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Article

Comparative Study of Colloidal and Rheological Behaviors of Mixed Palygorskite–Montmorillonite Clays in Freshwater and Seawater

by
Jiajun Zhang
1,
Guanzheng Zhuang
1,*,
Jinrong Chen
1,
Wenxiao Fan
1,
Jixing Fan
1,
Zhuhua Kuang
2 and
Dong Liu
3,*
1
School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou 510006, China
2
Department of Mineral Resources Survey, Guangzhou Geological Survey Institute, Guangzhou 510440, China
3
Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510460, China
*
Authors to whom correspondence should be addressed.
Minerals 2025, 15(3), 251; https://doi.org/10.3390/min15030251
Submission received: 31 January 2025 / Revised: 24 February 2025 / Accepted: 27 February 2025 / Published: 28 February 2025
(This article belongs to the Collection Clays and Other Industrial Mineral Materials)
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Abstract

:
This study systematically investigates the colloidal stability, rheological properties, and filtration behavior of palygorskite–montmorillonite mixed clays in both freshwater and seawater systems. By varying the mass content and dispersion medium (freshwater/seawater), we analyze the colloidal stability, zeta potential, flow curves, viscosity, shear-thinning behavior, thixotropy, and fluid loss of the dispersions. The results show that palygorskite exhibits good rheological performance in both freshwater and seawater, while montmorillonite performs better in freshwater but suffers a significant decline in seawater. However, palygorskite demonstrates high fluid loss, which is unfavorable for drilling fluid function. Mixed clays can mitigate the limitations of individual clays to some extent, but the specific performance depends on the clay mineral content and dispersion medium. In freshwater, a small amount of montmorillonite improves the viscosity and shear-thinning behavior of the dispersion, with optimal montmorillonite contents of 22% and 38%, respectively. The thixotropy and fluid loss reduction in the mixed clays are positively correlated with montmorillonite content. In seawater, the rheological performance inversely correlates with Mt content due to montmorillonite’s high sensitivity to electrolytes. The addition of Pal enhances the colloidal stability and rheological properties of the mixed clays in seawater. This work provides theoretical insights into the behavior of mixed clays in different media, offering valuable guidance for the design of seawater-based drilling fluids.

1. Introduction

Clay minerals are cost-effective, high-performance, and environmentally friendly colloidal materials with significant applications in industries such as coatings, biomedicine, and drilling fluids [1,2,3,4]. Among them, montmorillonite and palygorskite, due to their unique nanostructures and colloidal properties, have become essential rheological additives in water-based drilling fluids [5,6,7,8]. These minerals improve drilling efficiency and cuttings transport capacity while reducing costs and safety risks by modifying the viscosity, yield stress, thixotropy, shear-thinning behavior, and filtration properties of drilling fluids [9,10]. With the expanding exploration of oil and gas resources in deep and offshore environments, developing clay mineral-based rheological additives for drilling fluids is becoming increasingly important. Particularly, the colloidal and rheological behaviors of clay minerals in both freshwater and seawater systems have received growing attention [11].
Montmorillonite is a typical 2:1-type layered clay mineral, with its crystal unit consisting of two [SiO4] tetrahedral sheets sandwiching one [AlO6] octahedral sheet [12]. Due to the isomorphic substitution of Al3+ in the octahedral sheet by lower-valent ions such as Mg2+ and Fe2+, montmorillonite’s crystal structure generates permanent negative charges (layer charge range of 0.2–0.6) [12], which are balanced by exchangeable cations (e.g., Na+, Ca2+) in the interlayer space. This gives montmorillonite a high cation exchange capacity (CEC, usually 60–140 meq/100 g) [13]. The charge characteristics and weak van der Waals forces between the layers result in significant water absorption and swelling, with the basal spacing expanding from about 1 nm to several nanometers and even complete exfoliation, forming a stable colloidal dispersion [14,15,16,17]. Montmorillonite particles can form a “house-of-cards” network structure through electrostatic repulsion between negative faces and attraction between the negative surface and positive edges, which enhances the system’s viscosity and yield stress while exhibiting thixotropy and shear-thinning behavior [18,19,20]. Since the colloidal behavior of montmorillonite is charge-dependent, its colloidal and rheological properties are sensitive to ionic strength and pH conditions [21,22,23,24].
Palygorskite (also known as attapulgite) is a magnesium–aluminum silicate mineral with a crystal structure consisting of alternating continuous tetrahedral silica layers and magnesium (or aluminum) octahedral bands, forming a unique one-dimensional nanopore system [25,26]. The tetrahedral layers extend in bands, and the octahedral bands are centered around Mg2+; or Al3+, coordinated with oxygen or hydroxyl groups. Some Mg2+ ions are replaced by Al3+, resulting in smaller permanent negative charges (layer charge < 0.12 [27]), which are balanced by exchangeable cations (e.g., Na+, Ca2+) within the pores, giving it a low cation exchange capacity (typically less than 25 meq/100 g [28]). Palygorskite can form a three-dimensional network structure in water, exhibiting thixotropy and shear-thinning behavior [29,30]. Compared to montmorillonite, the rheological behavior of palygorskite is less influenced by electrolytes due to its lower surface charge density [31]. Therefore, palygorskite can be used in systems containing salts, such as brine- or seawater-based drilling fluids [32,33]. However, palygorskite is typically used in combination with montmorillonite rather than alone because palygorskite exhibits inferior filtration properties and is more expensive than montmorillonite [34]. Additionally, small amounts of montmorillonite are often present in natural palygorskite clay. Such clays are actually a mixture of the main mineral components of palygorskite and montmorillonite.
Therefore, it is essential to understand the colloidal and rheological behavior of mixed palygorskite–montmorillonite clays in aqueous systems. Although extensive research has been conducted on these individual clay minerals’ colloidal properties and rheological performance [29,35,36,37,38,39], studies on their mixed systems in different aqueous environments are still limited. However, the colloidal and rheological behavior of palygorskite–montmorillonite mixtures in different dispersion systems is not yet fully understood, especially in seawater systems, where the presence of salt ions may alter the hydration and electrical double layer of the minerals, thus affecting the rheological performance of the mixed clays. Neaman and Singer [40] reported that the incorporation of montmorillonite in a palygorskite dispersion in fresh water, at a total concentration of 3 wt.%, significantly affects the viscosity and yield stress of the dispersion. Later investigations confirmed that the viscosity and yield stress of mixed palygorskite–montmorillonite clay dispersions depend on the proportion of montmorillonite and the layer charge and charge localization [41]. A recent investigation has shown that the viscosity of a freshwater dispersion of palygorskite and montmorillonite with a mass ratio of 9:1 was affected by pH conditions and was maximally enhanced at pH = 13 [42]. However, these studies mostly focused on the rheological parameters, lacking discussions on flow behavior, thixotropy, shear thinning, filtration properties, and their relationship with colloidal properties. Moreover, the colloidal and rheological behavior of palygorskite–montmorillonite mixtures in seawater systems is not yet fully understood. In such a system, the presence of salt ions may alter the hydration and electrical double layer of the minerals, thus affecting the rheological performance of the mixed clays.
To address the limitations of existing research, this study systematically investigates the colloidal stability, rheological behavior, and filtration properties of mixed palygorskite–montmorillonite systems in freshwater and seawater media. By adjusting the mineral ratio and dispersion medium, the study aims to reveal the mechanisms by which salt ions affect mineral hydration, particle aggregation, and network formation. The results will provide theoretical support for the design of high-performance water-based drilling fluid formulations.

2. Materials and Methods

2.1. Materials

Palygorskite clay, marked as Pal, was provided by Active Mineral International LCC. The montmorillonite sample (marked as Mt) was collected from Kazuo, Liaoning, China. The X-ray diffraction (XRD) patterns of these two samples are presented in Figure 1a,b. The mineralogical compositions of these two clays were calculated based on the XRD patterns using simple quantitative calculations. The results are listed in Table 1 and Table 2. Although a small amount of montmorillonite was included in Pal, this did not substantially affect the results since the mixed clay is mainly composed of palygorskite and montmorillonite. However, the actual percentage of montmorillonite in mixed clays should be slightly higher than the experimental value. The cation exchange capacities (CECs) of Pal and Mt were recorded as 21 and 102 meq/100 g, respectively, using the saturated NH4Cl method in the lab. The soluble salt contents of Pal and Mt were 1.59 g/kg and 3.32 g/kg, respectively. Transmission electron microscopy (TEM) images show that Pal is in the form of nanofibers with fiber lengths around 1 μm (Figure 1c), whereas Mt is in a flexible layered structure (Figure 1d). Artificial seawater was prepared according to a simplified version of the ASTM D1141-98 standard [43]. Specifically, 24.53 g of sodium chloride (NaCl), 11.11 g of magnesium chloride hexahydrate (MgCl2·6H2O), 4.09 g of sodium sulfate (Na2SO4), 1.16 g of calcium chloride (CaCl2), and 0.70 g of potassium chloride (KCl) were added per liter of pure water. All chemicals used were of analytical grade and were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. The utilization of artificial seawater overcomes the disparities in the composition of natural seawater attributable to geographic location and depth, thereby facilitating replication or comparison in subsequent studies.

2.2. Preparation of Clay Dispersions

While it is acknowledged that real drilling fluids involve additional components and more complex interactions, we selected this simplified system to isolate the effects of the clays and the medium on their fundamental properties. This approach allows us to better understand the intrinsic behavior of clay minerals before considering the influence of other additives, such as polymers, surfactants, or weighting agents. A similar methodology can also be found in previous reports [44,45,46,47]. The procedure for the preparation of the clay dispersions, i.e., simplified drilling fluids, followed the American Petroleum Institute standard [48]. The dispersions were obtained by adding 20 g of clay to 400 mL of pure water or seawater and stirring at 10,000 rpm for 20 min. The clay samples were selected using the mass ratio of Pal to Mt, i.e., 10:0, 9:1, 7:3, 5:5, 3:7, 1:9, and 0:10. The mass percentages of palygorskite and montmorillonite in the mixed clay samples are presented in Table 3.

2.3. Characterization

X-ray diffraction (XRD) patterns were collected using a Rigaku MiniFlex 600-type X-ray diffractometer (Cu Kα, λ = 0.15046 nm) operating at 40 kV and 50 mA, with a scanning rate of 4°/min in the range of 3–70°. TEM was performed on an FEI Tecnai Spirit G2 F30 transmission electron microscope with a voltage of 120 kV. The samples were dispersed in ethanol using ultrasound for 30 min; then, the dispersions were dropped on copper supports with a carbon grid. The zeta potential of the sample was analyzed using a Malvern Zetasizer Nano ZS analyzer. The samples used for the zeta potential measurements were diluted to a concentration of 0.1 g/100 mL clay sample. Each sample was measured three times, and the average value was calculated as the final result. The gel volume was obtained by adding fresh clay dispersion to a 100 mL graduated cylinder with a stopper, and the gel volume was recorded after standing for 24 h. The rheological properties of the clay dispersions were determined using a Thermo Scientific Haake Mars iQ rotational rheometer with a C60/Ti rotor at 25 °C. The rheological behaviors were measured by applying an initial up–down cycle, which includes three consecutive steps: (1) shearing with the rate increasing from 0.1 s−1 to 1000 s−1 within 100 s (up step), (2) shearing at 1000 s−1 for 30 s, and (3) shearing with the rate decreasing from 1000 s−1 to 0.1 s−1 within 100 s (down step). Similar analyses can be found in previous reports [49,50,51,52]. The fluid loss test procedure follows the API SPEC 13A standards. This method involves measuring the amount of liquid (filtrate) that passes through a filter cake formed by drilling fluid under controlled conditions. A fluid sample is placed in a filter cell, and pressure (0.69 MPa) is applied to force the fluid through the filter paper for 30 min. The amount of filtrate collected is measured in milliliters. This test helps assess the fluid’s ability to form a proper filter cake and control seepage into the formation, which is crucial for wellbore stability and minimizing formation damage.

3. Results and Discussions

3.1. Colloidal Properties

The zeta potential and gel volume of clay samples in pure water and seawater systems are shown in Figure 2. In the pure water system, the zeta potentials of Pal and Mt are −16.2 mV and −33.8 mV, respectively (Figure 2a), which is consistent with reported values in the literature [53,54,55,56,57]. Generally, the surface charge density of palygorskite is significantly lower than that of montmorillonite, with the former having a layer charge, typically less than 0.12 [27], while the latter usually falls in the range of 0.2–0.6 [12]. The difference in surface charge densities between palygorskite and montmorillonite can primarily be attributed to the higher proportion of isomorphous substitution in the montmorillonite structure. In the mixed clay samples, as the mass ratio of Mt increases (Pal decreases), the absolute value of the zeta potential shows a gradual increase. This phenomenon can be attributed to the superimposition of the zeta potentials of palygorskite and montmorillonite [30]. However, the relationship between zeta potential and clay mineral contents is not strictly linear. This phenomenon suggests that the zeta potential of the mixed clay is not a simple average value of both components. Two factors may be considered: (1) the overlap of the electric fields of palygorskite and montmorillonite particles, and (2) the inhomogeneity of the surface charges of palygorskite and montmorillonite, which differ significantly in their morphology, size, and charge density.
In the seawater system, the trend in zeta potential changes is generally similar to that in the pure water system, but the zeta potentials are much lower (Figure 2b). Specifically, the zeta potential of Pal in seawater decreases to −2.1 mV, while that of Mt is only −11.1 mV. The zeta potential of the mixed clay samples in seawater also decreases accordingly. This change suggests that the ions in seawater significantly influence the surface charge properties of clay minerals. Since clay minerals typically carry a negative surface charge, the cations in seawater (such as Na+ and Mg2+) reduce the surface potential of the mineral particles through the charge screening effect, thus enhancing particle aggregation [29,58].
Regarding gel volume, in the pure water system, the volume remains almost constant at 100 mL regardless of the clay mineral compositions (Figure 2c). This indicates that the colloidal system is relatively stable in pure water, which is consistent with the zeta potential results. Generally, when the zeta potential is greater than 30 mV, the dispersion system is thermodynamically more stable. Although the zeta potential of Pal is only −16.2 mV, below the critical value of −30 mV, its nanoscale diameter allows it to maintain good dispersion stability. However, in the seawater system, as the montmorillonite content increases (Pal content decreases), a significant decrease in gel volume is observed (Figure 2d). The volume drops from 100 mL (14% of montmorillonite) to 26 mL (100% of montmorillonite). This phenomenon suggests that the stability of the montmorillonite dispersion is highly sensitive to electrolytes. In seawater, as the montmorillonite content increases, the electrostatic attraction between colloidal particles strengthens, leading to aggregation or precipitation, which reduces the gel volume.
The above results demonstrate that the mass ratio of palygorskite to montmorillonite has a significant effect on the colloidal properties of the mixtures in different solution systems. In pure water, the mixed clays exhibit good colloidal stability despite the decrease in negative surface charge. In seawater, the increased ionic strength has a pronounced effect on the surface charge and gel volume of montmorillonite, while the effect on palygorskite is less pronounced. As the montmorillonite proportion increases (palygorskite proportion decreases) for the mixed clays, the colloidal particles exhibit stronger aggregation and smaller gel volumes. This is due to the severe compression of the electrical double layer of montmorillonite, which decreases the electrostatic repulsion between montmorillonite particles. In fact, this colloidal behavior in the mixed clays arises from the different network formation mechanisms of palygorskite and montmorillonite. Montmorillonite forms a three-dimensional “house of cards” network structure through electrostatic attractions between its negative surface charges and positive charges on the edges [59,60,61,62]. Although the disordered cross-configuration of palygorskite fibers is considered to arise from a combination of electrostatic and van der Waals forces, the network structure of palygorskite is less affected by electrolytes due to its lower surface charge density [29,42]. Therefore, although ions influence the colloidal and rheological properties of mixed clay dispersions in seawater, the nature and proportion of Mt are more crucial in determining the colloidal and rheological behavior. This phenomenon provides important experimental data for the field of colloidal chemistry, particularly in dispersions containing different clay minerals, where the ionic strength of the solution and the interactions between minerals must be considered.

3.2. Rheological Behavior

The flow curves of the clay dispersions are shown in Figure 3. The flow curves of all dispersions in the high shear rate range (300–1000 s−1) approximate straight lines without passing through the origin, indicating Bingham plastic behavior. In the low shear rate range (0–300 s−1), a variation toward smaller shear stress is observed, indicating a tendency toward pseudoplastic behavior. These flow behaviors are similar to previous records [29,40,63]. In a pure water system, the shear stress values along these curves show that the pure Pal dispersion (14% of montmorillonite and 86% of palygorskite) exhibits a higher value than pure Mt dispersion (100% of montmorillonite), suggesting a higher viscosity of Pal than Mt. In addition, the shear stress value of Pal in seawater is much larger than that of Mt, confirming the better salt resistance of palygorskite than montmorillonite.
The viscosity results of all dispersions sheared at 1000 s−1 for 30 s are summarized in Figure 4. Pal (14% of montmorillonite and 86% of palygorskite) shows a viscosity of 11.6 mPa·s in a pure water system, while Mt (100% of montmorillonite) exhibits a value of 8.4 mPa·s. For mixed clays, the addition of montmorillonite increases the viscosity, and the viscosity reaches its maximum when the montmorillonite proportion increases to 22% (Pal:Mt = 9:1), similar to the previous report [40]. As the proportion of Mt increases to more than 22%, the viscosity of the mixed clay dispersion gradually decreases. When the montmorillonite proportion increases to more than 55%, the viscosity is smaller than Pal and approaches that of pure Mt dispersions. This phenomenon demonstrates that the proportion of montmorillonite in the mixed clay below 55% will contribute to the viscosity of the system, while when the proportion of montmorillonite is higher than 55%, the viscosity is essentially equivalent to that of pure Mt. However, in seawater systems, the situation changes. Compared to pure water dispersions, the viscosities of both Pal and Mt in seawater dispersions decrease to 9.4 and 1.5 mPa·s, respectively. The mixed clays’ viscosities decrease with the increase in montmorillonite proportion. The experimental findings indicate that Pal exhibits optimal viscosity in seawater. The mixed clay is found to be contingent upon the proportion of montmorillonite. It is evident that as the proportion of montmorillonite increases, the viscosity of mixed clay in seawater experiences a reduction.
The flow curves also indicate that the clay dispersions exhibit shear-thinning behavior. The shear-thinning index, which is defined as the ratio of viscosity at shear rates of 100 and 1000 s−1, is employed to characterize the shear-thinning property (see Figure 5). Pal exhibits a shear-thinning index of 5.2 in pure water dispersions, whereas Mt shows a value of 3.5, suggesting that Pal presents better shear-thinning ability. In the mixed clay samples, the increase in montmorillonite first increases the shear-thinning index and reaches the maximum when the montmorillonite proportion is less than 38%. Further increasing the montmorillonite proportion produces a decrease in the shear-thinning index. These results demonstrate that mixing palygorskite with montmorillonite should enhance the shear-thinning ability of clay dispersions when the montmorillonite proportion is below 38%. However, the seawater dispersions exhibit markedly different changes in the shear-thinning index. Compared to the pure water dispersions, the Pal seawater dispersion slightly increases its shear-thinning index to 5.5 while the Mt seawater dispersion decreases to 2.2. Moreover, the shear-thinning index gradually decreases with the increase in montmorillonite content in mixed clays. These phenomena suggest that palygorskite exhibits stable shear-thinning ability in seawater, while montmorillonite shows inferior shear thinning. Consequently, increasing montmorillonite in mixed clay contributes to poor shear-thinning behavior.
In addition to thickening and shear-thinning behaviors, the thixotropy of a drilling fluid is also critical. According to the flow curves (Figure 3), the shear-stress results of the up step (0.1–1000 s−1) and the down step (1000–0.1 s−1) do not overlap and thus form a hysteresis loop. This thixotropic loop area can be used to evaluate the thixotropy of clay dispersions [64,65]. In the pure water system, the Pal dispersion exhibits a thixotropic loop area of 318.5 Pa·s−1, while the Mt dispersion shows an area of 2058.3 Pa·s−1 (Figure 6). Unlike the viscosity and shear-thinning index, Mt exhibits better thixotropy than Pal in pure water. Consequently, the thixotropic loop area of mixed clay dispersions increases as the montmorillonite proportion increases. However, these clay dispersions in seawater present very small thixotropic areas, demonstrating a decrease in thixotropy. Particularly, higher montmorillonite content seems to result in a lower thixotropic loop area. This suggests that while the presence of montmorillonite enhances thixotropy in the pure water system, its sensitivity to electrolytes reduces thixotropic behavior in the seawater system.
By comparing the rheological properties of pure water and seawater systems, it is evident that there are notable differences between the two clay minerals. Such differences arise from the structure, morphology, and physical properties of these palygorskite and montmorillonite. Generally, montmorillonite particles form a “house-of-cards” structure by edge-to-face heterogeneous electrostatic charges (Figure 7a) [36,38]. Conversely, palygorskite tends to form a disordered “haystack” structure due to its nanofibrous morphology and excellent hydrophilicity [66]. This “haystack” structure is less affected by electrolytes as palygorskite particles show small surface charge density [29]. In the pure water system, when the montmorillonite proportion is less than 38%, the viscosity of mixed clays is improved, making it greater than that of pure palygorskite or montmorillonite dispersions. The presence of montmorillonite also enhances the shear-thinning and thixotropic behaviors of the dispersion, as the palygorskite nanofibers and montmorillonite flake particles enhance the strength and stability of the network structure through synergistic interaction (Figure 7a). The critical proportions of montmorillonite in the mixed clay are no more than 55% considering viscosity. The higher proportion of montmorillonite leads to a mismatch between the numbers of lamellar particles and nanofibers, resulting in reduced viscosity and shear-thinning performance. In seawater systems, the rheological performance of the dispersions is significantly reduced due to the regulatory effect of ionic strength on particle interactions. The seawater cations screen the negative surface charge of mineral particles, leading to enhanced particle aggregation (Figure 7b). This aggregation decreases viscosity, suppresses shear thinning, and reduces thixotropic performance. Furthermore, this screening effect is more pronounced for montmorillonite compared to palygorskite.

3.3. Filtration Properties

For drilling fluids, filtration properties are as crucial as rheological performance. During the drilling process, the fluid inevitably permeates into the formation and forms a mud cake on the wellbore wall. This mud cake prevents further fluid loss, but its effectiveness depends on the permeability of the cake [67]. Fluid loss is typically used as an indicator to evaluate the filtration properties of drilling fluids.
The fluid loss values for each of the clay dispersion systems are presented in Table 4. In pure water systems, fluid loss increases significantly with increasing palygorskite content. When montmorillonite is present alone (Pal:Mt = 0:10), the fluid loss is at its lowest (19 mL), indicating that montmorillonite has good hydration and expansion properties, as well as effective fluid loss control. As the palygorskite content increases, fluid loss gradually increases, reaching its highest value (134 mL) in the pure Pal system (86% of palygorskite), similar to the previous report [68]. This trend can be attributed to palygorskite’s lower expansion capacity and weaker hydration properties, which prevent the formation of stable colloidal structures in aqueous solutions, leading to increased fluid loss. Additionally, the nanofibrous palygorskite forms a porous mud cake that is ineffective in preventing fluid loss, whereas the layered Mt forms a dense cake that effectively controls fluid loss [8]. The mud cake images (Figure 8) confirm that montmorillonite produces the thinnest mud cake, while palygorskite produces the thickest. For mixed clay systems, the mud cake thickness increases with the increasing content of palygorskite.
In seawater dispersions, fluid loss is generally higher than in pure water systems, and the relationship with palygorskite content differs. The fluid loss in the pure Mt system (100% of montmorillonite) in seawater is 148 mL, significantly higher than that in pure water (19 mL), suggesting that seawater ions inhibit the hydration and expansion of montmorillonite, leading to reduced fluid loss control. In seawater, the fluid loss increases slightly with increasing palygorskite content, from 148 mL (0%) to 153 mL (45%), but decreases slightly at higher palygorskite proportions (78% and 86%). This phenomenon may be due to the high salt concentration in seawater causing the hydration layer of montmorillonite to shrink, thereby weakening its fluid loss control ability. In contrast, seawater does not significantly affect palygorskite’s fluid loss performance, resulting in more stable fluid loss values across different mixing ratios. These findings are further supported by the observed thick mud cake morphology (Figure 8).
In summary, in pure water systems, fluid loss is significantly influenced by the palygorskite–montmorillonite ratio, with higher montmorillonite content leading to better fluid loss control. In seawater systems, however, the high salt concentration inhibits montmorillonite’s hydration capacity, causing an overall increase in fluid loss with less sensitivity to the mixing ratio. These results suggest that increasing montmorillonite content in pure water systems can optimize fluid loss control in designing drilling fluid formulations, whereas in seawater systems, relying solely on montmorillonite is insufficient to reduce fluid loss effectively. Additional rheological modifiers or additives (e.g., polymers) to shield the effects of ions should be considered.

4. Conclusions

This study provides a comprehensive evaluation of the colloidal and rheological behavior of palygorskite–montmorillonite mixed clays in both pure water and seawater systems. The findings are as follows:
(1) Palygorskite and montmorillonite exhibit good colloidal stability and rheological properties in pure water, though their specific characteristics are different. Palygorskite dispersions demonstrate superior viscosity and shear-thinning behavior, while montmorillonite exhibits better thixotropy.
(2) Palygorskite maintains good colloidal and rheological performance in seawater, though with a slight reduction. In contrast, montmorillonite struggles to form stable colloids in seawater, and its rheological properties deteriorate significantly. This is attributed to montmorillonite’s higher surface charge density, which makes it more susceptible to the effects of seawater electrolytes, whereas palygorskite’s lower surface charge density gives it better salt resistance.
(3) The colloidal and rheological behavior of mixed clays depends on the ratio of palygorskite to montmorillonite and the dispersion medium. In pure water, as the proportion of montmorillonite increases, the dispersion’s viscosity and shear-thinning index first increase and then decrease, with maximum values occurring at montmorillonite contents of 22% and 38% (palygorskite contents of 78% and 62%), respectively. The thixotropy of the mixed clay is positively correlated with montmorillonite content—the higher the montmorillonite content, the greater the thixotropy. In seawater, due to the compression of the montmorillonite electric double layer by electrolytes, the rheological performance of the mixed clay decreases as the montmorillonite content increases.
(4) Montmorillonite exhibits lower fluid loss in pure water because its platelet-like particles form dense cakes, preventing liquid loss. Palygorskite forms a porous cake with its nanofibers, resulting in higher fluid loss. Thus, in pure water, the fluid loss of the mixed clay decreases as the montmorillonite content increases. However, due to the aggregation of montmorillonite particles, a dense filter cake is difficult to form in seawater, leading to higher fluid loss.
In conclusion, palygorskite demonstrates good rheological performance in pure water and seawater, while montmorillonite shows better rheological performance in pure water but significant deterioration in seawater. However, palygorskite has a very high fluid loss, which is undesirable for drilling fluids. Therefore, combining palygorskite and montmorillonite is a promising option for future drilling fluid design and development. From a rheological perspective, the montmorillonite content should be kept below 55%, while from a fluid loss perspective, the montmorillonite content should be as high as possible. The final formulation of drilling fluid should consider the overall balance between rheological properties, fluid loss, and cost. Particularly in seawater systems, the mixed clays’ rheological and fluid loss properties are suboptimal, making it necessary to introduce polymer-based rheological additives for enhancement.

Author Contributions

Conceptualization, G.Z. and D.L.; methodology, G.Z.; validation, J.Z. and J.C.; formal analysis, J.Z., G.Z. and J.C.; investigation, J.Z. and J.C.; resources, J.F. and W.F.; data curation, J.Z.; writing—original draft preparation, J.Z. and G.Z.; writing—review and editing, G.Z., D.L., J.F. and Z.K.; visualization, G.Z.; supervision, G.Z. and D.L.; project administration, G.Z.; funding acquisition, G.Z., W.F. and D.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Guangzhou Science and Technology Planning Project (SL2024A04J01144), the Guangdong Basic and Applied Basic Research Foundation (2023A1515110795), and the Science and Technology Planning Project of Guangdong Province, China (2023B1212060048), and The APC was funded by Guangdong University of Technology and Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed at the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 15 00251 g001
Figure 1. XRD patterns of (a) Pal and (b) Mt samples; TEM images of (c) Pal and (d) Mt samples.
Figure 1. XRD patterns of (a) Pal and (b) Mt samples; TEM images of (c) Pal and (d) Mt samples.
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Figure 2. Zeta potentials of mixed clays in (a) pure water and (b) seawater in relation to the mass content of montmorillonite; gel volumes (after 24 h) of mixed clays in (c) pure water and (d) seawater.
Figure 2. Zeta potentials of mixed clays in (a) pure water and (b) seawater in relation to the mass content of montmorillonite; gel volumes (after 24 h) of mixed clays in (c) pure water and (d) seawater.
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Figure 3. Flow curves of mixed clays in (a) pure water and (b) seawater dispersions in relation to the mass content of montmorillonite.
Figure 3. Flow curves of mixed clays in (a) pure water and (b) seawater dispersions in relation to the mass content of montmorillonite.
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Figure 4. Viscosities of mixed clays in (a) pure water and (b) seawater dispersions at a shear rate of 1000 s−1 in relation to the mass content of montmorillonite.
Figure 4. Viscosities of mixed clays in (a) pure water and (b) seawater dispersions at a shear rate of 1000 s−1 in relation to the mass content of montmorillonite.
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Figure 5. Shear-thinning indices of mixed clays in (a) pure water and (b) seawater dispersions in relation to the montmorillonite content.
Figure 5. Shear-thinning indices of mixed clays in (a) pure water and (b) seawater dispersions in relation to the montmorillonite content.
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Figure 6. A summary of the thixotropic loop areas of Pal–Mt clays in (a) pure water and (b) seawater dispersions (derived from the flow curves in Figure 3).
Figure 6. A summary of the thixotropic loop areas of Pal–Mt clays in (a) pure water and (b) seawater dispersions (derived from the flow curves in Figure 3).
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Figure 7. Interpretive diagrams of clay samples in (a) pure water and (b) seawater.
Figure 7. Interpretive diagrams of clay samples in (a) pure water and (b) seawater.
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Figure 8. Digital pictures of filter cakes of clay dispersions in relation to the clay mineral contents.
Figure 8. Digital pictures of filter cakes of clay dispersions in relation to the clay mineral contents.
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Table 1. A summary of the mineralogical composition of Pal.
Table 1. A summary of the mineralogical composition of Pal.
CompositionPalygorskiteMontmorilloniteMicroclineQuartzDolomiteSepiolite
Mass %75.312.55.53.31.81.5
Table 2. A summary of the mineralogical composition of Mt.
Table 2. A summary of the mineralogical composition of Mt.
CompositionMontmorilloniteMuscoviteCristobaliteQuartzAlbite
Mass %80.811.94.72.30.3
Table 3. Protocols for mixing clays and mass percentages of palygorskite and montmorillonite.
Table 3. Protocols for mixing clays and mass percentages of palygorskite and montmorillonite.
Pal:Mt (mass)10:09:17:35:53:71:90:10
Palygorskite86%78%62%45%27%9%0%
Montmorillonite14%22%38%55%73%91%100%
Table 4. Fluid loss of mixed clay in pure water and seawater fluids in relation to the clay mineral contents.
Table 4. Fluid loss of mixed clay in pure water and seawater fluids in relation to the clay mineral contents.
Pal:MtPalygorskiteMontmorilloniteFluid Loss (mL)
Pure WaterSeawater
10:086%14%134138
9:178%22%91143
7:362%38%47149
5:545%55%34153
3:727%73%21153
1:99%91%21150
0:100%100%19148
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Zhang, J.; Zhuang, G.; Chen, J.; Fan, W.; Fan, J.; Kuang, Z.; Liu, D. Comparative Study of Colloidal and Rheological Behaviors of Mixed Palygorskite–Montmorillonite Clays in Freshwater and Seawater. Minerals 2025, 15, 251. https://doi.org/10.3390/min15030251

AMA Style

Zhang J, Zhuang G, Chen J, Fan W, Fan J, Kuang Z, Liu D. Comparative Study of Colloidal and Rheological Behaviors of Mixed Palygorskite–Montmorillonite Clays in Freshwater and Seawater. Minerals. 2025; 15(3):251. https://doi.org/10.3390/min15030251

Chicago/Turabian Style

Zhang, Jiajun, Guanzheng Zhuang, Jinrong Chen, Wenxiao Fan, Jixing Fan, Zhuhua Kuang, and Dong Liu. 2025. "Comparative Study of Colloidal and Rheological Behaviors of Mixed Palygorskite–Montmorillonite Clays in Freshwater and Seawater" Minerals 15, no. 3: 251. https://doi.org/10.3390/min15030251

APA Style

Zhang, J., Zhuang, G., Chen, J., Fan, W., Fan, J., Kuang, Z., & Liu, D. (2025). Comparative Study of Colloidal and Rheological Behaviors of Mixed Palygorskite–Montmorillonite Clays in Freshwater and Seawater. Minerals, 15(3), 251. https://doi.org/10.3390/min15030251

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