Cellulose Nanocrystals and Polyanionic Cellulose as Additives in

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Cellulose Nanocrystals and Polyanionic Cellulose as Additives in Bentonite Water-Based Drilling Fluids: Rheological Modelling and Filtration Mechanisms Meichun Li, Qinglin Wu, Kunlin Song, Corneils F De Hoop, Sun-Young Lee, Yan Qing, and Yiqiang Wu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03510 • Publication Date (Web): 16 Dec 2015 Downloaded from http://pubs.acs.org on December 28, 2015

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Industrial & Engineering Chemistry Research

Cellulose Nanocrystals and Polyanionic Cellulose as Additives in Bentonite Water-Based Drilling Fluids: Rheological Modelling and Filtration Mechanisms

Mei-Chun Li1, Qinglin Wu 1,*, Kunlin Song 1, Corneils F De Hoop1, Sunyoung Lee2, Yan Qing3, and Yiqiang Wu3,* 1

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

Louisiana 70803, United States 2

Department of Forest Products, Korea Forest Research Institute, Seoul, 130-712, Korea

3

College of Materials Science and Engineering, Central South University of Forestry and

Technology, Changsha, 410004, China

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ABSTRACT

This research aims to develop low cost, sustainable, environmentally friendly and high performance water-based drilling fluids (WDFs) using bentonite (BT), polyanionic cellulose (PAC), and cellulose nanocrystals (CNCs). The effect of concentration of BT, PAC and CNCs on the rheological and filtration properties of PAC/CNC/BT-WDFs was investigated. Eight empirical rheological models were applied to quantitatively fit the fluid properties. Results showed that the presence of PAC, CNCs and BT improved the rheological and filtration properties of the WDFs. Among the eight empirical rheological models, the Sisko model performed the best in simulating the rheological behavior of the fluids. At the same concentration level of PAC and CNCs, CNCs had more impact on the rheological properties, while PAC had more influence on the filtration property. The incorporation of PAC resulted in very low permeable filter cakes, leading to the excellent filtration property. The combined use of PAC and CNCs yielded better rheological and filtration properties.

KEYWORDS: Cellulose nanocrystals, Polyanionic cellulose, Bentonite, Water-based drilling fluids, Rheology, Filtration. 2 ACS Paragon Plus Environment

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INTRODUCTION Bentonite (BT) is an aluminum phyllosilicate, primarily consisting of clay mineral montmorillonite (MMT). Structurally, MMT contains thin platelets with thickness less than 1 nm, and each platelet has a central aluminum octahedral layer covered by two silicon tetrahedral layers via Van der Waals forces. Two kinds of BT exist in nature, including sodium BT and calcium BT. Sodium BT has the ability to swell several times of its original volume in an aqueous solution. Calcium BT has non-swelling ability and forms colloids rapidly in an aqueous solution. Due to the intrinsically appealing characteristics, such as nano-scale dimension, large surface area, high aspect ratio and modulus, unique swelling ability, great ion-exchange capacity, and unusual rheological properties, sodium BT has been widely used in various industries as an additive for hydrogels,1 heavy metal absorbent,2 polymer composites,3 and drilling fluids.4-6 Drilling fluids are typically thixotropic shear-thinning fluids, which have low viscosity and good pumpability at high shear rates, but have high viscosity at low shear rates.7,8 There are mainly three types of drilling fluids, including synthetic drilling fluids (SDFs), oil-based drilling fluids (ODFs), and water-based drilling fluids (WDFs). WDFs are widely used, because they are cheaper than the other types of fluids and easy to prepare. WDFs are often a mixture of water, BT, rheology thickener, filtration control agent, and weighting materials. Drilling fluids, such as WDFs play an important role in the oil well drilling operation, including cleaning wellbore, carrying and suspending cuttings, cooling and lubricating drilling tools, and maintaining stability of the wellbore and formation.9-13 The transporting capacity of cuttings of WDFs is directly related to their rheological properties. Generally, WDFs with higher viscosity and yield stress have a superior transporting capacity. The stability of a wellbore is strongly affected by the filtration properties of WDFs. The invasion of water into porous formation weakens wellbore 3 ACS Paragon Plus Environment


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stability and causes serious problems, such as formation collapse and pipe stuck.13,14 Therefore, rheological and filtration properties of WDFs are two important aspects to ensure the safety and success of oil well drilling. Different natural polymers (e.g., starch,15 soy protein isolate,16 guar gum,17 xanthan gum,18 and cellulose derivative19,20), synthetic polymers (e.g., polyacrylamide,21 and polyacrylates22), and inorganic nanoparticles (e.g., graphene,13 nanosilica,23 carbon nanotube,24 and metal oxides25,26) have been applied to improve the rheological and filtration performances of WDFs. Considering economic, sustainable, and environmental effect, utilization of natural polymers as modifiers for rheology and fluid loss in WDFs is highly preferred. Cellulose nanocrystals (CNCs), as one of the most abundant, sustainable, and environmentally friendly cellulose derivatives, can be extracted from various natural cellulosic resources, such as wood, plants, marine animals, algae, and bacteria through acid hydrolysis and/or enzymatic treatments. Because of the elimination of amorphous domain in cellulosic materials, CNCs are highly crystalline and exhibit a rod or needle shape.27,28 The specific surface area of CNCs can reach up to several hundred square meters per gram, resulting in a large number of nanoparticle interactions as well as outstanding fluid properties at low concentrations. Moreover, recent investigations revealed that CNC suspensions exhibited marked thixotropic shear-thinning behavior.29-33 Therefore, CNCs are considered as an effective rheological modifier to improve the rheological performance of WDFs (i.e., more evident shear-thinning behavior and higher yield stress). Carboxymethyl cellulose (CMC), another cellulose derivative with carboxymethyl groups (CH2COOH) on the molecule backbone, is produced by alkali-catalyzed reaction of chloroacetic acid with natural cellulose.34 Polyanionic cellulose (PAC) is a water-soluble anionic cellulose ether, which is also synthesized using alkali-catalyzed method. The primary difference between 4 ACS Paragon Plus Environment

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CMC and PAC is the degree of substitution. PAC has a higher purity and degree of substitution compared with CMC.35 Besides, PAC has intrinsically outstanding characteristics, including superior resistance to heat (stable up to 150 ℃), excellent tolerance to the salt, and strong antibacterial activity.36-38 These features enable PAC to act as an effective fluid loss reducer in WDFs. Recently, Mahto and Sharma38 investigated the effect of tamarind gum and PAC on the viscosity, fluid loss and formation stability of BT-WDFs. Their results showed that the presence of both tamarind gum and PAC in BT-WDFs improved rheological properties, reduced fluid loss volume, and helped stabilize wellbore and formation. More importantly, the combined use of tamarind gum and PAC in BT-WDFs yielded more stable well formation. Therefore, it seems that the combined use of two or more additives can produce better performance than that of individual additives in BT-WDFs. Furthermore, the required performance can be also tailored by adjusting the concentration ratios of different components. Several complex WDFs with two or more additives, such as scleroglucan/xanthan gum/starch/BT,12 tamarind gum/PAC/BT,38 and xanthan gum/starch/BT,39 have been developed in recent years.

It is well known that most of WDFs are non-Newtonian fluids, and hence their rheological behaviors can be described using different empirical rheological models.4,12,26,39-44 Several modelling parameters, such as yield stress (𝜏0 ), fluid consistency (K), and flow behavior index (n) can be obtained through modelling. Specifically, yield stress (i.e., the minimum stress needed to move the fluids) is usually employed to evaluate the carrying capacity of cuttings of drilling fluids.4 Fluid consistency is a constant that presents the dependence of shear stress on the shear rate.39 A drilling fluid with a higher fluid consistency indicates that the shear stress and viscosity



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of the fluid are affected by the shear rate more significantly. Flow behavior index is the exponent of each modelling equation, which depicts the type of drilling fluid: Newtonian (n =1), nonNewtonian with a shear thinning behavior (n1).39,40,42 These parameters can provide drilling engineers more accurate information on the flow properties of drilling fluids as well as on the selection of appropriate drilling fluids, procedure, and tools for wellbore excavation, leading to the improvement in safety and efficiency of drilling operations and reduction of the excavation cost.39,40 A comprehensive investigation on rheology and filtration mechanisms of WDFs consisting of BT, CNCs and PAC was presented in this work. The specific objectives were 1) to examine the effect of the composition concentration on rheological and filtration properties of PAC/CNC/BTWDFs; 2) to understand rheological behaviors of the complex PAC/CNC/BT system using different statistical modelling technologies; and 3) to reveal the filtration mechanisms of different components in the PAC/CNC/BT-WDFs. Particularly, several WDFs consisting of varying compositions of PAC, CNCs and BT were formulated and tested for the rheological properties. Eight rheological models (i.e., Bingham-plastic, Casson, Power law, HerschelBulkley, Sisko, Heinz-Casson, Mizhari-Berk, and Robertson-Stiff models) were employed to quantify the relationship between measured shear stress and shear rate, and the goodness of fit for each model was evaluated. Standard American Petroleum Institute (API) filtration measurements were also carried out to obtain the fluid loss volume as a function of testing time as influenced by fluid composition. The filtration mechanisms were proposed and discussed based on the viscosity of fluids, and morphology and permeability of filter cakes.


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EXPERIMENTAL SECTION Materials. Two types of cellulose derivates (i.e., CNCs and PAC) were used as additives for BT-WDFs. CNCs were isolated from microfibrilated cellulose (Celish KY 100-S grade, 25% solid content, Daicel Chemical Industries, Ltd, Tokyo, Japan) using a combined preparation process of 64 wt% sulfuric acid hydrolysis and high-pressure homogenization.45 The resulatant CNCs had a needle-like morphology with an average width of 6.1±3.5 nm and length of 228.4±63.8 nm from a transmission electron microscopy (TEM) analysis (see Figure S1 in Supporting Information). Sulfuric acid-hydrolyzed CNCs carried negatively charged sulfate group on the surface. According to X-ray Photoelectron Spectroscopy (XPS) results, the mass concentration of sulfur element in sulfuric acid-hydrolyzed CNCs was 0.53 wt%, corresponding to a substitution degree of sulfate group of 2.7%.45 PAC (PAC-L, substitution degree of carboxylic group > 60%, white or tan powder, 40-55 lb/ft3) was provided by Hallibuton Company (Houston, TX, USA). The PAC particles had fiber-like morphology with width ranging from 10 to 20 𝜇𝑚 and length ranging from 100 to 200 𝜇𝑚 from a field emission scanning electron microscopy (FE-SEM) analysis (see Figure S2 in Supporting Information). Wyoming sodium BT (AQUAGEL GOLD SEAL, dry-powdered, 200 Mesh) was provided from Baroid Industrial Drilling Products (Houston, TX, USA). Formulation of PAC/CNC/BT-WDFs. To investigate the effect of the concentration of different components on the rheological and filtration properties of PAC/CNC/BT-WDFs, three groups of PAC/CNC/BT-WDFs were prepared according to the API standard (API Recommended Practice 13B-1, 2003).46 In Group I, five concentrations of BT were used, i.e., 0.0 (control), 1.0, 3.0, 4.5, and 6.0 wt%, while the concentrations of PAC and CNCs were fixed at 0.5 wt%. In Group II, four concentrations of CNCs were used, i.e., 0.0 (control), 0.1, 0.25, and 7 ACS Paragon Plus Environment


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0.5 wt%, while the concentrations of BT and PAC were fixed at 4.5 and 0.5 wt%, respectively. In Group III, four concentrations of PAC were used, i.e., 0.0 (control), 0.1, 0.25, and 0.5 wt%, while the concentrations of BT and CNCs were fixed at 4.5 and 0.5 wt%, respectively. Table 1 summarizes the formulation of PAC/CNC/BT-WDFs at various concentrations of PAC, CNCs and BT. Table 1. Formulation of PAC/CNC/BT-WDFs. Samples Group I

Group II

Group III

(wt%) BT

0

1.0

3.0

4.5

CNCs

0.5

0.5

0.5

PAC

0.5

0.5

0.5

6

4.5

4.5

4.5

4.5

4.5

4.5

4.5

4.5

0.5 0.5

0

0.1

0.25

0.5

0.5

0.5

0.5

0.5

0.5 0.5

0.5

0.5

0.5

0.5

0

0.1

0.25

0.5

Rheological property measurement. The rheological properties of the PAC/CNC/BT-WDFs were measured using a stress-controlled rheometer (AR 2000, TA Instrument, New Castle, DE, USA) equipped with a DIN concentric cylinder geometry, which consists of a rotator (28.03 mm in diameter) and a stainless steel cup (30.38 mm in diameter). Before each run, the formulated PAC/CNC/BT-WDF was vigorously agitated using a mixer for 20 min, and then about 20 mL fluid was carefully injected into the stainless steel cup using a syringe. The rheological curves were obtained by measuring the viscosity or shear stress as a function of shear rate ranging from 0.1 to 1200 s-1 at 25 ℃. Rheological modelling. The rheological data of the PAC/CNC/BT-WDFs were fitted to eight different rheological models, including Bingham-plastic, Casson, Power law, Herschel-Bulkley, 8 ACS Paragon Plus Environment

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Sisko, Heinz-Casson, Mizhari-Berk, and Robertson-Stiff models using the commercial software, Origin 8.0. Among the models, the Bingham-plastic model has the following form:47 𝜏 = 𝜏0 + 𝜇𝑝 𝛾̇

(1)

where 𝜏 is the shear stress, 𝜏0 is the yield stress, 𝜇𝑝 is the plastic viscosity, and 𝛾̇ is the shear rate. The linear model with two parameters (𝜇𝑝 and 𝜏0 ) is easy to use.43 However, for most of complex fluids, the relationship between shear stress and shear rate is nonlinear. Therefore, the Bingham-plastic model is not adequate, and its use for a complex fluid system often generates an extremely high value of 𝜏0 .40 Another common model used to describe the relationship between shear stress and shear rate is the Casson model:48 𝜏 0.5 = 𝜏0 + 𝐾𝛾̇ 0.5

(2)

where 𝜏0 is the yield stress, and K is the flow consistency coefficient. In comparison with the Bingham-plastic model, this model can predict the rheological behavior of a fluid at low shear rate more accurately. It was reported that this model fitted rheological data well for the BT suspensions.49,50 For some fluids, the plots of shear stress versus shear rate on double logarithmic coordinates are linear. Therefore, a Power-law model was developed:51 𝜏 = 𝐾𝛾̇ 𝑛

(3)

where K is the flow consistency coefficient, and n is the flow behavior index. This model has been successfully applied to describe the rheological behavior of liquid foods. However, because of the absence of 𝜏0 , this model did not fit the rheological curves well at the extremely low or 9 ACS Paragon Plus Environment


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high shear rates. Therefore, to overcome this drawback, more complex models with three or more parameters have been developed. One of the most popular and efficient three-parameter models is the Herschel-Bulkey model, which is a combination of the Bingham plastic and Power law models; and has the following form:52 𝜏 = 𝜏0 + 𝐾𝛾̇ 𝑛

(4)

where 𝜏0 is the yield stress, K is the flow consistency coefficient, and n is the flow behavior index. In comparison with the Power law model, this model includes the term 𝜏0 , and thus can yield better fitting results. It was reported that the rheological behaviors of most drilling fluids were well described using this model.4,12,26,39-42 The primary drawback of this model is that several sets of parameter values can give an equivalent fit of the rheological data (i.e., no unique fit for a given data set). Subsequently, several improved three-parameter models, i.e., the Sisko, Heinz-Casson, Robertson-Stiff, and Mizhari-Berk, were developed for more complex fluids. The Sisko model has the following form:53 𝜏 = 𝜇∞ 𝛾̇ + 𝐾𝛾̇ 𝑛

(5)

where 𝜇∞ is the viscosity at infinite shear rate, K is the flow consistency coefficient, and n is the flow behavior index. This model was originally proposed to describe the rheological properties of lubricating greases in 1958.53 The Heinz-Casson model, a modification of the Casson model, was first proposed in 1959, as given below:54 𝜏 𝑛 = 𝛾0̇ 𝑛 + 𝐾𝛾̇ 𝑛

(6)

where 𝛾0̇ is the shear rate correction factor, K is the flow consistency coefficient, and n is the flow behavior index. 10 ACS Paragon Plus Environment

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The Mizhari-Berk model was originally developed to characterize the flow behavior of orange juice in 1972, and has the following form:55 𝜏 0.5 = 𝜏𝑜 + 𝐾𝛾̇ 𝑛

(7)

where 𝜏0 is the yield stress, K is the flow consistency coefficient, and n is the flow behavior index. The Robertson-Stiff model was proposed for relating shear stress to shear rate in drilling fluids and cement slurries in 1976:56 𝜏 = 𝐾(𝛾0̇ + 𝛾̇ )𝑛

(8)

where 𝛾0̇ is the shear rate correction factor, K is the flow consistency coefficient, and n is the flow behavior index. Comparative performance of the above models was evaluated with the formulated fluids by comparing the values of derived statistical parameters, including squared correlation coefficient (R2), sum of square residuals (SSR), and root-mean-square deviation (RMSD). Filtration Property Measurement. Filtration measurements were performed at room temperature according to the API standard (API recommended Practice 13B-1, 2003).46 Test equipment and materials included a standard API filter press (Model No. 30201, Fann Ins. Co., Houston, TX), filter papers (Part No. 206051, Fann Ins. Co., Houston, TX), a timer and a graduated cylinder. Before the measurement, calibration of the filter press was carefully carried out. In each standard test, 100 mL of the WDFs were mechanically stirred for 10 min and poured into the filter cell. The filter cell was then sealed with a top cap and placed into the frame with a tightened T-screw. Subsequently, the pressure-relief value was closed and adjusted the regulator value to have a pressure of 100 psi (6.8 atm) using N2O gas chargers (Whip-it Brand, South San Francisco, CA). The timer was started immediately. The filtrate was collected using the 11 ACS Paragon Plus Environment


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graduated cylinder and its volume was recorded at 1.0, 7.5, 15, 20, 25 and 30 min to the nearest 0.1 mL. The filtrate rate was measured using the liquid flowing through already formed cake method, as reported previously.57 In detail, the test procedures included: (1) the formation of a filter cake according to the API guidelines as described above; (2) removing the remaining drilling fluid from the fluid chamber carefully without damaging the cake; (3) adding 100 mL of distilled water above the formed cake; (4) allowing the distilled water to flow through the already formed cake under 100 psi (6.8 atm) pressure; and (5) recording the volume of filtrate at each 180 s interval. Finally, the filtration rate was obtained from the slope of the fitted straight line of filtrate volume versus time. After each measurement, the remaining distilled water was carefully removed and a wet filter cake was obtained. Then, the filter cake was removed from the fluid chamber and its thickness was determined. Field emission - scanning electron microscopy (FE-SEM) analysis. The resultant filter cake was dried at room temperature for 5 days. FE-SEM (FEI QuantaTM 3D FEG dual beam SEM/FIB system, Hillsboro, OR) was used to study the surface and cross-sectional microstructure of the dried filter cake operating at an accelerating voltage of 5 kV. The samples were spin-coated with gold before observations.


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RESULTS AND DISCUSSION

Figure 1. Plots of (a) viscosity and (b) shear stress as a function of shear rate for PAC/CNC/BTWDFs at various BT concentrations. (Dash lines in (b) represent the fitted lines using the Sisko model.) Rheological properties of PAC/CNC/BT-WDFs. The plots of viscosity as a function of shear rate for PAC/CNC/BT-WDFs at different BT concentrations are depicted in Figure 1a. Without BT, the PAC/CNC suspension displayed the shear-thinning behavior, revealing the potential use of the combined PAC/CNC additives as effective rheological modifiers for drilling fluids. Specifically, three characteristic regions were observed in the rheological curve, indicating the gel-like microstructure of CNCs in PAC/CNC suspensions. It was reported that the microstructure of CNC suspensions could be isotropic, biphasic of isotropic and liquid crystalline, liquid crystalline, and gel-like, depending on the aspect ratio, surface properties and concentration of CNCs.29-33 Generally, at a high concentration, CNCs formed gel-like microstructure, displaying the observed three-region shear-thinning rheological curve. The three13 ACS Paragon Plus Environment


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region shear-thinning behavior was well maintained, until 4.5 wt% of BT was added. As shown in Figure 1a, PAC/CNC/BT-WDFs with 4.5 and 6.0 wt% of BT had nearly linear shear-thinning curves. This might be attributed to the transformation of CNC dispersion from anisotropic to isotropic as more BT was incorporated. Moreover, the presence of BT increased the viscosity and yielded more profound shear-thinning behavior, which were advantageous for drilling fluid application. For example, at the lowest shear rate of 0.1 s-1, the viscosities of PAC/CNC/BTWDFs with 0.0, 1.0, 3.0, 4.5, and 6.0 wt% of BT were 3.17, 6.37, 31.45, 77.01, and 137.70 Pa.s, respectively. Ivan et al.58 indicated that the low shear rate viscosity should be more than 50 Pa.s in order to efficiently suspend and carry cuttings. It can be seen that PAC/CNC/BT-WDFs with 4.5 and 6.0 wt% of BT met this specification well. The shear stress was strongly dependent on the shear rate and the concentration of BT (Figure 1b). The faster the shear rate, the higher the shear stress. As the concentration of BT increased from 0 to 6.0 wt%, the shear stress was gradually increased. For example, at the highest shear rate of 1200 s-1, the PAC/CNC/BT-WDFs with 0.0, 1.0, 3.0, 4.5, and 6.0 wt% of BT had shear stresses of 13.51, 20.04, 36.29, 59.35, and 86.67 Pa, respectively. The experimental data shown in Figure 1b were fitted to the eight rheological models as described in the experimental section, and the corresponding fit parameters are summarized (see Table S1 in Supporting Information). As expected, the Bingham-plastic model was impropriate to describe the rheological behavior for the complex PAC/CNC/BT-WDFs. As shown in Table S1, as the BT concentration increased from 0 to 6.0 wt%, the R2 gradually decreased from 0.9845 to 0.8798, and the SSR/RMSD values significantly increased from 6.2917/0.3966 to 1506.8/6.1376, respectively; indicating that the relationship between shear stress and shear rate was nonlinear. The Casson model seems to be well fitted for PAC/CNC/BT-WDFs at low BT concentrations (e.g., 0, and 1 wt%). However, 14 ACS Paragon Plus Environment

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at high BT concentrations (e.g., 3, 4.5 and 6 wt%), the Casson model also performed poorly with R2 values of 0.9796, 0.9702, and 0.9828, SSR values of 61.167, 210.39, and 252.75, and RMSD values of 1.2366, 2.2934, and 2.5137, respectively. In comparison with the Bingham-plastic model, the Power law model fitted better, revealing the nonlinear relationship between the shear stress and shear rate, again. However, the calculated R2 values are still lower than 0.99, and hence more complex rheological models with three parameters are needed to accurately describe the rheological behavior. Among the used three-parameter rheological models, except for the Roberston-stiff model, the other four models accurately fitted the rheological curves of PAC/CNC/BT-WDFs at all BT concentrations, which was evidenced by the values of R2 (> 0.99). The best model for describing the rheological behavior of PAC/CNC/BT-WDFs at different BT concentrations was determined according to the values of R2, SSR and RMSD. It can be seen that the rheological curves of PAC/CNC/BT-WDFs at 0.0, 1.0, 3.0 and 4.5 wt% of BT were best represented by the Sisko model with the highest R2 as well as the lowest SSR and RMSD. At 6.0 wt%, the Heinz-Casson model gave the best performance in predicting the rheological properties. It was reported that the Sisko model (a combination of Newtonian and Power law) performed well in describing the rheological behaviors of the complex fluids over the entire range of shear rate, such as Sarapar oil/glass bubbles/clay/emulsifier mixtures,41 lubricating greases,53 konjac glucomannan/styrene-acrylic

emulsion,59

and

laterite/gypsum/titani-um

dioxide/silica

flour slurries.60 The present study showed that the rheological behavior of PAC/CNC/BT-WDFs was best fitted by the Sisko model, demonstrating that the PAC/CNC/BT-WDFs had the similar rheological pattern as the above mentioned fluids, which consisted of a Power law region at low and intermediate shear rate region and an infinite plateau at high shear rate region. More useful information could be obtained from the fitted parameters (i.e., 𝜏0 , K, and n). As shown in Table



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S1, the 𝜏0 , K, and n values were strongly dependent on the concentration of BT. With the increase in the BT concentration, the 𝜏0 gradually increased. The improvement in the 𝜏0 benefits the cutting transportation. However, since the 𝜏0 also infers the minimum stress needed to move the drilling fluids, if the 𝜏0 is extremely high, much energy is required to pump the fluid down to the wellbore.35,37 Concerning the energy effect, 4.5 wt% was chosen as the optimum concentration of BT for PAC/CNC/BT-WDFs. The K value consistently increased as the BT concentration increased, suggesting that PAC/CNC/BT-WDFs at higher BT concentrations had a stronger dependence of shear stress on the shear rate than those at lower BT concentrations. On the contrary, the n value constantly decreased with the increase in the BT concentration, indicating that more significant pseudoplastic shear-thinning fluids were achieved with increase in the BT concentration.

Figure 2. Plots of (a) viscosity and (b) shear stress as a function of shear rate for PAC/CNC/BTWDFs at various CNC concentrations. (Dash lines in panel (b) represent the fitted lines using the Sisko model.) 16 ACS Paragon Plus Environment

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Figure 2a shows the plots of viscosity as a function of shear rate for PAC/CNC/BT-WDFs at different CNC concentrations. The absence of CNCs yielded a unique shear-thinning curve with two regions. In the first region, the viscosity decreased linearly as the shear rate increased from 0.1 to 100 s-1. In the second region, an evident plateau was observed as the shear rate increased from 100 to 1200 s-1. This phenomenon indicated that the vital role of CNCs on the enhancement in the viscosity of the PAC/CNC/BT-WDFs. Without the use of CNCs, the surface interaction among PAC, BT and water molecules was relatively poor. As a result, the viscosity quickly reached the plateau at a much lower shear rate. The incorporation of CNCs from 0.1 to 0.5 wt% gradually changed the pattern of the rheological curves. As shown in Figure 2a, the PAC/CNC/BT-WDFs with 0.1 wt% of CNCs displayed a similar shear-thinning curve as the control, whereas the PAC/CNC/BT-WDFs with 0.25 and 0.5 wt% of CNCs exhibited nearly linear shear-thinning curves due to the enhanced surface interaction. Furthermore, the viscosity of the PAC/CNC/BT-WDFs also increased with the increase in the CNC concentrations. For example, at the lowest shear rate of 0.1 s-1, the viscosities of the PAC/CNC/BT-WDFs increased from 0.85 Pa.s at 0 wt% of CNC to 3.79, 22.22, and 77.01 Pa.s at 0.1, 0.25 and 0.5 wt% of CNCs, respectively. The shear stress of PAC/CNC/BT-WDFs increased with increase in the CNC concentration (Figure 2b). For example, at the highest shear rate of 1200 s-1, the shear stress of PAC/CNC/BTWDFs increased from 18.95 Pa at 0 wt% of CNCs to 26.07, 40.01, and 59.35 Pa at 0.1, 0.25 and 0.5 wt% of CNCs, respectively. The corresponding model parameters are summarized (see Table S2 in Supporting Information). Interestingly, at low concentrations of CNCs (e.g., 0.0 and 0.1 wt%), except for Bingham-plastic model, the other seven rheological models gave well prediction in the rheological data of PAC/CNC/BT-WDFs, which was evidenced by the values of 17 ACS Paragon Plus Environment


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R2 (> 0.99), SSR (< 1) and RMSD (< 0.2). As the concentration of CNCs increased to 0.25 wt%, the Bingham-plastic and Power law models poorly performed in describing the fluid rheological behaviors. With further increase in the concentration of CNCs to 0.5 wt%, the Bingham-plastic, Casson, Power law, and Robertson-Stiff models became inappropriate, and the best model to describe the rheological behavior was the Sisko model. Furthermore, the concentration of CNCs also had a significant impact on the rheological parameters (i.e., 𝜏0 , K, and n). As shown in Table S2, with the increase in the CNC concentration, both the 𝜏0 and K values gradually increased; while the n value constantly decreased. These results are similar to the effect of BT on the rheological parameters, as discussed previously.

Figure 3. Plots of (a) viscosity and (b) shear stress as a function of shear rate for PAC/CNC/BT at various PAC concentrations. (Each dash line in (b) represents the fitted line using the Sisko model).


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Figure 3a shows the plots of viscosity as a function of shear rate for PAC/CNC/BT-WDFs at different PAC concentrations. The PAC/CNC/BT-WDFs exhibited nearly linear shear-thinning rheological curves at all concentrations of PAC used. The viscosity of PAC/CNC/BT-WDFs slightly increased with the increase in the PAC concentration. For example, at the lowest shear rate of 0.1 s-1, the PAC/CNC/BT-WDFs with 0.0, 0.1, 0.25, and 0.5 wt% of PAC had viscosities of 23.8, 24.54, 44.99, and 77.01 Pa.s, respectively. In comparison with CNCs (Figure 2a), PAC had less influence on the viscosity of PAC/CNC/BT-WDFs. The addition of PAC also increased the shear stresses of PAC/CNC/BT-WDFs (Figure 3b). For example, at the highest shear rate of 1200 s-1, the shear stresses of PAC/CNC/BT-WDFs with 0.1, 0.25 and 0.5 wt% of PAC were 25.31, 31.31, 42.43, and 59.35 Pa, respectively. The corresponding model parameters obtained are summarized (see Table S3 in Supporting Information). The Bingham-plastic, Casson, and Power law models are not suitable to describe the rheological of PAC/CNCs/BT-WDFs at any concentration of PAC, even for the control. These phenomena suggested that the complex rheological behavior of PAC/CNC/BT-WDFs was mainly resulted from the CNCs. Among these eight rheological models, the Sisko model also gave the best fitting the rheological behavior of PAC/CNC/BT-WDFs at all PAC concentrations. In terms of rheological parameters, the results in Table S3 revealed that higher concentrations of PAC also gave higher 𝜏0 and K. However, the n value less dependent on the concentration of PAC. For example, the n values derived from the Sisko model are in the range of 0.15 and 0.20. This finding indicated that the PAC/CNC/BT-WDFs well followed the shear-thinning behavior, regardless of the concentration of PAC; and the degree of shear-thinning behavior was not governed by the concentration of PAC. Moreover, in comparison with CNCs, PAC had less



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effect on the rheological parameters (i.e., 𝜏0 , K, and n), confirming the more effective role of CNCs in improving rheological properties of PAC/CNC/BT-WDFs.

Figure 4. API fluid loss volume versus time for PAC/CNC/BT-WDFs at various (a) BT, (b) CNC, and (c) PAC concentrations. Filtration property of PAC/CNC/BT-WDFs. The API fluid loss tests were carried out in order to measure the filtration property of PAC/CNC/BT-WDFs at various BT, CNC, and PAC concentrations. For all samples, the received filtrate was colorless and had very low viscosity (approximate 1 cP); indicating that the main component in the filtrate was water. Furthermore, in the first 5 min, the filtration rate was large. As time increased, the filtration rate gradually declined. The effect of BT concentration on the filtration property of PAC/CNC/BT-WDFs is shown in Figure 4a. It is worth noting that in the absence of BT, no filter cake was created in the filter paper. Consequently, all water rapidly flowed through within 30 seconds (not shown in Figure 4a). However, when the BT was added, the fluid loss volume was remarkably reduced due to the formation of thin and compact BT filter cake, which covered or even plugged the pores of filter paper, closing the fluid penetration channels. For example, the PAC/CNC/BTWDFs with 1.0, 3.0, 4.5, and 6.0 wt% of BT had the API fluid loss of 15.5, 11.2, 9.5, 7.8 mL/30 20 ACS Paragon Plus Environment

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min, respectively. Therefore, BT played an important role in preventing the fluid loss by the formation of high barrier filter cakes. Figure 4b shows the effect of CNC concentration on the filtration property of PAC/CNC/BT-WDFs. The PAC/BT-WDFs had very low API fluid loss of 9.8 mL/30 min, exhibiting the superior filtration property even in absence of CNCs. The incorporation of CNCs had no significant influence on the API fluid loss. As shown in Figure 4b, the PAC/CNC/BT-WDFs with 0.1, 0.25, and 0.5 wt% of CNCs had the API fluid loss of 9.9, 9.8, and 9.5 mL/30 min, respectively. The API fluid loss was independent of the CNC concentration, indicating that CNCs had little effect on the filtration property. By contrast, our previous study reported that pure CNCs were capable of improving the filtration property of CNC/BT-WDFs.4 The reason for the observed difference might be attributed to the use of 0.5 wt% of PAC in the present system. The contribution of PAC to the filtration property was so dominant that the contribution of CNCs to the filtration property was largely decreased. Nevertheless, when 0.5 wt% of CNCs was incorporated, the minimum API fluid loss (9.5 mL/30 min) was obtained, which was much less than the API recommended value (15.0 mL) for WDFs.61 The effect of PAC concentration on the fluid loss of PAC/CNC/BT-WDFs is shown in Figure 4c. Without the addition of PAC, the API fluid loss volume of CNC/BT-WDFs was as high as 18.4 mL/30 min, which was nearly 2 times of that of PAC/BT-WDFs (9.5 mL/30 min), further confirming that PAC performed more effective role in improving the filtration property. The PAC/CNC/BTWDFs with 0,0, 0.1, 0.25, and 0.5 wt% of PAC had the API fluid loss values of 18.4, 13.1, 11.3 and 9.5 mL/30 min, respectively; showing a good declining trend with increase in the PAC concentration. Discussion. Rheological measurements showed that BT, CNCs and PAC exhibited positive influence on the rheological properties of PAC/CNC/BT-WDFs. As known, BT is composed of 21 ACS Paragon Plus Environment


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superimposed thin platelets, having one central aluminum octahedral sheet sandwiching by two silicon tetrahedral sheets. The BT platelets had a permanent negative charge on the flat surface, but pH-dependent charge on the edge.25,62 In an aqueous suspension, BT platelets were linked together via several attractions, i.e., face-to-face, edge-to-edge, and edge-to-face. The electrostatic attraction between the positively charged edge and negatively charged flat surface (edge-to-face) formed a typical “house of card” structure, which was responsible for the rheological properties of BT suspension. With the increase in the BT concentration, much denser “house of card” structure was generated, resulting in higher viscosity and shear stress (Figure 1). CNCs and PAC are negatively charged due to the presence of sulfate and carboxylic groups, respectively. Thus, when they were mixed with BT in an aqueous solution, they were absorbed on the positively charged edge of BT platelets via the formation of electrostatic attraction. 4,19,38,63 Another possible surface interaction between CNCs/PAC and BT is hydrogen bond, since both of them contain a large number of hydroxyl groups on the surface. As a result, as more CNCs and PAC were incorporated into the BT suspension, much stronger surface interaction was created in the system, leading to a higher resistance of flow upon the shear force, reflecting the continuously increased viscosity and shear stress (Figures 2 and 3). At the same concentrations, CNCs had superior ability to improve the rheological properties over PAC. This might be ascribed to the different characteristics between CNCs and PAC, e.g., substitution degree, functional group, crystallinity, gel-forming ability, dimension and flexibility. Interestingly, although CNCs had much lower substitution degree than PAC, CNCs still had a profound effect on the rheological properties. It needs to be pointed out that the intrinsic features of CNCs, e.g., highly crystalline structure, low flexibility, strong gel-forming ability, nanoscale dimension, and high Young’s modulus (about 150 GPa)64 played crucial roles in enhancing the rheological


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properties. Furthermore, it is worth noting that the sulfate groups were preferentially ionized in solution in contrast to carboxylic groups, because carboxylate with pKa of 4.8 is less acidic than sulfate with pKa of -10 in solution. Thus, the electrostatic attraction between the sulfated CNCs and BT was stronger than that between the carboxylated PAC and BT in WDFs, leading to a profound influence of CNCs on the rheological properties, too. In case of the applicability of rheological models, it is concluded that the three-parameter models (e.g., Herschel-Bulkley, Sisko, Heinz-Casson, Mizhari-Berk and Robertson-Stiff) gave better fitting results than the two-parameter models (e.g., Bingham-plastic, Casson and Power law) for PAC/CNC/BT-WDFs in all formulations. These phenomena became more significant when the solid content is high. The limitations of two-parameter models, e.g., the absence of flow behavior index n for Bingham-plastic model and the absence of yield stress τ0 for Power law generally yielded the inaccurate fitting data. For example, as shown in Tables S1~S3, the Bingham-plastic model always led to the largest τ0 values as well as lowest R2 values, indicating the most inaccurate and unsatisfactory estimates. However, at the low solid content, the twoparameter models performed satisfactorily in the fitting. Considering the advantages of twoparameter models, i.e., easy usage and fast estimate,43 they can be also applied to simulate the rheological behavior of drilling fluids having low solid content and simple formulation. It is gratifying to see that the Sisko model gave the best simulation for PAC/CNC/BT-WDFs, regardless of the concentration of different components. These findings are valuable, allowing drilling engineers to predict the shear stress and viscosity at a certain shear rate for PAC/CNC/BT-WDFs in all formulations, leading to the improvement in the safety, efficiency and cost of the drilling operations using PAC and CNCs as additives.



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Filtration experimental results demonstrated that the API fluid loss of PAC/CNC/BT-WDFs remarkably decreased as the concentrations of BT and PAC increased, while CNCs had little influence on the fluid loss. According to the previous studies,9,21-23,65-71 the filtration property of drilling fluids was mainly governed by viscosity of drilling fluids and characteristics of filter cake. In general, increased fluid viscosity decelerates its filtration rate.67,68 If the viscosity is the predominant reason for the filtration property, there must be a strong correlation between the viscosity and API fluid loss. However, rheological studies (Figures 1a~3a) indicated that the viscosity of PAC/CNCs/BT-WDFs largely increased as the concentrations of BT and CNCs increased; whereas only slightly increased as the concentration of PAC increased. These trends were inconsistent with those of API fluid loss, especially for the PAC/CNC/BT-WDFs at various CNC and PAC concentrations, suggesting that the viscosity of drilling fluids had little influence on the filtration property of PAC/CNC/BT-WDFs, and the characteristics of filter cake, i.e., microstructure and permeability might be critical. FE-SEM observations were employed to visualize the change in the microstructure of filter cakes with the addition of CNCs or/and PAC. For the purpose of comparison, a filter cake from WDFs only containing 4.5 wt% BT (neat BT-WDFs) was also prepared and observed using FESEM. Figures 5a~5d show the surface morphologies of filter cakes from neat BT, CNCs/BT, PAC/BT and PAC/CNC/BT-WDFs, respectively. The presence of effective filtration control agent generally led to the occurrence of comparable differences (e. g., change in size and number of pore, and formation of polymer films as barrier)19,69-71 on the surface morphology of filter cake. Unexpectedly, by comparing Figures 5a~5d, no significant change in the surface morphologies of filter cakes was observed with the addition of CNCs or/and PAC. By contrast, in our previous study, the formation of CNC polymer films appeared on the surface of filter cake 24 ACS Paragon Plus Environment

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from CNC/BT-WDFs containing 1 wt% CNCs and 3 wt% BT.4 Such discrepancy might be ascribed to the different concentrations of CNCs and BT in WDFs. In the present study, the concentration of CNCs and BT in CNC/BT-WDFs is 0.5 and 4.5 wt%, respectively. The presence of a larger fraction of BT caused the deposition of a much thicker filter cake, strongly covering the assembled CNC polymer films. Therefore, it is highly interesting to investigate their inner microstructure.

Figure 5. Surface and cross-sectional FE-SEM morphologies of filter cakes from different WDFs: (a), (e), and (i) - neat BT-WDFs containing 4.5 wt% BT; (b), (f) and (j) - CNC/BTWDFs containing 0.5 wt% CNCs and 4.5 wt% BT; (c), (g) and (k) - PAC/BT-WDFs containing 0.5 wt% PAC and 4.5 wt% BT; and (d), (h) and (l) - PAC/CNC/BT-WDFs containing 0.5 wt%



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PAC, 0.5 wt% CNCs and 4.5 wt% BT. (a~d) – surface morphology; and (e~l) – cross-sectional morphology. Scale bar: (a~d, f~h) – 20 μm; (e) – 50 μm; and (i~l) – 5 μm. The cross-sectional morphologies of filter cakes from neat BT, CNC/BT, PAC/BT and PAC/CNC/BT-WDFs are shown in Figures 5e~5h, respectively; and their corresponding enlarged micrographs are shown in Figures 5i~5l. Gratifyingly, distinctive cross-sectional morphologies were observed. It is evident that the incorporation of CNCs or PAC created welloriented multilayer microstructures (Figures 5f, 5g, 5j and 5k) on the cross-section, which were not found on that of filter cake from neat BT-WDFs (Figures 5e and 5i). These phenomena suggested that CNCs or PAC acted as effective bridging agents. The absorption of CNCs or PAC on the surface of BT not only destroyed the “house of card” structure of BT platelets, but also effectively bridged among BT platelets.19,69 Upon the API measurements, these bridged BT platelets

layer-by-layer

deposited,

generating

the

observed

well-oriented

multilayer

microstructures on the cross-section. In addition, the formation of such multilayer microstructures greatly suppressed the penetration of fluid through filter cake, and therefore improved the filtration property. In comparison with CNC/BT-WDFs (Figures 5f and 5j), the PAC/BT-WDFs deposited more compact multilayer microstructures (Figures 5g and 5k) on the cross-section, which well interpreted our finding that PAC acted as more effective filtration control agent than CNCs. Interestingly, the further incorporation of 0.5 wt% CNCs into PAC/BT-WDFs seems to reduce the compactness of multilayer microstructure on the crosssection, as shown in Figures 5h and 5l. It is proposed that there was a competition between the absorption of CNCs and PAC onto the surface of BT. The presence of CNCs in PAC/BT-WDFs might reduce the amount of PAC absorbed on the surface of BT platelets, because CNCs had stronger surface interaction with BT platelets than PAC, as confirmed from rheological tests. 26 ACS Paragon Plus Environment

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To probe further into the filtration mechanism, the permeability (Kc) of filter cake was measured according to the Darcy’s Law:72 𝜇𝑡 𝑑𝑉

𝑐 𝐾𝑐 = ∆𝑃𝐴𝑑𝑡 =

𝜇𝑡𝑐 𝑞

(9)

∆𝑃𝐴

where 𝜇 is the viscosity of filtrate at 25 ℃ (1 cP); tc is the thickness of filter cake (cm); ∆𝑃 is the pressure difference (6.8 atm); A is the cross-section area (45.8 cm2); and 𝑞 =

𝑑𝑉 𝑑𝑡

is the filtrate

rate (cm3/s), which can be determined using the liquid flowing through already formed cake method, as described in the experimental section.

Figure 6. Filtrate rate (q) determination of the already formed filter cake from PAC/CNC/BTWDFs at various (a) BT, (b) CNC, and (c) PAC concentrations. The slope of each line indicates the filtration rate (q, cm3/s) of each filter cake. Figure 6 shows the filtrate volume through already formed filter cake from PAC/CNC/BTWDFs at various BT, CNC, and PAC concentrations as a function of time. By plotting the filtrate volume versus time, straight lines with different slopes, corresponding to the q values, were obtained. As shown in Table 2, with increase in the BT, CNC, and PAC concentrations, the q values gradually declined. In comparison with BT and PAC, CNCs showed less marked 27 ACS Paragon Plus Environment


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decreasing trend with increase in the CNC concentrations. The thickness (tc) of filter cake was also measured, as summarized in Table 2. The incorporation of BT and CNCs resulted in thicker filter cake, whereas the presence of PAC gave a little thinner filter cake. These results were in full agreement with the cross-sectional morphological results of filter cakes, which indicated that CNCs yielded less compact multilayer microstructure than PAC on the cross-section. By substituting these data into Equation 9, the Kc values were calculated (Table 2). It seems that the Kc values were independent of the BT concentration. Interestingly, the change in the CNC and PAC concentrations produced distinctive trends of Kc value. The Kc values gradually increased from 1.04×10-3 to 1.49×10-3 mD as the concentration of CNC increased from 0 to 0.5 wt%, while decreased from 3.12×10-3 to 1.49×10-3 mD as the concentration of PAC increased from 0 to 0.5 wt%. The continuous increased Kc values with increase in CNC concentration confirmed that the presence of CNCs can obstruct the absorption of PAC on the surface of BT platelets. Table 2. Filtrate rate (q), thickness (tc), and permeability (Kc) of filter cake from PAC/CNC/BTWDFs at various BT, CNC and PAC concentrations. Samples

BT concentration

(wt%)

1.0

q×10-3 (cm3/s)

5.43 3.43 2.60 2.16 3.61 3.57 3.15 2.60

4.96 3.85 3.07 2.60

tc (cm)

0.08 0.13 0.16 0.20 0.08 0.10 0.12 0.16

0.18 0.15 0.16 0.16

Kc ×10-3 (mD)

1.56 1.60 1.49 1.55 1.04 1.28 1.36 1.49

3.12 2.07 1.76 1.49

3.0

4.5

6

CNC concentration

PAC concentration

0

0

0.1

0.25 0.5

0.1

0.25 0.5

Based on the above findings, distinctive filtration mechanisms were proposed for PAC/CNC/BT-WDFs with the change in BT, CNC, and PAC concentrations. In case of


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PAC/CNC/BT-WDFs with increase in BT concentration, the increase in the viscosity was the main reason for the improved filtration property, since the permeability values were independent of the BT concentration. In case of PAC/CNC/BT-WDFs with increase in CNC concentration, the increased viscosity decelerated the filtration rate, while the increased permeability accelerated the filtration rate. These two effects might counteract each other, resulting in the little influence of CNC concentration on the filtration property. In case of PAC/CNC/BT-WDFs with increase in PAC concentration, both increased viscosity and decreased permeability yielded a synergistic effect in the reduction of the filtration rate, leading to significant development of filtration property. CONCLUSIONS The effect of the concentration of BT, CNC, and PAC on the rheological and filtration properties of PAC/CNC/BT-WDFs was investigated. Eight different rheological models were used to aid a better understanding of the rheological behaviors. The filtration mechanisms were proposed based on the viscosity, morphology and permeability investigations. On the basis of obtained results, the following conclusions were made: 1. The presence of BT and CNCs largely improved the rheological properties of PAC/CNC/BT-WDFs; whereas the effect of PAC on the rheological properties was relatively less. 2. Among the eight rheological models, the Sisko model performed the best in predicting the rheological properties of PAC/CNC/BT-WDFs, indicating that the rheological curves of PAC/CNC/BT-WDFs consisted of a Power law region at the low and intermediate shear rate, and a Newtonian region at the high shear rate.



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3. The API fluid loss of PAC/CNC/BT-WDFs remarkably decreased as the concentrations of BT and PAC increased, while CNCs had little influence on the API fluid loss of PAC/CNC/BT-WDFs. 4. There was a competition between the absorption of PAC and CNCs on the surface of BT platelets. The presence of CNCs in PAC/BT-WDFs hindered the absorption of PAC on the surface of BT platelets, leading to the reduction in the compactness and permeability of filter cake. 5. In case of PAC/CNC/BT-WDFs with increase in BT concentration, the increase in the viscosity was the main reason for the improved filtration property. For PAC/CNC/BTWDFs with increase in CNC concentration, the increased viscosity and permeability counteracted each other, resulting in the little influence of CNC concentration on the filtration property. In case of PAC/CNC/BT-WDFs with increase in PAC concentration, the increased viscosity and decreased permeability synergistically improved the filtration property. Overall, in the PAC/CNC/BT-WDFs, BT played an important role in the formation of highly compact filter cake; and CNCs and PAC acted as effective rheological modifier and filtration control agent, respectively. The combined use of CNCs and PAC in BT-WDFs not only yielded superior rheological properties, but also produced better filtration performance. It is believed that these findings give us more fundamental information and guidance in the utilization of CNCs and PAC as cheap, sustainable and environmentally friendly additives in drilling fluids, and the selection of optimum concentration of different components in order to ensure the safe, efficient, and economical drilling operations in the oil industry.


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Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: TEM micrograph of CNCs (Figure S1), FE-SEM micrograph of PAC (Figure S2), Calculated parameters for PAC/CNC/BT-WDFs at various BT concentrations using different rheological models (Table S1), Calculated parameters for PAC/CNC/BT-WDFs at various CNC concentrations using different rheological models (Table S2), Calculated parameters for PAC/CNC/BT-WDFs at various PAC concentrations using different rheological models (Table S3). AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. Phone: 225-578-8369. Fax: 225-578-4251. * E-mail: [email protected]. Phone: 731-85623301. Fax: 731-85623301. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This collaborative study is supported by the USDA National Institute of Food and Agriculture McIntire Stennis project [1000017], Louisiana Board of Regents [LEQSF-EPS (2014)-OPT-IN37, LEQSF(2015-17)-RD-B-01], Louisiana State University EDA program, Korea Forest Research Institute, and Central South University of Forestry and Technology, Changsha, China.



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For Table of Contents Use Only


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Figure 1. Plots of (a) viscosity and (b) shear stress as a function of shear rate for PAC/CNC/BT-WDFs at various BT concentrations. (Dash lines in (b) represent the fitted lines using the Sisko model.) 118x63mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2. Plots of (a) viscosity and (b) shear stress as a function of shear rate for PAC/CNC/BT-WDFs at various CNC concentrations. (Dash lines in panel (b) represent the fitted lines using the Sisko model.) 118x63mm (300 x 300 DPI)


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Figure 3. Plots of (a) viscosity and (b) shear stress as a function of shear rate for PAC/CNC/BT at various PAC concentrations. (Each dash line in (b) represents the fitted line using the Sisko model). 118x63mm (300 x 300 DPI)



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Figure 4. API fluid loss volume versus time for PAC/CNC/BT-WDFs at various (a) BT, (b) CNC, and (c) PAC concentrations. 118x40mm (300 x 300 DPI)


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Figure 5. Surface and cross-sectional FE-SEM morphologies of filter cakes from different WDFs: (a), (e), and (i) - neat BT-WDFs containing 4.5 wt% BT; (b), (f) and (j) - CNC/BT-WDFs containing 0.5 wt% CNCs and 4.5 wt% BT; (c), (g) and (k) - PAC/BT-WDFs containing 0.5 wt% PAC and 4.5 wt% BT; and (d), (h) and (l) PAC/CNC/BT-WDFs containing 0.5 wt% PAC, 0.5 wt% CNCs and 4.5 wt% BT. (a~d) – surface morphology; and (e~l) – cross-sectional morphology. Scale bar: (a~d, f~h) – 20 µm; (e) – 50 µm; and (i~l) – 5 µm. 169x116mm (300 x 300 DPI)



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Figure 6. Filtrate rate (q) determination of the already formed filter cake from PAC/CNC/BT-WDFs at various (a) BT, (b) CNC, and (c) PAC concentrations. The slope of each line indicates the filtration rate (q, cm3/s) of each filter cake. 118x40mm (300 x 300 DPI)


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Figure S1. TEM micrograph of CNCs. 84x84mm (300 x 300 DPI)



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Figure S2. FE-SEM micrograph of PAC.


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TOC 105x40mm (300 x 300 DPI)


Cellulose Nanocrystals and Polyanionic Cellulose as Additives in

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