The Interaction between Surfactants and Montmorillonite and its Influence on the Properties of Organo-Montmorillonite in Oil-Based Drilling FluIDS

Abstract

The increasing demands for oil and gas and associated difficult drilling operations require oil-based drilling fluids that possess excellent rheological properties and thermal stability. The objective of the present work was to investigate the rheological properties and thermal stability of organo-montmorillonite (OMnt) modified with various surfactants and under various loading levels in oil-based drilling fluids, as revealed by the interaction between organic surfactants and montmorillonite. The influence of the structural arrangement of surfactants on the thermal stability of organo-montmorillonite (OMnt) in oil-based drilling fluids was also addressed. OMnt samples were prepared in aqueous solution using surfactants possessing either a single long alkyl chain two long alkyl chains. OMnt samples were characterized by X-ray diffraction, high-resolution transmission electron microscopy, thermal analysis, and X-ray photoelectron spectroscopy. Organic surfactants interacted with montmorillonite by electrostatic attraction. The arrangements of organic surfactants depended on the number of long alkyl chains and the geometrical shape of organic cations. In addition to the thermal stability of surfactants, intermolecular interaction also improved the thermal stability of OMnt/oil fluids. A tight paraffin-type bilayer arrangement contributed to the excellent rheological properties and thermal stability of OMnt/oil fluids. The deterioration of rheological properties of OMnt/oil fluids at temperatures up to 200°C was due mainly to the release of interlayer surfactants into the oil.

Introduction

Montmorillonite (Mnt) belongs to the general family of phyllosilicates. An ideal Mnt layer is composed of two continuous [SiO₄] tetrahedral sheets (T) and an [AlO₆] octahedral sheet (O). Thus, the structure of Mnt is described as a TOT type (Bergaya et al. 2012). Due to isomorphic substitution, Mnt layers are often negatively charged. A negatively charged layer arises from the substitution of Mg²⁺ and other smaller-charge cations for Al³⁺ in octahedral sites (Brigatti et al. 2013; Jaber et al. 2014). Consequently, cations such as Na⁺ and Ca²⁺ present in the interlayer space counterbalance the deficit of positive charges. Significantly, these cations are exchangeable (Lagaly 1981) with organic cations such as quaternary ammonium salts and quaternary phosphonium salts.

The preparation of organo-montmorillonite (OMnt) with cationic surfactants (Paiva et al. 2008; He et al. 2010; Lagaly et al. 2013), non-ionic surfactants (Shen 2001; Bertuoli et al. 2014; Guégan et al. 2015), anionic surfactants (Sarier et al. 2010; Zhang et al. 2010), and a mixture of different kinds of surfactants (Chen et al. 2008; Gunawan et al. 2010; Zhang et al. 2013; Wu et al. 2014) is reported frequently. OMnt prepared with cationic surfactants (often quaternary ammonium salts) are used widely in industrial and scientific applications. A substantial industry has been established to develop the utilization of OMnt in paint, adsorbents, greases, cosmetics, and nanocomposites, etc. (Jaber et al. 2002; Paiva et al. 2008; He et al. 2010; Lee and Tiwari 2012).

An important use of OMnt is as a rheological additive in oil-based drilling fluids (Caenn & Chillingar 1996; Caenn et al. 2011). Quaternary ammonium salts in which the alkyl chain has 12–22 carbon atoms are usually used to prepare OMnt for oil-based drilling fluids (Dino & Thompson 2002; Frantz 2014); for example, cetyl trimethyl ammonium (Zhuang et al. 2016; Ratkievicius et al. 2017), octadecyl trimethyl ammonium chloride (Zhuang et al. 2017a), octadecyl benzyl dimethyl ammonium (Hermoso et al. 2014, 2017), and dimethyl dioctadecyl ammonium chloride (Hermoso et al. 2014, 2017). With the increasing demands of the oil and gas industry, drilling operations have been undertaken in many difficult wells, such as high-temperature, high-pressure, high-angle, and offshore wells. Oil-based drilling fluids are more popular due to their excellent lubricity, high rate of penetration, shale inhibition, wellbore stability, and good thermal stability (Caenn & Chillingar 1996; Khodja et al. 2010).

Previous studies identified that the rheological properties of oil-based drilling fluids are affected by the concentration and nature of OMnt (Hermoso et al. 2014, 2015; Zhuang et al. 2016, 2017a). The lipophilicity of surfactants contributes to the compatibility between oil and OMnt. Furthermore, for the same surfactant, more surfactant usually results in a larger basal spacing and further improves the swelling ability or even exfoliation. Exfoliation of OMnt in oil improves the rheological properties (Zhuang et al. 2017a,c). The dissolution of organic surfactants into oil might be a crucial reason for the deterioration of rheological properties at high temperature. Such previous studies mostly reported the relationship between the structure and properties of OMnt and the properties of oil-based drilling fluids.

Some question remain unanswered, however: (1) how do organic surfactants remain stable on the exfoliated OMnt layers? (2) what is the reason for the deterioration of rheological properties at high temperatures? and (3) the influence of the arrangements of interlayer surfactants on the properties of oil-based drilling fluids is unresolved. The purpose of the present study was to try to answer these questions, using two typical organic surfactants to modify Mnt, by determining the rheological properties and thermal stability of various OMnt samples in oil-based drilling fluids, thus revealing the interaction between organic surfactants and Mnt, and to measure the attendant structural changes in OMnt at the molecular scale.

Materials and Methods

Materials

Mnt was obtained from the Kazuo Shuanglong Mining Co., Ltd., Liaoning Province, China. The mass percentage of montmorillonite included in the Mnt sample was calculated from X-ray diffraction (XRD) patterns (Chinese standard SY/T 5163–2010: analytical method for clay minerals and ordinary non-clay minerals in sedimentary rocks by X-ray diffraction). This method was explained in a previous study (Zhuang et al. 2018). The calculation follows the formula: $X_i=\left(\frac{I_i}{K_i}/\left(\sum \frac{I_i}{K_i}\right)\right)\times 100\%$ , where X _i is the mass percent of phase i; K _i is the intensity ratio of phase i to corundum with the mass ratio i/corundum = 1:1. For the current work, the K values of minerals are listed in Table 1. The XRD pattern of Mnt (Fig. 1) indicated the presence of montmorillonite (88%), quartz (7%), calcite (2%), albite (2%), and pyrite (1%) (Table 2). The cation exchange capacity (CEC) of the Mnt was 120 cmol₍₊₎/kg. Cationic surfactant octadecyl trimethyl ammonium chloride (C18) and dimethyl dioctadecyl ammonium chloride (DC18) were purchased from Anhui Super Chemical Technology Co., Ltd., Hefei, Anhui, China. The ideal structures of these two organic cations (Figure 2) were optimized by ChemBio 3D using the molecular mechanics (MM2) minimization program (Bowen et al. 1987). The purity of the surfactants was 99%. The base oil, No. 5 white oil, was obtained from the China National Petroleum Corporation.

Table 1 K values of selected minerals

Fig. 1 XRD pattern of Mnt with the JCPDS cards of montmorillonite, quartz, calcite, albite, and pyrite

Table 2 A summary of the components in the Mnt sample

Fig. 2 Structural diagrams of organic cations with optimized geometrical shapes and molecular sizes

Preparation of OMnt

OMnts were prepared in aqueous solution as reported previously (Zhuang et al. 2017a): 100 g of Mnt was added to 1 L of deionized water and stirred for 0.5 h; surfactant was then added to the previous dispersion and the resulting dispersion stirred for 1 h. Finally, after centrifugation, drying at 60°C for 24 h, and milling and sieving with a 200-mesh sieve, OMnt was obtained. C18-modified OMnts were named C18-Mnt-1.0 and C18-Mnt-2.0, where 1.0 and 2.0 indicated that the amounts of C18 were equivalent to 1.0 CEC or 2.0 CEC of Mnt, respectively. Correspondingly, OMnts prepared with DC18 (0.5 CEC and 1.0 CEC of Mnt) were marked as DC18-Mnt-0.5 and DC18-Mnt-1.0.

Preparation of Oil-based Fluids

12 g of OMnt was added to 400 mL white oil (concentration of 30 kg/m³) and blended for 20 min at 8000 rpm. A drilling fluid should be aged at different temperatures to model the real drilling operation. The blended fluids were placed in a rotary oven heated to 66, 150, 180, and 200°C in which they were aged for 16 h. All of the operations followed the standards of the American Petroleum Institute (API), i.e. API SPEC 13A (Specification for Drilling Fluid Materials, 2010) and API RP 13B-2 (Recommended practice for field testing oil-based drilling fluids, 2014). The oil-based fluids were named following the template of OMnt/oil-temperature. For example, C18-Mnt-1.0/oil-66 was prepared from C18-Mnt-1.0 and white oil aged at 66°C.

Characterization

The XRD analysis was conducted using a Bruker D8 Advance X-ray powder diffractometer (Germany), using CuKα radiation at 40 kV and 40 mA and a scan speed of 0.05 s per step (step size of 0.02°2θ). The XRD data points covered the range 1.5 to 70°2θ. The transmission electron microscope (TEM) analysis was conducted using a Tecnai G2 F20 TEM instrument (Hillsboro, Oregon, USA) operated at a voltage of 200 kV. Thermogravimetry (TG) analysis was carried out using a NETZSCH STA 449 F3 type DTA-TG instrument (Selb, Bavaria, Germany) from room temperature to 900°C in air, with a heating rate of 10°C/min. The X-ray photo electron spectroscopy (XPS) analysis was carried out using a Thermo escalab 250Xi instrument (Waltham, Massachusetts, USA). Bombardment of the surface with X-rays (monochromated AlKα radiation, 1486.6 eV) resulted in the emission of photoelectrons with element-specific binding energies (BE). Firstly, a survey scan in the energy range of 1350–0 eV was recorded at a resolution of 1 eV. Then, high-resolution O 1s, Si 2p, Al 2p, C 1s, and N 1s scans were obtained. The rheological properties (apparent viscosity (AV), plastic viscosity (PV), and yield point (YP)) of aged oil-based drilling fluids were determined at 20°C, using a FANN 35A viscometer (Qingdao HaiTongDa Special Purpose Instrument Co., Ltd., China). AV = 1/2θ₆₀₀ (θ₆₀₀ is the dial reading at 600 rpm, corresponding to a shear rate of 1021.8 s⁻¹). PV = θ₆₀₀–θ₃₀₀ and YP = 1/2(θ₃₀₀–PV). The dynamic rheological behavior of oil-based fluids was measured using a Thermo Scientific HAAKE Roto Visco 1 rotational viscometer (Pittsburgh, Pennsylvania, USA). The programmed measurement regime was: the shear rate increased linearly from 0 s⁻¹ to 100 s⁻¹ in 5 min (up step), and then decreased linearly from 100 s⁻¹ to 0 s⁻¹ in 5 min (down step).

Results and Discussion

XRD of OMnt Powders

The basal reflection of Mnt occurred at 7.05°2θ, corresponding to d ₀₀₁ = 1.25 nm (Fig. 1). After organic modification, the basal spacing of OMnt increased (Fig. 3), giving d ₀₀₁ values for C18-Mnt-1.0, C18-Mnt-2.0, DC18-Mnt-0.5, and DC18-Mnt-1.0 of 2.12 nm, 4.06 nm, 3.51 nm, and 3.68 nm, respectively. The d ₀₀₁ of C18-Mnt-2.0 is almost double that of C18-Mnt-1.0. In the case of DC18-modified OMnt, however, the d ₀₀₁ of DC18-Mnt-1.0 increased by ~5% over DC18-Mnt-0.5. This phenomenon indicated that C18 and DC18 occupied very different structural arrangements in OMnt. The (002) and (003) reflections emerged in the XRD patterns of C18-Mnt-2.0, DC18-Mnt-0.5, and DC18-Mnt-1.0, whereas no peaks can be referred to the (002) and (003) reflections in the XRD pattern of C18-Mnt-1.0. The basal reflection intensity showed the sequence DC18-Mnt-1.0 > C18-Mnt-2.0 > DC18-Mnt-0.5 > C18-Mnt-1.0. Thus, the order of the degree of layer stacking (along the c axis) follows DC18-Mnt-1.0 > C18-Mnt-2.0 > DC18-Mnt-0.5 > C18-Mnt-1.0. DC18 was probably arranged in a more ordered manner in the interlayer space than C18.

Fig. 3 XRD patterns of OMnt samples

TEM Analysis

High-resolution TEM images (Figure 4) gave information about the basal spacing and the thickness of platelets. The TEM images of raw Mnt showed tightly stacked aluminosilicate layers. The thickness of the platelets of raw Mnt was >50 nm and the lamellae contained >50 layers. The lamellae of OMnt were thicker than those of raw Mnt and contained fewer layers. For both C18- and DC18-modified OMnt, more surfactant led to thicker lamellae. The thickness of C18-Mnt-1.0 lamellae was a little larger than that of DC18-Mnt-0.5 lamellae, and the thicknesses of C18-Mnt-2.0 and DC18-Mnt-1.0 lamellae were similar. This indicates that 1.0 CEC DC18 and 2.0 CEC C18 resulted in similar effects on the thickness of OMnt lamellae.

Fig. 4 TEM images of Mnt and OMnt samples

The TEM images also revealed the basal spacing directly. The layers in C18-Mnt-1.0 were not arranged neatly and the basal spacing ranged from 1.44 to 1.79 nm. Ordered stacking of layers was observed in the TEM images of C18-Mnt-2.0, DC18-Mnt-0.5, and DC18-Mnt-1.0. DC18-modified OMnt samples were more likely to exhibit an ordered arrangement of layers. More surfactant also led to ordered layer stacking. The basal spacing derived from the TEM images, however, was smaller than the results derived from XRD (Table 3) possibly due to radiation damage from the high voltage (200 kV). The lattices of clay minerals are easily damaged by high voltage in high-resolution TEM (Kogure 2013). Surfactants would degrade under high voltage, resulting in the decrease in basal spacing. Δd ₀₀₁ indicated the change of arrangement of the interlayer surfactants. C18-Mnt-2.0 exhibited the largest Δd ₀₀₁ value, demonstrating the dramatic re-organization of interlayer surfactants under high voltage. The similar Δd ₀₀₁ values of DC18-Mnt-0.5 and DC18-Mnt-1.0 suggested similar arrangements of interlayer surfactants in these two OMnt samples.

Table 3 Summary of basal spacings derived from XRD and TEM

Δd ₀₀₁ = d ₀₀₁ (XRD) – d ₀₀₁ (TEM)

Thermal Analysis

Mnt showed two steps in terms of mass loss (Fig. 5). The first step (<150°C), corresponding to a mass loss of 6.8%, was attributed to the loss of the water molecules on the surface and in the interlayer space of Mnt (He et al. 2005; Zhuang et al. 2015). The second mass-loss step (500–745°C, mass loss of 6.3%) represented the dehydration of hydroxyl groups coordinated by the structural cations in tetrahedral and octahedral sites (Greene-Kelly 1957; Hedley et al. 2007). The organic surfactants decomposed completely above 500°C. The onset temperatures (T _onset) corresponding to the thermal decomposition of C18 and DC18 were 202 and 145°C, respectively, indicating that C18 is more thermally stable than DC18.

Fig. 5 TG and corresponding DTG curves of Mnt, organic surfactants, and OMnt samples

In summary, dehydration of adsorbed water (below 150°C), oxidation of organic surfactants (150–430°C), continuous oxidation of organic surfactants (430–650°C), and dehydration of hydroxyl groups (650–800°C) can be observed in the TG and DTG curves of OMnt. The percentage water loss from C18-Mnt-1.0, C18-Mnt-2.0, DC18-Mnt-0.5, and DC18-Mnt-1.0 was 2.2%, 1.8%, 0.8%, and 0.0%, respectively. OMnt samples contained less water than Mnt. In addition, C18-modified OMnt samples contained more adsorbed water than DC18-modified OMnt samples. The main factors affecting the interlayer hydration of montmorillonite include: (1) hydration energy of the interlayer cations; (2) polarization of the water molecules by interlayer cations; (3) variation of the electrostatic surface potentials because of differences in layer-charge locations; (4) activity of water; and (5) size and morphology of the clay particles (Brigatti et al. 2013). Although C18 and DC18 cations had the same positive charges with Na⁺ cations, the organic cations showed a larger size and lower polarity due to the alkyl chains. In addition, the hydrophobicity of organic cations could prevent the adsorption of water. DC18 cations exhibited a larger size and better hydrophobicity than C18 cations, resulting in less interlayer water in the DC18-modified OMnt..

The T _onset corresponding to the thermal decomposition of organic surfactants in OMnt samples revealed the thermal stability of the samples. The T _onset values of C18, C18-Mnt-1.0, and C18-Mnt-2.0 were 202, 185, and 175°C, respectively. The T _onset values of DC18, DC18-Mnt-0.5, and DC18-Mnt-1.0 were 145, 180, and 159°C, respectively. C18-modified OMnt, therefore, showed better thermal stability than DC18-modified OMnt. For C18-modified OMnt, the T _onset value was lower than that of C18 because organic surfactants not only intercalated into the interlayer space, but also occupied the outer surface (He et al. 2005; Hedley et al. 2007; Zhu et al. 2011). The interlayer surfactants were protected by the Mnt layers. However, the surfactants exposed on the external surface were more susceptible to thermal degradation without the protection of the Mnt interlayers. Evidently, surfactants were mostly intercalated in the interlayer space when the surfactant loading level was <1.0 CEC of Mnt. When more surfactant was used, more should have adsorbed on the external surface, resulting in the decrease of T _onset (Zhuang et al. 2016). For DC18-modified OMnt, most of the surfactant was intercalated into the interlayer space due to the smaller amount of surfactant (≤1.0 CEC). Accordingly, DC18-Mnt exhibited better thermal stability than the pure surfactant.

XPS Analysis

The XPS survey scans of Mnt (Figure 6) showed the presence of O, Si, Al, Mg, Fe, Na, and C in Mnt. The presence of C in Mnt was assigned to calcite. After surfactant modification, the signals of N and Cl emerged in the spectra of OMnt samples. In addition, the intensity of the C 1s signal in OMnt was much greater than that in Mnt, demonstrating the adsorption and intercalation of organic surfactants. The signals of Cl 2 s and Cl 2p in C18-Mnt-2.0 were more intense than those in other OMnt samples. This phenomenon indicated that more Cl⁻ ions are included in C18-Mnt-2.0, because the excess surfactant (more than 1.0 CEC of Mnt) cannot intercalate into the interlayer space via cation exchange but remains neutral as an ion pair with Cl⁻ (He et al. 2007).

Fig. 6 XPS survey scans of Mnt and OMnt samples

Oxygen is the element most exposed on the surface of Mnt and TOT layers. Interaction between Mnt and surfactants should, therefore, first affect the binding energy of O. The binding energy of O 1s in Mnt was 532.5 eV (Fig. 7), representing oxygen in Si-O(H) and Al(Mg, Fe)-O(H) groups. Compared with Mnt, the binding energy of O 1s in OMnt samples was smaller, indicating greater electron density around O atoms in OMnt. The high-resolution XPS scans of Si 2p (Figure 8) and Al 2p (Figure 9) also showed a decrease in binding energy, suggesting that the [SiO₄] tetrahedra and [Al(Mg, Fe)O₆] octahedra, as a whole, exhibited higher electron densities after organic modification.

Fig. 7 O 1s high-resolution XPS spectra of Mnt and OMnt samples

Fig. 8 Si 2p high-resolution XPS spectra of Mnt and OMnt samples

Fig. 9 Al 2p high-resolution XPS spectra of Mnt and OMnt samples

The C 1s spectra (Figure 10) of surfactants can be distinguished as two parts: C-C groups corresponding to a binding energy of 284.8 eV and C-N groups corresponding to 285.9 eV (C18) and 286.0 eV (DC18) (He et al. 2007; Schampera et al. 2015). The binding energy of C 1s involving C-C groups maintained a constant value of 284.8 eV after organic modification, indicating no interaction involving the long alkyl chains. The binding energy of C 1s spectra involving C-N groups, however, shifted to larger values, demonstrating the decline of electron density around C atoms connecting with N. From the high-resolution XPS spectra of N 1s (Figure 11) in organic surfactants, the binding energy of N 1s was 402.1 eV. However, the binding energy of N 1s in OMnt increased slightly, demonstrating a decrease of electron density around N. The decrease in binding energies of N 1s and C 1s in the C-N groups proved that the reduction of electron density occurs only in the polar heads of surfactants, without the long alkyl chains.

Fig. 10 C 1s high-resolution XPS spectra of surfactants and OMnt samples

Fig. 11 N 1s high-resolution XPS spectra of surfactants and OMnt samples

From the high-resolution XPS spectra, the interaction between Mnt and organic surfactants occurred between the TOT layers of Mnt and polar heads of surfactants. TOT layers were electron acceptors and the polar heads of organic surfactants were electron donors. The shift of the binding energy (ΔBE) (Table 4) value of O 1s was in the range of −1.2 to −1.0 eV. The ΔBE value of C 1s (C-N) was between 0.3 and 0.5 eV and that of N 1s was in the range 0.1–0.3 eV. Two conclusions can be drawn from the XPS results: (1) no new signals emerged in the XPS spectra, except for small changes in binding energy; and (2) compared with Mnt and surfactants, the binding energy of Mnt elements (O 1s, Si 2p, and Al 2p) in OMnt decreased while the surfactant elements (C 1s and N 1s) in OMnt increased. Considering the negatively charged Mnt layers and organic cations, the XPS results demonstrate electrostatic attraction between the TOT layers and the polar heads of surfactants, without chemical bonds. The binding energy of O 1s in C18-Mnt-2.0 showed the smallest shift because extra surfactants intercalate into the interlayer space in the form of ion pairs (Cl⁻ anions and C18 cations). DC18-modified OMnt samples showed smaller ΔBE values than C18-modified OMnt samples, due to the conjugated effect of two long alkyl chains.

Table 4 Summary of ΔBE values

ΔBE (O 1s) = BE (O 1s, OMnt) – BE (O 1s, Mnt); ΔBE (C 1s) = BE (C 1s, OMnt) – BE (C 1s, surfactant); and ΔBE (N 1s) = BE (N 1s, OMnt) – BE (N 1s, surfactant)

Arrangements and Interactions of Interlayer Surfactants

Electrostatic attraction between Mnt and the polar heads of surfactants is affected by the quantity of electricity only. Thus, electrostatic attraction is irrelevant to the molecular size or conformation. Previous reports revealed that OMnt modified with various surfactants exhibited various properties and thermal stability. Hence, the interaction between organic surfactants should be considered. C18 cations, having a single long alkyl chain, can be considered as having a ‘linear shape’ and DC18 cations can be regarded as being ‘V-shaped’ due to its two long alkyl chains (Figure 2). Interlayer surfactants arrange themselves as lateral-monolayer, lateral-bilayer, pseudo-trimolecular layer, paraffin-type monolayer. or paraffin-type bilayer (Vaia et al. 1994; Lagaly et al. 2013). The arrangements of interlayer surfactants are influenced by the loading level, conformation of surfactants, the length of the alkyl chain, and even the charge of Mnt. Short-chain alkylammonium cations are arranged in monolayers and longer-chain alkylammonium ions in bilayers with the alkyl chain axes parallel to the silicate layers (Lagaly et al. 2013); a pseudo-trimolecular arrangement is often observed with highly charged smectites and/or long surfactant cations. The periodicity along the c axis of Mnt (without cations and water) is 0.96 nm (Brigatti et al. 2013). Considering the size of the C18 cation (Fig. 2), C18-modified OMnts ideally exhibit a basal spacing of 1.33 nm with a monolayer arrangement, 1.73 nm with a bilayer arrangement, and 2.19 nm with a pseudo-trimolecular arrangement. Lagaly et al. (2013) concluded that the monolayer arrangement had a basal spacing of 1.4 nm, the bilayer, 1.8 nm, and the pseudomolecular arrangement, 2.2 nm. Thus, the basal spacing (2.12 nm) of C18-Mnt-1.0 suggests that C18 molecules were arranged as a pseudo-trimolecular layer (Fig. 12). The positive heads of C18 were attached the silicate layers, whereas the alkyl chains assumed a trimolecular arrangement by the formation of kinks. The pseudo-trimolecular arrangement of C18 cannot result in ordered arrangements of C18 cations in the interlayer space. In addition, the octadecyl chains could kink by formation of gauche bonds at different C atoms (Lagaly 1976). Hence, the arrangement of C18 molecules was not sufficiently homogeneous to form very ordered stacks of layers. Only the low-intensity (001) reflection, therefore, emerged in the XRD patterns of C18-Mnt-1.0; the TEM image also testified to the non-uniform basal spacing. With the increase in loading level or alkyl chain length, organic cations tended to be arranged as a paraffin-type in a tilted to vertical arrangement (Lagaly 1986). Based on the basal spacing of 4.06 nm, C18-Mnt-2.0 nm is proposed to be arranged as a tilted paraffin-type bilayer (Fig. 12). The tilting angle, θ, is correlated positively with the amount of intercalated organic surfactants. In the case of C18-Mnt-2.0, θ is 52°. 1.0 CEC organic cations were assumed to exchange all the inorganic cations and to occupy all the negative sites. The extra 1.0 CEC surfactants cannot intercalate into the interlayers completely because all of the exchangeable sites had been occupied. They should be adsorbed in the form of ion pairs (with anions). More surfactant molecules resulted in a tight arrangement, which made every single surfactant molecule difficult to move. Consequently, C18-Mnt-2.0 showed a more ordered structure and displayed (002) and (003) reflections.

Fig. 12 Schematic diagram of the various arrangements of surfactants in the interlayer space of OMnt

Quaternary alkylammonium ions with two or more long alkyl chains often form paraffin-type arrangements in the interlayer space of smectites (Lagaly et al. 2013). DC18 has two long octadecyl chains. Considering the size of the DC18 and the basal spacing of OMnt, DC18 molecules in the interlayer space of DC18-Mnt-0.5 and DC18-Mnt-1.0 must arrange themselves in the form of a paraffin-type bilayer (Figure 12). The angle between the two octadecyl chains varied with the loading level. The most stable conformation of DC18 corresponded to an angle of ~118.9°. This angle, however, must gradually reduce in order to accommodate more DC18 cations, i.e. α > φ > γ. Finally, an almost parallel orientation of the chains was attained by formation of gauche bonds near the ammonium group. The conformation of DC18 cations allowed a denser packing of these surfactants in mono- and bimolecular films (Favre & Lagaly 1991). This intensive arrangement with a strong interaction between DC18 cations bound individual cations together.

Rheological Properties of OMnt in Oil

Drilling fluids are often evaluated using the Bingham plastic flow model and are often required to work in high-temperature conditions with low viscosity. Generally, AV is used as the effective viscosity to evaluate the viscosity of drilling fluids. PV is not expected to be too high, because extremely high PV would make it difficult to start to drill. The rheological properties of OMnt/oil drilling fluids (AV, PV, and YP) vary with temperatures of ageing (Table 5). A commercial OMnt (DG-Mnt), as used by Mud Service Company, Bohai Drilling Engineering Co. Ltd., China, was used as a reference. DG-Mnt/oil, C18-Mnt-1.0/oil, and DC18-Mnt-0.5/oil fluids all showed quite low viscosities and yield points. Their yield points were zero or very close to zero, indicating that these OMnt/oil fluids possessed no gel strength. Although the values of AV, PV, and YP were stable, the thermal stabilities of DG-Mnt/oil, C18-Mnt-1.0/oil, and DC18-Mnt-0.5/oil fluids were meaningless because of their poor rheological properties. Compared to the rheological properties of DG-Mnt/oil, C18-Mnt-1.0/oil, and DC18-Mnt-0.5/oil fluids, the rheological properties of C18-Mnt-2.0/oil and DC18-Mnt-1.0/oil fluids were increased dramatically. This result demonstrates that more surfactants lead to better rheological properties. C18-Mnt-2.0 showed larger d ₀₀₁ than DC18-Mnt-1.0; however, DC18-Mnt-1.0/oil fluids presented better rheological properties than C18-Mnt-2.0/oil fluids. This phenomenon testifies that a larger basal spacing does not necessarily result in better rheological properties. The rheological properties of OMnt in oil-based drilling fluids should not be influenced by surfactant loading level and basal spacing only, but also by the arrangements of interlayer surfactants. The AV and YP of both C18-Mnt-2.0/oil and DC18-Mnt-1.0/oil fluids first increased and ultimately decreased with rising temperature. For example, the viscosity of C18-Mnt-2.0/oil fluid increased from 16.5 mPa s at 66°C to 31.0 mPa·s at 150°C, then decreased to 26.5 mPa s at 180°C and 24.0 mPa s at 200°C. The AV of DC18-Mnt-1.0/oil fluid and the YP of C18-Mnt-2.0/oil and DC18-Mnt-1.0/oil fluids are affected similarly. Viscosity and gel strength improved with increasing temperature because higher temperatures promote the swelling and even exfoliation of OMnt in oil (Zhuang et al. 2017a, c). Temperature increase above 180°C, however, was harmful for rheological properties. Focusing on the rheological properties of OMnt/oil fluids aged at 150 to 200°C, DC18-Mnt-1.0/oil fluid was more stable than C18-Mnt-2.0/oil fluid. The AV of DC18-Mnt-1.0/oil fluid decreased from 47.0 mPa s to 40.0 mPa s and the YP decreased from 18.0 Pa to 15.0 Pa. But the AV of C18-Mnt-2.0/oil fluid decreased from 31.0 mPa s to 24.0 mPa s and the YP decreased from 15.0 Pa to 3.0 Pa.

Table 5 Rheological properties of OMnt/oil fluids aged at different temperatures

The dynamic rheological curves of OMnt/oil fluids (Figure 13) revealed rheological behavior and thixotropy. The DG-Mnt/oil, C18-Mnt-1.0/oil, and DC18-Mnt-0.5/oil fluids showed non-linear curves and presented low shear stress, in agreement with the results in Table 5. The rheological curves of DG-Mnt/oil, C18-Mnt-1.0/oil, and DC18-Mnt-0.5/oil can be divided into two parts: (1) the Bingham plastic model (a line which does not cross the zero point) in the range of 20–100 s⁻¹; and (2) deviation from the Bingham plastic model to the zero point. DC18-Mnt-1.0/oil fluid exhibited greater shear stress than C18-Mnt-2.0/oil fluid. The shear stress of C18-Mnt-2.0/oil decreased dramatically from 150 to 200°C. The rheological curve of DC18-Mnt-1.0/oil aged at 180°C nearly coincided, however, with that of DC18-Mnt-1.0/oil aged at 150°C. When the temperature increased to 200°C, the shear stress decreased a little.

Fig. 13 Dynamic rheological curves of OMnt/oil fluids aged at 66, 150, 180, and 200°C

Thixotropy is another important rheological property. It is a reversible isothermal transformation of a colloidal sol to a gel. In drilling practice, low resistance (low viscosity) is expected for the bit to ensure a rapid drilling rate, while high viscosity is needed for carrying cuttings. Excellent thixotropy is, thus, an essential property of an oil-based drilling fluid. The areas of thixotropic loops (Figure 13) were applied to evaluate thixotropy of OMnt/oil fluids. The areas were calculated by integration (Table 6). Similarly with the viscosity results, DG-Mnt/oil, C18-Mnt-1.0/oil, and DC18-Mnt-0.5/oil fluids showed very small areas, indicating almost no thixotropy of these fluids. The area of C18-Mnt-2.0/oil aged at 66°C is 2.61 Pa s⁻¹. It increased to 193.43 Pa s⁻¹ at 150°C, then decreased to 27.50 Pa s⁻¹ at 200°C, declining by 86% from the area at 150°C. The area of DC18-Mnt-1.0/oil aged at 66°C was 26.21 Pa s⁻¹ and then increased to 424.68 Pa s⁻¹, indicating that high temperature below 150 °C promotes thixotropy. With increasing temperature, the area decreased to 324.85 Pa s⁻¹ at 200°C, down by 19% from that at 150°C. This result demonstrated that the thixotropy of DC18-Mnt-1.0/oil fluid was more stable than that of C18-Mnt-2.0/oil fluid.

Table 6 Areas of thixotropic loops derived from Figure 13

In conclusion, the rheological properties and thermal stability followed the order DC18-Mnt-1.0/oil > C18-Mnt-2.0/oil > DC18-Mnt-0.5/oil ≈ C18-Mnt-2.0/oil. Two possible reasons for the decrease in rheological properties at high temperatures can be proposed: (1) thermal decomposition of surfactants; and (2) dissolution of interlayer surfactants into oil. Based on the thermal analysis results, DC18-Mnt-1.0 started to decompose below 180°C (in air). But DC18-Mnt-1.0/oil fluid showed very stable rheological properties at 200°C, indicating that the thermal stability of OMnt in oil was improved due to the lack of oxygen. Thus, the decline of other OMnt/oil fluids below 200°C was not caused by thermal decomposition. The only possibility is the dissolution of interlayer surfactants into oil at high temperature. The HLB values of C18 and DC18 are 14.9 and 6.8. respectively. DC18 showed more lipophilicity than C18. DC18-Mnt-1.0, however, had more stable rheological properties than C18-Mnt-2.0, indicating that the paraffin-type bilayer of DC18 in OMnt can resist high temperatures better than the paraffin-type bilayer of C18.

XRD of OMnt/Oil Gels

To reveal the relationship between the thermal stability of OMnt/oil fluids and the arrangement of surfactants in the interlayer space of OMnt, the structure of OMnt in oil must be known. The structural change of OMnt in oil can be determined by XRD of OMnt/oil gel (Figure 14). All the samples showed a wide and low-intensity reflection at 17°2θ, which is assigned to the oil (Zhuang et al. 2017b). Two reflections, corresponding to d values of 2.05–2.06 nm and 1.38–1.43 nm, emerged in the C18-Mnt-1.0/oil aged at 66, 150, and 180°C. The d values of these two reflections were smaller than the basal spacing of C18-Mnt-1.0 (2.12 nm), suggesting that the d ₀₀₁ of C18-Mnt-1.0 in oil was reduced. Thermal analysis results proved that C18-Mnt-1.0 was stable up to 180°C. Therefore, the decrease in basal spacing must be due to the surfactants dissolving in oil. These two reflections cannot be attributed to (001) and (002) reflections, because the d value of the second reflection is not half that of the first. The two reflections, therefore, represented different basal spacings, indicating that the interlayer surfactants dissolved into oil gradually. The surfactants on the surfaces and edges dissolved first, then the internal surfactants dissolved, resulting in two reflections. Finally, aged at 200°C, most of the surfactants in the interlayer space were lost, leading to one reflection with a d value of 1.36 nm. This phenomenon also demonstrated that high temperature promoted the dissolution of interlayer surfactants, possibly because high temperature facilitated the thermal motion of oil molecules and surfactant molecules. A similar phenomenon took place for DC18-Mnt-0.5/oil fluid. The shrinkage of basal spacings of C18-Mnt-1.0 and DC18-Mnt-0.5 in oil demonstrated that loose arrangements resulted in the easy loss of interlayer surfactants. The basal spacing of C18-Mnt-2.0 increased gradually as the temperature increased to 180°C. The basal spacing of C18-Mnt-2.0 in oil reached a maximum value of 4.33 nm when aged at 150°C, corresponding to the best rheological properties of C18-Mnt-2.0/oil fluid. Aged at 200°C, the basal spacing of C18-Mnt-2.0 in oil declined to 1.39 nm. Although C18 cations and molecules are arranged tightly, the interlayer C18 was still lost at high temperature because no strong interaction force exists among the surfactants.

Fig. 14 XRD results for OMnt/oil gels aged at 66, 150, 180, and 200°C

DC18-Mnt-1.0/oil-66 showed a similar reflection to DC18-Mnt-1.0 powder, indicating that no swelling happened, and no surfactants were lost. Because of the tight arrangement of DC18 in OMnt, no extra space was available to accept oil molecules. Below 15°2θ, no reflection is observed in the XRD patterns of DC18-Mnt/oil gels aged at high temperatures, while the (100) reflection remained. Thus, DC18-Mnt-1.0 was exfoliated in oil at high temperatures because of thermal motion and interaction among surfactants. Surfactants in the interlayer space were protected by silicate layers, resulting in stabilization. DC18 cations could still remain stably on the surface of exfoliated layers because the strong interaction between DC18 cations fixed them tightly on the nanolayers. Hence, tight arrangement and strong interaction are necessary for the stability of OMnt in oil based-drilling fluids.

Conclusion

Based on the results and discussions above, several conclusions can be drawn. Organic surfactants occupy the surface and interlayer space of Mnt by means of electrostatic attraction. Interaction happens between the Mnt layers and polar heads of surfactants only. Interaction between organic cations has a critical influence on the stability of OMnt in oil. The style of arrangement of a surfactant with a single long alkyl chain changed from a pseudo-trimolecular layer to a paraffin-tape bilayer with the increase in the surfactant’s loading level. Surfactants with two long alkyl chains arranged as a paraffin-type. Paraffin-type arrangements were more ordered than pseudo-trimolecular layers. A paraffin-type bilayer arrangement of DC18 resulted in more ordered layer stacking than the same arrangement of C18. Tight paraffin-type bilayer arrangements generally led to excellent rheological properties and thermal stability. Loose paraffin-type, pseudo-trimolecular layer and tilted bilayer arrangements resulted in easy dissolution of interlayer organic cations into oil at high temperature. A tight paraffin-type bilayer arrangement of DC18 led to exfoliation of OMnt in oil at high temperatures, improving rheological properties. Organic cations can remain stable in the interlayer space or even on the exfoliated Mnt layers because of the strong interaction force among cations, in addition to the electrostatic attraction. In conclusion, to improve the rheological properties and thermal stability of OMnt in oil-based drilling fluids, more than 1.0 CEC surfactants with two or three long alkyl chains are advised.