polymers Article
High Performance Clean Fracturing Fluid Using a New Tri-Cationic Surfactant Jinzhou Zhao *, Jinming Fan, Jincheng Mao * and Wenlong Zhang
ID
, Xiaojiang Yang *, Heng Zhang
State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China;
[email protected] (J.F.);
[email protected] (H.Z.);
[email protected] (W.Z.) * Correspondence:
[email protected] (J.Z.);
[email protected] (J.M.);
[email protected] (X.Y.); Tel.: +86-28-8303-3546 (J.Z. & J.M. & X.Y.)
Received: 20 April 2018; Accepted: 5 May 2018; Published: 16 May 2018
Abstract: In order to improve the heat resistance of current clean fracturing fluids, a novel cationic surfactant (VES-T), composed of three single-chains and a spacer group, was designed and synthesized as thickener for the fluids. Various performances of such VES-T fluid in the presence of NaSal were evaluated carefully. Study of the rheological properties demonstrated that the fluids with varying concentrations (3–5 wt %) of VES-T have excellent thermal stabilities under ultra-high temperatures ranging from 140 to 180 ◦ C. Until now, this is the highest temperature that the VES fracturing fluid could bear. The VES-T/NaSal fluid exhibited good viscoelasticity and proppant-suspending capability, which was attributed to the three-dimensional network formed by entangled wormlike micelles. Furthermore, the VES fracturing fluids can be completely gel broken by standard brines within 2 h. Thus, the VES-T synthesized in this work has a good prospect for utilization during the development of ultra-high temperature reservoirs. Keywords: tri-cationic surfactant; wormlike micelles; network; fracturing fluid
1. Introduction As an effective practice for increasing the rate of production of low-permeability reservoirs, hydraulic fracturing technology has been applied worldwide for decades [1]. Fracturing fluids have been utilized to generate artificial fractures in reservoir and transport proppant particles into the fracture, enhancing the conductivity of formation [2,3]. Polymer fluids such as polyacrylamide and guar gum have been widely used as thickeners for fracturing fluids. However, polymer fluids have the shortcoming of leaving non-removable residue in formation, which could plug the pore throats and cause serious formation damage [4,5]. In addition, due to the inherent high viscosity properties, the polymer-based fluids force the fracture to extend in height rather than in length [6,7]. Thus, other alternative fracturing fluids have gained great attention. Being a substitute for conventional polymer fracturing fluid, the concept of viscoelastic surfactant (VES) fracturing fluid was first put forward by Schlumberger in 1997 [8]. Since then, VES fluids have been employed in the development of low-permeability gas and oil reservoirs for nearly 20 years [9]. Unlike polymer fracturing fluids, the sand suspension capability of the VES fluid depends largely on the elasticity instead of viscosity [10,11]. This unique property is rendered by the entangled wormlike micelles [12]. In general, the breaking of VES fracturing fluid after fracturing construction relies on two primary external conditions (reservoir conditions): (i) contact with hydrocarbons produced in the reservoir and (ii) dilution of reservoir brines [8,13]. Nevertheless, the VES fracturing fluid cannot break efficiently if the two reservoir conditions are not met in some cases, particularly in Polymers 2018, 10, 535; doi:10.3390/polym10050535
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dry-gas reservoirs [14]. Usually, the internal breakers for VES fluid are used to address this problem. In comparison to traditional polymer fracturing fluid, VES fracturing fluid has a wide range of advantages, such as no insoluble residue, easy preparation in the well site, good sand-carrying capability, extremely low damage to formation, and low friction, etc. [15–17]. Furthermore, the VES foam is also applied as a treatment for ultralow-permeability reservoirs since it can reduce not only the interfacial tension but also the water consumption in the fracturing fluid [18]. In spite of the above mentioned advantages, the conventional VES fracturing fluid still has many deficiencies, which would limit its further application. For example, the traditional single head/single tail surfactants have high critical micelle concentrations (CMC) and exhibit poor surface-active behaviors. Thus, with these surfactants as thickener, the VES fracturing fluids are used at a much higher concentration than polymer fracturing fluids. Moreover, the capability of the known VES fracturing fluids to maintain high temperature and high shearing is quite limited. Meanwhile, with the rapid development of modern drilling technology, the exploitation and utilization of the ultra-high temperature oil and gas reservoirs attract tremendous attention. Accordingly, there is a great demand for VES fracturing fluids capable of resisting such high temperature. Gemini surfactant is composed of two single head/single tail surfactants linked by a spacer between the head groups [19]. The spacer group restricts the electrostatic repulsion between hydrophilic groups through strong chemical bonds [20]. Hence, Gemini surfactants have higher surface activities and lower critical micelle concentration (CMC), exhibiting better rheological behaviors than single-chain surfactants [21,22]. Benefiting from the superiority of molecular design, the temperature resistant capacities of Gemini surfactant fracturing fluids are greatly promoted compared with single-chain surfactants. However, most of them still cannot have satisfactory performances (less than 150 ◦ C). Therefore, the development of VES fracturing fluid with higher temperature resistance is the popular topic of research. In view of the excellent properties brought by the combination of two single-chain monomeric surfactants, a question that emerges is whether a surfactant molecule consisting of more monomeric surfactants could have the better performance when compared to Gemini surfactants? In this work, a novel surfactant composed of three single head/single tail surfactants was synthesized using N,N 0 -Dimethyl-1,3-propanediamine, epichlorohydrin, and erucamidopropyl dimethylamine as raw materials. The performances of the fracturing fluid based on this surfactant, including surface activity, microstructure, viscoelasticity, thermo-shear resistance, sand suspending capability, and gel breaking property were investigated. In this way, a possible practical clean fracturing fluid could be developed for the various oilfields. It is noteworthy that, to our knowledge, this novel VES fracturing fluid could bear the highest temperature (up to 180 ◦ C) till now, which is potentially useful for the exploration of deep or super deep reservoirs. 2. Materials and Experiments 2.1. Materials The product named as VES-T was synthesized as follows. Erucamidopropyl dimethylamine was provided by Shanghai Winsono New Material Tech Co., Ltd. (Shanghai, China). N,N0 -Dimethyl1,3-propanediamine, epichlorohydrin, ethanol, concentrated hydrochloric acid (HCl), n-hexane, acetone, ethyl acetate, potassium chloride (KCl), and sodium salicylate (NaSal) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water was used in all measurements. 2.2. Synthesis of VES-T 2.2.1. Synthesis of Intermediate Ethanol (50 mL) and N,N 0 -dimethyl-1,3-propanediamine (6.13 g, 0.06 mol) were added into a round-bottom flask. Epichlorohydrin (27.76 g, 0.3 mol) was then added dropwise to this stirring
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mixture at 25 ◦ C. Subsequently, concentrated hydrochloric acid (HCl) (6 g,vigorously 0.06 mol) was added to at provide hydrogen atoms for this reaction. Then, the mixture was stirred under reflux Polymers 2018, 10, x FOR PEER REVIEW 3 of 13 provide hydrogen atoms for this reaction. Then, the mixture was stirred vigorously under reflux 60 °C for 6 h (Scheme 1). The solvent ethanol was removed by vacuum rotary evaporator. The at 60 ◦ C was for 6extracted h (Schemefor1).three Thetimes solvent ethanol was removed by vacuum rotary evaporator. intermediate with athe mixture deionized water and n-hexane provide hydrogen atoms for this reaction. Then, mixtureofwas stirred vigorously under reflux(1:1) at to The intermediate was extracted for three times with a mixture of deionized water and n-hexane remove residual 60 °C for 6 hepichlorohydrin. (Scheme 1). The solvent ethanol was removed by vacuum rotary evaporator. The (1:1) to remove residual epichlorohydrin. intermediate was extracted for three times with a mixture of deionized water and n-hexane (1:1) to 2.2.2.remove Synthesis of VES-T residual epichlorohydrin. 2.2.2. Synthesis of VES-T
The The intermediate (24.97 g, g, 0.06 inethanol ethanol(60 (60 mL). Then, erucamidopropyl intermediate (24.97 0.06mol) mol)was was dissolved dissolved in mL). Then, erucamidopropyl 2.2.2. Synthesis of VES-T dimethylamine (76.14 mol)was was added the round-bottom flask.theDuring reaction dimethylamine (76.14g,g,0.18 0.18 mol) added into into the round-bottom flask. During reactionthe process, The intermediate (24.97 g, 0.06 mol) was dissolved ◦in ethanol (60 mL). Then, erucamidopropyl process, the mixture was stirred vigorously underatreflux 8518 °Chfor 18 h (Scheme Next,was ethanol the mixture was stirred vigorously under reflux 85 Catfor (Scheme 2). Next,2). ethanol dimethylamine (76.14 g, 0.18 mol) was added into the round-bottom flask. During the reaction evaporated under reduced pressure. The crude product obtained was repeatedly washed with acetone was evaporated under reduced pressure. The crude product obtained was repeatedly washed with process, the mixture was stirred vigorously under reflux at 85 °C for 18 h (Scheme 2). Next, ethanol and and thenthen recrystallized from afrom mixture of ethyl acetate ethanol (7:1) for three times yieldtimes the to acetone recrystallized a mixture of ethyland acetate and ethanol (7:1) fortothree was evaporated under reduced pressure. The crude product obtained was repeatedly washed with pure VES-T. yieldacetone the pure andVES-T. then recrystallized from a mixture of ethyl acetate and ethanol (7:1) for three times to yield the pure VES-T.
Scheme of intermediate. intermediate. Scheme1.1.Synthesis Synthesis of Scheme 1. Synthesis of intermediate.
Scheme 2. Synthesis of VES-T.
Scheme2.2.Synthesis Synthesis of Scheme of VES-T. VES-T.
2.3. Measurement
2.3. Measurement 2.3. Measurement
1H nuclear magnetic resonance (1H NMR) spectral analysis of VES-T was performed using The 1 H nuclear magnetic resonance (1 H NMR) spectral analysis of VES-T was performed using a The 1 The H nuclear magnetic resonance (1H NMR)(Bruker, spectralKarlsruhe, analysis Germany). of VES-T was performed using aBRUKER BRUKER AVANCE HD400 400MHz MHzspectrometer spectrometer AVANCE IIIIIIHD (Bruker, Karlsruhe, Germany). Fourier-transform Fourier-transform a BRUKER III HD 400 MHz spectrometer (Bruker, Karlsruhe, infraredAVANCE (FT-IR) spectrum was obtained on a Nicolet MAGNA IR 560Germany). ESP FT-IRFourier-transform spectrometer infrared (FT-IR) spectrum was obtained on a Nicolet MAGNA IR 560 ESP FT-IR spectrometer 1 (Nicolet, Madison, WI, USA). FT-IR and NMR spectroscopic results shown in Figures 1 infrared (FT-IR) spectrum was The obtained on 1aH MAGNA IR 560 are ESP FT-IR (Nicolet, Madison, WI, USA). The FT-IR and HNicolet NMR spectroscopic results are shown in spectrometer Figures 1 1 and 2. (Nicolet, and 2.Madison, WI, USA). The FT-IR and H NMR spectroscopic results are shown in Figures 1 tension measurement of VES-T was conducted using KRUSS DSA30S tensiometer and 2. Surface Surface tension measurement of VES-T was conducted using KRUSS DSA30S tensiometer (KRUSS, Hamburg, Germany). Scanning electron were obtained Surface Hamburg, tension measurement of VES-T was microscope conducted (SEM) using micrographs KRUSS DSA30S tensiometer (KRUSS, Germany). Scanning electron microscope (SEM) micrographs were obtained from a Quanta 450 scanning SEM (FEI, Hillsboro, OR, USA). The samples were prepared by freezing from aHamburg, Quanta 450Germany). scanning SEM (FEI, Hillsboro, USA). The samples were prepared were by freezing (KRUSS, Scanning electronOR, microscope (SEM) micrographs obtained a drop of VES-T fracturing fluid with liquid nitrogen (−185 °C), the microstructure of the ◦ C), where a drop of VES-T fracturing fluid with liquid nitrogen ( − 185 where the microstructure of the from a Quanta 450 scanning SEM (FEI, Hillsboro, OR, USA). The samples were prepared by freezing solutions could be retained. Subsequently, the frozen samples were dried and then coated with gold. solutions could be retained. Subsequently, the frozen samples were dried and then coated with gold. a drop of VES-Tproperties fracturing fluid with liquid °C), the (Thermo microstructure of the Rheological were evaluated using nitrogen a HAAKE(−185 MARS III where Rheometer Scientific, Rheological properties were evaluated using a HAAKE MARS III Rheometer (Thermo Scientific, solutions could be retained. Subsequently, the frozen by samples andMCR then301 coated with gold. Munich, Germany). Viscoelasticity was determined Antonwere Paar dried physical Rotational Munich, Germany). Viscoelasticity was determined by Anton Paar physical MCR 301 Rotational Rheological properties were evaluated Rheometer (Anton Paar, Graz, Austria).using a HAAKE MARS III Rheometer (Thermo Scientific, Rheometer (Anton Paar, Graz, Austria).
Munich, Germany). Viscoelasticity was determined by Anton Paar physical MCR 301 Rotational Fracturing FluidPaar, Preparation Rheometer (Anton Graz, Austria). The VES fracturing fluid mainly contains two components, wherein the VES acts as a thickener,
Fracturing Fluid Preparation and counter-ion additive screens the electrostatic repulsion between the hydrophilic head groups and promotes the growth of wormlike micelles. The fracturing fluids with varying concentrations (3 and The VES fracturing fluid mainly contains two components, wherein the VES acts as a thickener, 5 wt %) of VES-T were prepared to evaluate their rheological properties, viscoelasticity, sand-carrying
and counter-ion additive screens the electrostatic repulsion between the hydrophilic head groups and promotes the growth of wormlike micelles. The fracturing fluids with varying concentrations (3 and
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Fracturing Fluid Preparation The VES fracturing fluid mainly contains two components, wherein the VES acts as a thickener, and counter-ion additive screens the electrostatic repulsion between the hydrophilic head groups and Polymers 2018, 10,growth x FOR PEER REVIEW micelles. The fracturing fluids with varying concentrations (34 of 13 promotes the of wormlike and 5 wt %) of VES-T were prepared to evaluate their rheological properties, viscoelasticity, sand-carrying abilities, and gel breaking properties. The concentrations of NaSal in the two fracturing fluids were 1 abilities, and gel breaking properties. The concentrations of NaSal in the two fracturing fluids were and 1.2 wt %, respectively. Additionally, the fluid sample with KCl as counter-ion was also prepared 1 and 1.2 wt %, respectively. Additionally, the fluid sample with KCl as counter-ion was also prepared to investigate its effect on the formation of the network. The samples were centrifuged before tests to to investigate its effect on the formation of the network. The samples were centrifuged before tests to remove the air bubbles. remove the air bubbles.
3. 3. Results Results and and Discussion Discussion 3.1. Structural Characterization of VES-T
725.0
1096.6
1637.8 1556.1 1466.3
2923.0 2852.7
3376.3
The structure of VES-T was confirmed FT-IR and and 1H confirmed by FT-IR H NMR. NMR. Figure Figure 11 shows shows the the FT-IR FT-IR spectrum spectrum of VES-T. The The absorption absorption peak peak resulting resulting from from –O–H –O–Hisispresented presentedatat3376.3 3376.3cm cm−−11 and the absorption −1 bands corresponding correspondingtotothe theCH CH 3 and CH 2 stretch acylamino group are found at 2923.0 CH onon thethe acylamino group are found at 2923.0 cm−1cm and 3 and 2 stretch − 1 − 1 −1 −1 and 2852.7 , respectively. The absorption at cm 1637.8is cm is the dueC=O to the C=O stretching 2852.7 cm cm , respectively. The absorption peak atpeak 1637.8 due to stretching vibration −1 . Moreover, vibration absorptions. N–H stretching peak appears 1556.1 cm−1. Moreover, theatpeak at 1466.3 absorptions. The N–HThe stretching peak appears at 1556.1atcm the peak 1466.3 cm−1 -1 cm corresponds to C–N stretching vibration. corresponds to C–N stretching vibration.
Figure spectra of of VES-T. VES-T. Figure 1. 1. Fourier-transform Fourier-transform infrared infrared spectroscopy spectroscopy (FT-IR) (FT-IR) spectra
Figure 2 is about the 1H1 NMR (400 MHz, CDCl3) for VES-T: 0.88 (t, 9H, CH3), 1.26 (m, 84H, CH2), Figure 2 is about the H NMR (400 MHz, CDCl3 ) for VES-T: 0.88 (t, 9H, CH3 ), 1.26 (m, 84H, 1.58 (s, 6H, COCCH2), 2.03–1.98 (m, 18H, C=CCH2, COCH 2), 2.23 (m, 8H, N+CCH2), 2.70 (s, 6H, NCH2), CH2 ), 1.58 (s, 6H, COCCH ), 2.03–1.98 (m, 18H, C=CCH , COCH ), 2.23 (m, 8H, N+ CCH2 ), 2.70 (s, 6H, 2 2 2 3.47–3.35 (m, 32H, N+CH3, N++CH2), 3.72 (s, 6H, CONCH2), 3.90 (s, 8H, CN+CH2), 4.39 (s, 3H, COH), + + NCH2 ), 3.47–3.35 (m, 32H, N CH , N CH2 ), 3.72 (s, 6H, CONCH 2 ), 3.90 (s, 8H, CN CH2 ), 4.39 (s, 3H, 5.07 (s, 2H, N+CCH), +5.38–5.30 (m,3 6H, HC=CH), 6.83 (s, 1H, N+CCH), 7.83 (m, 3H, CONH). Thus, the COH), 5.07 (s, 2H, N CCH), 5.38–5.30 (m, 6H, HC=CH), 6.83 (s, 1H, N+ CCH), 7.83 (m, 3H, CONH). desired product VES-T was successfully synthesized. Thus, the desired product VES-T was successfully synthesized. 3.2. Surface Tension Measurement 3.2. Surface Tension Measurement As mentioned earlier, the surface tension of VES-T was measured ◦at 25 °C using a KRUSS As mentioned earlier, the surface tension of VES-T was measured at 25 C using a KRUSS DSA30S DSA30S tensiometer. The measurement process continued until the surface tension value obtained tensiometer. The measurement process continued until the surface tension value obtained was almost was almost constant. Shown in Figure 3 is the variation of surface tension versus the concentration constant. Shown in Figure 3 is the variation of surface tension versus the concentration of VES-T. of VES-T. The variation tendency of surface tension could be described with two straight lines. The The variation tendency of surface tension could be described with two straight lines. The surface surface tension decreases with the increasing concentration of VES-T, and then reaches a clear tension decreases with the increasing concentration of VES-T, and then reaches a clear inflection point, inflection point, where the intersection of the two lines is the CMC. The surface tension at CMC is where the intersection of the two lines is the CMC. The surface tension at CMC is recorded as γCMC. recorded as γCMC. As shown in Figure 3, the CMC of the corresponding product VES-T is 1.44 × 10−4 mol/L and the γCMC is 25.98 mN/m. Akram et al. [19] designed a Gemini surfactant (16-E2-16), whose CMC and γCMC value are 1.2 × 10−3 mol/L and 42.6 mN/m, respectively. Apparently, the CMC and γCMC of VES-T are both smaller than that of 16-E2-16, which is consistent with the previous assumption that
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As shown in Figure 3, the CMC of the corresponding product VES-T is 1.44 × 10−4 mol/L and the γCMC is 25.98 mN/m. Akram et al. [19] designed a Gemini surfactant (16-E2-16), whose CMC and γCMC value are 1.2 × 10−3 mol/L and 42.6 mN/m, respectively. Apparently, the CMC and γCMC of VES-T are both smaller than that of 16-E2-16, which is consistent with the previous assumption that the surfactant composed of three hydrophobic groups has a better performance than general Gemini2018, surfactants. Polymers 10, x FOR PEER REVIEW 5 of 13
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Figure 2. 1H NMR spectrum of VES-T in CDCl3. H NMR NMR spectrum spectrum of of VES-T VES-T in in CDCl CDCl33. . Figure 2. 11H
m Surface m-1)-1) Surfacetension tension(mN (mN
70 70 60 60 50 50 40 40 30 30 20 20
0.0000 0.0000
0.0002 0.0002
0.0004 0.0004
0.0006 0.0006 -1 -1
0.0008 0.0008
0.0010 0.0010
C (molL ) C (molL ) Figure 3. Surface tension plot for VES-T tri-cationic surfactant. Figure Figure 3. 3. Surface Surface tension tension plot plot for for VES-T VES-T tri-cationic tri-cationic surfactant. surfactant.
3.3. Scanning Electron Microscopy (SEM) 3.3. Scanning Electron Microscopy (SEM) As SEM visualizes the micellar morphology directly, it was employed to investigate the As SEM visualizes the micellar morphology directly, it was employed to investigate the microstructure of the VES-T fracturing fluid, which could also associate the microstructure with microstructure of the VES-T fracturing fluid, which could also associate the microstructure with macroscopic properties, such as the rheology and viscoelasticity of the VES-T fracturing fluid [23]. macroscopic properties, such as the rheology and viscoelasticity of the VES-T fracturing fluid [23]. According to Candau et al. [20], the self-aggregation and entanglement of surfactant micelles largely According to Candau et al. [20], the self-aggregation and entanglement of surfactant micelles largely depend on the nature and concentration of salts. The salts would help transform spherical micelles depend on the nature and concentration of salts. The salts would help transform spherical micelles
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3.3. Scanning Electron Microscopy (SEM) As SEM visualizes the micellar morphology directly, it was employed to investigate the microstructure of the VES-T fracturing fluid, which could also associate the microstructure with macroscopic properties, such as the rheology and viscoelasticity of the VES-T fracturing fluid [23]. According to Candau et al. [20], the self-aggregation and entanglement of surfactant micelles largely depend on the nature and concentration of salts. The salts would help transform spherical micelles into rod-like or wormlike micelles [24,25]. For this reason, the SEM measurements of the VES-T fracturing fluids without any counter-ion, with NaSal and KCl were conducted using a Quanta 450 scanning SEM (FEI, Hillsboro, OR, USA), Polymers 2018, 10, x FOR PEER REVIEW 6 of 13 respectively. The results are shown in Figure 4a–c. Compared with Figure 4a, b and c both show cross-linked networks networks composed composed of of entangled This result result confirms confirms that that the the cross-linked entangled wormlike wormlike micelles. micelles. This presences of of counter-ions counter-ions (NaSal (NaSal and and KCl) KCl) greatly greatly assist assist the the formation formation of of 3D 3D networks, networks, and and the the presences shielding effects by overhead counter-ions are remarkable. The viscoelastic and rheological characters shielding effects by overhead counter-ions are remarkable. The viscoelastic and rheological of the VES-T fracturing can befluid explained the formation the 3D network. characters of the VES-T fluid fracturing can be as explained as the of formation of the 3D network.
(a)
(b)
(c)
Figure electron microscopy microscopy(SEM) (SEM)images imagesofof(a)(a)VES-T VES-T solution without any counterFigure 4. 4. The The Scanning Scanning electron solution without any counter-ion ion (served as a blank control); (b) VES-T solution with NaSal; (c) VES-T solution with KCl. The cross(served as a blank control); (b) VES-T solution with NaSal; (c) VES-T solution with KCl. The cross-linked linked networks could be observed in both (b) and (c) in comparison to (a). networks could be observed in both (b) and (c) in comparison to (a).
3.4. and High High Shear Shear 3.4. Resistance Resistance to to High High Temperature Temperature and Both the formation formation of of entangled entangled wormlike wormlike micelles Both KCl KCl and and NaSal NaSal have have significant significant effects effects on on the micelles according to the SEM morphologies of Figure 4a–c. For the purpose of investigating the influence according to the SEM morphologies of Figure 4a–c. For the purpose of investigating the influence of of the the counter-ions counter-ions on on the the macroscopic macroscopic rheological rheological properties properties of of the the VES-T VES-T fluids, fluids, the the thermal-stability thermal-stability tests for the the VES-T VES-Tfluids fluidswith withKCl KCland andNaSal NaSal were performed at 160 at the constant shear ◦ C °C tests for were performed at 160 andand at the constant shear rate −1 for 80 min. The results are shown in Figures 5 and 6. As depicted in Figure 5, the rate of 170 s − 1 of 170 s for 80 min. The results are shown in Figures 5 and 6. As depicted in Figure 5, the viscosity viscosity of the prepared solution prepared 5 wt % VES-T and 1.4 wt % decreases KCl first decreases to·s, 433 mPa·s, of the solution with 5 wtwith % VES-T and 1.4 wt % KCl first to 433 mPa and then and then increases to 458 mPa·s. Subsequently, the viscosity decreases again and finally maintains at increases to 458 mPa·s. Subsequently, the viscosity decreases again and finally maintains at about about 50 mPa·s. Similarly, for a solution consisting of 5 wt % VES-T and 1.2 wt % NaSal, the viscosity 50 mPa·s. Similarly, for a solution consisting of 5 wt % VES-T and 1.2 wt % NaSal, the viscosity of of fluid decreases sharply the increase of testing temperature, and maintains then maintains at 78 about fluid decreases sharply withwith the increase of testing temperature, and then at about mPa78 ·s. mPa·s. The results indicate that NaSal has a superior performance in enhancing the thermal stability The results indicate that NaSal has a superior performance in enhancing the thermal stability of the of the system compared with KCl. system compared with KCl. In view of the excellent rheological property that VES-T/NaSal solution exhibited at 160 ◦ C, 700test was further conducted at 180 ◦ C. As depicted in Figure 7, the VES-T/NaSal the thermal stability 160 solution displays a similar rheological behavior to that at 160 ◦ C, and the viscosity is constant at about 600 s 42 mPa·s till the end of test. Obviously, NaSal is more suitable toViscosity be themPa counter-ion of the system. Temperature C
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viscosity of the solution prepared with 5 wt % VES-T and 1.4 wt % KCl first decreases to 433 mPa·s, and then increases to 458 mPa·s. Subsequently, the viscosity decreases again and finally maintains at about 50 mPa·s. Similarly, for a solution consisting of 5 wt % VES-T and 1.2 wt % NaSal, the viscosity of fluid decreases sharply with the increase of testing temperature, and then maintains at about 78 Polymers 2018, 535 7 of 13 mPa·s. The10,results indicate that NaSal has a superior performance in enhancing the thermal stability of the system compared with KCl. 700 160
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160 Figure 5. Shear resistance of 5 wt % VES-T with 1.4 wt % KCl fracturing fluid at 160 170 s−1s. −1 . ◦ C, Figure 5. Shear500 resistance of 5 wt % VES-T with 1.4 wt % KCl fracturing fluid at 160°C, 170 Viscosity mPa.s
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−1 100 Figure 6. Shear resistance of 5 wt % VES-T with 1.2 wt % NaSal fracturing fluid at 160 40 °C, 170 s .
0 In view of the excellent rheological property that VES-T/NaSal solution exhibited at 160 °C, the 0 10 20 30 40 50 60 70 80 thermal stability test was further conducted Time/min at 180 °C. As depicted in Figure 7, the VES-T/NaSal solution displays a similar rheological behavior to that at 160 °C, and the viscosity is constant at about −1. −1 ◦ C, 170 Figure Shear resistance of with 1.2 %% NaSal fracturing fluid at 160 °C, Figure 6.6.the Shear resistance of 55 wt wt% %VES-T VES-T with 1.2wt wt NaSal fluid at 160 s . 42 mPa·s till end of test. Obviously, NaSal is more suitable tofracturing be the counter-ion of 170 thessystem.
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In view of the at 160 °C, the 600excellent rheological property that VES-T/NaSal solution exhibited 200 thermal stability test was further conducted at 180 °C. As depicted in Figure 7, the VES-T/NaSal 180 solution displays500 a similar rheological behavior to that at 160 °C, and the viscosity is constant at about mPa.s 42 mPa·s till the end of test. Obviously, NaSal is more suitable toViscosity be the counter-ion 160 of the system. o
60 100 Figure Shear resistanceofof5 5wt wt%%VES-T VES-Twith with 1.2 1.2 wt wt % 170 s−1s. −1 . ◦ C, 40 Figure 7. 7. Shear resistance % NaSal NaSal fracturing fracturingfluid fluidatat180 180°C, 170 20 To reduce the 0cost, the10concentration was to703 wt %, 0 20 30 of VES-T 40 50 reduced 60 80 and the rheological property of the system was evaluated at 140 Time/min °C. As shown in Figure 8, the viscosity of the 3 wt % VES-T/1 wt % NaSal solution remains at 70 mPa·s, which could meet the viscosity requirement (>25 7. Shear resistance 5 wt % VES-T with 1.2 wt % NaSal fracturing fluid at 180 °C, 170 s−1.results mPa·s) Figure for VES fracturing fluidofaccording to industry standard. The thermal-stability testing demonstrate that the VES-T fracturing fluids with the above formula could have good performance To reduce the cost, the concentration of VES-T was reduced to 3 wt %, and the rheological in the development of reservoirs under ultra-high temperature. property of the system was evaluated at 140 °C. As shown in Figure 8, the viscosity of the 3 wt %
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To reduce the cost, the concentration of VES-T was reduced to 3 wt %, and the rheological property of the system was evaluated at 140 ◦ C. As shown in Figure 8, the viscosity of the 3 wt % VES-T/1 wt % NaSal solution remains at 70 mPa·s, which could meet the viscosity requirement (>25 mPa·s) for VES fracturing fluid according to industry standard. The thermal-stability testing results demonstrate that the VES-T fracturing fluids with the above formula could have good performance in the development of reservoirs under ultra-high temperature. Polymers 2018, 10, x FOR PEER REVIEW
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Figure 8. Shear resistance 3 wt VES-Twith with 11 wt wt % fluid at 140 °C, ◦170 . s−1 . Figure 8. Shear resistance of of 3 wt %%VES-T % NaSal NaSalfracturing fracturing fluid at 140 C, s170 0
−1
20
Some studies reported chlorides compounds, such as KCl and NaSal, 0 that10both 20 30 and 40 aromatic 50 60 70 80 Time/min Someinduce studies reported that both chlorides and aromatic compounds, such KCl and NaSal, could micelle morphology transition for cationic surfactant from spherical to as wormlike [26– − could34]. induce micelle morphology transition for cationic surfactant from spherical to wormlike [26–34]. However,Figure there8.are some differences between the1 wt effects of KCl and fluid NaSal. Chlorine anions (Cl ) Shear resistance of 3 wt % VES-T with % NaSal fracturing at 140 °C, 170 s−1 . are adsorbed onsome the surface of the cationic group form an electric double However, there are differences betweenhydrophilic the effectshead of KCl andtoNaSal. Chlorine anionslayer (Cl− ) are Some studies reported that both chlorides and aromatic compounds, such as KCl and NaSal, around which screens the electrostatic repulsion. Alternatively, addition to the adsorbed onthe themicelle surfaceshell, of the cationic hydrophilic head group to form an electricindouble layer around could induce micelle morphology transition for cationic surfactant from spherical to wormlike [26– −, the aromatic −) penetrates similar surface adsorption of Cl anion (Sal the surface of the micelle, whose the micelle 34]. shell, which screens thedifferences electrostatic repulsion. in addition similar −) However, there are some between the effects ofAlternatively, KCl and NaSal. Chlorine anionsto(Clthe aromatic nucleus strong hydrophobic interaction with the surfactant, increasing the whose surfactant−a, the − ) penetrates surface adsorption of has Clon aromatic anion (Sal surface the micelle, are adsorbed the surface of the cationic hydrophilic head the group to formofan electric double layer aromatic packing parameter by shell, efficiently decreasing the area repulsion. per surfactant monomer (a0) [28,29,31]. the micelle which screens thewith electrostatic addition to the nucleus hasaround a strong hydrophobic interaction the surfactant,Alternatively, increasing in the surfactant-packing − is a far more effective driving force to the growth of the worm-like micelles. The Therefore, Sal − − similar surface adsorption of Cl , the aromatic anion (Sal ) penetrates the surface of the micelle, whose parameter by efficiently decreasing the area per surfactant monomer (a0 ) [28,29,31]. Therefore, Sal− is a mechanism by which Cl-hydrophobic affect the micelle shape is schematically illustrated in aromatic nucleusSal has− aand strong interaction withtransition the surfactant, increasing the surfactantfar more effective driving to the growth of the micelles. The mechanism which packing parameterforce by efficiently decreasing the worm-like area per surfactant monomer (a0) [28,29,31]. Figure 9a,b, respectively. Additionally, the sphere-rod transition of micelles generally occurs by with − affect the −micelle Sal− the andlower ClTherefore, shape transition is schematically illustrated in Figure 9a,b, respectively. − − Salof is more drivingwhich force to the growth of the micelles. The addition Sala far than Cl effective [28,30,32–34], agrees well with theworm-like experimental results.
mechanism by which transition Sal− and Cl- affect the micelle shape transition schematically illustrated in of Sal− Additionally, the sphere-rod of micelles generally occursiswith the lower addition Figure 9a,b, respectively. Additionally, the sphere-rod transition of micelles generally occurs with − than Cl [28,30,32–34], which agrees well with the experimental results. − − the lower addition of Sal than Cl [28,30,32–34], which agrees well with the experimental results.
Figure 9. (a) Schematic diagram for Sal− to affect micelle shape transition; (b) Schematic diagram for − to affect micelle shape transition; (b) Schematic diagram for Figure 9. (a) Schematic diagram for to(a) affect micelle shape transition. Cl-9. Figure Schematic diagram for Sal− Sal to affect micelle shape transition; (b) Schematic diagram for Cl- to affect micelle shape transition.
Cl− to affect micelle shape transition. 3.5. Viscoelasticity Evaluation 3.5. Viscoelasticity Evaluation
Compared with traditional viscous fluids, thetheproppant VESfracturing fracturing fluid Compared with traditional viscous fluids, proppant suspension suspension ofofVES fluid is is stronglystrongly affected by both viscosity and elasticity, that is, the viscoelasticity. Viscoelasticity is an affected by both viscosity and elasticity, that is, the viscoelasticity. Viscoelasticity is an essential property to evaluate the the performance ofofVES fluid,which whichis isattributed attributed to the essential property to evaluate performance VESfracturing fracturing fluid, to the formation of network. a 3D network. Therefore, viscoelasticityof of the the VES-T VES-T solution thethe presence of of formation of a 3D Therefore, thethe viscoelasticity solutioninin presence
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3.5. Viscoelasticity Evaluation Compared with traditional viscous fluids, the proppant suspension of VES fracturing fluid is strongly affected by both Polymers 2018, 10, x FOR PEER viscosity REVIEW and elasticity, that is, the viscoelasticity. Viscoelasticity is an essential 9 of 13 property to evaluate the performance of VES fracturing fluid, which is attributed to the formation of a 3Dwas network. Therefore, the viscoelasticity the 301 VES-T solutionRheometer, in the presence of NaSal was NaSal measured by an Anton Paar physical of MCR Rotational as shown in Figure measured by an Anton Paar physical MCR 301 Rotational Rheometer, as shown in Figure 10. 10. In each modulus G0 and loss loss modulus G00 both increaseincrease with thewith increase In each solution, solution,storage storage modulus G′ and modulus G″ slightly both slightly the 0 is greater than loss of frequency during the range of frequency scanning, and storage modulus G increase of frequency during the range of frequency scanning, and storage modulus G′ is greater than modulus G00 throughout the test. the twothe solutions both exhibit elastic behaviors, loss modulus G″ throughout theAccordingly, test. Accordingly, two solutions bothtypical exhibit typical elastic indicating indicating the formation of entangled wormlikewormlike micelles. micelles. Moreover, it is apparent that the that storage behaviors, the formation of entangled Moreover, it is apparent the 0 and loss modulus G00 of the VES solution both increase with the increasing concentration modulus G storage modulus G′ and loss modulus G″ of the VES solution both increase with the increasing of VES-T fromof 3 wt % to from 5 wt %, which demonstrates that the increasing VES-T concentration in the concentration VES-T 3 wt % to 5 wt %, which demonstrates that the increasing VES-T presence of NaSal the entanglements. The increase of VES concentration results inofa more concentration in augments the presence of NaSal augments the entanglements. The increase VES obvious viscoelasticity of the VES fracturing fluid [35]. concentration results in a more obvious viscoelasticity of the VES fracturing fluid [35].
G', G" (Pa)
100
5 wt% G' 5 wt% G"
3 wt% G' 3 wt% G"
10
1
0.1
0.01
0.1
1
10
Frequency (Hz) Figure asas a function of of frequency forfor thethe two (3 00 varying Figure 10. 10. Storage Storagemodulus modulusG′ G0and andloss lossmodulus modulusG″Gvarying a function frequency two wt %% and 5 wt %)%) VES-T samples ◦ C.G′Gis 0 isgreater (3 wt and 5 wt VES-T samplesatat2525°C. greaterthan thanG″ G00throughout throughoutthe thetest testin inboth both the the two two samples. G′ and G″ of the 5 wt % VES-T sample are greater than that of the 3 wt % VES-T sample. 0 00 samples. G and G of the 5 wt % VES-T sample are greater than that of the 3 wt % VES-T sample.
3.6. 3.6. Proppant Proppant Suspension Suspension Measurement Measurement Good sand-carryingcapability capability is essential to fracturing the fracturing fluid, which it proppant to carry Good sand-carrying is essential to the fluid, which allows itallows to carry proppant evenly into the fractures [36]. If the sand-carrying capability of the fracturing fluid is poor, evenly into the fractures [36]. If the sand-carrying capability of the fracturing fluid is poor, the proppant the will fallcannot rapidly cannot be transported into deep theThe reservoir [37]. The willproppant fall rapidly and beand transported into deep fractures in thefractures reservoirin [37]. static proppant static proppant settling tests for the VES-T fracturing fluids with abovementioned formula ◦ C to settling tests for the VES-T fracturing fluids with abovementioned formula were carried out at 80 were carried out at 80 °C to evaluate the sand-carrying capabilities. 50 mL of VES fluid and 10 g of ceramics evaluate the sand-carrying capabilities. 50 mL of VES fluid and 10 g of ceramics of mesh size 20/40 of mesh size uniformly 20/40 wereand mixed anda then poured into acylinder. 50 mL graduated After were mixed thenuniformly poured into 50 mL graduated After 120 cylinder. min under the 120 min under the experimental conditions, almost no ceramics settled in 5 wt % VES-T/1.2 wt % experimental conditions, almost no ceramics settled in 5 wt % VES-T/1.2 wt % NaSal fluid. For the NaSal fluid. For the fluid 3 wt % VES-T, a small amount of proppant settled and was the fluid sample with 3 wt % sample VES-T, awith small amount of proppant settled and the settling velocity settling velocity was 0.007 mm/s. The proppant settlement phenomenon was more obvious in fluid 0.007 mm/s. The proppant settlement phenomenon was more obvious in fluid with lower VES-T with lower VES-T concentration. Whenwere the experiments conducted at 25 settled °C, fewinproppant concentration. When the experiments conducted atwere 25 ◦ C, few proppant both the settled in both the two fluid samples after 2 d. Thus, the fracturing fluids with above formula could two fluid samples after 2 d. Thus, the fracturing fluids with above formula could meet the demand meet the demand for suspending proppant during the fracturing construction. The proppant for suspending proppant during the fracturing construction. The proppant suspension of the VES suspension of the VES fracturing depends on both viscosity and elasticity the system, fracturing fluid depends on both fluid viscosity and elasticity of the system, which isofquite differentwhich from is quite different from that of the polymer and guar gum fracturing fluids. that of the polymer and guar gum fracturing fluids. 3.7. Gel Breaking Test The gel breaking process must be completed rapidly and thoroughly after fracturing construction to minimize the damage to the reservoir. Generally, the fracturing fluid is considered to be completely broken once the viscosity of the breaking fluid is less than 5 mPa·s. VES fracturing
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3.7. Gel Breaking Test The gel breaking process must be completed rapidly and thoroughly after fracturing construction to minimize the damage to the reservoir. Generally, the fracturing fluid is considered to be completely broken once the viscosity of the breaking fluid is less than 5 mPa·s. VES fracturing fluid usually relies on reservoir conditions such as standard brines and hydrocarbons to achieve the purpose of gel breaking. Hence, the VES-T fracturing fluids with the above formula were mixed separately with different proportions of standard brine and kerosene at 90 ◦ C. As shown in Tables 1 and 2, when the weight ratios are higher than 1:4 (standard brine to 3 wt % VES fluid) and 1:10 (standard brine to 5 wt % VES fluid), the fracturing fluids are completely broken and the viscosities of the breaking fluids are both less than 5 mPa·s within 2 h. Moreover, it is found that the VES-T fracturing fluid breaks quicker with the higher volume ratio of brine. For the reservoir hydrocarbon condition, when 30 wt % of kerosene is added, the viscosities of fluids with above formula are below 3 mPa·s within 30 min. Table 1. 3 wt % viscoelastic surfactant (VES) fluid gel breaking at different brine ratios. Ratio (Standard Brine to Fluid)
Breaking Time (h)
Viscosity of Breaking Fluid (mPa·s)
1:5 1:4 1:3
2.3 1.8 1.1
4.9 4.1 3.1
Table 2. 5 wt % VES fluid gel breaking at different brine ratios. Ratio (Standard Brine to Fluid)
Breaking Time (h)
Viscosity of Breaking Fluid (mPa·s)
1:12 1:10 1:8
2.6 1.9 1.3
4.3 3.8 2.9
These results confirm that the gel breaking properties of the VES fluids of the above formula are sufficient to fulfill the requirements for flowback treatment of fracturing fluid. 3.8. General Description The tri-cationic surfactant (VES-T) developed as a thickener for clean fracturing fluid was synthesized in a simple way with high yield (>96%). As far as we know, this is the first example of tri-cationic surfactant for the fracturing fluid. The main feedstock used in synthesis of the VES-T is erucic acid, which comes from the rapeseed. Thus, it is environmental friendly. The heat resistance of the VES-T fracturing fluid far exceeds that of the majority of the existing VES fracturing fluids. For instance, the 2% erucyldimethyl amidopropyl amine oxide (EMAO) solution cannot have good performance over 90 ◦ C [38]. A VES-HT fracturing fluid could only fulfill the requirement of 275 ◦ F (135 ◦ C) according to Schlumberger [39]. Nevertheless, compared with the generally used single-chain and Gemini surfactants, the molecule of VES-T consists of more monomeric surfactants, and the price of erucic acid is higher than commonly used raw materials such as oleic acid, which would slightly increase the cost. However, if the rapeseed could be directly employed as the starting material, the cost will be reduced sharply. Work is ongoing in our lab to further optimize the formula to reduce the amount of VES. 4. Conclusions A novel surfactant consisting of three single head/single tail surfactants was synthesized as thickener for VES fracturing fluid. The product was characterized by 1 H NMR and FT-IR. The two optimized formula of VES fracturing fluids were determined to be 5 wt % VES-T + 1.2 wt % NaSal and 3 wt % VES-T + 1 wt % NaSal, and both of them have excellent performances in various tests.
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(1). The surface tension measurement data confirm that the combination of three single-chain surfactants leads to a lower CMC and higher surface activity than the conventional Gemini surfactants. (2). Microstructure study suggests that the organic (NaSal) and inorganic (KCl) salts both have significant effects on the micellar transition from spherical micelles to wormlike micelles. (3). The fracturing fluids with the two concentrations (3 and 5 wt %) of VES-T both exhibit excellent rheological properties under high temperature. Especially, the viscosity of fluid sample containing 5 wt % VES-T and 1.2 wt % NaSal maintains at about 42 mPa·s at 180 ◦ C, which is the highest temperature recorded till now. (4). Viscoelasticity evaluation shows typical elastic properties of the two fracturing fluids. Additionally, the storage modulus G0 and loss modulus G00 of the VES solution both increased with the increasing concentration of VES-T, indicating a more obvious viscoelasticity of the VES fracturing fluid. In other words, a higher VES concentration leads to a stronger network structure. The two developed VES fracturing fluids also have many other advantages, such as strong sand suspending abilities and easy gel breaking, etc. These benefits demonstrate that the synthesized product can be a promising application in the development of gas and oil reservoirs with ultra-high temperature. Author Contributions: J.Z., J.M. and H.Z. conceived and designed the experiments; J.F., X.Y. and W.Z. performed the experiments; J.Z. and J.F. analyzed the data; J.M. contributed reagents/materials/analysis tools and supervised all the works; all authors wrote the paper. Acknowledgments: The authors are grateful to Sichuan Youth Science & Technology Foundation (2017JQ0010), National High Technology Research & Development Program (2016ZX05053), Key Fund Project of Educational Commission of Sichuan Province (16CZ0008), Explorative Project Fund (G201601) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), the Major Program of the National Natural Science Foundation of China (51490653) and 973 Program (2013CB228004). Conflicts of Interest: The authors declare no conflict of interest.
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