Surface Chemistry Tuned Cellulose Nanocrystals in Bentonite


2 days ago - Bentonite (BT) suspension is extensively used as water-based drilling fluids (WDFs) for well ... candidates to improve the rheology and f...
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Surface Chemistry Tuned Cellulose Nanocrystals in Bentonite Suspension for Water-based Drilling Fluids Mei-Chun Li, Suxia Ren, Xiuqiang Zhang, Lili Dong, Tingzhou Lei, Sun-Young Lee, and Qinglin Wu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01830 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

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Surface Chemistry Tuned Cellulose Nanocrystals in Bentonite Suspension for Water-based Drilling Fluids Mei-Chun Li1, Suxia Ren2, Xiuqiang Zhang2, Lili Dong2, Tingzhou Lei2, Sunyoung Lee3, and Qinglin Wu1,*

1

School of Renewable Natural Resources, Louisiana State University AgCenter, Baton Rouge,

Louisiana 70803, United States 2

Key Biomass Energy Laboratory of Henan Province, Zhengzhou, 450008, Henan, China

3

Department of Forest Products, National Institute of Forest Science, Seoul 130-712, Korea

*

Corresponding Author:

Qinglin Wu. E-mail: [email protected]

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ABSTRACT

Bentonite (BT) suspension is extensively used as water-based drilling fluids (WDFs) for well excavation; however, it suffers from insufficient rheology and poor filtration performance at low solid content. Cellulose nanocrystals (CNCs) with nanoscale dimension, large surface area, high stiffness, and reactive functional groups are promising candidates to improve the rheology and filtration performance of BT-WDFs through surface interactions. Consequently, a fundamental understanding of surface interaction between CNCs and BT platelets becomes critical. This work aims to reveal the crucial role of surface characteristics of CNCs on their interaction with BT platelets as well as the rheological and filtration performance of CNC/BT-WDFs. Two types of CNCs with distinctive surface characteristics (i.e., carboxylated CNCs - cCNCs and cationic CNCs - caCNCs) were rationally prepared and applied as modifiers in BT-WDFs. The cCNCs were shown to attach to the edge surface of BT platelets, and acted as “bridges”, connecting BT platelets via “edge-to-edge” association. On the contrary, the caCNCs were absorbed to the face surface of BT platelets, and acted as “crosslinking agents”, stacking numerous layers of BT platelets through “face-to-face” association. These differences led to distinctive dispersion state of BT platelets and overall performance of CNC/BT-WDFs. The cCNCs served as more effective rheological and filtration agents in the developed fluids compared with caCNCs, highlighting the importance of surface characteristic of CNCs in the development of low solid content, high-performance CNC/BT-WDFs for well excavation.

KEYWORDS: Drilling fluids, Cellulose nanocrystals, Interface, Rheology, Filtration

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INTRODUCTION Drilling fluids play vital roles in the entire well drilling operations for successful oil and gas exploration.1 The main functions of drilling fluids include: suspending/carrying drill cuttings from wellbore, cooling/lubricating the bit and drilling assembly, stabilizing the wellbore, and avoiding formation collapse.2,3 Generally, drilling fluids can be classified into four categories: water-based drilling fluids (WDFs), oil-based drilling fluids (ODFs), synthetic-based drilling fluids (SDFs) and gas-based drilling fluids (GDFs). WDFs, composed of water, bentonite (BT), and various functional additives (e.g., weighting materials, thickeners, filtration control agents, deflocculants, and heat/salt resistant agents), are preferred over others due to low cost and environmentally friendliness. In recent years, the development of low solid content BT-WDFs or even BT-free WDFs have attracted great attention because of their numerous advantages, e.g., high drilling rate, low friction in all drilling equipment, thin filter cake, and less pipe sticking.4-7 However, the disadvantages, e.g., poor suspending/carrying capacity for drill cuttings and rapid fluid penetration into formation, strongly limit their utility. Much effort has been devoted to overcoming these drawbacks through adding various thickeners and filtration control agents, mainly including natural polymers,2,6,8-10 synthetic polymers,7,11-13 inorganic particles,3-5,14-17 and inorganic particle/polymer composites.18-20 Over the past decade, the energy industries have been taking benefits from the advances in nanotechnology in the areas of exploration, excavation, reservoir protection, and enhanced oil recovery. Because of the extremely high surface area and outstanding physico-chemical properties, nanomaterials are considered as the most promising candidates for the formulation of low solid content BT-WDFs with tailorable rheological and filtration performance.1 Recently, one of the most emerging and promising nanomaterials is nanocellulose due to numerous 3 ACS Paragon Plus Environment

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appealing features, e.g., sustainability, nanoscale dimension, high aspect ratio, large surface area, extraordinary mechanical performance, ease of functionalization, and notable shear thinning behaviors.21,22 In general, depending on the preparation strategy and morphology, nanocellulose can be categorized into three families: (i) cellulose nanocrystals (CNCs), (ii) cellulose nanofibers (CNFs), and (iii) bacterial cellulose (BC). In the past few years, there has been a growing interest in utilizing CNCs and CNFs to formulate low solid content, high-performance BT-WDFs. For example, in our previous work, it was demonstrated that both CNCs and CNFs can act as effective rheology and filtration modifiers in BT-WDFs.23 Owing to the highly negatively charged surface and good mobility, CNCs with a specific surface charge were anchored on the edge surface of BT, yielding superior rheological and filtration properties of CNC/BT-WDFs over CNF/BT-WDFs. The influence of CNCs and a commercial filtration control agent – polyanionic cellulose (PAC) on the rheological and filtration performance of BT-WDFs was also compared. 24 It was found that CNCs and PAC acted as rheological modifiers and fluid loss reducers in BT-WDFs, respectively. The combined use of them led to synergistic improvement in rheological and filtration performance of BT-WDFs. Furthermore, the thermos-stability of nanocellulose suspension was studied by Heggset et al.25 They demonstrated that both CNCs and CNFs were stable after thermal aging at 140 ℃ for 3 days, which is superior over the conventional rheological modifiers used in WDFs, such as guar gum and xanthan. The application of nanocellulose in different well servicing fluids (e.g., drilling fluids, fracturing fluids, treatment fluids, gravel pack carrier fluids) has also gained great attention from different energy industries, e.g., Rhodia Chimie, Halliburton Energy Service Inc., Schlumberger Canada Limited, American Process Inc., Marquis Alliance Energy Group Inc., and Alberta Innovates judged by a growing patent literature.26-30

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It is well known that the BT particle is comprised of a large number of plate-like layers called “platelet” with two types of surfaces, namely face and edge. The face surface is permanently negatively charged caused by the substitution of lattice cations. However, the edge surface charge is pH dependent, i.e., positive under acidic condition and negative under alkaline condition owing to protonation or deprotonation of amphoteric Al−OH and Si-OH groups on the edge. Under the neutral condition, the edge surface could possibly bear a positive charge due to the exposed octahedral Al layers as well.31,32 As a result, three typical association modes, i.e., face-to-face, edge-to-face , and edge-to-edge can be created via different surface interactions, including Van der Walls forces, hydrogen bonds, electrostatic repulsion forces and electrostatic attraction forces, which greatly governs the rheology and filtration performance of BT-WDFs. Therefore, it is anticipated that the surface characteristics of CNCs play critical role in the surface interaction with BT platelets as well as the association mode of BT platelets, and finally affect the overall performance of CNC/BT-WDFs. Herein, two types of CNCs with distinctive surface features, i.e., carboxylated CNCs (cCNCs) and cationic CNCs (caCNCs) were prepared and then used as modifiers in order to formulate low solid content, high-performance BT-WDFs. The rheological and filtration performances of CNC/BT-WDFs were examined, and correlated with surface interactions between CNCs and BT platelets depending on the surface characteristic of CNCs. The goals of this work were 1) to understand the fundamental surface interactions between BT platelets and CNCs with tailorable surface characteristics, and 2) to probe the relationship between surface interaction and performance of CNC/BT-WDFs. We believe that the findings from the present work serves as an important step forward toward the development of low solid content, high-performance BT-WDFs using sustainable CNCs as modifiers.

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EXPERIMENTAL SECTION Materials. Wyoming sodium bentonite (BT, AQUAGEL GOLD SEAL, dry-powdered, 200 Mesh) was purchased from Baroid Industrial Drilling Products Inc. (Houston, TX, USA). Carboxylated cellulose nanocrystals (cCNCs, BGB Ultra, 8 wt% gel) were obtained from Blue Goose Biorefineries Inc. (Saskatoon, SK, CANADA). Sodium hydroxide pellets and 2,3-epoxypropyl trimethylammonium chloride (EPTMAC) were supplied from Sigma-Aldrich Company (St. Louis, MO, USA). Polyanionic cellulose (PAC) was purchased from Halliburton Company (Houston, TX, USA). All chemicals were used without further purification. Preparation of cationic CNCs (caCNCs). About 50 g of 8 wt% cCNC gel was activated in a concentrated NaOH solution at 65 ℃ for 30 min. Next, 0.074 mol EPTMAC monomer was dropped into the activated cCNC gel. The reaction was carried out in a 50 mL round-bottom flask with vigorous stirring at 65 ℃ for 6 h. The reaction was quenched by addition of excess deionized water. The obtained suspension was poured into the regenerated cellulose dialysis tube. The tube was then dialyzed against excess deionized water in a water tank for 2 weeks in order to eliminate impurities. Characterization of CNCs. The carboxylate content of CNCs was measured by conductometric titrations.33 Typically, 10 g of 0.5 wt% CNC suspension was vigorously stirred for 30 min in order to achieve uniform dispersion state. Next, 15 mL of 0.01 M HCl solution was added, followed by vigorous stirring for another 10 min. The resultant suspension was titrated using 0.005 M NaOH solution, and their corresponding conductivity was recorded using a conductivity and pH meter (PC 700, OAKION).

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Fourier Transform Infrared Spectroscopy (FTIR) spectra were recorded using a Bruker FTIR analyzer (Tensor-27, Bruker Optics Inc., Billerica, MA) equipped with a Zn/Se attenuated total reflectance crystal accessory in the transmittance mode. The measured wavenumbers are in the region 600~4000 cm−1 with a resolution of 4.0 cm−1. Solid-state

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C nuclear magnetic resonance (NMR) measurements were performed on a

Bruker Avance 400 WB instrument operated at a magnetic field of 9.39 T under room temperature. The magic angle spinning (MAS) rate, cross-polarization (CP) contact time, and relaxation delay time was 12 Hz, 2 ms, and 2 s, respectively. The Hartman-Hahn matching procedure was performed based on glycine. The chemical shift values were related to adamantane (C10H16) with its low-field signal set to 38.48 ppm. X-ray photoelectron spectroscopy (XPS) experiments were conducted on a Specs PHOIBOS-100 spectrometer (SPECS, Berlin, Germany) at 10 kV under a current of 10 mA. Survey spectra were recorded from 1200 to 0 eV at 40 eV pass energy and a 1.0 eV scan step. High-resolution N 1s spectrum was acquired at 40 eV pass energy and a 0.1 eV scan step. The element composition was determined using the SpecLab software. Morphology of CNCs were visualized using a Transmission Electron Microscope (TEM, JEM 1400, JEOL) operated at an accelerating voltage of 120 kV. A droplet of highly diluted CNC suspension was deposited on a glow-discharge treated copper grid. Then, the samples were stained with 2 wt% uranyl acetate solution for 3 min. Zeta potential values were determined using a ZetaTrac analyzer (MicroTrac Inc., Largo, FL, U.S.A.). CNC suspensions were diluted to 0.1 wt% using deionized water at pH of ~8. Five replicates were tested and the average values were reported. 7 ACS Paragon Plus Environment

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Formulation of CNC/BT-WDFs. Typically, CNC suspensions were diluted into 0.5, 1 and 2 wt% using tap water (pH 8.5±0.2, salinity 206±3 ppm) with vigorous stirring for 1 h using a mixer (Model 936, Hamilton Beach, Southern Pines, NC), and then 3 wt% BT was slowly added. The mixture was stirred for another 1 h at a high speed of 10000 rpm in order to fully disperse BT (Figure S1). The formulated WDFs were designated as cCNCx/BTy and caCNCx/BTy, where x and y represent the weight percent of CNCs and BT in WDFs, respectively. For the purpose of comparison, various BTz and PACx/cCNCy/BTz-WDFs were also formulated, where x, y and z represent the weight percent of PAC, cCNC and BT in WDFs. Characterization of BT and CNC/BT-WDFs. The surface interaction between CNC and BT was studied using several techniques, including zeta potential, precipitation experiments and TEM observations. Zeta potential and particle size were measured using the ZetaTrac analyzer (MicroTrac Inc., Largo, FL). Five replicates were carried out and the average value was reported. Precipitation experiments were conducted in 20 mL vials for 24 h. Digital photos were taken at various intervals, i.e., 0, 5 min, 1 h, 6 h, 12 h and 24 h. Morphological observation was also conducted using the TEM (JEM 1400, JEOL) operated at an accelerating voltage of 120 kV. Rheological properties were evaluated using a stress-controlled rheometer (AR 2000, TA Instrument, New Castle, DE, USA), equipped with a DIN concentric cylinder geometry. Approximately 20 mL of fluids were injected into a stainless steel cup using a syringe. Steady-state viscosity and shear stress curves were recorded as a function of shear rate ranging from 0.1 to 1200 s-1 at 25 ℃. Temperature sweep tests were performed in the region 25~100 ℃ at a fixed shear rate of 10 s-1. 8 ACS Paragon Plus Environment

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The low-temperature low-pressure (LTLP) filtration performance was examined under 100 psi pressure at ambient temperature following the API guideline (API recommended Practice 13B-1, 2003).34 Experiments were performed in a standard LPLT API filter press (Model No. 30201, Fann Instrument Co., Houston, TX) using the specially hardened filter paper (Part No. 206051, Fann Instrument Co., Houston, TX). Approximately 200 mL of fluids were used, and the filtrate volume was carefully recorded in mL at 1.0, 7.5, 15, 20, 25 and 30 min after the test began. Once the measurement completed, the filter paper containing a layer of deposited muds called “filter cake” was carefully collected. Image of fresh filter cake was directly taken using a digital camera, and the thickness (Tc) of filter cake was measured as well. The filtration rate (q) and permeability of already formed filter cake was determined according to the previously reported method.9,35 The high-temperature high-pressure (HTHP) filtration performance was evaluated under various differential pressures (i.e., 500, 650 and 800 psi) and temperatures (i.e., 100, 125 and 150 ℃). Experiments were conducted in a 175 mL HTHP filter press (Part No. 170181, OFITE, Houston, TX) with the specially hardened filter paper (Part No. 170-19, OFITE, Houston, TX). In a standard run, about 138 mL of fluids were poured into the cell, which was then placed in the heating jacket at desired temperature (Figure S1). While the cell was heating, 100 psi was applied to both top and bottom as the back pressure in order to prevent vaporization of the filtrate. After 1 h of heating, the top pressure was increased to 600, 750 or 900 psi. The filtrate volume was then carefully collected and recorded in mL at the intervals of 1.0, 7.5, 15, 20, 25 and 30 min. The 175 mL HTHP filter press has half filtration area of a standard filtration test. Therefore, to correct the results to a standard API filtration test, doubling the filtration volume was applied and reported. 9 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION CNCs are hydrophilic, highly crystalline, and rod-shaped nanomaterials derived from renewable resources through acid hydrolysis or chemical oxidation methods. Different preparation approaches yield CNCs bearing distinctive surface functional groups. For example, hydrolysis with sulfuric acid introduces sulfate groups on the surface of CNCs;36 while oxidation with 2,2,6,6-tetramethyl-1-piperidinyloxy radical (TEMPO),37 periodate–chlorite,38 ammonium persulfate (APS),39 and transition metal/hydrogen peroxide40 generate carboxyl groups. In our previous work, laboratory-made, sulfuric acid-hydrolyzed CNCs (sCNCs) with width of 6.1±3.5 nm, length of 228.4±63.8 nm, and sulfate group of 2.72 per 100 anhydroglucose units (i.e., ~0.17 mmol/g) were examined as rheological and filtration modifiers in BT-WDFs, and very promising findings were obtained.23 Nowadays, as the rapid growth in the research and commercialization of CNCs, several industrially manufactured CNCs are available on the market globally. The industrially manufactured CNCs are produced on a large scale in pilot plant, ensuring the uniformity in the size and surface chemistry. As a consequence, they are becoming more prevalent than laboratory-made CNCs for various practical applications.41 In order to continuously grow the community of CNCs as well as to ultimately realize the commercial application, the use of industrially manufactured CNCs in drilling fluids is strongly preferred. Herein, commercially available, surface carboxylated CNCs (cCNCs) from Blue Goose Biorefineries Inc. (Saskatoon, SK, CANADA) were used as the starting material. According to the information provided from manufacturer, these cCNCs were isolated from western hemlock through a transition metal catalyzed oxidation method.42 During 10 ACS Paragon Plus Environment

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the oxidation process, the hydroxyl groups on the surface of cellulose were partially oxidized to carboxyl groups, producing cCNCs having carboxyl content of 0.15±0.03 mmol/g (Figure S2) and a negative zeta potential value of -34.5±0.6 mV. Morphologically, the individual cCNC is in rod shape with 9–14 nm in width and 100– 150 nm in length (Figure 1b). Because of the large specific surface area as well as the formation of hydrogen bonding and van der Walls forces, several cCNCs aggregated transversely and longitudinally to form clusters and bundles, respectively.

Figure 1. Preparation and morphology of CNCs: (a) schematic illustration of cationic modification of cCNCs, and TEM micrographs of (b) cCNCs and (c) caCNCs. Cationic CNCs (caCNCs) were synthesized by surface grafting of a cationic surfactant (2,3-epoxypropyl)trimethylammonium chloride (EPTMAC) through alkaline-activated nucleophilic etherification (Figure 1a). Particularly, the alkaline pretreatment activated the hydroxyl groups of cCNCs to be highly nucleophilic; whereas the epoxide groups of EMTMAC are inherently electrophilic due to the strained ring of cyclic ether. As a consequence, at elevated temperature, a nucleophilic etherification occurred through an 11 ACS Paragon Plus Environment

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SN2 mechanism, i.e., the nucleophilic hydroxyl group attacked the less substituted carbon in the electrophilic epoxide group of EMTMAC. After cationization, the dimension of individual nanoparticle seems little changed; while the resultant caCNCs are better individualized and more uniformly dispersed than cCNCs (Figure 1c) due to the presence of larger amount of charged groups on the surface, which was confirmed by zeta potential and XPS analyses (discussed later). Zeta potential measurements showed that the cationic surface modification reversely changed the zeta potential value from -34.5±0.6 to +36.4±2.0 mV. It is also worth noting that the dispersion of caCNCs is highly dependent on the cationic surface charge density. Indeed, both poor and uniform state of dispersion was observed, when the cationic surface charge densities were 0.325 and 2.05 meq/g, respectively.43 It is reasonably proposed that with a low cationic surface charge density, aggregation is likely to take place through electrostatic attractions, since caCNCs bear both cationic groups and negatively charged carboxyl groups on the surface.44 However, when the cationic surface charge density is high enough to exert electrostatic repulsion dominant over electrostatic attraction, homogeneous dispersion can be realized as well. This was further confirmed by suspension stability evaluation through static precipitation experiments. As shown in Figure S3, both cCNCs and caCNCs were able to well suspend in aqueous solution at concentrations of 0.1 and 1 wt% for 7 days without precipitation. caCNC suspension showed milky-white appearance due to the coating of layers of EPTMAC substitutes. The superior suspension stability of CNCs ensured them to function well in drilling fluids without losing their properties as a function of time.

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Figure 2. Chemical structure characterization of CNCs: (a) FTIR spectra; (b) solid-state 13

C NMR spectra, (c) wide scan XPS spectra, and (d) high-resolution N 1s photoelectron

spectrum of caCNCs. The structure of cCNCs and caCNCs was investigated through FTIR, solid-state 13C NMR and XPS analyses (Figure 2). As shown in Figure 2a, cCNCs exhibited characteristic absorption peaks at 1741, 1644 and 1606 cm-1, assigning to carboxyl group (C=O) in carboxylic acid (–COOH), absorbed moisture, and carboxyl group (C=O) in sodium carboxylate group (–COONa), respectively.45 The sodium carboxylate groups in cCNCs were mostly formed during the alkaline extraction, whereby the generated carboxylic acids were soluble and able to be washed from cCNCs.42 After cationization, the absorption peak at 1741 cm-1 completely disappeared, while the absorption peak at 1644 and 1606 cm-1 still remained. This might be ascribed to the fact that the alkaline 13 ACS Paragon Plus Environment

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pretreatment further converted the carboxylic acid to sodium carboxylate groups. In addition, two new absorption peaks at 1480 and 1450 cm-1 appeared after cationic modification (Figure S4), corresponding to the CH2 and CH3 bends of EPTMAC substituents, respectively. Solid-state 13C NMR spectra of cCNCs and caCNCs are presented in Figure 2b. cCNCs showed typical resonance signals of cellulose type I at 104.6 (C1), 88.4 (C4, crystalline), 84.1 (C4, amorphous), 74.3 (C2, C3 and C5), and 65.3 (C6, crystalline) and 63.6 (C6, amorphous) ppm.46 Furthermore, a tiny resonance signal corresponding to carbonyl group (C=O) appeared at around 174 ppm, evidencing the presence of carboxyl groups on the surface of cCNCs.47 After cationization, those resonance signals arising from cCNCs were well preserved. Additionally, a new resonance signal at 55.2 ppm was observed, attributing to the methyl carbons in the EPTMAC substituents as well.48 These observations are in full agreement with FTIR results, indicating the successful cationic modification of cCNCs. The crystallinity index (CI) was further determined from the C4 region where the signals of crystalline and amorphous domain were clearly observed.49 Therefore, the C4 region from 80 to 93 ppm was further deconvoluted into two peaks centered at 84.1 and 88.4 ppm through two Gaussians (Figure S5). Then, the CI values were calculated by dividing the area of crystalline domain by the total area of crystalline and amorphous domains. The obtained CI values are 58.4 and 58.2% for cCNCs and caCNCs, respectively; suggesting that the surface cationic modification had little influence on the crystalline structure of cCNCs. XPS analyses further gave more accurate information on the chemical structure and element composition of cCNCs and caCNCs (Figure 2c and Table S1). In the case of 14 ACS Paragon Plus Environment

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cCNCs, two notable peaks at 533 and 285 eV are associated with the contributions of O 1s and C 1s atoms, respectively. The C%, O% and O/C ratio in cCNCs are determined as 66.81%, 33.19% and 0.50, respectively. It is well known that the chemical formula of cellulose is (C6H10O5)n, and hence it has a theoretical O/C ratio of 0.83. The determined lower O/C ratio for cCNCs might be ascribed to the introduction of contaminants on the surface of cCNCs (e.g., adsorbed molecules because of air exposure and non-cellulosic residues from wood resources).50 In the case of caCNCs, the introduction of EPTMAC substitutes resulted in the occurrence of a new N 1s peak at around 400 eV (Figures 2c,d). Moreover, since EPTMAC substitutes contain higher fraction of C atom than cCNCs, the C% in caCNCs increased from 66.81% to 72.12%. By virtue of the N element composition in caCNCs (Table S1), the EPTMAC content in caCNCs was calculated as 0.43 mmol/g, which is about 2.4-fold of carboxyl content in cCNCs. The deconvolution of C 1s signal from high-resolution spectra further provided insight into the structure of carbon bonds. As shown in Figure S6a, the C 1s signal of cCNCs can be resolved into four component peaks, i.e., C-C or C-H at 285 eV, C-O at 286.5 eV, O-C-O at 288 eV and O-C=O at 289 eV.51 After grafting of EPMTAC on the surface of cCNCs, the composition of C-C (C-H) and C-O that are present in EPMTAC substituents increased; whereas the composition of O-C-O and O-C=O that are absent in EPMTAC substituents declined, further confirming the successful substitution of EPMATAC on the surface of cCNCs. Based on these observations, it is concluded that two types of CNCs with distinctive surface features (i.e., cCNCs with anionic carboxyl content of 0.15±0.03 mmol/g and caCNCs with cationic EPTMAC content of 0.43 mmol/g) were successfully obtained. With the completely distinctive surface characteristics but analogous dimension,

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cCNCs and caCNCs provided an excellent platform to elucidate the indispensable role of CNC surface characteristic on the surface interaction with BT platelets as well as on the rheological and filtration performance of CNC/BT-WDFs.

Figure 3. Stability and particle size of highly diluted CNC/BT-WDFs with different CNC/BT weight ratios: (a) zeta potential; (b) average particle size; and (c) digital precipitation photos as a function of standing time. In the highly diluted CNC/BT-WDFs, the concentration of BT was fixed as 0.1 wt%, and weight ratio of CNC/BT varied from 0 to 1/10, 1/2 and 1/1. To probe the surface interaction between BT and CNCs, zeta potential analyses of highly diluted CNC/BT-WDFs with different CNC/BT weight ratios (i.e., 0, 1/10, 1/2 and 1/1) were performed (Figure 3a). The neat BT suspension had the zeta potential value of 31.71±2.81 mV. Since the edge surfaces only occupy a tiny fraction of the total surfaces of BT platelet (< 1%), the measured negative zeta potential value of BT was mainly 16 ACS Paragon Plus Environment

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resulted from the negatively charged face surfaces of BT platelets regardless of the charge of edge surface.52 Interestingly, the incorporation of CNCs gave rise to distinctive change trends in zeta potential values of CNC/BT-WDFs depending on the surface charge of CNCs. In the case of cCNC/BT-WDFs, their zeta potential values were slightly shifted to more negative values, as more cCNCs were included. When the weight ratio was 1/1, its zeta potential value reached -34.1±3.96 mV, which was close to that of neat cCNC (i.e., 34.5±0.6 mV). On the contrary, the addition of caCNCs led to gradual increase in the zeta potential values of BT-WDFs. Particularly, with increase in the caCNC/BT weight ratio from 0 to 1/10, their zeta potential values increased sharply from -31.71±2.81 to 17.04±5.64 mV, which almost reached a neutral value of -1.68±3.56 mV at the weight ratio of 1/2, and ultimately came to a positive value of 12.77±1.88 mV at the weight ratio of 1/1. Additionally, particle size analyses revealed that the presence of cCNCs gradually declined the average particle size of BT-WDFs; whereas notable aggregation occurred when caCNCs were included (Figure 3b). At the highest weight ratio of 1/1, the average particle size of caCNC/BT-WDFs is 5.34±1.32 μm, which is 3.1- and 7.4-fold of BT- and cCNC/BT-WDFs. To be complementary to the zeta potential and particle size results, their colloidal stability was also investigated through static precipitation experiments (Figure 3c). For the neat BT suspension, BT platelets can well suspend in aqueous solution within 1 h. It is believed that the presence of a large number of negative charges on the face surfaces of BT platelets endowed strong repulsion forces among BT platelets, leading to the formation of exfoliated structure with good colloidal stability in an aqueous solution. However, the visible precipitation took place after standing for 6 h. The addition of 17 ACS Paragon Plus Environment

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cCNCs was conducive to improving the colloidal stability of BT suspension. Especially, when the cCNC/BT weight ratio reached 1/1, the state dispersion state lasted for 12 h. Similar phenomena have been recently reported that the negatively charged CNCs can be utilized to stabilize aqueous graphene solution with more exfoliated sheets and better stable dispersion state.53,54 On the contrary, the incorporation of caCNCs in BT suspension (especially for caCNC/BT weight ratio of 1/2 and 1/1) caused rapid sedimentation, generating the clear supernatant within only 5 min. On the basis of the above observations, two hypotheses on the surface interactions between CNCs and BT were proposed depending on the surface characteristics of CNCs (Figure 4). First, the highly negatively charged cCNCs were attached to the possibly positively charged edge surfaces of BT platelets via electrostatic attractions (Figure 4c). Another driving force might arise from the strong repulsion between the highly negatively charged cCNCs and face surfaces of BT platelets, which pushed cCNCs to be located on the edge surface of BT platelets. This led to neutralization or compensation of cCNCs, resulting in a slight reduction in the absolute zeta potential value of cCNC suspension in the presence of BT. Additionally, the absorption of cCNCs on the edge surface of BT platelets screened the possibly positively charged sites of BT platelets. This greatly refrained BT platelets from the formation of flocculated structure through “face-to-edge” association model, and hence facilitated their dispersion state and colloidal stability. Second, the caCNCs were significantly absorbed to the negatively charged face surfaces of BT platelets via electrostatic attractions (Figure 4d). This caused charge overcompensation of BT and the formation of severe aggregations, resulting in extremely poor colloidal stability, which were evidenced from the dramatic shift in the zeta potential 18 ACS Paragon Plus Environment

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ACS Applied Nano Materials

value from negative to positive as well as the occurrence of rapid sedimentation phenomena within only 5 min. In order to intuitively prove the above hypotheses, TEM observations were carried out. As expected, the attachment of cCNCs on the edge surface of BT platelets was clearly seen (Figure 4a,S7a). By contrast, the caCNCs were extensively bound on the face surface of BT, forming large, thick aggregates consisting of several layers of BT platelets (Figure 4b,S7b). Overall, these TEM micrographs are in good agreement with zeta potential, particle size and static precipitation results, strongly confirming our assumptions as illustrated in Figure 4c,d. It is believed that these distinctions can induce different rheology and filtration performance of CNC/BT-WDFs.

Figure 4. TEM micrographs and proposed surface interactions of CNC/BT-WDFs: (a,c) cCNC/BT and (b,d) caCNC/BT. 19 ACS Paragon Plus Environment

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Figure 5. Rheological properties of BT and CNC/BT-WDFs: (a,b) influence of CNCs on the viscosity and shear stress of BT3-WDFs, respectively; and (c,d) comparison on the viscosity and shear stress of cCNC0.5/BT3 with BT-WDFs at different BT loadings, respectively. Note: Dash lines in panels b and d represent the fitted lines using the Nasiri−Ashrafizadeh model. Rheology is considered as the most important property for drilling fluids, which directly reflects the ability of drilling fluids to carry, suspend and transport drill cuttings from wellbore to surface. In addition, drilling fluids should have excellent shear-thinning behavior, i.e., high viscosity at rest in order to carry drill cuttings, but low viscosity at

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high shear rates in order to be rapidly pumped into wellbore. 23 Figure 5a shows the steady-state viscosity of BT3 and CNC0.5/BT3-WDFs as a function of shear rate. The BT3-WDFs exhibited the nearly Newtonian behavior in the region 0.1~200 s -1, i.e., the viscosity is independent on the shear rate. However, when the shear rate exceeded 200 s-1, the shear-thickening behavior occurred, i.e., the viscosity increased as the shear rate increased due to the formation of oriented microstructure under high shear stresses. The occurrence of Newtonian and shear-thickening behaviors indicated that the suspensions with low BT concentrations (e.g., 3 wt%) are inappropriate as drilling fluids. The incorporation of cCNCs and caCNCs in BT-WDFs effectively overcome those drawbacks, yielding a pronounced shear thinning behavior as well as much higher viscosity values within the entirely investigated shear rate region. For example, the viscosity value of BT3-WDFs at a low shear rate of 0.1 s-1 was improved by more than three orders of magnitude by including 0.5 wt% CNCs, suggesting the better drill cuttings removal performance. The surface characteristic of CNCs had notable influence on the viscosity of CNC/BTWDFs. cCNCs are considered as better rheological modifiers for BT-WDFs than caCNCs based on the following two observations. First, cCNC0.5/BT3-WDFs exhibited a progressive, smooth shear-thinning curve, i.e., the viscosity gradually decreased with increase in the shear rate. It is believed that such progressive shear-thinning pattern allowed drilling engineers to predict the flow behavior of fluids more accurately, and therefore ensured the efficiency and safety of drilling operations. However, caCNC0.5/BT3-WDFs displayed non-progressive, unstable shear-thinning behavior. For instance, a sudden plateau appeared in the shear rate region from 1 to 10 s-1 due to the 21 ACS Paragon Plus Environment

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formation of oriented aggregates under the applied shear force.55,56 The aggregates could be created more easily through the complexation of caCNCs with BT platelets via “faceto-face” association, which was validated from particle size analysis (Figure 3b) as well as TEM observation (Figure 4b,S7b). Second, in comparison with caCNC0.5/BT3-WDFs, cCNC0.5/BT3-WDFs had higher viscosity at low shear rates, but lower viscosity at high shear rates. These merits allowed cCNC/BT-WDFs not only to carry more drill cuttings at rest, but also to be pumped to wellbore more easily when drilling was applied. Figure 5b shows the shear stress of BT3 and CNC0.5/BT3-WDFs as a function of shear rate. The plots of shear stress (𝜏) versus shear rate (𝛾̇ ) were then fitted using the HerschelBulkley model, which has the following form:57 𝜏 = 𝜏0 + 𝐾 × 𝛾̇ 𝑛

(1)

where 𝜏0 is the yield stress or yield point, 𝐾 is the flow consistency coefficient, and n is the flow behavior index. Particularly, the yield stress is the minimum stress required to induce movement, indicating the ability of drilling fluids to carry and suspend drill cuttings from wellbore to surface. Fluid consistency depicts the shear rate-dependence of shear stress of drilling fluids. Flow behavior index reflects the flow behavior of drilling fluids, i.e., n=1 (Newtonian), n < 1 (shear thinning), and n > 1 (shear thickening).24 As given in Table 1, the addition of 0.5 wt% cCNCs led to substantial increase in the yielding stress of BT3-WDFs from 0.02 to 1.04 Pa, indicating a notable improvement in the carrying capacity. On the contrary, the flow behavior index decreased from 1.35 to 0.83, demonstrating the transition of shear behavior from shear thickening to shear thinning, which was in good agreement with the viscosity results.

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Unexpectedly, caCNC0.5/BT3-WDFs derived a physically meaningless negative value, i.e., 0.98 probably due to the limitation of the Herschel-Bulkley model. Table 1. Derived rheological parameters of BT and CNC/BT-WDFs using the HerschelBulkley (H-B) and Nasiri-Ashrafizadeh (N-A) models. Models/Samples

H-B

N-A

BT3

caCNC0.5/BT3

cCNC0.5/BT3

cCNC1/BT3

cCNC2/BT3

BT6

BT9

BT12

𝜏𝑜

0.0192

-0.9789

1.0372

3.7619

11.825

0.1442

0.6999

6.0727

K

0.0006

1.9947

0.0299

0.2157

0.8012

0.0004

0.0392

0.2663

n

1.3515

0.2864

0.8310

0.5714

0.4227

1.4301

0.8597

0.7538

𝑅2

0.9995

0.9691

0.9866

0.9756

0.9105

0.9923

0.9976

0.9984

SSR

0.0680

20.891

3.7482

10.509

73.523

1.2450

1.7188

12.312

MSWD

0.0017

0.5357

0.0961

0.2695

1.885

0.0319

0.0441

0.3157

𝜏𝑜

0.0132

0.3376

0.7779

3.6846

12.260

0.0774

0.5471

6.3290

b

0.0015

0.7263

0.1803

0.2796

0.5155

0.0050

0.1256

0.1407

K

0.0001

-0.2761

-0.0693

-0.1017

-0.1934

0.0000

0.0416

0.0125

n-m

1.3275

0.8575

0.8507

0.8605

0.8578

3.2764

0.8616

1.0102

𝑅2

0.9995

0.9802

0.9979

0.9818

0.9127

0.9986

0.9991

0.9987

SSR

0.0644

13.075

0.5689

7.6470

69.892

0.2248

0.6298

9.4371

MSWD

0.0002

0.3441

0.0150

0.2012

1.8393

0.0059

0.0166

0.2483

To avoid the derivation of physically meaningless negative value for yield stress, Nasiri and Ashrafizadeh developed a novel four-parameter equation by combining the Binghamplastic, power-law, and Herschel-Bulkley models with a logarithmic correction factor, which has the following form:58 𝜏 = 𝜏0 + 𝑏 × 𝛾̇ + 𝐾 × 𝛾̇ 𝑛−𝑚 ln(1 + 𝛾̇ )

(2)

𝜏0 ≥ 0; 𝑏 ≥ 0; 0 ≤ 𝑛 ≤ 1; 0 ≤ 𝑚 ≤ 1 23 ACS Paragon Plus Environment

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Therefore, we applied this novel model to fit the shear stress versus shear rate plots for BT3 and CNC0.5/BT3-WDFs, and the derived parameters are given in Table 1 as well. According to the values of derived statistical parameters, including squared correlation coefficient (R2), sum of square residuals (SSR), and mean square weighted deviation (MSWD), the Nasiri-Ashrafizadeh model performed better in simulating the shear stress versus shear rate plots for all samples compared with the Herschel-Bulkley model. Furthermore, as expected, no negative yield stress value was obtained using the Nasiri-Ashrafizadeh model. For instance, caCNC0.5/BT3-WDFs gave a positive yield stress value of 0.3375 Pa, which is physically meaningful. Nevertheless, the derived yield stress of WDFs using the Nasiri-Ashrafizadeh model increased in the order of BT3 < caCNC0.5/BT3 < cCNC0.5/BT3, which was completely consistent with that obtained using the Herschel-Bulkley model. Based on the above results as well as the TEM observations, it was demonstrated that the attachment of cCNCs on the edge surface of BT platelets (Figure 4a,c) helped create more rigid network, and hence yielded higher viscosity and yield stress. The cCNCs might act as “bridges”, connecting several BT platelets via “edge-to-edge” association to create the rigid network (Figure S7a). However, the attachment of caCNCs on the face surface of BT platelets (Figure 4b,d) caused severe aggregation of BT platelets, resulting in reduction in viscosity and yield stress. The caCNCs might act as “crosslinking agents”, stacking numerous layers of BT platelets through “face-to-face” association to generate aggregates (Figure S7b). Conventionally, natural polymers, e.g., xanthan and guar gum, are widely applied to enhance the rheological properties of BT-WDFs due to their unique high viscosity at low shear rates arising from the high molecular weight and highly branched microstructure.59 However, those biopolymers suffer from poor thermal stability, i.e., the viscosity and 24 ACS Paragon Plus Environment

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ACS Applied Nano Materials

yielding stress decline at elevated temperatures.25,60 Various thermal degradation mechanisms, including hydrolysis, oxidation and radical depolymerization have been proposed.61,62 This drawback strongly limited their usage in deep well excavation, since the temperature gradually elevated as the wellbore went deeper. The influence of temperature on the viscosity of BT and CNC/BT-WDFs was studied. As shown in Figure S8, as the temperature increased from 25 to 100 ℃, the viscosity barely changed, indicating the superior thermal stability of CNC/BT-WDFs. This observation is in good agreement with the findings from Heggset et al., who demonstrated the superior thermal stability of CNCs over xanthan and guar gum.25 Filtration is another critical performance for WDFs, which is responsible for the cost, safety and efficiency of drilling activities. During the drilling operation, the water in drilling fluids easily penetrated into the formation, leading to rapid swelling of clay, and subsequent formation damage and wellbore collapsing, which largely increased drilling cost and period.1,3 Meanwhile, the solids in drilling fluids deposited under high pressure, creating a layer of solid on the face of formation called as “filter cake”, which could give rise in differential pressure sticking and stuck pipe issues.63,64 Therefore, the low fluid loss volume and the deposition of thin, compact, impermeable filter cake are also required for high-performance WDFs. The filtration performance of BT3 and CNC0.5/BT3-WDFs under LTLP conditions are given in Figure 6. As shown in Figure 6a, BT3-WDFs had a fluid loss volume of 31.2 mL/30 min. When 0.5 wt% cCNCs and caCNCs were incorporated, the fluid loss volume was changed to 25.7 and 61.3 mL/30 min, respectively. The remarkable reduction in the fluid loss volume of BT-WDFs was achieved by adding cCNCs, but the presence of caCNCs caused 2-fold increase in fluid penetration volume. 25 ACS Paragon Plus Environment

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Figure 6. Filtration properties of BT and CNC/BT-WDFs under LTLP conditions: (a) fluid loss as a function of time, (b) filtration rate (q) determination of the formed filter cakes, and (c), (d), and (e) thickness (Tc), filtration rate (q), and permeability (Kc) of deposited filter cakes, respectively. Note: In panel b, the slope of dash line denotes the filtrate rate (q, cm3/s) of filter cake. In panel c, the inserted digital photos show the appearance of fresh filter cakes. The fluid loss volume is directly related to the quality of deposited filter cake. Generally, low fluid loss is accompanied by the deposition of thin, compact and impermeable filter cake; whereas high fluid loss comes along with the formation of thick, loosen, highly permeable filter cake. The permeability (Kc) of filter cake was then determined using the Darcy’s Equation:65 26 ACS Paragon Plus Environment

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Kc =

μTc dV ∆PAdt

=

μTc q ∆PA

(3)

where 𝜇 is the viscosity of filtrate at 25 ℃ (1 cP); Tc is the thickness of filter cake (cm); ∆𝑃 is the pressure drop across the filter cake (6.8 atm); A is the cross-sectional area to fluid flow (45.8 cm2); and 𝑞 =

𝑑𝑉 𝑑𝑡

is the filtration rate (cm3/s). The filtration rate was evaluated using the water

flowing through already formed filter cake method following the literature reported previously,9,35 and the corresponding results is given in Figure 6b. As expected, a linear relationship between filtrate volume and time was observed. From the slope of linear regression line, the filtration rate was obtained (Figure 6d). On the other hand, after each filtration measurement, the appearance of deposited filter cake was recorded with a digital camera and its thickness was also measured using a caliper (Figure 6c). By substituting these values into Equation 3, the permeability of filter cakes was calculated (Figure 6e). The filter cake deposited from BT3-WDFs had the thickness of 0.12 cm, filtration rate of 8.110-3 cm3/s, and permeability of 3.1210-3 md; which respectively became 0.13 cm, 6.810-3 cm3/s, 2.8410-3 md by including 0.5 wt% cCNCs, suggesting the improved quality of filter cake. On the contrary, the presence of 0.5 wt% caCNCs deteriorated the quality of filter cake. The thickness, filtration rate, and permeability of filter cake deposited from caCNC0.5/BT3-WDFs were 0.75 cm, 14.110-3 cm3/s, and 33.96 10-3 md, respectively. The above phenomena strongly demonstrated that the surface characteristic of CNCs played critical role on the filtration performance of CNC/BT-WDFs, and cCNCs acted as more effective filtration control agents in BT-WDFs than caCNCs as well. The formation of different dispersion state of BT platelets (e.g., exfoliation and aggregation) arising from the distinctive surface interactions between CNCs and BT platelets was responsible for

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these observations. In the case of cCNC/BT-WDFs, the absorption of cCNCs on the edge surfaces of BT platelets shielded the possibly positive sites of BT platelets (Figure 4a,c), and thus prevented BT platelets from the formation of flocculation through “face-to-edge” association. This led to uniform dispersion state and superior colloidal stability of BT platelets via “edge-to-edge” association in cCNC/BT-WDFs (Figure 3). As a result, the fluid barrier capacity of BT platelets was substantially enhanced, resulting in a notable reduction in the fluid loss volume. Furthermore, the enhanced colloidal stability effectively prevented BT platelets from precipitation, and hence a thin, compact, low permeable filter cake was produced. In the case of caCNC/BT-WDFs, the attachment of caCNCs on the face surface of BT platelets caused severe BT aggregates via “face-to-face” association (Figure 4b,d). This greatly diminished the fluid barrier capacity of BT platelets, resulting in a dramatic increase in fluid loss volume. Furthermore, the formed large BT aggregates with poor colloidal stability precipitated significantly (Figure 3), generating the thick, loosen, high permeable filter cake. It was demonstrated that cCNC0.5/BT3-WDFs had better rheological and filtration performance than caCNC0.5/BT3-WDFs. The concertation influence of cCNCs on the rheological and filtration performance of BT3-WDFs was further investigated. As shown in Figures 5a, 5b and 6, both the rheological and filtration properties of cCNC/BT3WDFs were substantially enhanced as more cCNCs were incorporated. Particularly, with the increase in the concentration of cCNCs from 0 to 0.5, 1 and 2 wt%, the yield stress values derived using the Nasiri-Ashrafizadeh model (Table 1) gradually increased from 0.0132 to 0.7779, 3.6846 and 12.260 Pa; whereas the fluid loss under the LTLP conditions (Figure 6a) progressively decreased from 31.2 to 25.7, 21.6 and 15.3 mL/30 28 ACS Paragon Plus Environment

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min, respectively. These results strongly validated that the presence of cCNCs in BTWDFs was advantageous for enhancing the drill cuttings removal performance as well as maintaining the wellbore stability. Furthermore, the suspension stability of drilling fluid system was studied through aging experiments. Typically, the cCNC2/BT3-WDFs were stood under room temperature for different days. Then, the influence of aging time on rheological and filtration performance of cCNC2/BT3-WDFs was studied (Figure S9). Both the viscosity and fluid loss volume slightly increased as the aging time increased from 0 to 1 and 3 days due to the gelation of cCNCs and BT platelets. The timedependent gelation might slightly change the dispersion state of BT platelets from exfoliation to aggregation, leading to the deterioration on the fluid barrier capacity of BT platelets.

Figure 7. Filtration properties of BT, cCNC/BT and PAC/cCNC/BT-WDFs under HTHP conditions: (a) fluid loss volume performed at 100 ℃ and 500 psi, and (b) influence of temperature and pressure on the fluid loss volume of PAC0.5/cCNC0.5/BT3-WDFs.

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It is also of great interest to investigate whether the developed cCNC/BT-WDFs display good filtration performance under the HTHP conditions. Figure 7a shows the fluid loss volume of BT3 and cCNC/BT3-WDFs under the conditions of 100 ℃ and 500 psi. As shown in Figure 7a, cCNCs exhibited very limited capacity to reduce the fluid loss of BTWDFs under the HTHP conditions. For instance, the BT3-WDFs had API fluid loss of 65.5 mL/30 min, which slightly dropped to 64.2, 60.1 and 58.8 mL/30 min in the presence of 0.5, 1, and 2 wt% cCNCs, respectively. Polyanionic cellulose (PAC) is a commercial filtration controlling agent used in WDFs. The addition of 0.5 wt% PAC effectively reduced the fluid loss volume of cCNC0.5/BT3-WDFs from 64.2 to 30.8 mL/30 min. In addition, the influence of temperature and pressure on the HTHP filtration performance of PAC0.5/cCNC0.5/BT3-WDFs was further studied (Figure 7a). It was revealed that their HTHP filtration performance was dominated by temperature rather than pressure. As depicted in Figure 7b, when the temperature was elevated from 100 to 125 and 150 ℃, the API fluid loss volume notably increased from 30.8 to 39.9 and 69.7 mL, respectively; whereas there was no apparent change in the API fluid loss volume with increase in pressure from 500 to 650 and 800 psi. Under the HTHP conditions, on one hand, BT platelets underwent hydration, flocculation and aggregation; on the other hand, polymer additives were decomposed. The flocculation or aggregation of BT platelets and the thermal degradation of polymer additives were responsible for the observed poor HTHP filtration performance. Further chemical modification on the CNC surface through grafting thermal-resistant polymers can help improve the HTHP performance of CNCs. Finally, in order to demonstrate the effectiveness of cCNCs as rheological and filtration modifiers on the preparation of low solid content BT-WDFs, comparative investigations 30 ACS Paragon Plus Environment

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were performed with BT-WDFs with higher BT loadings in the fluids. As shown in Figure 5c,d and Table 1, cCNC0.5/BT3-WDFs exhibited comparable rheological properties to BT9-WDFs. Thus, 6 wt% BT can be reduced using only 0.5 wt% cCNCs to formulate the low solid content muds. In the case of filtration performance, cCNC0.5/BT3-WDFs exhibited comparable filtration performance to BT4.5-WDFs (Figure 6). Namely, 1.5 wt% of BT can be substituted by 0.5 wt% cCNCs, while still maintaining analogous fluid loss volume and quality of filter cake. It is believed that several benefits, e.g., rapid drilling penetration, low friction coefficient between drill string and wellbore, prolonged drill bit life, and high quality filter cake, would be resulted from the application of such low solid content muds in drilling operations. Moreover, the use of various nanoparticles (e.g., starch, SiO2, Fe2O3, Fe3O4, ZnO, CuO, TiO2, Al3O4, carbon nanotube, graphene oxide, and their hybrids) to enhance the rheological and filtration performance of BT-WDFs has been widely reported in recent years. Comparative investigation on the effectiveness of cCNCs and other nanoparticles in improving rheological and filtration performance of BT-WDFs were performed. As shown in Table S2, CNCs possess outstanding capacity to enhance the yield stress of BTWDFs; whereas lower ability to reduce the fluid loss of BT-WDFs than some sphereshaped nanoparticles (e.g., colloidal SiO2 and starch). It is believed that the sphere-shaped nanoparticles can enter into the pores and subsequently plug their throat more effectively over the rod-shaped CNCs. Furthermore, in compared with the laboratory-made sCNCs reported previously,23 the industrially manufactured cCNCs showed lower capacity to enhance the rheological and filtration performance of BT-WDFs. This is probably because of the much lower aspect ratio of cCNCs (~ 10) compared with sCNCs (~ 37),

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since sCNCs and cCNCs had similar content of surface charged groups (0.17 vs 0.18 mmol/g). This finding highlighted the critical role of morphology of CNCs on the overall performance of CNC/BT-WDFs as well.

CONCLUSIONS CNCs with two distinctive surface characteristics (i.e., cCNCs with anionic carboxyl content of 0.18 mmol/g and caCNCs with cationic EPTMAC content of 0.43 mmol/g) were prepared and used as modifiers to improve the rheological and filtration performance of low solid content BT-WDFs. It was found that the surface characteristic of CNCs played a key role in providing the BT-WDFs with improved rheological and filtration performance. In comparison with caCNC/BT-WDFs, cCNC/BT-WDFs exhibited superior rheological properties (e.g., more smooth shear-thinning behavior, higher viscosity, and higher yield stress), as well as better LTLP filtration performance (e.g., lower fluid loss volume, and thinner, more compact, and less permeable filter cake). The differences in rheological and filtration between these two developed fluids were ascribed to the formation of distinctive dispersion states of BT platelets (e.g., exfoliation and aggregation) arising from the different surface interactions between CNCs and BT platelets. Particularly, the attachment of cCNCs on the edge surface of BT platelets was favorable, which not only generated more rigid network to induce better rheology, but also facilitated the dispersion state and colloidal stability of BT platelets to generate superior LTLP filtration performance. However, under HTHP conditions, cCNCs showed very limited capability to improve the filtration performance of BT-WDFs. Although the

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incorporation of PAC effectively reduced the fluid loss, the filtration performance of PAC/cCNC/BT-WDFs was still highly temperature-dependent, signaling needs for further surface chemical modification of the CNCs. Finally, comparative investigations revealed that 6 and 1.5 wt% BT can be substituted by only 0.5 wt% cCNCs, while maintaining comparable rheological and LTLP filtration performance, respectively; demonstrating the effectiveness of cCNCs in the formulation of low solid content CNC/BT-WDFs. This work highlighted the importance of surface characteristic of CNCs on their surface interaction with BT platelets, and eventually on the rheological and filtration performance of CNC/BT-WDFs, providing us design rules for the development of low solid, highperformance CNC/BT-WDFs in oilfield applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Flow chart for the formulation and characterization of CNC/BT-WDFs (Figure S1); Typical conductometric titration curve (Figure S2); Appearance and static precipitation phenomena (Figure S3); Enlarged FTIR spectra (Figure S4); Deconvolution of C4 region from XRD pattern (Figure S5); Deconvolution of the high-resolution C 1s peak from XPS curves (Figure S6); TEM micrographs of CNC/BT-WDFs (Figure S7); Viscosity curves as a function of temperature (Figure S8); Influence of aging time on performance of cCNC2/BT3-WDFs (Figure S9); Atomic surface composition from wide scan and quantification of differently bonded carbon atoms from C 1s high-resolution scan (Table 33 ACS Paragon Plus Environment

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S1); Comparison on the effectiveness of different nanoparticles in improving rheological and filtration performance of BT-WDFs under LTLP conditions (Table S2). AUTHOR INFORMATION Corresponding Author * E-mails: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This collaborative study was carried out with support from the US Endowment and USDA Forest Service [E17-23] and the Louisiana Board of Regents [LEQSF (2016-2017)-ENH-TR-01, LEQSF(2015-17)-RD-B-01, and LEQSF(2018-19)-ENH-DE-06], and Henan Academy of Science (Project No: 18TP06007 and 18JK1007).

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For Table of Contents Use Only Surface Chemistry Tuned Cellulose Nanocrystals in Bentonite Suspension for Water-based Drilling Fluids Mei-Chun Li1, Suxia Ren2, Xiuqiang Zhang2, Lili Dong2, Tingzhou Lei2, Sunyoung Lee3, and Qinglin Wu1,*

Synopsis: Low-solid bentonite-water-based drilling fluids were developed using sustainable, commercially available, and surface chemistry tunable cellulose nanocrystals as rheological and filtration modifiers.

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Figure 1. Preparation and morphology of CNCs: (a) schematic illustration of cationic modification of cCNCs, and TEM micrographs of (b) cCNCs and (c) caCNCs. 169x95mm (300 x 300 DPI)

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Figure 2. Chemical structure characterization of CNCs: (a) FTIR spectra; (b) solid-state 13C NMR spectra, (c) wide scan XPS spectra, and (d) high-resolution N 1s photoelectron spectrum of caCNCs. 169x165mm (300 x 300 DPI)

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Figure 3. Stability and particle size of highly diluted CNC/BT-WDFs with different CNC/BT weight ratios: (a) zeta potential; (b) average particle size; and (c) digital precipitation photos as a function of standing time. In the highly diluted CNC/BT-WDFs, the concentration of BT was fixed as 0.1 wt%, and weight ratio of CNC/BT varied from 0 to 1/10, 1/2 and 1/1. 169x143mm (300 x 300 DPI)

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Figure 4. TEM micrographs and proposed surface interactions of CNC/BT-WDFs: (a,c) cCNC/BT and (b,d) caCNC/BT. 169x177mm (300 x 300 DPI)

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Figure 5. Rheological properties of BT and CNC/BT-WDFs: (a,b) influence of CNCs on the viscosity and shear stress of BT3-WDFs, respectively; and (c,d) comparison on the viscosity and shear stress of cCNC0.5/BT3 with BT-WDFs at different BT loadings, respectively. Note: Dash lines in panels b and d represent the fitted lines using the Nasiri−Ashrafizadeh model. 169x168mm (300 x 300 DPI)

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Figure 6. Filtration properties of BT and CNC/BT-WDFs under LTLP conditions: (a) fluid loss as a function of time, (b) filtration rate (q) determination of the formed filter cakes, and (c), (d), and (e) thickness (Tc), filtration rate (q), and permeability (Kc) of deposited filter cakes, respectively. Note: In panel b, the slope of dash line denotes the filtrate rate (q, cm3/s) of filter cake. In panel c, the inserted digital photos show the appearance of fresh filter cakes. 169x126mm (300 x 300 DPI)

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Figure 7. Filtration properties of BT, cCNC/BT and PAC/cCNC/BT-WDFs under HTHP conditions: (a) fluid loss volume performed at 100 ℃ and 500 psi, and (b) influence of temperature and pressure on the fluid loss volume of PAC0.5/cCNC0.5/BT3-WDFs. 169x90mm (300 x 300 DPI)

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