluid optimization, especially the surfactant, salt and alkali concentration in viscous suspension medium, and thermal response of this medium F
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°C, but at the extreme synthesis conditions (∼70 °C), an unstable gel results. The flow behavior pattern of the gel at the optimized fabrication temperature (55 °C) was also found to be a time dependent parameter (Figure S2). The increases in the fabrication time (for 6−9 h) increase the viscosity of the gel, and this observation is quite evident at the lower shear rates. Increase in the fabrication temperature enhances the thermal mobility; hence, the thermal energy of the wormlike micelles which in effect accentuate the network alignment. This leads to the other temperature independent micelle aggregates, hence accentuates shear and thermal stability. The viscosity changes narrow down as the shear rate changes to the higher values, finally the shear history was same at all fabrication temperatures at 1000 s−1. Extreme thermal conditions like 70 °C may possibly impart the excess thermal energy, in another way induce the micelle instability by perturbing the network alignments; hence results in a low viscosity system.37 Temperature induced micellar alignments dominant at the low shear conditions and it majorly influence the gel threedimensional network reinforcement, hence the viscosity too. At high shear rate (∼200−500 s−1), where the flow induced structural effects dominates, viscosity is no longer dependent on the time interval. Gels kept under long thermal exposure (fabrication or experimental) resulted in disruption of inter micelle network, hence decreasing the gel viscosity. Thermal stability tests performed for the gels (I and II) is in accord with above results (see Figure 5b). The gel synthesized at ideal fabrication temperature, the viscosity drop is minimal (0.121 → 0.104 Pa. s) at 11 s−1. The temperature independent micelle structures add to the shear stability of the fluid I in a desirable manner. Effect of the Surfactant, Salt, and Temperature on the Viscoelastic and Suspension Properties of the NaOL Gel. Dynamic shear rheological studies now emerged as a secondary characterization method for evaluating the performance of the fracturing fluids. Experimental studies show that modulus functions (especially G′), relaxation time along with viscosity of gel were found to play critical role in governing settling rate of proppants inside viscoelastic gels.37 More information on the network structure of the gel phases can be obtained from the
Proppant settling studies done on NaOL/KCl hydrogel showed that suspension performance (inverse relation with settling rate) depends on rheology of medium (η values); viscous 13 wt % sample generally exhibits good suspension nature evident from low settling rate (VS ∼ 0.127 cm/s) at its optimize maximum temperature (Tmax) as shown in Table 1. Table 1. Important Parameters of Sodium Oleate Gel η@ 111 s−1 (mPa s)
G′ (Pa) at ωc
Ea (kJ/mol)
Vs (cm/s)
0.0024 2.6 9.8
154.2 173.4 210.6
6.46 0.127 0.068
236.3
0.0074
189.9
0.186
composition
Tmax (°C)
10 wt % 13 wt % 13 wt % NaOA + 0.5 wt % KCl (pH 7.9) 13 wt % NaOA + 1 wt % KCl (pH 9.7)* 13 wt % NaOA + 1 wt % KCl (pH 10.5)
50 60 66
103.4 108.8 113.4
75
118 0.7
60
88.6
32.5
0.043
The settling velocity of the sand particles in NaOL gel show a steady improvement when the alkali and salt concentrations were maintained where the high shear stability noted during the steady shear rheological studies at high temperature (70−75 °C). Further additions of alkali (pH > 10) however exhibit a negative impact on the shear viscosity, suspension performance of the NaOL gel at corresponding Tmax (Table 1). Reasonable explanation for this tend can be obtained after studying its viscoelastic behavior given in later section. Effect of Fabrication Temperature on Rheology and Thermal Stability of NaOL Hydrogel. The viscosity of micelle solutions generally affected by the fabrication conditions too. Rheometrical measurements were conducted for the optimized NaOL/(KCl + NaOH) system synthesized at high fabrication temperatures keeping the other conditions ideal and the results as shown in the Figure 5a. A significant improvement in the gel viscosity particularly at the low shear rates (at 1.1 s−1) was noted when the fabrication temperature kept in the range 55
Figure 5. (a) Shear viscosity changes of 13 wt % NaOL gel at different fabrication temperature Figure 7. (b) Shear viscosity changes of 13 wt % NaOL gel with time at 90 °C. G
DOI: 10.1021/acs.iecr.6b00158 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research oscillatory shear frequency sweep measurements. The dynamic rheological properties of viscoelastic fluids are represented in terms of complex shear modulus38 G*(ω) = G′(ω) + iG″(ω)
In this equationn, the storage modulus, G′(ω), describes the elastic properties of the material, while the loss modulus, G″(ω), is proportional to its viscous resistance. The shear thinning fluids in the semidilute regime exhibit distinct viscoelastic behavior depending upon the experimental conditions like concentration of additives, temperature, etc. The NaOL gels in the presence of KCl exhibit a dynamic rheological pattern as shown in Figure 6. These samples were observed with G′ exceeding over G″ and both the moduli being independent of the frequency with finite relaxation time.
Figure 7. Schematic diagrams of morphological transitions in NaOL. (a) Spherical micelles above cmc. (b) Cylindrical micelles in region I. (c) Wormlike micelles (WLMs) in region II. (d) 3D network of WLMs. (e) Disruption of 3D network of WLMs.
Paar Physica Rheometer MCR-301. Gels kept under long thermal exposure (experimental) give loose gels and relative shift in relaxation time, τR a low value is observed (Figure 8). It
Figure 6. Frequency sweep plot of NaOL hydrogel in the presence of 1 wt % KCl at 50 °C.
The frequency independence with finite relaxation time implies that NaOL solution shows viscoelastic response typical of gel-like materials. The origin of elasticity in the colloidal crystal phase has been reported in the literature and can be attributed to entanglement of surfactant chains while the value of loss modulus could be due to friction between micelles.39 The viscoelastic pattern depicted here is reminiscent of an entangled network of wormlike chains rather than cross-linked gel which are characterized by the absence of a finite relaxation Sample responds like elastic gel (G′ ≫ G″) with a high relaxation time as surfactant concentration changed to high values. The high elasticity reflects stronger interactions among cylindrical aggregates that favors the formation of structural network that act as support basis for proppant suspension (Figure 7). The relative shift in crossover frequency, ωc, was observed as NaOL concentration changes to higher values which is testified by long relaxation time (τR = 1/ωc).40 This clearly indicates the presence of more reinforced 3D network in gel at high surfactant concentration and these dense networks undergo stress relaxation very slowly. This type of network aggregation is absent in NaOL solution at low concentrations and testified by very low τR values. Frequency sweep data of 13 wt % NaOL in the presence of 1.1% KCl at different temperature were collected using Anton
Figure 8. Arrhenius plot of 10 wt % NaOL hydrogel.
indicates the progressive destruction of the reinforced 3D network aggregate like structures (due to decrease in average length of micellar chains); which undergo stress relaxation very easily. The activation energy (Ea) of NaOL gel with change in experimental conditions like concentration, salinity, and pH of medium was given in Table 1. Activation energies can be calculated from ln τR vs 1/T plot (Figure 8), and results are given in Table 1. The gel in saline and alkaline environments give (positive slopes were observed), resulting in apparent positive activation energy (Eapp), from an Arrhenius-type theory. The values of Ea obtained from the slope falls into the wide range of Ea values (70−300 kJ/mol) reported for wormlike micellar systems.41 The NaOL hydrogel in the presence of 1.1 wt % KCl at pH 9.5−9.7 can keep viscosity above 100 mPa·s even at a high temperature range 70−75 °C. Addition of alkali improves the storage moduli parameter of gel better than viscosity; it reflects network strength being increased (internal structure is becoming denser and more stable and gives a stable proppant H
DOI: 10.1021/acs.iecr.6b00158 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 9. (a) Plot of NaOL break time vs concentration of sodium oleate solutions at different temperature. (b) Partition coefficients of NaOL after break test at different concentrations.
medium). Experimental results show that at optimized surfactant, salt, alkaline concentrations, static proppant settling is significantly impacted by fluid elasticity (G′) than viscosity. At high temperature (>85 °C), the suspension medium possesses low shear viscosity (below 0.1 Pa s) and G′ value (
