Carboxylated cellulose nanocrystals as environmental-friendly and multi-functional additives for bentonite water-based drilling fluids under high-temperature conditions

During the oil and gas drilling engineering, the selection of drilling fluids must take account of the technical and environmental factors. This study investigated the effectiveness of carboxylated cellulose nanocrystals (denoted as CNCs) as environmentally friendly additives for improving the rheological, filtration, and inhibitive properties of water-based drilling fluids (WBDFs). CNCs used in this study were modified by carboxylation reaction, displaying small size, negative surface charge, good colloidal stability, and prominent shear-thinning behavior. Experimental results indicated that BT/CNC suspensions had superior rheological properties, low fluid loss volumes, and effective inhibition, even at 140 °C. Microstructure analysis demonstrated that CNCs could attach to the surface of BT via hydrogen bond and ionic bond. CNCs, BT, and vicinal water molecules could form a stiff gel network, which had a strong resistance to flow under shear force, leading to a significant improvement in the rheological properties. Moreover, under the differential pressure, BT/CNC suspensions formed thin and less hydrophilic filter cakes with compact layered structure, thereby efficiently decreasing the fluid loss volume. Finally, due to the gel network and filtration ability, BT/CNC suspensions performed low water activity, which was beneficial for preventing the penetration of free water into the shales and borehole well. Thus, CNCs also exerted satisfactory inhibition on hydration and dispersion of BT and shales. As a result, CNCs showed great potential to be used as efficient, multi-functional, and environmentally friendly additives in WBDFs.


Introduction
Drilling fluids are vital functional fluids in oil and gas drilling operations (Caenn and Chillingar 1996). They can cool and lubricate drilling tools, carry cuttings from the bottom of borehole to the surface, stabilize borehole, and thereby ensure the success of the drilling operations (Apaleke et al. 2012;Agwu et al. 2021). With the increasing strict technical and environmental requirements in complex formations, the selection of drilling fluids must take account of the thermal stability, salt tolerance, cost, health safety, and environmental factor, especially in shale areas (Amanullah et al. 2016). In these easy-hydrated areas, oil-based drilling fluids (OBDFs) have been often applied due to their excellent technical performance, such as good inhibition, lubricity, filtration, and hightemperature resistant (Patel et al. 2007). However, OBDFs are expensive, disadvantageous to well logging, and easy to pollute environment (Chegny et al. 2008). Therefore, high-performance WBDFs with environmental consideration are being emphatically studied to replace the OBDFs (Attia et al. 2010).
To achieve ideal properties, water-based drilling fluids (WBDFs) are usually composed of water, clay, fluid loss additive, rheology modifier, inhibitor, and other organic and inorganic additives (Yao et al. 2014;Akpan et al. 2019). Effective fluid loss additives include copolymers of acrylamide (AM), 2-Acrylamido -2-methylpropane sulfonic acid (AMPS) and cationic monomers (Liu et al. 2016;Yang et al. 2017), sulfonated phenolic resin (Sun et al. 2013;Liu et al. 2018), modified asphalt (Igwilo et al. 2020), and others. Xanthan gum (Villada et al. 2017) and polyanionic cellulose (Li et al. 2016) (PAC) are commonly used as rheological modifiers. These additives are helpful to obtain desired properties. However, few materials among these additives can simultaneously meet the demands of both technical and environmental factors. For example, these copolymer fluid loss additives might raise toxicity issues and could only be degraded at high temperatures (Aftab et al. 2020). The temperature resistance of Xanthan gum and PAC is limited with a temperature of lower than 120 °C (Zhu et al. 2021). Thus, in order to minimize the environmental hazard and health risk as well as to meet the demand of drilling engineering, developing highperformance additives with environmentally friendly, biodegradable, and renewable properties is still a continuous effort and research focus, and even that one materials can exhibit multiple functions.
Cellulose has been always favored by researches due to their wide range of sources, low cost, facile modification, and suitable performance. They have nanofibrillar structures and can form self-assembled systems in the nano and macro scale. In recent years, various cellulose nanoparticles, primarily including microfibrillated cellulose (MFC) (Kumar et al. 2021), microcrystalline cellulose (MCC) (Ventura-Cruz and Tecante 2021), and cellulose nanocrystals (CNCs) (Ng et al. 2021), have been isolated from many cellulosic resources, such as wood, plants, marine animals, algae, and bacteria using different preparation methods. MFC can be produced by mechanical heating method (Pääkkö et al. 2007;Saito et al. 2006). MFC has dimensions of 10-50 nm in width and several micrometers in length. CNCs, belonging to rod nanoparticles, can be fabricated using a strong acid hydrolysis method. Because of their nanoscale dimensions, high surface area, large aspect ratio, superior water retention ability, and self-assembling ability, these nanoparticles can form stable and viscous gels at a relatively low concentration, with good dispersion, viscosity and formability (Li et al. 2021). Thus, these nanoparticles have been commonly applied in food, coatings, cosmetics, pharmaceuticals and materials industries. These intrinsically appealing features also enable cellulose nanoparticles to act as potential effective additives in oil industry. MFC and CNCs could perform rheological regulation and fluid loss control properties at room temperatures (Li et al. 2015;Villada et al. 2021). High quality cement materials could be prepared by adding MCC or CNCs (Montes et al. 2020;Kamasamudram et al. 2021). Some modified nanocelluloses have been also expected to be an alternative for enhancing oil recovery (Combariza et al. 2021). Carboxylated cellulose nanocrystals (CNCs) are a kind of cellulose nanocrystal derivatives, which can be obtained through organic acid hydrolysis, enzymolysis, oxidation, mechanical processing, or a combination of these means (Lam and Hemraz 2021). In addition to high thermal stability and high crystallinity, CNCs possess many carboxyl groups that can promote the electrostatic repulsion between neighboring CNCs, presenting more outstanding colloidal stability. One particularly enticing feature of CNCs is the ability for these carboxyl groups to undergo further reactivity for surface modification to tailor the properties of the nanomaterial for a diverse range of applications. These advantages make a wide gateway for CNCs to be used for nano-biocomposite (Lu et al. 2019), cationic dye adsorbent (Yu et al. 2016), wastewater treatment (Fan et al. 2019), emulsion stability (Mikulcova et al. 2018), and rheological control (Li et al. 2021). In drilling fluid industry, CNC derivatives have been also used as fluid loss additives or rheological modifiers (Li et al. 2018(Li et al. , 2020a. However, few studies have investigated in detail the interaction between CNCs and clay, especially at high temperatures. In this study, CNCs were developed as novel, renewable, biodegradable, effective, and environmentally friendly additives for bentonite water-based drilling fluids (BT-WBDFs). CNCs showed multiple functions on improving the rheological, filtration, and inhibitive properties of BT-WBDFs at room and high temperatures. The interaction behaviors between CNCs and BT have been also studied in detail through measuring the colloidal stability of the fluids, observing the microstructure of BT/CNC suspensions and filter cakes, determining the wettability and water activity. Finally, the underlying mechanism of the performance improvement of BT-WBDFs was summarized.

Preparation of CNC suspensions and BT/CNC suspensions
The process of preparing CNC suspensions: CNCs (0.5 g or 1.5 g) was added in deionized water (100 ml) and the fluid was vigorously stirred at 8000 rpm for 30 min to prepare CNC suspensions.
The process of preparing BT/CNC suspensions: BT (6 g) was added in deionized water (300 ml) and vigorously stirred for 24 h to prepare BT suspensions. The BT concentration was always fixed at 2.0 wt% for all BT suspensions. Then, CNCs were added into BT suspensions and the dispersion was stirred at 8000 rpm for 30 min. The concentration of CNCs was varied at 0, 0.25, 0.5, and 1.0 wt%. In order to evaluate the influence of high temperatures on the property variations of BT suspensions after adding CNCs, an aging test was conducted by hot-rolling in a roller oven at a set temperature for 16 h.

Rheology analysis
The rheological properties of BT/CNC suspensions were performed by a Haake Mars rheometer (Thermo Electron Corporation, Germany) with a cone-plate model. Before each measurement, the suspensions were vigorously stirred for 20 min. Approximately 1.5 ml of the samples were carefully placed on the plate. The gap between the cone rotor and the plate is 53 μm, and the diameter of the cone rotor is 35 mm, containing a cone angle of 1.0°. All tests began with a preshearing procedure (2 min, γ = 2000.0 s −1 ). Then, the apparent viscosity as a function of shear rate from 1000.0 to 0.1 s −1 was measured at 25 °C.
The rheological model of drilling fluids has an important and direct effect on cutting carrying capability, borehole cleaning, wellbore stabilization, and subsurface safety (Liu et al. 2021). For non-Newtonian drilling fluids, Bingham, power-law, and Herschel-Bulkely models are commonly used to fit the relationship between shear stress and shear rate (Nasiri and Ashrafizadeh 2010). The Bingham model is given by Eq. (1).
where τ is the shear stress, τ 0 is the yield stress, μ PV is the plastic viscosity, and γ is the shear rate. In the Bingham plastic model, the relationship between the shear stress and shear rate is linear. This model is simple and is adequate for some simple dispersions with high solid content and uniform particles. Afterwords, a power-law model is developed, as expressed by Eq. (2).
where K is the consistency coefficient, and n is the flow behavior index. Many polymer drilling fluids conform to the power-law model. The Herschel-Bulkely model is a power-law model with a yield point, as given by Eq. (3).
Based on the shear stress and shear rate obtained from the rotating viscometer, the rheological curves of the set WBDF were fitted through a data processing system in Origin software. (1)

Filtration measurement
Fluid loss for freshly prepared BT-WBDFs and those aged for 16 h were measured according to the American Petroleum Institute (API) guidelines for drilling fluids. About 240 ml of the fluids were poured into a standard SD6 filter press (Qingdao Tongchun Petroleum Instrument Co., Ltd, China) equipped with quantitative filter paper. Afterwards, the fluids were pressed under a 0.69 MPa by using N 2 gas chargers for 30 min and the fluid loss volumes were recorded.

Linear swelling test
The inhibitive effect of CNCs on the swelling behavior of BT was evaluated by a CPZ-2dualchannel linear swelling meter (Tongchun, Qingdao). BT (5 g) was pressed into a pellet under 10 MPa pressure for 5 min. Then, the BT pellet was placed on the instrument. 20 ml filtrate from filtration tests was added to immerse the BT pellet. Finally, the swelling height with time was recorded for 24 h.

Hot-rolling recovery test
The inhibition performance of BT/CNC suspensions on the dispersion of shale cuttings at high temperatures was evaluated by hot-rolling recovery test. In this test, shale cuttings (20 g) between 6 and 10 mesh and 350 ml BT/CNC suspensions were poured into a sealed jar together. Then, the jar was hotrolled in a BGRL-5 roller furnace (Qingdao, China) at 140 °C for 16 h. After cooling to room temperature, the remaining shale cuttings were screened with a 40-mesh sieve and washed repeatedly and gently with pure water. Finally, the recovered cuttings were dried at 75 °C for 48 h and weighed. The shale recovery percentage was calculated by the following equation: where m 1 was the mass of shale cuttings before hot-rolling and m 2 was the mass of shale cuttings remained after hot-rolling.

Microstructural analysis of BT/CNC suspensions
The morphology of the surface and cross section of the filter cakes from BT/CNC suspensions was observed using a SU8010 scanning electron microscopy (SEM) at an accelerating voltage of 5.0 kV. The structure of BT in CNC suspension was observed by a JEM-2100 transmission electron microscopy (TEM) at an accelerating voltage of 10 kV.
The water contact angles of filter cakes obtained form different BT/CNC suspensions were also measured. Deionized water was dropped onto the membrane with a micro-injector and the images were captured by a JC2000C contact angle tester.
The particle size distribution of BT/CNC suspensions was conducted using a Malvern Mastersizer 2000 particle size analyzer. The concentration of all samples was approximately 0.1 g l −1 . Each of the measurements was carried out at 25 °C.
The ζ potential of BT dispersion was measured by Malvern Zetasizer Nano series. The concentration of all samples in this test was approximately 2.0 g l −1 . Each of the measurements was repeated two times and the average value is used for accuracy.
The water activity (a w ) of various inhibitor solutions was measured by HygroLab C1 at 25 °C. In this test, a 30 ml sample was added into a disposable sample cup. Each measurement was repeated two times and the average was recorded.

Results and discussion
The detailed characteristics of CNCs used in this study are summarized in Table 1, and their TEM micrographs are displayed in Fig. 1. CNCs exhibited a short rod-like morphology with a width range of 5-20 nm and length of 50-100 nm. Its aspect ratio was approximately between 2.5 and 20. After the carboxylation treatment, negatively charged carboxy groups were introduced on the surface of CNCs, which contributed to the colloidal stability under the electrostatic repulsion. The ζ potential also proved this result. CNCs performed ζ potential values of − 22.1 mV at pH 6.95 and − 31.5 mV at pH 9.16. Figure 1b showed the apparent viscosity versus shear rate for CNC suspensions at concentrations of 0.5 and 1.5 wt%. Due to the nanometer dimensions and carboxyl group, CNCs in water could form connective networks and stiff gels. So all CNC suspensions exhibited low viscosity at high shear rates, but high viscosity at low shear rates, which was commonly called as "shear-thinning" behavior . Moreover, the viscoelastic behavior of CNC suspensions was strongly affected by the mass fractions. It could be seen that the shear-thinning phenomenon became more significant as the concentration of CNCs increased from 0.5 to 1.5%. The higher viscosity at high concentration indicated a higher resistance against deformation and a better ability of recovery.
BT-WBDFs are greatly expected to be typical shear-thinning non-Newtonian fluids, which have high viscosity at low shear rates to suspend or carry cuttings from downhole, but low viscosity at high shear rates to reduce friction and assist rock breaking (Fagundes et al. 2018). CNCs were incorporated with BT suspensions to explore how profitable CNCs modified the rheological properties of BT-WBDFs. The concentrations of CNCs were varied in the range of 0.25 to 1.0 wt%, while that of BT was fixed at 2.0 wt%. Figure 2a,b showed the plots of viscosity versus shear rate for BT/CNC suspensions at different CNC concentrations and temperatures. All samples revealed a shear-thinning behavior in the whole range of shear rates. But BT and BT/CNC suspensions presented distinctive shearthinning behaviors after high temperature treatment. At room temperature, BT suspension exhibited a predictable shear-thinning, thereby being commonly used in WBDFs (Farag et al. 2019;Zou et al. 2019). The addition of CNCs effectively increased the viscosity of BT suspension and produced more remarkable shear-thinning behavior, which was conducive to optimizing drilling fluid performance. For example, at the shear rate of 1.0 s −1 , the addition of 0.25, 0.5, and 1.0 wt% CNCs into BT suspension increased the apparent viscosity values from 201.59554 to 740.77179, 1602.26233, and 4702.94775 mPa s, respectively. In comparison with the result at room temperature, the high temperature processing at 140 °C sharply decreased the viscosity of pure BT suspension, but the viscosity of BT/ CNC suspensions maintained highly values, and the shear-thinning behavior was more outstanding. Moreover, it can been seen that the shear-thinning behavior for BT/CNC suspensions at room temperature was progressive, whereas that for BT/CNC suspensions at 140 °C was nonprogressive, in which all BT/CNC suspensions displayed similar viscosity values at low shear rates, and progressive viscosity variation at high shear rates. For example, at a shear rate of 1.0 s −1 , BT/CNC suspensions with 0.25, 0.5, and 1.0 wt% of CNCs had viscosity values of 2353.05176, 2851.86499, and 3578.5708 mPa s, respectively, while at a shear rate of 119 s −1 , that were 21.98516, 48.28335, and 65.98486 mPa s, respectively.
It was postulated that CNCs fully interacted with BT particles and adsorbed onto their surfaces after aging at 140 °C. When the shear rate was low, the contribution of surface interactions on the viscosity was inapparent. At this time, BT content might play a dominant role for the viscosity, resulting in similar viscosity of BT/CNC suspensions with different concentrations of CNCs. As the shear rate increased, the surface interactions between CNCs and BT acted as crucial role in the maintenance of viscosity. As a result, the viscosity of BT/CNC suspensions increased with increasing the concentration of CNCs. Based on these observations, it could be concluded that CNCs efficiently improved the shear-thinning properties of BT suspensions, even under high-temperature conditions.
Further, the curves of shear stress versus shear rate for BT/CNC suspensions at different CNC concentrations were shown in Fig. 2c,d. Similar to the viscosity results, the shear stress also enhanced with the increase in the concentration of CNCs. The Bingham plastic, power-law and Herschel-Bulkey models were applied to fit their shear stress-shear rate curves, and the corresponding fit parameters were listed in Table 2. Whether room temperature or 140 °C, the Herschel-Bulkey model was more befitting for all shear stress-shear rate curves, compared with the Bingham plastic and power-law models, which was evidenced by the higher values of R 2 . In detail, after the addition of CNCs, the yield point values increased and the the flow behavior n index decreased with the increase of CNC concentration. The yield point indicated the stress required to start the flow of drilling fluid (Hussaini and Azar 1983), which could reflect the strength of grid structure between clay particles and CNCs (Ismail et al. 2016). The rheological modeling results further proved that CNCs had superior rheological modification ability, beneficial for transporting the cuttings and cleaning the wellbore (Liu et al. 2017).
As shown in Fig. 3, the influence of pH on the viscosity of BT/CNC suspensions was investigated. At the low shear rate, both of BT/CNC suspensions at pH 7 and 9 showed high viscosity, while the viscosity of BT/CNC suspension at pH 7 was higher than that at pH 9. It was inferred that more positively charged sites appeared on the edge of BT layers at an appropriately low pH, leading to stronger ionic interaction between CNCs and BT layers. However, when the pH was as low as 4, the viscosity of BT/CNC dispersion sharply decreased, demonstrating that the electrostatic repulsion weakened and some BT particles occurred coalescence instead of edge-face structure.
These results proved that the interaction between CNCs and BT was affected by the pH condition. Based on above measurement results, it can be speculated that there must be particular surface interactions between CNCs and BT layers. To verify the interactions and driving force between CNCs and BT, the macroscopic phenomenon and microstructures of BT/CNC suspensions were examined, respectively. As displayed in Fig. 4a,b, pure BT suspension could easily flow after being placed for 10 h, while BT/ CNC suspension with 1.0 wt% CNCs added formed a whole piece of gel, indicating that BT/CNC had higher viscosity and stress. Moreover, after shaking for 10 s, the formed BT/CNC gel could flow again. Specific variation on viscosity was shown in Fig. 4c. BT/CNC suspesnions performed quite higher viscosity at a low shear rate of 0.1 s −1 . After increasing shear rate from 0.1 to 1000 s −1 , the viscosity of BT/CNC suspensions could sharply decrease.  Furthermore, compared to that of BT suspension, the viscosity of BT/CNC suspensions could recover and reach high values quickly after the shear rate decraesed from 1000 to 0.1 s −1 . These distinctive phenomena between pure BT and BT/CNC suspensions demonstrated that CNCs had a strong gel formation capacity and a stiff network among BT layers, CNCs, and water molecules was created, which had a strong resistance to flow under shear force (Wang et al. 2018). This conclusion could also be proved through the viscoelastic properties of these suspensions. As shown in Fig. 5, the elastic (G′) and viscous (G″) moduli of BT/CNC suspensions were obviously larger than that of BT suspensions and grew gradually with inceasing the concentration of CNCs, which further proved the interaction between BT and CNCs.
The phase interactions between CNCs and BT layers were also observed by TEM (as shown in Fig. 6). BT hydrated in water and presented typical sheet-like structures (Fig. 6a). In BT/CNC suspensions, CNCs were homogeneously dispersed in BT layers and fully interacted with them ( Fig. 6b-d). The observations meant that CNCs were adsorbed onto the surface of BT particles and a stable colloidal structure was formed.
It was clear that BT was composed of a large number of plate-like crystal layers with permanent negative charges due to isomorphic substitutions on the faces and pH-dependent charges developed on the surface hydroxyls at the edges (Anderson et al. 2010). Due to the broken bonds of the octahedral Al-OH and tetrahedral Si-OH groups on the edges, The concentration of CNCs in BT suspension was 1.0 wt%, and then the fluids were diluted 10 times using deionized water some amphoteric sites also exist and variable (either positive or negative) charges can develop at the edges depending on the pH. Besides, a positive charged edge could be also created due to the exposed octahedral Al layers when the pH value is less than 9 (Tombácz and Szekeres 2006; Avena et al. 2003). In pure BT suspension, the ionic attraction between the negatively charged face and positively charged edge (edgeto-face attraction) could form a "house-of-cards" structure, which was responsible for the viscosity of BT suspensions (Li et al. 2020c). Therefore, BT was commonly used as a ideal additive in drilling fluid.
In BT/CNC suspension, CNCs would adsorb to the surface of BT layers via hydrogen bond between their hydroxyl groups, and ionic bond between the positively charged edges of BT layers and the negatively charged carboxy groups of CNCs. Meanwhile, in addition to the hydration and colloidal properties of BT itself, CNCs also had strong gel formation capacity, due to the high specific surface area and a large number of hydroxyl groups on the nanoparticle surface (Crawford et al. 2012). Then a large number of water molecules were bounded at the vicinity of BT and CNCs and a compact network was created, leading to a strong resistance to flow under shear force, even at high temperatures. Based on above interaction, a significnt improvement in the rheological properties was achieved. Filtration is one of the most important properties for drilling fluids. The penetration of fluids into the formation always causes shale swelling and wellbore collapse. Furthermore, high temperatures may also destroy the hydration of clay and induce the flocculation of BT particles. Poor filter cakes with large thickness and high porosity formed on the wall of wellbore, resulting in large fluid loss volume and poor filtration (Wang et al. 2021). As shown in Fig. 7, the API fluid loss volumes of BT and BT/CNC suspensions were measured at different temperatures. After treatment at high temperature of 140 °C, the fluid loss volume of pure 2.0 wt% BT suspension increased from 41.0 to 50.0 ml, revealing the detriment of hightemperature environments. By contrast, whether at room temperature or high temperatures, the addition of CNCs always improved the filtration performance of BT suspensions. The fluid loss volume of BT/CNC suspensions decreased as the concentration of CNCs increased, whereas the filter cakes also became thinner. For example, compared to the filter cake thickness of 1.96 mm from pure BT suspension at 140 °C, the BT/CNC suspensions with 0.25. 0.5, and 1.0% of CNCs had filter cake thickness values of 1.02, 0.64 and 0.56 mm, respectively. No matter how CNCs worked, the filter cake was the final barrier to prevent filtration. It could be inferred that more suitable structure was formed in the filter cakes after adding CNCs as fluid loss reducer.
For drilling fluids, it is always thought that filtration is mainly determined by fluid viscosity and filter cake quality (Elkatatny et al. 2011). Generally, high viscosity improves the fluid with the ability to against fluid loss (Borges et al. 2021). As analyzed in Fig. 2a,b, the viscosity increased with increaseing the concentration of CNCs. However, the viscosity of BT/CNC suspensions at low shear rates under 140 °C processing varied mildly, which was mainly related to the API fluid loss volume under static filtration condition. Therefore, increasing viscosity might be not the predominant factor affecting the filtration control of CNCs. Then, the microstructure of filter cakes was further detected by SEM observation (as shown in Fig. 8). At room temperature, it could be seen that both of the BT and BT/CNC presented satisfactory filter cakes with smooth and hydrated surface. With increasing the concentration of CNCs, the surface of filter cakes obtained from BT/CNC suspensions was much smoother and more compact, like a "film". In comparison, after aging at 140 °C, the viscosity of pure BT suspension decreased (Fig. 2b) and the obtained filter cake appeared cracks and irregular accumulation, due to the dehydration effect under high temperatures. Similar phenomena on the variation of filter cakes after adding CNCs also went at 140 °C. In order to observe the deposit structure of BT layers more clearly, the cross sections of these filter cakes were also investigated, as displayed in  Fig. 9. The section of filter cake from pure BT suspension showed a relatively clear pattern, but not orderly. After aging, layered structure of the accumulation became thick and irregular, which was related to the coalescence and settlement of BT at 140 °C. In comparsion, the sections from BT/CNC suspension with 1.0 wt% of CNCs were more regular and compact than that from pure BT, especially at high temperature. BT layers in BT/CNC deposited regularly under differential pressure and formed a clear lamellar structure, which was beneficial for producing thin and compact filter cakes.
Interestingly, we also found that the addition of CNCs improved the hydrophobicity of the filter cakes. As exhibited in Fig. 10, filter cakes from pure BT displayed strong hydrophilcity and had a water contact angle of approximate 27° at room temperature and 140 °C. Adding CNCs dramatically enhanced the water contact angle of the filter cakes. For example, at a CNC concentration of 1.0 wt%, the water contact angle of the filter cake obtained from BT/CNC suspensions was increased to 57.78 and 66.67° at room temperature and 140 °C, respectively. Performance evaluation results mentioned above showed that CNCs had a temperature resistance of 140° in BT suspension. So it was inferred that CNCs fully interacted with BT particles and more CNCs adsorbed onto the surface of BT particles after aging at 140 °C. CNCs covering the surface of BT enhanced the water contact angle.
The enhanced hydrophobic effect was favorable for preventing the invasion of water (Saparti et al. 2018). Besides, according to Wenzel's equation (Wang et al. 2015), the decrease of roughness can increase the contact angle measured on hydrophilic surface. The contact angle measurement also proved that the the addition of CNCs caused a more smooth surface on the filter cakes. In short, based on the gel network among BT layers, CNCs, and immobilized water molecules, and the adsorption of CNCs on BT surface, BT deposited on the filter paper in a regular and compact layered structure, forming a more hydrophobic and smooth CNC polymer film on the surface. Finally, a high-quality filter cake was built and the fluid loss volume was sharply decreased.
In comparison with OBDFs, one of the deficiencies of WBDFs is the insufficient inhibition. There are more water in WBDFs and the fluids easily penetrate into the formation, resulting in clay hydration, swelling, and dispersion. Therefore, efficient shale inhibitors must be added in WBDFs (Muhammed et al. 2021). The inhibition of CNCs was evaluated by linear swelling measurement and shale recovery test. As shown in Fig. 11a, the swelling height curves of all samples exhibited a similar tendency with a dramatic increase rate within the initial period and a gradual slower growth as time proceeds, which was the typical swelling behavior of BT in aqueous solution. However, there was a gradual decrease in swelling height after CNCs were added as inhibitors. In detail, after immersing in water for 24 h, the swelling height of BT pellet in pure water reached as high as 6.16 mm. In comparison, the linear swelling heights of the BT pellets in filtrate obtained from BT/CNC suspensions at a CNC concentration of 0.25, 0.5, and 2.0 wt% CNCs were 5.77, 5.23, and 4.00 mm, respectively, displaying the inhibition of BT swelling. Shale recovery test, as a standard method for evaluating the hydration dispersion of shale cuttings after hotrolling at a set temperature, was also conducted. As illustrated in Fig. 11b, after aging at 140 °C, the shale recovery value of pure BT suspension was the lowest (29.4%), which indicated the strong water sensitivity and dispersion of shales. Under the same conditions, the shale recovery values for BT/CNC suspensions with 0.25, 0.5, and 1.0 wt% of CNCs were 35.8, 48.6, and 55.25%, respectively, suggesting the capability of CNCs for inhibiting the dispersion of the shales. Moreover, the results from hot-rolling shale recovery tests also demonstrated the temperature resistance of CNCs. The findings from linear swelling tests and shale recovery experiments were consistent. CNCs could used as valid inhibitors in WBDFs.
Generally, shale inhibitors achieved efficient inhibition through decreasing the interlayer spacing, such as KCl, the oligomeric and polymeric amines with cationic groups (Anderson et al. 2010), or encapsulation effect, such as various acrylamide copolymers . Visibly, CNCs, as negatively charged solid-phase nanoparticles, might not inhibit BT by these approaches, presumably due to the interactions (gel network and adsorption) between CNCs and BT particles analyzed above, or whether there were other mechanisms. With these questions, the colloidal stability of BT was examined by measuring the ζ potential and particle size distribution. As shown in Fig. 12, the addition of CNCs decreased the ζ potential efficiently. When the concentration of CNCs was 1.0 wt%, the ζ potential of BT decreased from −23.05 mV to −32.4 mV at room temperature. The variation tendency of the ζ potential at 140 °C was similar with that at room temperature. However, the whole values of the ζ potential at 140 °C were higher, echoing the increase of fluid loss volume from 25 to 140 °C in Fig. 5. Anyway, the cooperation of CNCs could increase the negative charges of BT particles and improve their stability.
Meanwhile, the particle size distribution of BT/ CNC was also determined. As exhibited in Fig. 13, the addition of CNCs didn't increase the particle size of BT, even showing a dispersion effect at small size range. These results demonstrated that CNCs didn't inhibit the hydration and swelling of BT by encapsulating the BT particles. The ζ potential and particle size measurements indicated that CNCs could promote the stability of BT suspensions, which was contrary to the general inhibition mechanisms mentioned above. How does CNCs exert competent inhibition? In order to figure this out, the water activity of these samples were further evaluated (as listed in Table 3). Water activity is an important index in estimating shale/drilling fluid interactions, which can evaluate the hydration state of shale and its potential to adsorb or lose water. Drilling fluids with lower water activity than shale can reduce the osmotic pressure between drilling fluids and the formation fluids, finally preventing water from invading shales (Chenevert. 1970;Zhang et al. 2008). From the Table 3, the water activity of BT samples decreased gradually with increasing the concentration of CNCs from 0 to 1.0%. In fact, these results should be foreseeable. The rheological tests concluded that BT, CNCs and immobilized water molecules formed stiff gel network. This network could efficiently bound the water molecules and reduce the free water, thereby decreasing the water activity. Moreover, the filtration measurements indicated that CNCs could Fig. 12 The ζ potential of BT/CNC suspensions at different CNC concentrations after treatment at 25 °C and 140 °C contribute to creating a thin, compact, and lowhydrophilicity filter cake. The plugging and wettability modification abilities of CNCs aslo restrained the penetration of free water into the clay or shales. Under these comprehensive actions, CNCs presented the inhibition well.
In summary, the modification effect of CNCs on the properties of BT-WBDFs and the interaction mechanism were concluded and listed in Table 4, which was also clearly described by the schematic diagrams, as shown in Fig. 14. BT platelet was negatively charged on the flat surface and positively charged on the edge. These plates could be linked together via face-to-face (FF), edge-to-edge (EE), and edge-to-face (EF) attractions. The edge-toface attraction formed a "house-of-cards" structure, which was responsible for the viscosity of BT suspensions. At 140 °C, the high temperature passivation and dehydration played the dominant roles for BT suspensions, which could result in coalescence of BT particles and decrease of viscosity.When the CNCs were employed in BT suspensions, one hand, CNCs adsorbed on the surface of BT through hydrogen bond and electrostatic interaction. The BT platelet and CNCs interwove with each other and formed stable "house of cards" structure at low shear rates and dispersed structure at high shear rates. On the other hand, due to the strong gel formation capacity of CNCs, a stiff network between BT layer, CNCs, and water molecules was created. Finally, BT/CNC suspensions exhibited excellent "shear-thinning". Based on the prominent rheological properties and plugging function of CNCs, BT/CNC deposited  The addition of 1.0 wt% CNCs decreased the API fluid loss volume of BT suspension from 50.0 to 16.0 ml at 140 °C Thin and compact filter cakes with layered structure Improve the hydrophobicity of filter cakes Plugging effect of CNCs Inhibition The addition of 1.0 wt% CNCs decreased the swelling height of BT in filtrate from 6.16 to 4.00 mm at room temperature. The addition of 1.0 wt% CNCs increased the shale recovery of BT suspension from 29.40 to 55.25% at 140 °C Adsorption of CNCs on BT Stiff network between BT layer, CNCs, and water molecules Low water activity Plugging effect on the filter paper with a compact layered structure and constituted a "film" with improved hydrophobic ability, thereby resulting in a thin and compact filter cake and low filtration volume. Meanwhile, under the actions of CNCs, BT/CNC suspensions kept colloidal stability with high viscosity, displayed lower water activity, and prevented the penetration of free water, exerting competent inhibition on hydration and dispersion of clays. Moreover, compared to MFC and MCC, the crystallinity of CNCs was higher, which was associated with the total removal of amorphous non-cellulosic compounds and leaving more crystalline domains unaltered. As a result, CNCs possessed higher thermal decomposition temperature, displaying outstanding thermal stability. CNCs showed great potential to be used as efficient and environmentalfriendly rheological modifier, fluid loss agent, and clay inhibitor for BT-WBDFs at 140 °C. However, as a kind of natural materials, CNCs might degrade at a higher temperature, and antioxidants might be needed to alleviate this situation.

Conclusion
Carboxylated cellulose nanocrystals (CNCs) were selected as efficient and environmentally friendly additives to improve the rheological, filtration, and inhibitive properties of BT suspension. CNCs were modified by carboxylation reaction, displaying small size, negative surface charge, good colloidal stability, and prominent shear-thinning behavior. The experimental results indicated that BT/CNC suspensions had superior rheological properties, low fluid loss volumes, and effective inhibition, even at 140 °C. CNCs could attach to the surface of BT via hydrogen bond and ionic bond. A stiff gel network between CNCs, BT, and vicinal water molecules was observed and proved, which had a strong resistance to flow under shear force, leading to a significant improvement in the rheological properties, including the viscosity, shear force, and yield point. Moreover, under the differential pressure, the improved viscosity, the created gel network as well as the formation of CNC films remarkably reduced the fluid loss volume, forming a thin, compact, and less hydrophilic filter cake with layered structure. Finally, benefiting from the gel network and outstanding filtration, BT/CNC provided low water activity and plugging effect, which could prevent the penetration of free water into the shales and borehole wall, therefore exerting satisfactory inhibitive performance for shale formations. CNCs could be used as multi-functional, and environmentally friendly additives in BT-WBDFs. This research demonstrates the effectiveness of CNCs in enhancing the performances of BT suspensions, offering a pathway for designing a new generation of additives in drilling fluid applications.