Article pubs.acs.org/EF
Supramolecular-Structure-Associating Weak Gel of Wormlike Micelles of Erucoylamidopropyl Hydroxy Sulfobetaine and Hydrophobically Modified Polymers Guancheng Jiang,† Qihui Jiang,*,† Yunlong Sun,‡ Ping Liu,§ Zhihang Zhang,∥ Xiaoxiao Ni,† Lili Yang,† and Chunlei Wang† †
College of Petroleum Engineering, China University of Petroleum (Beijing), Beijing 102249, People’s Republic of China University of Chinese Academy of Sciences, Beijing 100039, People’s Republic of China § Langfang Branch of Research Institute of Petroleum Exploration and Development, PetroChina, Langfang, Hebei 065007, People’s Republic of China ∥ Sinopec Jiangsu Petroleum Engineering Company, Limited, Yangzhou, Jiangsu 225261, People’s Republic of China ‡
ABSTRACT: The properties of a new supramolecular-structure-associating weak gel (SAWG) are discussed in this paper. On the basis of a hydrophobically modified polymer thickener (HMPT), the thickening system formed a gel with wormlike micelles of the erucoylamidopropyl hydroxy sulfobetaine (EHSB) surfactant by experimental design. The resultant gel contained very efficient physical cross-links between the HMPT and EHSB, generating viscoelasticity. To describe the performance of the SAWG, we investigated its high-temperature and shear-rate tolerance properties, suspension behavior, and rheological and structural thermal stabilities. Furthermore, scanning electron microscopy and transmission electron microscopy images of the solution microstructure revealed a supramolecular network honeycomb structure in the gel, which was formed with strong hyperbranched compound structures through extremely efficient non-covalent interactions. Meanwhile, an envisioned gelation mechanism model was proposed to facilitate rational discussion of the results. Moreover, the gel-breaking properties and formation damage were studied. The results of the steady shear viscosity and static-column tests showed that the optimized formula of the SAWG was obtained at 0.5 wt % EHSB, which can effectively satisfy the tight gas reservoir at approximately 150 °C. The results of the rheological experiments demonstrated that the salt-absorption capacity and charge-shielding effect of the surfactant micelles co-determined the solution structure and rheological properties. Differential scanning calorimetry data showed that temperatures reached their maximum during the SAWG structure transition point. The structural thermal stability evaluation of the SAWG showed that the temperature disaggregates the long rod wormlike micelles into spherical micelles and results in the tendency toward a supramolecular structure that was connected by spherical micelles containing on average two hydrophobic monomers suspended in the copolymer at 0.5 wt % EHSB. The thermodynamic study also demonstrated that this supramolecular structure had the lowest activation energy, generated the highest thermal stability, and had the most stable network structure of the optimized formulation. The SAWG could be completely broken with almost no water-insoluble residue, and its formation damage was 20% less than that of the guar fracturing fluid. throats and cause formation damage.15,16 Because of these issues, additives, such as alkali metal hypochlorite, persulfate, and peroxide, have been widely used to decrease the fluid viscosity after fracturing. These additives are extensively described in the literature; however, they only slightly reduce the formation damage.17 In addition, the price of guar gum over the past few decades has fluctuated drastically depending upon the market, causing companies worldwide to incur high costs.18,19 As a result, viscoelastic surfactant (VES) fracturing fluids have recently been used as alternatives to traditional guar polymers because they possess sufficient viscoelasticity to create fractures and have a good proppant-transporting capacity in the reservoir.20 These fluids exhibit excellent rheological properties for proppant transport and cause highly conductive stimulation treatments with no polymer damage.21−23 The VES fluids offer
1. INTRODUCTION Hydraulic fracturing is an extremely efficient method for enhancing the productivity of low- and ultralow-permeability reservoirs.1−4 The recent increase in tight gas production has created a huge demand for fracturing fluids with high performance and low formation damage.5−8 Fracturing fluids play a major role in providing a sufficient fracture width to ensure easy proppant entry into the fracture. They carry the proppant from the wellbore to the fracture tip and generate the desired net pressure to control the height growth. These processes directly and substantially affect the post-fracture production response in tight reservoirs.9−11 The fluid used to generate the desired viscosity must be safe to handle, thermally stable, resistant to shearing, low-damaging to the reservoir permeability, environmentally friendly, and easy to prepare for field operation.12,13 Traditionally, guar and its modified derivatives are widely used as thickeners in hydraulic fracturing because of their high viscosity.14 However, guar has a fatal defect in that it leaves insoluble residues in the formation that extensively plug pore © XXXX American Chemical Society
Received: December 21, 2016 Revised: March 19, 2017 Published: April 10, 2017 A
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. FTIR Spectra of (a) QASM, (b) HPAM, and (c) HMPT (a) FTIR (cm−1) of QASM
(b) FTIR (cm−1) of HPAM
(c) FTIR (cm−1) of HMPT
3430 (ν N−H) 3090 (ν CH) 2920 and 2850 (ν C−H) 945 and 910 (γ CH) 719 (γ C−H)
3430 (ν N−H) 2922 and 2854 (ν C−H) 1630 (ν CO) 1400 (ν C−O)
3420 and 3200 (ν N−H) 2925 and 2854 (ν C−H) 1676 (ν CO) 1618 (δ N−H) 1405 (ν C−O)
was dried at 80 °C under vacuum and the HMPT was ground to a powder. 2.3. Characterization of QASM and HMPT. Fourier transform infrared (FTIR) spectroscopy tests of the QASM and HMPT, with purification by acetone, were recorded by a Magna-IR 560 spectrometer with a resolution of 4 cm−1 and a wavenumber range from 4000 to 400 cm−1 (Vertex 70 from Bruker, Karlsruhe, Germany). 1H nuclear magnetic resonance (NMR) analysis is the most common method used to determine molecular structures and functional groups. The 1H NMR analysis was conducted using a NMR spectrometer (JEOL JNMECA600, Tokyo, Japan); the samples were diluted in dimethyl sulfoxide (DMSO) and deuterium oxide (D2O). 2.4. Preparation of the Fluid and Gel. The devised SAWG was formed using the HMPT and EHSB. A predetermined weight of HMPT was dissolved for 20 min using a mixer at a rotational speed of 1000 rpm. EHSB formed a gel with potassium chloride and sodium salicylate when subjected to the same treatment.23,24,28 The fluid was then centrifuged at a rotational speed of 3500 rpm for 30 min to remove air bubbles. All of the sets of the SAWG were prepared by weight percentage. All of formulations contained 1 wt % KCl. 2.5. Static-Column Tests. The static-settling behavior of the fracturing gel was determined by a static-column test in 100 mL glass cylinders.29−31 The fracturing fluids were tested using 20/40 mesh ceramsite sand with a 20 wt % concentration. Solutions were stirred using a mechanical stirrer at 500 rpm for 10 min. Each solution was then transferred to a 100 mL measuring cylinder, and a stopwatch was started at the same time. A 100% suspension was defined as no proppants settling to the bottom, and a 0% suspension was defined as thorough sedimentation of all of the proppants. 2.6. Rheological Measurement. A HAAKE MARS 60 was used for all rheological tests in our experiments (Thermo Fisher Scientific Corp.). The steady shear viscosity test was conducted for 2 h and at 170 s−1 at the experimental temperature (SY/T5107, 2005). The fluid consistency coefficient and flow behavior index were investigated at 90 °C (SY/T5107, 2005). The dynamic rheological properties, frequency sweep measurements, and yield stress values were measured using a cone-and-plate geometry. The viscoelastic properties of the fluid were acquired from small-amplitude oscillatory shear tests at 0.5 Pa and 1 Hz.32 Frequency sweep measurements were performed to evaluate the characteristics of the viscoelastic material.33 The cone had a diameter of 30 mm and a cone angle of 1°. Approximately 0.5 mL of the sample was required for one test. The viscosities of the samples were measured at 100 s−1 using the concentric-cylinder geometry. Four different temperatures (30, 50, 70, and 90 °C) were selected for investigating how to positively influence the rheology of the weak gel fracturing fluid.34 2.7. SEM. SEM observations were conducted to examine the surface morphology of the samples (Quanta 200 FEG, FEI Corp.). To prepare samples for SEM, fresh samples were rapidly submerged in liquid nitrogen and then quickly transferred to a cryo unit into a frozen state, where samples were frozen at −80 °C. All of the prepared samples were fixed onto SEM stubs using double-sided carbonized adhesive tape and were soon afterward coated with a gold/palladium alloy. The accelerating voltage was 15 kV. 2.8. TEM. TEM was used to observe samples of the SAWG (F20, FEI Corporation). Samples were coated onto a microgrid and dried. 2.9. Differential Scanning Calorimetry (DSC). The influence of the temperature on the structure transition point of the weak gel was determined by DSC (Thermo Fisher Scientific Corp.) from room temperature to 120 °C at a heating rate of 3 °C/min under an argon
several general advantages: their preparation is simple, with fewer numbers of chemicals and equipment required at the well site compared to conventional fluids, and no polymer hydration, cross-linkers, or other chemical additives are required. However, these ClearFRAC fluids can only be used for the fracturing treatment below 120 °C, and the concentration of VES usually exceeds 3%.24,25 Recently, a new supramolecular-structureassociating weak gel (SAWG) based on associative polymers and erucoylamidopropyl hydroxy sulfobetaine (EHSB) was developed in our laboratory. The SAWG could be a total or partial alternative to VES and guar for hydraulic fracturing and satisfy the tight gas reservoir effectively at approximately 150 °C, whose price is subjected to harvest areas.26 In the present work, the formulation and physicochemical characterization of a new SAWG fracturing fluid was described. The formula of the SAWG was optimized through steady shear viscosity tests and static-column tests. To investigate the properties of the SAWG, we carried out rheological tests and structural thermal stability experiments. Next, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the microstructures of these fluids. Furthermore, on the basis of the data related to the rheological properties, structural thermal stability properties, microstructures, and proppant suspension behavior of the SAWG, we proposed a physical model of the gelation mechanism. This paper aims to provide a better understanding of associating weak gels with a supramolecular structure.
2. EXPERIMENTAL SECTION 2.1. Materials. Acrylamide (AM), N,N-dimethylacrylamide (DMAM), and ammonium persulfate (98%) were obtained from Sigma-Aldrich Corp. (Shanghai, China). NaOH (99%), KCl (99%), and sodium salicylate (99%) were purchased from Alfa Aesar (Shanghai, China). Partially hydrolyzed polyacrylamide (HPAM) and the hydroxyl sulfobetaine surfactant EHSB were acquired from the Changqing Oilfield, China. The electrospray ionization high-resolution mass spectrometry (ESI-HRMS) spectrum for EHSB is shown in Figure 4A, and the result is consistent with the molecular structure of a previous report, as shown in Figure 4B.27 The quaternary ammonium surfactant monomer (QASM) and hydrophobically modified polymer thickener (HMPT) were prepared in our laboratory. QASM was synthesized using industrial raw material. HMPT was prepared with QASM and analytical reagents. HMPT and HPAM were purificated by the mixture of methanol and acetone, which were dried at 70 °C for 24 h. The viscosity average molecular weight (Mη) of the polymers was measured and calculated: MηHPAM was 1700 × 104 g mol−1, and MηHMPT was 350 × 104 g mol−1. 2.2. Synthesis of the HMPT. The HMPT is an associative polymer synthesized from monomers, such as AM, sodium acrylate, DMAM, and a homemade surfactant monomer. A certain mole ratio of monomer/ reactants was dissolved in 500 mL of distilled water at 30 °C (the pH was adjusted to 7 with NaOH), and the total monomer mass ratio was maintained at 25%. The solution was placed in a reactor and deoxygenated with purging nitrogen for 30 min. After the solution was heated to the reaction temperature, the ammonium persulfate solution (the initial ratio was based on the total monomer mass) was quickly added within 30 s. After the reaction was completed, the product B
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels atmosphere (50 mL/min); the gel samples (40−45 mg) were weighed on a balance.
3. RESULTS AND DISCUSSION 3.1. Characteristics of the QASM and HMPT. Table 1 shows the FTIR spectra of the QASM, HPAM, and HMPT. To demonstrate that the quaternary copolymer was composed of AM/AA/DMAM/QASM, 1H NMR spectra of the QASM and HMPT were recorded in DMSO and D2O, respectively. Figure 1
Figure 3. 1H NMR spectra of HMPT in D2O.
Figure 1. 1H NMR spectra of QASM in DMSO.
shows the 1H NMR spectrum of QASM. 1H NMR (600 MHz, DMSO): 6.09−5.98 (m, 1H), 5.63 (s, 1H), 5.60 (d, J = 3.0 Hz, 1H), 3.93 (d, J = 7.2 Hz, 2H), 3.26−3.15 (m, 2H), 2.97 (s, 6H), 1.67 (s, 2H), 1.24 (s, 32H), and 0.85 (t, J = 6.7 Hz, 3H). In comparison of Figure 3 to the unmodified HPAM (Figure 2), the spectrum of HMPT shows two additional peaks as a result of DMAM and QASM. There were no peaks for the vinyl protons in Figure 3.
Figure 4. (A) ESI−HRMS spectrum of hydroxyl-sulfobetaine surfactant EHSB. ESI−HRMS: calculated, 583.40942 (MC30H60O5N2S + Na+); found, m/z 583.40942; calculated, 445.41138 (MC27H54ON2 + Na+), found, m/z 445.41138; calculated, 378.37190 (MC25H48ON), found, m/z 378.37190; calculated, 205.04519 (MC5H12O4NS + Na+), found, m/z 205.04519. (B) Molecular structure of hydroxyl-sulfobetaine surfactant EHSB.
3.2. Optimization of the Associating Weak Gel Fracturing Fluid. 3.2.1. Optimizing the Formula of the SAWG through Steady Shear Viscosity Tests. As Figure 5a shows, steady shear viscosity tests were conducted with the conditions of the test duration of 2 h and a shear rate of 170 s−1 at the experimental temperature (SY/T5107, 2005). The plot shows the viscosity (average viscosity of the final 10 min) and temperature as a function of the EHSB concentration (Figure 5b). Figure 5a shows that the three fluid samples (0.4, 0.6, and 0.8 wt % HMPT) obtained a maximum viscosity at 0.5 wt % EHSB and different temperatures (90, 120, and 150 °C, respectively). Furthermore, the final viscosity maintained was greater than 50 mPa s. The mechanism inferred from data analysis was as follows: high temperatures transformed the long rod wormlike micelles into spherical micelles (panels a and b of Figure 16). Furthermore, sufficient amounts of spherical micelles solubilized each single hydrophobic monomer suspended in the copolymer and led to a decrease in the viscosity (Figure 16e). However, hydrophobic monomers accounted for a very small proportion of
Figure 2. 1H NMR spectra of HPAM in D2O. C
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 5. (a) Steady shear viscosity test (sample 1, 0.4 wt % HMPT + 0.5 wt % EHSB; sample 2, 0.6 wt % HMPT + 0.5 wt % EHSB; and sample 3, 0.8 wt % HMPT + 0.5 wt % EHSB) and (b) average viscosity of the last 10 min of the sample by the steady shear viscosity test compared to the EHSB concentration.
Figure 6. Static-column test of several formulas of SAWG at 30 °C.
increased to 1.0 wt %, every two hydrophobic monomers were embedded into a wormlike segment (Figure 16c). The two hydrophobic monomers of the HMPT were in a wormlike micelle and formed a network structure, which led to the minimum proppant settling rate. With the continuous increase in EHSB, the wormlike micelles increased. Therefore, each long rod micelle could only be embedded by one hydrophobic monomer, which was formed by spherical micelles with positive charges facing inside and negative charges facing outside. The spherical micelles of EHSB were observed by TEM, as shown in panels a and b of Figure 17; the microvisual model of the spherical micelle for EHSB was shown in Figure 17c, which was the same with the description from refs 24 and 37. The HMPT contained numerous negative charges because of its carboxylic acid segments. Furthermore, the worm-chain side groups of the copolymer showed a charge-shielding effect, which resulted in an increase in the grid structure radius for the fluid because of excessive repulsive forces. Therefore, the strength of the solution structure decreased (Figure 16d). The proppant settling rate of the HPAM solution was greater than that of the HMPT solution at the same concentration of 0.8 wt %, which resulted in a HMPT solution with a greater proppant suspension capability. 3.3. Property Evaluations. 3.3.1. Rheological Properties of the Associating Weak Gel. 3.3.1.1. Rheological Parameters. Rheological parameters of the SAWG were described by the power law (μ = kζn − 1).32 The fluids behaved as non-Newtonian fluids (n < 1) according to the laboratory data. Figure 8a shows
the total monomer content compared to polymerizable monomers. Therefore, we clearly observed the maximum point of viscosity at 0.5 wt % EHSB. 3.2.2. Optimizing the Formula of the SAWG by StaticColumn Tests. Figures 6 and 7 show that EHSB micelles could effectively reduce the proppant settling rate in a fluid containing 0.4 or 0.6 wt % HMPT. The proppant suspension data show that the two fluids obtained the lowest velocity at 30 °C with 1.0 wt % EHSB. However, in the case of a temperature of 60 °C, Figure 7 indicates that the SAWG reached the lowest proppant settling rate at 0.5 wt % EHSB; the mechanism deduced from the data analysis also conforms to the data in Figure 16b. The proppant settling rate of all fluids appeared to reach a minimum point with an increasing EHSB content. The lower the amount of polymer, the more definitive the trend. The attributed reasons were as follows: First, the HMPT main chain curled because of the salt-shielding effect in the pure polymer solution, increasing the proppant settling rate. Second, EHSB with a saltabsorption capacity, as shown in Figure 17, could decrease the salt-shielding effect, which resulted in increasing entanglement of the polymer chains, with the polymer chains stretching.35 Thus, the proppant settling rate sharply decreased because of the strengthened solution structure. In addition, the two hydrophobic monomers could form a spherical micelle at 0.5 wt % EHSB (Figure 16b). Third, when the concentration of EHSB was D
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 8. Rheological parameter test: (a) k and n of samples (a1, 0.8 wt % HMPT without 1 wt % KCl; a2, 0.8 wt % HMPT; a3, 0.8 wt % HMPT + 0.5 wt % EHSB; a4, 0.8 wt % HMPT + 1.0 wt % EHSB; a5, 0.8 wt % HMPT + 1.5 wt % EHSB; and a6, 0.8 wt % HMPT + 2.0 wt % EHSB) and (b) k and n of samples (b1, 0.8 wt % HPAM without 1 wt % KCl; b2, 0.8 wt % HPAM; b3, 0.8 wt % HPAM + 0.5 wt % EHSB; b4, 0.8 wt % HPAM + 1.0 wt % EHSB; b5, 0.8 wt % HPAM + 1.5 wt % EHSB; and b6, 0.8 wt % HPAM + 2.0 wt % EHSB).
Figure 7. Static-column test of several formulas of SAWG at 60 °C.
copolymer, which led to a continuously decreasing fluid consistency coefficient and increased flow behavior index. When the EHSB concentration was greater than 1.5 wt %, a sufficient number of spherical micelles (as shown in Figure 17) with a negatively charged outside and positively charged inside produced the charge-shielding effect. Therefore, the chargeshielding effect curled the polymer main chains to a certain extent, resulting in partially hydrophobic side groups associating by cross-linking to exhibit the increased fluid consistency coefficient and decreased flow behavior index. Figure 8b shows that the salt-shielding effect of the pure HPAM solution substantially reduced the fluid consistency coefficient and increased the flow behavior index. However, when the EHSB concentration was in the range from 0.5 to 1.5 wt %, the spherical micelles were not numerous enough to produce an obvious charge-shielding effect. Furthermore, the fluid consistency coefficient substantially decreased, and the flow behavior index sharply increased, until the EHSB concentration was greater than 1.5 wt %. 3.3.1.2. Elasticity and Viscosity Modulus. As illustrated in Figure 9a, the modulus values continued to increase with the addition of EHSB at 30 °C. However, the data in Figure 9b for experiments conducted at 60 °C differ from those in Figure 9a,
that the salt-shielding effect on the polymer segments can significantly reduce the fluid consistency coefficient and increase the flow behavior index. When the EHSB concentration was less than 0.5 wt %, the flow behavior index slightly decreased with an increasing EHSB concentration. However, the fluid consistency coefficient continued to decrease until the EHSB concentration was greater than 1.5 wt %. In contrast, the fluid consistency coefficient increased, and the flow behavior index slightly decreased. First, the HMPT main chain curled because of the saltshielding effect in the pure polymer solution. Therefore, the fluid consistency coefficient decreased, and the flow behavior index increased. Second, EHSB with a salt-absorption capacity decreased the salt-shielding effect, which resulted in increased chain entanglement because of the polymer chain stretching. In addition, the two hydrophobic monomers could then embed in a spherical micelle at 0.5 wt % EHSB, as shown in Figures 16b and 17c, which conforms to the result in ref 36. Thus, the flow behavior index slightly decreased with the enhancement of the solution structure. Third, the spherical micelles increased as the EHSB concentration was increased to 1.0 wt % (Figure 16e). Moreover, a sufficient number of spherical micelles could solubilize each single hydrophobic monomer suspended in the E
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 9. Elasticity modulus (G′) and viscosity modulus (G″): (a) tested G′ and G″ at 30 °C (a1, 0.8 wt % HMPT; a2, 0.8 wt % HMPT + 0.5 wt % EHSB; a3, 0.8 wt % HMPT + 1.0 wt % EHSB; and a4, 0.8 wt % HMPT + 1.5 wt % EHSB), (b) tested G′ and G″ at 60 °C (b1, 0.8 wt % HMPT; b2, 0.8 wt % HMPT + 0.5 wt % EHSB; b3, 0.8 wt % HMPT + 1.0 wt % EHSB; and b4, 0.8 wt % HMPT + 1.5 wt % EHSB), (c) tested G′ and G″ at 30 °C (c1, 0.8 wt % HPAM; c2, 0.8 wt % HPAM + 0.5 wt % EHSB; c3, 0.8 wt % HPAM + 1.0 wt % EHSB; and c4, 0.8 wt % HPAM + 1.5 wt % EHSB), and (d) tested G′ and G″ at 60 °C (d1, 0.8 wt % HPAM; d2, 0.8 wt % HPAM + 0.5 wt % EHSB; d3, 0.8 wt % HPAM + 1.0 wt % EHSB; and d4, 0.8 wt % HPAM + 1.5 wt % EHSB).
which displays the maximum modulus at 0.5 wt % EHSB. However, the modulus data obtained at both 30 and 60 °C for the HPAM fluid show that the maximum modulus occurred at 0.5 wt % EHSB. This result was due to the long rod wormlike micelles cross-linking with the HMPT at 30 °C. Moreover, the network structure of the wormlike micelle and HMPT was destroyed as the long rod micelles shortened at 60 °C. As a result, a maximum modulus was observed at 0.5 wt % EHSB, as shown in Figure 9b. EHSB could improve the modulus value of the weak gel because of its salt-absorption capacity. However, the linear polymer did not form a strong network structure through crosslinking with wormlike micelles, which only increased the length of the rod micelles or the number of sphere micelles to enhance the charge-shielding effect of the HPAM polymer. Therefore, as shown in panels c and d of Figure 9, the polymer chains curled, leading to a decreasing modulus value, which resulted in a maximum modulus at 0.5 wt % EHSB. 3.3.1.3. Frequency Sweep. Figure 10 shows that the elasticity modulus (G′) was greater than the viscosity modulus (G″) from 0.1 to 10 Hz and that the SAWG was a highly elastic fluid because the HMPT formed a highly elastic structure by cross-linking with the wormlike micelles. Moreover, the modulus from the laboratory data decreased along with the increment of the temperature. The elasticity modulus (G′) of the HPAM fluid was
Figure 10. Frequency sweep measurement: sample 1, 0.8 wt % HPAM + 0.5 wt % EHSB; and sample 2, 0.8 wt % HMPT + 0.5 wt % EHSB.
greater than the viscosity modulus (G″) under the same experimental conditions (Figure 10). However, the viscosity modulus increased along with the increasing temperature. Therefore, the linear polymer did not form a strong network F
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
hydrophobic monomers suspended in the copolymer. Consequently, we clearly observed a maximum point of the yield stress at 0.5 wt % EHSB. Surprisingly, Figure 11 indicates that the yield stress values of the HPAM fluid remained unchanged under the test conditions. This result is attributed to the linear polymer not forming a strong network structure by cross-linking with wormlike micelles, which resulted in the HPAM solution not displaying a maximum point of yield stress. Meanwhile, the yield stress values in the figure similarly decreased along with an increasing temperature for the HMPT fluid. 3.3.2.2. Structure Transition Point of the SAWG. The influence of the temperature on the structure transition point of the weak gel was studied by DSC in Figure 12. Figure 12a
structure by cross-linking with the wormlike micelles, which resulted in a viscosity-dominant HPAM fluid. Thus, Figure 10 indicates that the modulus values of the SAWG were greater than those of the HPAM solution, even though the viscosity average molecular weight of the HPAM is 5 times greater than that of the HMPT. As a result, the SAWG was a highly elastic fluid because of the weak supramolecular network honeycomb structure of the gel (Figure 14h). 3.3.2. Structural Thermal Stability of the Associating Weak Gel. 3.3.2.1. Yield Stress. Figure 11a shows that the fluids
Figure 11. Yield stress of samples: (a) test at 30 °C and (b) test at 60 °C.
obtained the maximum value of yield stress at 1.0 wt % EHSB, which contained 0.4 and 0.6 wt % HMPT. Moreover, the fluid containing 0.8 wt % HMPT obtained the maximum value of the yield stress at 0.5 wt % EHSB. For this reason, the weak gel required a quantity of long rod micelles to form a network structure because it contained a relatively low HMPT concentration. Thus, the fluid with a lower HMPT concentration reached its maximum value of yield stress at 1.0 wt % EHSB, and the fluid with a higher HMPT concentration reached its maximum value of yield stress at 0.5 wt % EHSB. Moreover, Figure 11b shows that all of the fluids, except the HPAM fluid, reached their maximum value of yield stress at 0.5 wt % EHSB and 60 °C. The high temperature disaggregated the long rod wormlike micelles into spherical micelles, which led to the solution structure tending to be the supramolecular structure connected by spherical micelles containing on average two
Figure 12. Test structure transition point of SAWG by DSC: (a) SAWG containing 0.6 wt % HMPT and (b) SAWG containing 0.8 wt % HMPT.
shows that the fluid containing 0.6 wt % HMPT reached a maximum temperature for the structure transition point of the gel at 1.0 wt % EHSB. However, Figure 12b indicates that the fluids containing 0.8 wt % HMPT obtained a maximum temperature for the structure transition point of the gel at 0.5 wt % EHSB. This was because the temperature could disassemble the honeycomb network structure of the gel, which required the higher temperature to disaggregate the long rod wormlike micelles. Therefore, the SAWG containing 0.6 wt % HMPT G
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Figure 13. Heat-shearing resistance test and thermodynamics: (a) heat-shearing resistance test of samples (0.6 wt % HMPT + 0.5 wt % EHSB, 0.6 wt % HMPT + 1.0 wt % EHSB, and 0.6 wt % HMPT + 1.5 wt % EHSB), (b) thermodynamic data of samples (0.6 wt % HMPT + 0.5 wt % EHSB, 0.6 wt % HMPT + 1.0 wt % EHSB, and 0.6 wt % HMPT + 1.5 wt % EHSB), (c) heat-shearing resistance test of samples (0.8 wt % HMPT + 0.5 wt % EHSB, 0.8 wt % HMPT + 1.0 wt % EHSB, and 0.8 wt % HMPT + 1.5 wt % EHSB), and (d) thermodynamic data of samples (0.8 wt % HMPT + 0.5 wt % EHSB, 0.8 wt % HMPT + 1.0 wt % EHSB, and 0.8 wt % HMPT + 1.5 wt % EHSB).
Table 2. Values of the Activation Energy sample of SAWG
activation energy, Ea (kJ mol−1)
sample of SAWG
activation energy, Ea (kJ mol−1)
0.6 wt % HMPT + 0.5 wt % EHSB 0.6 wt % HMPT + 1.0 wt % EHSB 0.6 wt % HMPT + 1.5 wt % EHSB
11.682 15.183 16.927
0.8 wt % HMPT + 0.5 wt % EHSB 0.8 wt % HMPT + 1.0 wt % EHSB 0.8 wt % HMPT + 1.5 wt % EHSB
8.995 13.094 14.572
the absolute temperature (K), and Ea is the activation energy (J mol−1).38 Panels b and d of Figure 13 show plots of the logarithm for viscosity versus the reciprocal temperature. The linear plots in the figure show data obtained from the heat-shearing resistance tests, which are in agreement with the Arrhenius equation. The values of the activation energy were calculated using the slope value (Ea/RT) from the Arrhenius-type equation; the results are tabulated in Table 2. The fluid exhibited lower values of the activation energy, 11.682 and 8.995 kJ mol−1, when the SAWG contained 0.6 and 0.8 wt % HMPT, respectively. Thus, the low activation energy values resulted in greater thermal stability of the fluid and in a more stable network structure compared to the others. 3.4. Proposed Model of the Microstructure and Mechanism Analysis. Figure 14a shows that the polymer chains were connected to the hydrophobic domains in a branched manner and that potassium chloride crystals existed in the hydrophobic microscale areas. Figure 14b indicates that strong hyper-branched compound structures replaced the hydrophobic domains and that the amount of potassium chloride crystals was reduced.
required a higher maximum temperature than the SAWG with 0.8 wt % HMPT to reach the structure transition point. 3.3.2.3. Heat-Shearing Resistance and Thermodynamic Properties. The apparent viscosity was studied at 100 s−1 to further understand the structure changes of the weak gel as a function of the temperature and shear rate. Panels a and c of Figure 13 show that the apparent viscosity decreased with an increasing temperature; the micelle volume decreases, thereby yielding smaller micelles (panels a and b of Figure 16), which indicates that the network structure of the SAWG is partly destroyed with the diminishment of non-covalent interactions. Furthermore, the weak gel tended toward the supramolecular structure connected by spherical micelles containing on average two hydrophobic monomers suspended in the copolymer (Figures 16b and 17c). Therefore, we clearly observed a maximum viscosity at 0.5 wt % EHSB, consistent with the results of the steady shear viscosity tests. The relationship between the viscosity and temperature can be expressed by an Arrhenius-type equation, η = A exp(Ea/RT), where η is the viscosity, A is a characteristic constant of the material, R is the universal gas constant (8.314 J K−1 mol−1), T is H
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
through the laminated network structure of the associated polymer. Furthermore, the wormlike micelles of EHSB and associated polymer chains strongly interweaved with each other through non-covalent interactions, which contributed to a threedimensional honeycomb network structure. Remarkably, a large amount of potassium chloride crystals disappeared. As a result, EHSB surfactant micelles significantly reduced the salting-out phenomenon to enhance the hyper-branched capacity and the strength of the solution structure. That is, this effect generated a gel by the formation of extremely efficient physical cross-linking between wormlike micelles and polymer chains, which had advantageous viscous and elastic fluid properties because of the supramolecular network honeycomb structure. A physical model of the gelation mechanism is proposed in Figure 15; this model is based on polymer chains and VES
Figure 15. Proposed model of SAWG for the gelation mechanism.
Figure 14. TEM and SEM images of samples: (a) TEM images of the sample for SMPT solution (0.15 wt % HMPT + 0.25 wt % KCl), (b) TEM images of the sample for SAWG (0.2 wt % HMPT + 1 wt % EHSB + 0.25 wt % KCl), (c) 1000 times magnification SEM image of the 1.0 wt % HMPT solution, (d) 5000 times magnification SEM image of the 1.0 wt % HMPT solution, (e) 100 times magnification SEM image of the 1.0 wt % HMPT solution, (f) 300 times magnification SEM image of the 1.0 wt % HMPT solution, (g) 100 times magnification SEM image of the SAWG (0.8 wt % HMPT + 0.5 wt % EHSB), and (h) 300 times magnification SEM image of the SAWG (0.8 wt % HMPT + 0.5 wt % EHSB).
Figure 16. Envisioned gelation mechanism model of SAWG at different conditions. The SAWG was designed with both HMPT and EHSB in weight percentage: (a) 0.6 wt % HMPT + 0.5 wt % EHSB, 30 °C; (b) 0.6 wt % HMPT + 0.5 wt % EHSB, 90 °C; (c) 0.6 wt % HMPT + 1.0 wt % EHSB, 30 °C; (d) 0.6 wt % HMPT + 1.5 wt % EHSB, 30 °C; and (e) 0.6 wt % HMPT + 1.5 wt % EHSB, 90 °C.
SEM micrographs of the samples are shown from panels c to h of Figure 14. Panels c and d of Figure 14 show images of samples with the dimensions of 50 and 10 μm, respectively. They reveal that the polymer grids interweaved together to form a laminated network structure. Panels e and f of Figure 14 show pictures of samples with dimensions of 500 and 200 μm, respectively. They explicitly show a broad weak grid structure. Moreover, many potassium chloride crystals are present. However, panels g and h of Figure 14 show a “honeycomb structure”, which is completely different from the structures shown in panels c−f of Figure 14. VES micelles penetrated
micelles entangled through reversible non-covalent interactions.39 These interaction mechanisms were as follows: The presence of the hydrophobic monomer strengthened the hydrophobic interactions by the association of cross-linking bonds. The hydrophobic monomers of the copolymer penetrated into the wormlike micelles, which connected the HMPT and EHSB to form a network structure that increased the viscosity of the system. Some free stickers were formed between the wormlike micelles and polymer chains or among themselves to build a temporary network. I
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
The data demonstrate that the formation damage of the SAWG was less than that of the guar fracturing fluid because of the high concentration residues of guar blocking the formation pore throats. After the SAWG was broken with an oxidative breaker, it only formed wormlike micelles. Thus, the SAWG exhibited properties similar to those of the VES fracturing fluid for the formation damage.
4. CONCLUSION In this work, the supramolecular SAWG had the following properties during hydraulic fracturing: The optimized formula of the SAWG was obtained at 0.5 wt % EHSB and satisfied the tight gas reservoir effectively at approximately 150 °C. The rheological results demonstrated that the SAWG formed a strong network structure through physical cross-linking with EHSB micelles. Furthermore, the salt-absorption capacity and charge-shielding effect of EHSB micelles co-determined the strength of the solution structure and rheological properties of the SAWG. The structural thermal stability evaluation of the SAWG showed that the temperature disaggregates the long rod wormlike micelles into spherical micelles, resulting in a solution structure tending to be the supramolecular structure; this supramolecular structure was connected by single spherical micelles containing on average two hydrophobic monomers suspended in the copolymer of the optimized formulation. Moreover, the thermodynamic study also demonstrated that this supramolecular structure had the lowest activation energy, generated the highest thermal stability, and had the most stable network structure. SEM micrographs directly displayed a supramolecular network honeycomb structure in the weak gel. TEM micrographs indicated that the supramolecular network honeycomb structure was formed by a strong hyperbranched compound structure because of extremely efficient non-covalent interactions. A physical model of the microstructure from the gelation mechanism was proposed to rationally discuss the reversible non-covalent interactions of the SAWG. The gel-breaking fluid of the SAWG resulted in almost no water-insoluble residue; therefore, it caused low formation damage.
Figure 17. TEM images and microvisual model of spherical micelle for EHSB.
The supramolecular complex structures in the gels were directly confirmed by TEM and SEM (panels b and h of Figure 14). The images displayed a supramolecular network honeycomb structure formed by strong hyper-branched compound structures through non-covalent interactions. 3.5. Formation Damage. The fracturing fluids leaked off and increased the irreducible water saturation, damaging the formation. In this paper, formation damage was measured using a core-flow test on a sandstone core of the Sulige reservoir. The core was 5.972 cm in length and 2.518 cm in diameter, with 22.52% porosity and 5.86 mD permeability. We performed the core damage test with the supramolecular viscoelastic gel (SY/ T5107, 2005). Tests showed that the permeability damage rate of the core-flow test was 12.2%. Formation damage of VES was 9.1%,25 which is a drop by 3.1%. However, that of the guar fracturing fluid is usually greater than 30%.25 The results of these experiments reveal that the SAWG with an oxidative breaker was clear after breaking, as shown in Figure 18, whereas the boratecross-linked guar was turbid and contained insoluble residues.
■
AUTHOR INFORMATION
Corresponding Author
*Telephone: +86-177-0130-3893. E-mail:
[email protected]. ORCID
Qihui Jiang: 0000-0003-1155-7771 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was financially supported by the Special Project on Oil and Gas of the National 863 Foundation of China (2013AA064800), the Funds for National Major Projects (2017ZX05023-003), the National Natural Science Foundation of China (Grants 51521063 and 51474231), and the New Method and Technology Foundation of China National Petroleum Corporation (2014A-4212). The authors thank the help of the Research Institute of Petroleum Exploration and Development (RIPED)Langfang Branch, PetroChina.
■
Figure 18. Picture of gel-breaking fluid: (a) SAWG and (b) boratecross-linked guar.
REFERENCES
(1) Rahm, D. Regulating hydraulic fracturing in shale gas plays: The case of Texas. Energy Policy 2011, 39 (05), 2974−2981.
J
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Europec/EAGE Conference and Exhibition; Rome, Italy, June 9−12, 2008; DOI: 10.2118/113533-MS. (22) Huang, T.; Crews, J. B. Nanotechnology applications in viscoelastic-surfactant stimulation fluids. SPE Prod. Oper. 2008, 23 (04), 512−517. (23) Crews, J. B.; Huang, T.; Wood, W. R. New technology improves performance of viscoelastic surfactant fluids. SPE Drill. Completion 2008, 23 (01), 41−47. (24) Samuel, M. M.; Card, R. J.; Nelson, E. B.; Brown, J. E.; Vinod, P. S.; Temple, H. L.; Qu, Q.; Fu, D. K. Polymer-free fluids for fracturing applications. SPE Drill. Completion 1999, 14 (04), 240−246. (25) Khair, E. M. M.; Shicheng, Z.; Shanbo, M.; et al. Performance and application of new anionic d3f-as05 viscoelastic fracturing fluid. J. Pet. Sci. Eng. 2011, 78 (01), 131−138. (26) Jiang, Q.; Jiang, G.; Wang, C.; Zhu, Q.; Yang, L.; Wang, L.; Zhang, X.; Liu, C. A New High-Temperature Shear-Tolerant Supramolecular Viscoelastic Fracturing Fluid. Proceedings of the IADC/SPE Asia Pacific Drilling Technology Conference; Singapore, Aug 22−24, 2016; DOI: 10.2118/180595-MS. (27) Wang, Y.; Zhang, Y.; Liu, X.; et al. Effect of a Hydrophilic Head Group on Krafft Temperature, Surface Activities and Rheological Behaviors of Erucyl Amidobetaines. J. Surfactants Deterg. 2014, 17 (02), 295−301. (28) Lin, Z.; Cai, J. J.; Scriven, L. E.; et al. Spherical-to-Wormlike Micelle Transition in CTAB Solutions. J. Phys. Chem. 1994, 98 (23), 5984−5993. (29) Wang, S.; Zhang, Y.; Guo, J.; et al. A study of relation between suspension behavior and microstructure and viscoelastic property of guar gum fracturing fluid. J. Pet. Sci. Eng. 2014, 124 (12), 432−435. (30) Harris, P. C.; Walters, H. G.; Bryant, J. Prediction of proppant transport from rheological data. SPE Prod. Oper. 2009, 24 (04), 550− 555. (31) Shah, S. N. Proppant settling correlations for non-Newtonian fluids under static and dynamic conditions. SPEJ, Soc. Pet. Eng. J. 1982, 22 (02), 164−170. (32) Jiang, Q. H.; Jiang, G. C.; Wang, C. L.; et al. The influence of fiber on the rheological properties, microstructure and suspension behavior of the supramolecular viscoelastic fracturing fluid. J. Nat. Gas Sci. Eng. 2016, 35 (09), 1207−1215. (33) Gouveia, L. M.; Paillet, S.; Khoukh, A.; et al. The effect of the ionic strength on the rheological behavior of hydrophobically modified polyacrylamide aqueous solutions mixed with sodium dodecyl sulfate (SDS) or cetyltrimethylammonium p-toluenesulfonate (CTAT). Colloids Surf., A 2008, 322 (03), 211−218. (34) Baruah, A.; Chauhan, G.; Ojha, K.; et al. Phase behavior and thermodynamic and rheological properties of single-(SDS) and mixedsurfactant (SDS + CAPB)-based fluids with 3-methylbutan-1-ol as the cosurfactant and pine oil as the organic phase. Ind. Eng. Chem. Res. 2014, 53 (51), 19765−19774. (35) Vasserman, A. M.; Motyakin, M. V.; Yasina, L. L.; et al. Effect of salts on local mobility and rheological properties of micelles of new longchain surfactant. Colloid J. 2011, 73 (04), 453−457. (36) Biggs, S.; Selb, J.; Candau, F. Effect of surfactant on the solution properties of hydrophobically modified polyacrylamide. Langmuir 1992, 8 (03), 838−847. (37) Pasquali, M. Gelation: Grow with the flow. Nat. Mater. 2010, 9 (05), 381−382. (38) Hassan, P. A.; Candau, S. J.; Kern, F.; et al. Rheology of wormlike micelles with varying hydrophobicity of the counter ion. Langmuir 1998, 14 (21), 6025−6029. (39) Couillet, I.; Hughes, T.; Maitland, G.; Candau, F. Synergistic effects in aqueous solutions of mixed wormlike micelles and hydrophobically modified polymers. Macromolecules 2005, 38 (12), 5271−5282.
(2) Pirzadeh, P.; Lesage, K. L.; Marriott, R. A. Hydraulic fracturing additives and the delayed onset of hydrogen sulfide in shale gas. Energy Fuels 2014, 28 (08), 4993−5001. (3) Dai, C.; Wang, K.; Liu, Y. L.; et al. Reutilization of fracturing flowback fluids in surfactant flooding for enhanced oil recovery. Energy Fuels 2015, 29 (04), 2304−2311. (4) McIlvaine, R.; James, A. The potential of gas shale. World Pumps 2010, 2010, 16−18. (5) Bulgakova, G. T.; Kharisov, R. Ya.; Pestrikov, A. V.; Sharifullin, A. R. Experimental study of a viscoelastic surfactant-based in situ selfdiverting acid system: Results and interpretation. Energy Fuels 2014, 28 (3), 1674−1685. (6) Bell, C. E.; Brannon, H. D. Redesigning fracturing fluids for improving reliability and well performance in horizontal tight gas shale applications. Proceedings of the SPE Hydraulic Fracturing Technology Conference; The Woodlands, TX, Jan 24−26, 2011; DOI: 10.2118/ 140107-MS. (7) Voneiff, G. W.; Robinson, B. M.; Holditch, S. A. The effects of unbroken fracture fluid on gas well performance. SPE Prod. Facil. 1996, 11 (04), 223−229. (8) Gupta, D. V. S.; Leshchyshyn, T. T. CO2-Energized hydrocarbon fracturing fluid: History and field application in tight gas wells. Proceedings of the SPE Latin American and Caribbean Petroleum Engineering Conference; Rio de Janeiro, Brazil, June 20−23, 2005; DOI: 10.2118/95061-MS. (9) Yuan, B.; Wood, D. A.; Yu, W. Q. Stimulation and hydraulic fracturing technology in natural gas reservoirs: Theory and case studies. J. Nat. Gas Sci. Eng. 2015, 26, 1414−1421. (10) Pang, W.; Du, J.; Zhang, T.; et al. Actual and optimal hydraulicfracture design in a tight gas reservoir. SPE Prod. Oper. 2016, 31 (01), 60−68. (11) Zhao, H. F.; Shi, J.; Meng, S. Z.; Li, Y. W. Steady-state optimization model and practical design of the fracture network system in tight sand gas reservoirs. J. Pet. Sci. Eng. 2016, 139, 185−190. (12) Wang, X. L.; Sun, H.; Zhou, J.; Guerin, M.; Li, L. A non-damaging polymer fluid system for conventional and unconventional formations. Proceedings of the SPE European Formation Damage Conference and Exhibition; Budapest, Hungary, June 3−5, 2015; DOI: 10.2118/174280MS. (13) Yoshimura, K.; Matsui, H.; Morita, N. Development of Polyglycolic-and polylactic-acid fluid-loss-control materials for fracturing fluids. SPE Drill. Completion 2016, 30 (04), 295−309. (14) Ihejirika, B.; Dosunmu, A.; Eme, C. Performance evaluation of guar gum as a carrier fluid for hydraulic fracturing. Proceedings of the SPE Nigeria Annual International Conference and Exhibition; Lagos, Nigeria, Aug 4−6, 2015; DOI: 10.2118/178297-MS. (15) Xu, B.; Hill, A. D.; Zhu, D.; et al. Experimental evaluation of guarfracture-fluid filter-cake behavior. SPE Prod. Oper. 2011, 26 (04), 381− 387. (16) Maloney, D. R.; Gall, B. L.; Raible, C. J. Non-Darcy gas flow through propped fractures: Effects of partial saturation, gel damage, and stress. SPE Prod. Eng. 1989, 4 (04), 417−422. (17) Lei, C.; Clark, P. E. Crosslinking of guar and guar derivatives. SPE J. 2007, 12 (03), 316−321. (18) Vaughn, S. F.; Kenar, J. A.; Felker, F. C.; et al. Evaluation of alternatives to guar gum as tackifiers for hydromulch and as clumping agents for biodegradable cat litter. Ind. Crops Prod. 2013, 43 (05), 798− 801. (19) Gresta, F.; Sortino, O.; Santonoceto, C.; et al. Effects of sowing times on seed yield, protein and galactomannans content of four varieties of guar (Cyamopsis tetragonoloba L.) in a Mediterranean environment. Ind. Crops Prod. 2013, 41 (01), 46−52. (20) Nasr-El-Din, H. A.; Samuel, E.; Samuel, M. Application of a new class of surfactant in stimulation treatments. Proceedings of the SPE International Improved Oil Recovery Conference; Kuala Lumpur, Malaysia, Oct 20−21, 2003; DOI: 10.2118/84898-MS. (21) Crews, J. B.; Huang, T. Performance enhancement of viscoelastic surfactant stimulation fluids with nanoparticles. Proceedings of the SPE K
DOI: 10.1021/acs.energyfuels.6b03415 Energy Fuels XXXX, XXX, XXX−XXX
