Investigating the Role of Alkyl Chain Length of the Inhibitors

May 27, 2019 - ... the alkyl chain length inhibitors should be considered to design a better class of inhibitors when using the water-based drilling f...
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Article Cite This: Energy Fuels 2019, 33, 5182−5190

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Investigating the Role of Alkyl Chain Length of the Inhibitors on Its Intercalation Inhibiting Mechanism in Sodium Montmorillonite Gang Xie,* Danchao Huang, Mingyi Deng, and Pingya Luo* State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, Sichuan 610500, China

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S Supporting Information *

ABSTRACT: Shale hydration and swelling are the major problems in using water-based drilling fluids in developing shale gas. In this work, the intercalation inhibiting mechanism of alkyl chain length on the swelling of sodium montmorillonite (NaMMT) has been innovatively investigated using isothermal adsorption, adsorption kinetics, scanning electron microscopy, X-ray diffraction, cation exchange capacity, elemental analysis, and X-ray photoelectron spectroscopy techniques. The results indicate that an increase in the alkyl chain length of the inhibitor reduces the saturated adsorption capacity of the inhibitor on Na-MMT accordingly, thus reducing the dosage of the inhibitor. The adsorption rate constant increased rapidly when the length of the alkyl chain was increased, which also reduced the degree of hydration of the clay minerals and improved the inhibitory potential. When a flat-lying monolayer of alkyl chain containing four to seven carbon atoms was inserted, the d-spacing of Na-MMT was minimized compared to the hydrated Na-MMT, which in turn replaced the sodium ions in the interlayer space of Na-MMT. The inhibitor was strongly attached in the Na-MMT interlayer between the silicon−oxygen tetrahedron and the primary amine groups. Finally, the water molecules were expelled from the interlayer of Na-MMT. Therefore, the alkyl chain length inhibitors should be considered to design a better class of inhibitors when using the water-based drilling fluid system.

1. INTRODUCTION Research and development of water-based drilling fluids with performance identical to that of oil-based drilling fluids has become an urgent task because of the rapid development of shale gas in China. Even though it is a recent trend in the development of drilling fluid technology, water-based drilling fluid theories and technologies both at home and abroad still have difficulties in inhibiting the hydration of clay minerals obtained from shale formations. This is a basic problem that has not been effectively solved for reservoir protection. Shale is made up of many clay minerals, such as montmorillonite, illite, illite/smectite formation, and kaolinite. Montmorillonite, as a water-sensitive clay mineral, has a higher swelling ability than other clay minerals, resulting in borehole instability in the drilling operation. Montmorillonite can make clay polymer nanocomposites as adsorbents for some organic pollutants7−12 and has a wide range of properties used in engineering and various industries (drilling fluids, antisettling agents, ointments, cosmetics, etc.).1−3 Yet, it is in petroleum engineering where it bears a significant impact. Modifying the rheology and controlling the filtration of drilling fluid systems are important applications.4−6 The expected properties of clay polymer nanocomposites require the clay particles to have a good dispersion that expands the interlayer space maximally, ensuring that large polymer molecules are involved in the clay interlayers.13−15 Drilling engineering in the oil exploration industries uses water-based drilling fluids, instead of oil-based drilling fluids that are limited by environmental regulations. Therefore, some serious problems, such as borehole collapse and tight holes, can occur while drilling through formations containing a high content of swelling clay minerals. To maintain the borehole stability during drilling operations, the © 2019 American Chemical Society

use of polymers has been increased, serving as clay swelling inhibitors, which can minimize the interlayer space. The proposed method is on the opposite side of that employed in preparing clay polymer nanocomposites, in which the interlayer space requires enlargement. Clay swelling inhibitors should ensure that the cation molecules are water-soluble16−19 and a hydrophobic carbon backbone, which is extended properly and applied to shape a flat-lying and dense monolayer in the interlayer space.20 Xie has explored the inhibition system of a primary alkylamine over montmorillonite and found a reduction in the saturated adsorption capacity of the alkylamines over sodium montmorillonite. At the same time, the adsorption rate constant has rapidly grown while increasing the number of primary amine groups.21,22 Nevertheless, there has been no literature exploring the inhibition influence of the alkyl chain length of the inhibitor in the water-based drilling fluid for the decrease in the swelling of clay minerals so far. The results not only have important academic research value for the fundamental theory of replacing oil-based drilling fluids with water-based drilling fluids but can also be used to guide the development of various water-based working fluids in oil wells to prevent reservoir damage. In this paper, the intercalation inhibiting mechanism of inhibitors with two primary amine groups and various alkyl chain lengths has been innovatively explored by isotherm adsorption, adsorption kinetics, scanning electron microscopy (SEM), X-ray diffraction (XRD), cation exchange capacity Received: March 29, 2019 Revised: May 21, 2019 Published: May 27, 2019 5182

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3. RESULTS AND DISCUSSION 3.1. Analysis of the Inhibition Mechanism. 3.1.1. Isotherm Adsorption. Figure 1 illustrates the change in the

(CEC), elemental analysis (EA), and X-ray photoelectron spectroscopy (XPS) techniques. This study was aimed at verifying the hypothesis supporting the argument that the saturated adsorption capacity of the inhibitor on sodium montmorillonite would decrease with the increase in the alkyl chain lengths, and simultaneously, the adsorption rate constant would increase and thus improve the inhibition performance. These parameters are helpful to establish the molecular structure design of an inhibitor and develop a highly effective intercalation inhibitor used in the water-based drilling fluids.

2. MATERIALS AND METHODS 2.1. Materials. Sodium montmorillonite (Na-MMT) was provided by Nanocor Company. The theoretical structural formula of sodium montmorillonite is Na(Al, Mg, Fe)2(Si, Al)4O10(OH)2.23 Na-MMT contained 13.22% Al2O3, 71.30% SiO2, 7.10% MgO, 4.79% Na2O, and 3.59% Fe2O. First, the samples were dried at 150 °C for 24 h and then sieved through a 200-mesh sieve. CEC, that is, the cation exchange capacity, of Na-MMT is 145 mequiv/100 g. Inhibitors with two primary amine groups were water-soluble; thus, there was a hydrophobic backbone of various carbon atoms being applied to investigate the influence of the alkyl chain length on the hindering of the swelling of Na-MMT. Inhibitors of different lengths were employed, such as decamethylenediamine (C10), nonamethylenediamine (C9), octamethylenediamine (C8), heptyldiamine (C7), hexamethylenediamine (C6), pentamethylenediamine (C5), butanediamine (C4), propanediamine (C3), and ethanediamine (C2). All of them were purchased from J&K Scientific Ltd. It was very difficult to dissolve the inhibitors C9 and C10 in water; therefore, they were not further studied. 2.2. Characterization. A UV−vis spectrophotometer from Shanghai Onlab Instruments Co., Ltd., Shanghai, China measured the UV−vis absorbance spectra of the inhibitor solutions. The differences in the interlayer space for the inhibitor intercalated into Na-MMT were gauged through XRD. Powder X-ray diffraction patterns were measured on an X Pert PRO MPD diffractometer using Cu (Kα) radiation. The microstructures of the Na-MMT−inhibitor complexes were observed via SEM. The elemental analysis for nitrogen was analyzed via a Var10EL-III analyzer from Elementar Analysensysteme GmbH. A Thermo Scientific K-Alpha X-ray photoelectron spectrometer confirmed the XPS of the samples (Al Kα, 1486.8 eV, 15 kV, and 10 mA). 2.3. Sample Preparation. Na-MMT was heated at 150 °C for 24 h. Afterward, 1.0 g of Na-MMT was added to 25 mL of distilled water and left stirring at 30 °C for 3 h to form a dispersion. Subsequently, 25 mL of the inhibitor solution was added to the Na-MMT dispersion and mixed at 30 °C for 24 h. The inhibitor concentration in the mixture was between 0.7 and 3.0 CEC. Later, the dispersion was centrifuged at 5000 rpm for 20 min on average. The supernatant was collected in a volumetric flask. Afterward, 25 mL of deionized water was added to the centrifugal sediment, and the mixture was stirred. The prepared dispersion was washed and centrifuged three times. The filtrate was diluted to a constant volume and combined with the supernatant. Ultimately, the UV−vis absorbance of this solution was measured, and the concentration of the inhibitors in the liquid supernatant was determined based on the standard curve (linearly dependent coefficient R2 > 0.9999). The concentration of the inhibitors adsorbed on Na-MMT was obtained by subtracting the total concentration from the concentration of the inhibitors in the liquid supernatant. The adsorption capacity at equilibrium of the inhibitor adsorbed on Na-MMT was also calculated, which was then employed to determine the saturated adsorption capacity of the inhibitors on Na-MMT. The centrifuged precipitate was studied via XRD and SEM. The dried samples were analyzed with XPS, EA, and CEC.

Figure 1. Adsorption isothermal diagram of inhibitors on Na-MMT.

adsorption capacity of the inhibitors over Na-MMT at different temperatures. The adsorption follows a Langmuir adsorption isotherm and a modified Langmuir isotherm, which are shown in Table 1. Langmuir isotherm24 Γ = Γ∞kc /(1 + kc)

(1) 25

Modified Langmuir isotherm Γ = Γ∞kc n/(1 + kc n)

(2)

When the concentration of C2 and C3 is below 1.8 CEC, the equilibrium adsorption capacity of C2 and C3 adsorbed over Na-MMT showed a dramatic growth with increasing amounts of inhibitor added. When the concentration of C2 and C3 is larger than 1.8 CEC, there is a slow growth in the Na-MMT adsorption capacity, which is subsequently saturated with a further increase in the additional volume of inhibitor. Thus, the 1.8 CEC level serves as the inflection point of C2 and C3. There is a decrease in the inflection point of the adsorption capacity with an increase in the chain length, for example, a CEC of 1.5 is the inflection point of C4, whereas 1.3 CEC is the inflection point of the inhibitor from C5 to C8. Moreover, the saturated adsorption capacity of C5 to C8 over 5183

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Energy & Fuels Table 1. Parameters for Adsorption Isotherms at Different Temperaturesa Langmuir

modified Langmuir

inhibitor

temp (°C)

Γ∞ (mmol·g−1)

k (L·mmol−1)

R2

Γ∞ (mmol·g−1)

k (L·mmol−1)

n

R2

C2

30 60 90 30 60 90 30 60 90 30 60 90 30 60 90 30 60 90 30 60 90

2.72 2.65 2.57 2.57 2.45 2.40 2.02 1.92 1.85 1.89 1.77 1.73 1.69 1.50 1.41 1.59 1.48 1.39 1.57 1.47 1.38

4.28 2.27 2.24 2.14 2.29 2.16 2.75 2.85 2.58 3.03 2.83 2.82 4.14 3.72 3.04 3.51 6.77 8.42 3.66 4.10 2.86

0.9895 0.9753 0.9822 0.9690 0.9615 0.9609 0.9771 0.9730 0.9706 0.9823 0.9885 0.9912 0.9975 0.9997 0.9996 0.9940 0.9999 0.9993 0.9968 0.9993 0.9987

2.69 2.53 2.47 2.44 2.34 2.29 1.96 1.87 1.79 1.83 1.73 1.69 1.67 1.50 1.40 1.63 1.48 1.39 1.57 1.46 1.37

4.95 3.75 3.32 3.91 4.46 3.61 5.12 5.61 5.12 5.96 4.24 4.01 5.66 3.99 3.14 5.90 6.32 6.53 4.74 4.73 3.00

2.57 2.49 2.36 2.48 2.41 2.32 2.32 2.11 2.04 2.26 2.01 1.92 1.43 1.26 1.12 1.28 1.15 1.04 1.19 1.08 1.01

0.9898 0.9950 0.9983 0.9974 0.9980 0.9955 0.9993 0.9992 0.9994 0.9996 0.9996 0.9996 0.9999 0.9999 0.9999 0.9999 0.9999 0.9996 0.9997 0.9999 0.9999

C3

C4

C5

C6

C7

C8

Γ∞saturated adsorption capacity; kadsorption constant; R2correlation coefficient; nmicelle aggregation number.

a

theoretical characteristics of the hard/soft acid/base theory of interlayer cation and the functional groups of guest molecules. Amines can replace the compensated cations (Na+, Mg2+, and Ca2+) in the interlayer space of clay minerals. In addition, coordination bonds and ionic bonds are often observed when amines are protonated because of the pH of the acidic solutions or the increased acidity of water molecules between layers.29 According to electrostatics, van der Waals forces and ionic bonds that balance ions between the adjacent layers of montmorillonite are the main forces of the interlayer interaction, hence making it easy to separate along the bottom.30 As amines are strong Brønsted bases, they strongly interact with the silica tetrahedron, which verifies that the functional groups of the inhibitors have a strong adsorption effect on the upper and lower layers of the two adjacent silicate crystals in the clay minerals, tightening the adjacent crystal layers of clay minerals, minimizing the d-space, and forming stable intercalated compounds that cannot be destroyed by heating or washing with other polar molecules. Γadsorption capacity of inhibitors at equilibrium; c concentration of inhibitors in solutions at adsorption equilibrium. 3.1.2. Adsorption Kinetics. Both pseudo-first- and pseudosecond-order equations were commonly used to describe the adsorption behavior of an adsorbate on the adsorbent.31 Pseudo-first-order equation is based on the concentration of one reactant remaining essentially constant over time. In the pseudo-second-order equations, it is assumed that the adsorption rate is controlled by the chemical adsorption mechanism. This chemical adsorption involves electron sharing or electron transfer between the adsorbate and the adsorbent. The adsorption of amines on montmorillonite is driven by various chemical interactions: hydrogen bonds, ion−dipole interaction, coordination bonds, acid−base reactions, chargetransfer, and van der Waals forces.32−39 Some published works reported that the amine compounds adsorbed on the clay

Na-MMT slightly decreases along with an increase in temperature, indicating that the inhibitors with two primary amine groups would be adsorbed strongly over Na-MMT and have good temperature resistance. On the basis of Table 1, the correlation coefficient R2 of the isothermal adsorption models is larger than 0.9615, suggesting that the inhibitors follow the modified and unmodified Langmuir models for adsorption in Na-MMT as the calculated value of the modified Langmuir model should match the experimental value. The difference in the adsorption capacity of different inhibitors in Na-MMT under different temperatures is shown in Table 1. The saturated adsorption capacities of C8, C7, C6, C5, C4, C3, and C2 over Na-MMT are 1.57, 1.63, 1.67, 1.83, 1.96, 2.44, and 2.69 mmol/g at 30 °C, respectively, suggesting that there is a decrease in the saturated adsorption capacity of the inhibitors in Na-MMT corresponding with the increase in the alkyl chain length. EA experiments were conducted (see Supporting Information), and the results show that longer alkyl chain lengths correspond to a decrease in the saturated adsorption capacity. It also indicates that the dosage of the inhibitors in the water-based drilling fluid system is reduced and the cost is effectively decreased. The saturated adsorption capacity of the inhibitors was found to exceed the cation exchange capacity by isotherm adsorption and EA techniques. Swartzenallen26 considered that amine molecules are adsorbed onto Na-MMT primarily by ion exchange. When more amine molecules are added to the amine−Na-MMT system, Na-MMT can absorb to a maximum of about twice the CEC which makes the excess amine molecules difficult to be removed with water or heating. The adsorption of polar molecular amines on 2:1 type clay mineral is driven by various chemical forces: hydrogen bonds, ion−dipole interactions, coordination bonds, acid−base reactions, charge transfer, and van der Waals forces.27,28 The substitution of interlayer water molecules depends on the 5184

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Figure 2. Fitting of the pseudo-second-order kinetic model of inhibitors on Na-MMT.

minerals follow the pseudo-second-order model.21,40 Therefore, this paper mainly discusses the pseudo-second-order equations. On the basis of solid phase adsorption, it is suggested by the pseudo-second-order kinetic model that the capacity is proportional to the number of active sites that the adsorbent occupies. Consequently, the kinetic rate law for the reaction is defined as41 1 1 t = + t qt qe kqe 2

(g/mmol·min), whereas qe represents the number of solvated adsorbates adsorbed at equilibrium (mmol/g). qt represents the number of solvated adsorbates on the adsorbent surface at any time t (mmol/g). Figure 2 and Table 2 demonstrate the analysis of the pseudo-second-order reaction from the data obtained, and the plots are generated for the linearized form. Equation 3 presents the analysis of the pseudo-second-order model of the inhibitor in the adsorption systems. According to the pseudo-second-order rate law, the correlation coefficients are very high for the linear plots of t/qt against time, indicating that the expression of the pseudo-second-order adsorption rate is suitable for the adsorption systems. At the low dose of 1.0 CEC adsorbed on Na-MMT, inhibitors with various alkyl chain lengths have higher

(3)

As for the pseudo-second-order reaction, it is integrated with the rate law, where k represents the constant of adsorption rate 5185

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Energy & Fuels Table 2. Kinetic Parameters for the Pseudo-Second-Order Kinetic Model of Inhibitors on Na-MMT 1.0 CEC

2.0 CEC

inhibitor

temp (°C)

qe (mmol·g−1)

kg/(mmol min)

R2

qe (mmol·g−1)

kg/(mmol· min)

R2

C2

30 60 90 30 60 90 30 60 90 30 60 90 30 60 90 30 60 90 30 60 90

1.43 1.42 1.41 1.42 1.41 1.40 1.41 1.40 1.39 1.39 1.38 1.37 1.38 1.36 1.30 1.36 1.34 1.29 1.35 1.33 1.28

61.70 41.49 37.56 83.19 71.07 56.20 343.04 315.99 298.31 389.84 350.88 331.63 434.53 413.87 388.22 493.18 477.02 458.80 546.34 511.00 454.34

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

2.59 2.52 2.44 2.44 2.34 2.30 1.96 1.86 1.79 1.82 1.72 1.68 1.67 1.50 1.40 1.64 1.47 1.37 1.62 1.46 1.36

48.56 41.49 28.71 68.83 59.10 46.08 181.29 169.29 154.57 195.17 179.91 164.00 233.36 218.31 201.11 268.77 245.86 221.66 316.36 283.86 244.27

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

C3

C4

C5

C6

C7

C8

adsorption rates, which indicates that the affinity of Na-MMT for the inhibitor is strong. At a higher dosage of 2.0 CEC adsorbed on Na-MMT, the inhibitor shows lower adsorption rates compared with 1.0 CEC, which demonstrates that NaMMT has a lower affinity for the inhibitor. The adsorption rate constant decreases with an increase in temperature. The order of the adsorption rate constant of inhibitors is C8 > C7 > C6 > C5 > C4 > C3 > C2, which indicates that the adsorption rate constant increases rapidly with an increase in the alkyl chain length. Therefore, greater the alkyl chain length, faster is the adsorption rate. Moreover, the adsorption rate constant of C4 is greater than that of C2 and C3, which indicates that when there is an inflection point in the four carbon atoms, the hydrophobicity is improved. With the increase in the alkyl chain length, both the adsorption rate and the affinity between the inhibitor and montmorillonite increase. On the one hand, because of the increase in the number of hydrophobic groups (−CH2), the van der Waals force between the inhibitor and montmorillonite increases. On the other hand, the solubility of the inhibitor decreases with the increase in the alkyl chain length. Inhibitor adsorption from the liquid to the solid phase is the result of a mutual competition of two forces in the adsorption system: the interaction between the solid surface and inhibitor and the interaction between the inhibitor and water molecules. Inhibitors with short carbon chain have a higher solubility in water and a strong attraction to water molecules, which increase the resistance of the inhibitor toward bonding with the silica surface. The rate of diffusion of water in clay minerals is higher than that of the inhibitor; hence, an increase in the rate of diffusion of the inhibitors (i.e., increasing alkyl chain length) can reduce the degree of hydration/swelling of the clay minerals and thus improve the inhibition performance. 3.2. Microstructure Analysis. 3.2.1. X-ray Diffraction Analysis. Figure 3 shows the XRD patterns of Na-MMT and its inhibitor with different alkyl chain lengths (C2 ≈ C8). In hydrated Na-MMT, the d-spacing was enhanced from 1.01 to

1.92 nm, showing that Na-MMT was extensively hydrated and expanded after adsorbing the water molecules. After the addition of diesel into the dry Na-MMT, the d-spacing of NaMMT was determined as 1.26 nm. This value indicates that the diesel molecules can penetrate the interlayer space of NaMMT by the “wedging effect” and increase its d-spacing. As diesel molecules are hydrophobic, their presence in the interlayer space of hydrophilic Na-MMT can reduce the interaction between the clay crystal layers. Therefore, the borehole instability can be observed in drilling operations with oil-based drilling fluids. When of C2 or C3 was added, the d-spacing declined rapidly from 1.92 to 1.43 nm, respectively, indicating that C2 and C3 were intercalated into the interlayer space of Na-MMT. For 2.0% (w/w) C2 or C3, the d-spacing decreased to 1.37 nm or 1.35 nm, respectively. When the inhibitor concentration was increased further, the d-spacing of Na-MMT did not change. However, the corresponding d-spacing of dry Na-MMT with C2 and C3 was 1.32 nm, indicating that a tilted monolayer of the inhibitor was intercalated within the interlayer spaces. C2 and C3 could not diminish the d-spacing of hydrated NaMMT as the inhibition potentials of C2 and C3 were weak. When 0.5% (w/w) of the inhibitor (C4, C5, C6, and C7) was added, the d-spacing declined significantly from 1.92 to 1.32 nm, showing that the inhibitors are intercalated within the interlayer space of Na-MMT. When the inhibitor concentration was further increased, the d-spacing of Na-MMT did not change. The d-spacing of dry Na-MMT containing inhibitor was 1.32 nm, indicating that a flat monolayer of the inhibitors was intercalated in the interlayer space. When the molecular diameter of the primary amine groups was 0.338 nm, the molecular diameter of protonated primary amines was 0.37 nm,42,43 which was smaller than that of the water molecules (0.4 nm). Therefore, the molecular size of the amine was smaller than that of the water molecules, indicating that the water molecules could be removed from the clay layer by the primary amine groups. The inhibitors (C4, C5, C6, and C7) 5186

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Figure 3. XRD patterns of Na-MMT under different conditions for inhibition performance.

among the tilted monolayer, flat-lying monolayer, and tilted bilayer. For minimizing the d-spacing of Na-MMT, the inhibitors should be intercalated into Na-MMT as a flat-lying monolayer. Therefore, four to seven equivalent carbon atoms were the best alkyl chain lengths for the inhibitor. Therefore, the saturated adsorption capacity of the inhibitor reduced and the adsorption rate constant increased while increasing the length of the alkyl chain. In conclusion, long chains of hydrophobic alkyl groups should be selected for structurally designing the inhibitor molecules. 3.2.2. Scanning Electron Microscopy. SEM can monitor the morphological changes in hydrated Na-MMT or Na-MMT with different inhibitors. The SEM images (Figure 4) show that the microstructures of the hydrated Na-MMT resembled a crimped leaf. After the addition of 1.0% (w/w) of C2 or C3, the majority of Na-MMT remained scattered and curly, indicating the weak inhibition potentials of C2 and C3. When 1.0% (w/w) inhibitors (C4, C5, C6, C7, and C8) were added,

can minimize the d-spacing of the hydrated Na-MMT. This result indicates that the inhibition potentials of the inhibitors (C4, C5, C6, and C7) were strong. When 0.5% (w/w) of C8 was added, the d-spacing of Na MMT declined significantly from 1.92 to 1.34 nm, showing that C8 was intercalated into the interlayer space of Na-MMT. When the concentration of C8 was further increased, the dspacing of Na-MMT did not decrease, but increased to 1.65 nm. The corresponding d-spacing of the dry Na-MMT containing C8 was 1.31 nm. When the C8 concentration was further increased, the d-spacing of Na-MMT did not change, showing that the tilted monolayer of C8 was intercalated within the interlayer space, and C8 cannot minimize its dspacing. In summary, the alkyl chain length of the inhibitor showed a significant influence on the intercalation arrangement of the inhibitor in Na-MMT. With the increase in the alkyl chain length, the arrangement of the inhibitor in Na-MMT varied 5187

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can attach to the exchange sites on the surface of Na-MMT by replacing the exchangeable cations. The measured CEC value of the Na-MMT samples was 113 mmol/100 g, which was lower than the measured value (145 mmol/100 g) of the supplier specifications. Table 3 shows that the CEC values of C2, C3, and C4 intercalated into Na-MMT remain unchanged compared to that of Na-MMT, indicating that C2, C3, and C4 cannot displace the exchangeable cations in the interlayer space of Na-MMT. However, C4 can minimize the d-spacing of clay minerals at low dosages, and a flat-lying monolayer is intercalated into Na-MMT (according to the XRD results), indicating that the inhibitors with the alkyl chain lengths of four carbon atoms are the inflection point of Na-MMT transition from hydrophilicity to hydrophobicity. The CEC values of C5, C6, C7, and C8 intercalated in Na-MMT decreased significantly. The CEC value of C6 intercalated within Na-MMT is lower than that of the other inhibitors, indicating that the different alkyl chain lengths possess different CEC values for the inhibitors, and an alkyl chain length of six carbon atoms can provide the best charge distribution in Na-MMT. Figure 5 shows a schematic diagram of the inhibitor intercalated in Na-MMT. Inhibitors are intercalated within the interlayer space of clay replacing the sodium ions.45,46 Because of hydrogen bonding and electrostatic interactions between the silicon−oxygen tetrahedron and the primary amine groups, the inhibitor molecules (C4, C5, C6, are C7) are embedded in the interlayer space of Na-MMT as flat-lying monolayers. By expelling the water molecules from the clay interlayer space, the interlayer space of Na-MMT was diminished. Inhibitors with longer alkyl chain length showed lower saturated adsorption capacity and higher adsorption rate constant. 3.2.4. X-ray Photoelectron Spectroscopy. XPS analysis was conducted to investigate the surface composition of Na-MMT and the Na-MMT−inhibitor complex (Figure 6).47 The NaMMT spectrum shows an obvious Na 1s signal, and no N 1s signal, which indicates that Na-MMT contains many sodium atoms but no nitrogen atoms. In C2 and C3, the areas of Na 1s remain unaltered compared to that of Na-MMT, which indicates that C2 and C3 cannot replace the sodium ions in Na-MMT. However, the binding energy of Na 1s decreases from 1073.5 to 1071.8 eV; it indicates that the primary amine groups can occupy the interaction sites of Na-MMT and decrease the interaction between the sodium ions and clay sheets. Compared to Na-MMT, the Na 1s signal of C4−C8modified Na-MMT decreases, and the N 1s signal increases, indicating that the inhibitor is intercalated within the interlayer space of Na-MMT by displacing parts of the sodium ions. The results indicate that the combinations of long alkyl chains and primary amine groups can collaboratively replace the sodium ions. The order of the peak areas of the N 1s signal in the inhibitors is C2 > C3 > C4 > C5 > C6 > C7 > C8, which is an agreement with the saturated adsorption capacity of inhibitors over Na-MMT. The results indicate that the primary amine groups can displace the interlayer cations of Na-MMT. The interlayer cationic hydration resulted in the crystal expansion of montmorillonite.48,49 If the inhibitor could entirely displace the interlayer cations of Na-MMT, the crystal expansion of NaMMT would be inhibited.

Figure 4. SEM images of hydrated Na-MMT and Na-MMT− inhibitor complexes: (a) hydrated Na-MMT, (b) Na-MMT−C2 complexes, (c) Na-MMT−C3 complexes, (d) Na-MMT−C4 complexes, (e) Na-MMT−C5 complexes, (f) Na-MMT−C6 complexes, (g) Na-MMT−C7 complexes, and (h) Na-MMT−C8 complexes.

the surface morphologies of the Na-MMT−inhibitor complexes showed more aggregates than that of C2 and C3. Moreover, the surface structure and edge curling phenomenon were improved in the Na-MMT−inhibitor complexes. The combined results of the SEM and XRD experiments showed that the inhibitors (C4, C5, C6, and C7) could inhibit the intercalation in an excellent manner. 3.2.3. Cation Exchange Capacity. The ammonium chloride−ammonium acetate method was utilized for measuring the cation exchange capacity value of the inhibitor intercalated in Na-MMT44 (Table 3). The ammonium ions Table 3. CEC Values of Na-MMT−Inhibitor Complexes samples

CEC (mmol/100 g)

Na-MMT C2 C3 C4 C5 C6 C7 C8

113 113 112 112 102 45.8 89.8 82.3 5188

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Figure 5. Schematic diagram of the alkyl chain length of inhibitors intercalated into Na-MMT.

(2016A-3903). The project was funded by China Postdoctoral Science Foundation, “The Study on Inhibiting Surface Hydration of Clay Minerals Based on the Principle of Intercalation” (2018M643524).

■

Figure 6. XPS of Na-MMT and the Na-MMT−inhibitor complex.

4. CONCLUSIONS The results show that the inhibitors with various alkyl chain lengths agree with the Langmuir and modified Langmuir models. Moreover, the increase in the alkyl chain length results in a decrease in the saturated adsorption capacity of the inhibitor. The adsorption kinetics of inhibitors follow the pseudo-second-order kinetic model. Longer the alkyl chain length, smaller would be the saturated adsorption capacity, and faster would be the adsorption rate constant, with the precondition of hydrophobicity and water solubility. The dspacing of Na-MMT was minimized when the inhibitors were intercalated into Na-MMT in a flat monolayer. Therefore, the alkyl chain length inhibitors should be considered to design a better class of inhibitors when using the water-based drilling fluid system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00969. Nitrogen proportion of an inhibitor adsorbed on NaMMT at different concentrations (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.X.). *E-mail: [email protected] (P.L.). ORCID

Gang Xie: 0000-0002-1608-1103 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the science and technology research and technology development projects of the China National Petroleum Corporation, “The basic research of new material and new system of wellbore liquid” 5189

DOI: 10.1021/acs.energyfuels.9b00969 Energy Fuels 2019, 33, 5182−5190

Article

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DOI: 10.1021/acs.energyfuels.9b00969 Energy Fuels 2019, 33, 5182−5190