Colloidal Properties of Clays from Ventzia Basin Enhanced with Chemical Additives and Subjected to Dynamic Thermal Aging Suitable for Water-Based Drilling Fluids

Abstract

This work examines the colloidal properties of clays sampled from two different locations in Ventzia basin processed as low-density solid additives for water-based drilling fluid applications. The obtained samples were mechanically processed to reach a size less than 2 cm. The material was then activated with 3 wt% soda ash without oven drying, keeping the moisture in environmental conditions to simulate industrial activation conditions. After laying for one month curing time, samples were oven dried at 60 °C and further ground to <120 μm. Two groups of samples were created mixing clays from Ventzia basin and additives. The first group contained clay, xanthan gum and sodium polyacrylate (PAA), while the second group contained clay, xanthan gum and sodium hexametaphosphate (SHMP). Standard tests were performed for the rheological behavior and filtration properties prior to and after dynamic thermal aging. Results obtained were compared with commercial clays from Milos and Wyoming used in drilling fluid systems, after thermally deteriorating also their properties. The obtained results revealed that the enhanced clays under study maintain excellent thermal stability. Notably, the top-performing formulation met the critical American Petroleum Institute (API) benchmark for filtrate loss (<15 mL) and exhibited a robust rheological profile at temperatures up to 105 °C, demonstrating its suitability for water-based fluid (WBF) applications.

1. Introduction

In recent decades, the Eastern Mediterranean has emerged as a globally significant frontier for energy exploration and production. Recent developments in energy exploration and production, including major discoveries of hydrocarbon reserves in the Levant Basin in Israel, Cyprus and Egypt have reshaped the region’s geopolitical and economic dynamics, fostering new collaborations and intensifying the search for further resources. This renewed activity is evident across several nations, with landmark projects such as Egypt’s Zohr supergiant gas field, Israel’s Leviathan and Karish fields, and Cyprus’s Aphrodite, Calypso, Cronos and recently Pegasus discoveries driving extensive drilling campaigns. Concurrently, Greece has reactivated its exploration programs, issuing new onshore and offshore licenses in Western Greece and the Ionian Sea, areas with complex and promising geological potential but also the latest Chevron request for Hydrocarbons exploration offshore Crete. These activities are not limited to fossil fuels alone, but they are also integral to the region’s energy transition. For instance, the Prinos Basin in Northern Greece is the site of a pioneering project to repurpose a depleted oil reservoir for carbon dioxide (CO₂) sequestration and storage, a critical technology for achieving national and European decarbonization targets [1,2,3].

This surge in drilling activity, whether for hydrocarbons [1], geothermal energy [4], or carbon storage [5], pushes operations into increasingly challenging environments. Formations in the Eastern Mediterranean are frequently characterized by deep water, high pressures, and high temperatures (HPHT), alongside complex geologies that include mobile salt bodies (evaporites), fractured carbonates, and reactive shales. These conditions place extreme demands on all aspects of drilling engineering, particularly on the formulation and performance of drilling fluids. As the primary medium for maintaining wellbore stability, transporting drill cuttings, and managing downhole pressure, the drilling fluid is a critical component for operational success, safety, and efficiency [1,2,3]. Consequently, there is an urgent and growing need for advanced, reliable, and environmentally compliant fluid systems tailored to the specific geological challenges that delineate the region’s emerging energy landscape.

In the formulation of modern drilling fluids, water-based fluids (WBFs) have become the system of choice for a majority of applications, especially in environmentally sensitive offshore areas like the Mediterranean [1]. Compared to their oil-based counterparts, WBFs offer significant advantages in terms of lower toxicity, reduced environmental impact, and more cost-effective disposal of drill cuttings. However, their performance is inherently more complex to manage. The effectiveness of a WBF hinges on a delicate balance of rheological and filtration properties, which are controlled through the synergistic interaction of a base clay material, which is typically bentonite, and a suite of specialized chemical additives to serve specific purposes [6]. The primary function of bentonite is to build viscosity to carry the cuttings but also form a thin, low-permeability filter cake on the wellbore wall, preventing filtrate losses into the formation and maintaining stability in terms of pressure control [7].

Despite their versatility, conventional WBFs face significant limitations, particularly their susceptibility to downhole contaminants. The presence of salts (e.g., NaCl from salt domes) and divalent cations (Ca²⁺ and Mg²⁺ from carbonate or anhydrite formations) can severely degrade the performance of standard sodium bentonite. These ions disrupt the clay’s hydration mechanism, causing particles to undergo electrostatic aggregation through edge-to-face association, which leads to a loss of desired viscosity, poor cuttings suspension, and excessive fluid loss. It is worth noting that the term “flocculation” is used to describe that the clay platelets form the gel-like “house-of-cards” structure. However, we reserve this term for polymer-bridging mechanisms, and the more appropriate and precise term “electrostatic aggregation” will be used from this point onwards. This vulnerability necessitates the use of robust additives to preserve fluid properties. Furthermore, the global drilling industry has historically relied on a limited number of high-quality sodium bentonite sources, primarily from Wyoming, USA, and Milos in Greece. While effective, this reliance on imported materials introduces logistical complexities, higher costs, and a potential mismatch with local geological conditions. This context has motivated a growing interest in evaluating and optimizing locally sourced clays as a more sustainable and economically sound alternative [8,9,10]. This approach aligns with sustainability goals by reducing the economic costs and carbon footprint associated with the long-distance transportation of commercial materials, thereby promoting regional resource self-sufficiency.

The scientific literature contains extensive research focused on enhancing the performance of bentonite clays for drilling applications. The fundamental challenge often lies in upgrading lower-grade calcium bentonites (Ca-bentonites) to mimic the superior properties of naturally occurring sodium bentonites (Na-bentonites). This is achieved through a process of chemical activation, most commonly with sodium carbonate (Na₂CO₃). This process facilitates an ion exchange reaction where the divalent calcium ions in the clay’s interlayer space are replaced by monovalent sodium ions, significantly enhancing the clay’s capacity to swell and disperse in water [7,8,10]. Variations in this method include the use of other alkalis like sodium hydroxide (NaOH) or combinations with agents like magnesium oxide (MgO) [11,12], sometimes coupled with mechanical processing to improve activation efficiency [13].

Beyond initial activation, the fine-tuning of WBF properties is accomplished through a range of specialized additives. To control the tendency of clay particles toward electrostatic aggregation, dispersants or thinners are employed. Polyphosphate compounds, particularly sodium hexametaphosphate (SHMP), are highly effective in this role. They absorb onto the positively charged edges of clay platelets, neutralizing the charge attraction and promoting a dispersed, low-viscosity state [14,15]. This dispersion is critical for maintaining fluid pumpability and managing viscosity under dynamic conditions.

In parallel, water-soluble polymers are indispensable for controlling filtration and enhancing viscosity, especially in HPHT wells. Filtration control agents, such as polyanionic cellulose (PAC) and carboxymethyl cellulose (CMC), are long-chain polymers that plug the pores of the filter cake, reducing fluid invasion into the formation [7]. For viscosity modification, biopolymers like xanthan gum, which is known for its biodegradability and low environmental impact, are widely used. Due to its rigid, rod-like molecular structure, xanthan gum imparts a highly desirable shear-thinning rheological profile. A high viscosity at low shear rates is capable of suspending cuttings when circulation stops, and a low viscosity at high shear rates reduces friction during pumping [16]. The thermal stability of these polymers is a key area of research, as degradation at high temperatures can lead to catastrophic fluid failure [17,18]. The stability and performance of any given formulation are rigorously tested in the laboratory using standardized procedures, most notably dynamic thermal aging (hot rolling), which simulates the combined effect of temperature and circulation on fluid properties over time.

The use of palygorskite and smectite in drilling fluids is well-established, with each mineral offering distinct and often complementary benefits. Bentonite, which is predominantly smectite, remains the most common mineral due to its excellent swelling and filtration control properties after standard activation with Na₂CO₃ [8,10,11,19,20,21,22]. However, its performance degrades severely in high-salinity environments, where charge screening leads to particle aggregation and a catastrophic loss of viscosity [23]. In contrast, palygorskite, often termed “saltwater clay,” builds viscosity through the mechanical entanglement of its fibrous particles, a mechanism that is largely unaffected by water chemistry [24,25]. Studies have shown that the rheological properties of palygorskite can be further enhanced through chemical and physical treatments [26,27] and are dependent on factors such as particle morphology and concentration [28,29]. Given these complementary characteristics, mixed palygorskite–smectite systems have been investigated to create fluids that combine the salt-tolerant rheology of palygorskite with the superior filtration control of smectite. Prior work has examined the rheology of these mixed systems in freshwater [30,31,32] and seawater environments [33], consistently showing that their behavior is complex and often synergistic, not merely an average of the individual components. While these investigations have established a strong foundation, the performance of naturally occurring, untreated smectite-palygorskite deposits, such as those from the Ventzia basin, remains a largely unexplored area. Their specific response to modern chemical additives and their stability under dynamic thermal aging represent a significant gap in the literature that this study aims to address.

While extensive research exists on modifying conventional bentonites, the unique clay resources of the Ventzia basin in Western Macedonia, Greece, remain largely unexplored in the context of advanced drilling fluid design. These deposits are geologically distinct, characterized by a mixed assemblage of Mg-Fe-smectite and the fibrous clay mineral palygorskite (also known as attapulgite). This mineralogical composition sets them apart from typical monomineralic bentonites. Palygorskite, with its needle-like crystal morphology, is known for its exceptional ability to build viscosity in high-salinity environments, a property that arises from the mechanical entanglement of its fibers rather than electrostatic interactions. Initial studies by [30,31,34] provided a foundational characterization of the mineralogy and basic rheological behavior of these mixed clay suspensions. Their work revealed complex interactions between the platy smectite and fibrous palygorskite particles, suggesting that their combined rheological profile is not merely an average of the individual components. However, this foundational research did not extend to a systematic evaluation of how these clays respond to the suite of modern chemical and polymeric additives used in drilling fluids. There is a clear and significant gap in the literature regarding the enhancement of these specific clays. Key unanswered questions remain like (i) how do these mixed-mineralogy clays react to standard activation and dispersion treatments? (ii) can their unique properties be synergistically combined with polymers like xanthan gum to create stable, (iii) high-performance WBFs, and (iv) how do they perform under simulated downhole conditions of dynamic thermal aging compared to industry-standard materials? The novelty of this study lies in being the first to systematically investigate the performance of the unique smectite–palygorskite clays from the Ventzia basin when modified with modern drilling fluid additives. This work moves beyond basic characterization to assess their rheological and filtration stability under simulated downhole conditions, providing a direct performance benchmark against industry-standard materials [30,31,34].

The scientific literature widely employs dynamic thermal aging to assess the efficacy of new additives and formulations especially under hard conditions of Pressure and Temperature. Numerous studies demonstrate the degradation of conventional water-based muds when subjected to hot rolling, showing increases in fluid loss and undesirable changes in viscosity [17]. Consequently, researchers use this method as a benchmark to validate the performance of novel polymers, nanoparticles, and other stabilizing agents. For instance, ref. [18] utilized dynamic aging to confirm that a specific acrylamide copolymer enhanced rheological characteristics and reduced filtration under HPHT conditions. Similarly, research on biopolymers like xanthan gum often includes hot rolling tests to prove their thermal stability compared to less robust additives [35]. These investigations consistently measure properties before and after aging to quantify the performance improvement offered by the additive in question, making dynamic aging a cornerstone of experimental validation in the field.

This study aims to address the identified knowledge gap by conducting the first comprehensive investigation into the formulation of WBFs using the unique clays of the Ventzia basin. The research is designed to move beyond basic characterization and systematically assess their potential as a primary material for modern drilling applications. Specifically, this work aims to evaluate the effect of selected chemical additives on the rheological and filtration properties of Ventzia clay suspensions, assess their thermal stability under dynamic thermal aging, and benchmark their performance against established American Petroleum Institute (API) standards for commercial bentonite. By achieving these objectives, this research will provide novel insights into the behavior of smectite-palygorskite clay systems in the presence of modern fluid additives. The findings are expected to furnish a scientific basis for the development of high-performance, sustainable WBFs derived from local Greek resources, contributing to the operational efficiency, economic viability, and environmental goals of the wider regional energy industry.

This work is organized as follows: in Section 2 we present the geology of the Ventzia basin by emphasizing the minerology of the region with respect to the clays under study. Section 3 presents the materials and methods used for the purposes of this work, while Section 4 analyzes the experimental results, with a focus on performance after dynamic thermal aging. Finally, Section 5 presents the conclusions and key contributions to this work.

2. Geology and Mineralogy of the Ventzia Basin

2.1. Regional Geological Setting and Basin Formation

The clays investigated in this study were sourced from the Ventzia Basin, a Neogene-Quaternary continental basin situated within the Grevena region of Western Macedonia, Greece (Figure 1). Geotectonically, it lies within the Pelagonian zone, a continental microplate that forms a key part of the Hellenides orogenic belt. The basin’s formation and subsequent sedimentary fill are intrinsically linked to the region’s complex tectonic history. The bedrock upon which the basin sediments were deposited consists primarily of ultramafic rocks of the Vourinos ophiolite complex, a remnant of obducted Neo-Tethyan oceanic crust. The basin is dated to the Plio-Pleistocene, an age constrained by both structural analysis of faulting and paleontological evidence from mammalian fossil assemblages discovered within its clastic deposits [36]. Figure 1 depicts the locations that the samples were taken on the Knidi geological map [37].

2.2. Stratigraphy and Depositional Environment

The sedimentary succession of the Ventzia Basin, reaching an average thickness of approximately 200 m, lies with angular unconformity over the ophiolitic basement. The stratigraphic sequence begins with a high-energy basal conglomerate, indicating the initial phase of basin development. This is succeeded by intermediate members characterized by alternating layers of conglomerates and reddish-brown clays. This cyclic pattern reflects a dynamic depositional environment, likely a fluctuating fluvial or lacustrine system where periods of high-energy transport (depositing gravels) alternated with quiescent, low-energy periods (allowing for the settling of fine-grained clays). The uppermost part of the stratigraphic sequence consists of the most fine-grained material, where the economically significant deposits of palygorskite and Mg-Fe-smectite are located. These argillaceous layers, which are the focus of this research, are locally overlain by more recent ultramafic colluvium or calcareous talus deposits derived from adjacent highlands. The general stratigraphy of the basin and the specific disposition of the clay-rich beds have been documented in several studies [38,39], with representative geological sections and profiles shown in Figure 2 and Figure 3.

2.3. Petrogenesis and Unique Mineralogy of the Ventzia Clays

The unique mineralogical composition of the Ventzia clays is a direct result of their geological origin. As first documented by [40], the clays are of sedimentary origin, formed through the low-temperature chemical weathering of the ultramafic rocks of the Vourinos ophiolite complex [41]. The parent ophiolitic rocks are inherently rich in magnesium (Mg) and iron (Fe), which created a geochemical environment conducive to the formation of a distinct clay mineral assemblage. The two primary components are:

Mg-Fe-Smectite is a member of the smectite group, characterized by a 2:1 layered silicate structure. Its platy morphology and ability to undergo cation exchange and swell in the presence of water are responsible for the viscosity-building and fluid-loss-control properties typical of bentonites.

Palygorskite (Attapulgite) is a fibrous clay mineral with a needle-like (acicular) crystal morphology. Unlike smectites, whose rheological properties are governed by electrostatic surface hydration, palygorskite builds viscosity through the mechanical entanglement of its microscopic fibers. This mechanism is highly effective and notably resistant to the flocculating effects of electrolytes, making palygorskite an exceptional viscosifier in high-salinity fluids. This naturally occurring, synergetic mixture of a platy, swelling smectite and a fibrous, salt-tolerant palygorskite is what makes the Ventzia deposits a compelling material for advanced drilling fluid applications. This study seeks to harness the potential of this unique mineralogical blend.

3. Materials and Methods

3.1. Materials Characterization

The two primary clay samples were sourced from the Ventzia Basin (see Figure 1), one from the Pefkaki location, identified as an Mg-Fe-smectite which we will refer to as P15, and one from the Piloroi location, identified as a mixed-layer smectite–palygorskite clay that we will refer to as KM. For comparative benchmarking, two commercial API-grade bentonites were used. Namely, Zenith’s Bentonite (ZB) from Milos Island in Greece owned by Imerys LTD and AquaGel (AQZ) which is a bentonite from Wyoming, USA provided from Bentonite Performance Minerals, LLC (BPM). The mineralogical composition of the key materials used in this study was determined by XRD analysis. The X-Ray Diffraction (XRD) mineralogical characterization was performed using a Bruker D8 Advance diffractometer equipped (Bruker Corporation, Billerica, MA, USA) with a nine-position Da Vinci autosampler and a silicon detector. The analysis utilized Cu-Kα radiation with a Ni filter, operating at 40 kV and 40 mA. Randomly oriented powder samples were scanned from 2° to 70° 2θ with a step size of 0.019° and a dwell time of 0.2 s per step. Mineral phase identification was conducted using the ICDD PDF4-2023 database provided with the instrument’s software.

Figure 4 presents the diffractograms that were performed on the P15 sample compared against the two commercial bentonites from Milos and Wyoming. On the other hand, Figure 5 presents the comparison of the XRD diffractograms between the other sample (KMG) and the two commercials. From the analysis performed, the following material characterization is obtained.

P15-based products (P15B8, P15D4): This group of activated bentonites are predominantly composed of Mg-Fe-smectite, identified by its characteristic reflection at d ≈ 13.06–13.09 Å (2θ ≈ 6.75°). The associated minerals include palygorskite (d ≈ 10.58 Å), serpentine (mainly lizardite), quartz, plagioclase, and minor carbonates (calcite or dolomite).

KMG1 (activated KM clay): This clay containing both smectite (d ≈ 12.69 Å) and a significant amount of palygorskite (d ≈ 10.53 Å), along with serpentine (mainly lizardite), quartz, plagioclase, and dolomite.

Zenith Bentonite (ZB-Milos): The primary mineral is smectite (d ≈ 12.42 Å), with accessory minerals including cristobalite, muscovite, zeolite, dolomite, and calcite.

AquaGel (AQZ-Wyoming): This is the classic API sodium bentonite with smectite as the main phase (d ≈ 12.26 Å). It also contains muscovite, zeolite, kaolinite, quartz, plagioclase, and dolomite.

As mentioned above, the two products derived from the Ventzia Basin, P15B8 and P15D4, are confirmed to be predominantly composed of Mg-Fe-smectite. This is evidenced by the characteristic primary smectite peak observed at a 2θ angle of approximately 6.75° (d = 13.09 Å). A key distinguishing feature, consistent with the geology of the Ventzia Basin, is the presence of palygorskite in both samples, identified by its peak near 8.35° 2θ (d = 10.58 Å). In addition to the primary clay minerals, several accessory phases were identified, including quartz (with major peaks at 20.88° 2θ (d = 4.25 Å) and 26.67° 2θ (d = 3.34 Å), lizardite (a serpentine mineral indicative of its ophiolitic origin), plagioclase, and minor amounts of carbonate minerals (calcite and/or dolomite). For comparison, the two commercial samples exhibit mineralogical composition typical of high-grade bentonites from different geological settings. The AquaGel sample, a classic Wyoming bentonite, consists almost entirely of sodium-montmorillonite (smectite), identified by a very intense and sharp primary reflection at approximately 7.20° 2θ (d = 12.28 Å). Its accessory minerals include minor amounts of quartz, plagioclase, muscovite, kaolinite, and dolomite. The Zenith bentonite also shows smectite as its principal component, with a main peak at 7.11° 2θ (d = 12.43 Å). Its accessory mineral suite is distinct, containing cristobalite, muscovite, zeolite, and carbonates.

The XRD analysis was extended to the KMG1 product, derived from the smectite-palygorskite clay (sm-pal), and its diffractogram is compared with the commercial benchmarks in Figure 5. The results confirm that KMG1 is a complex, mixed-mineralogy material, distinct from both the P15-based products and the commercial bentonites. The pattern for KMG1 displays a clear smectite reflection at approximately 6.96° 2θ (d = 12.70 Å), but more significantly, it features a sharp and intense reflection corresponding to palygorskite at 8.38° 2θ (d = 10.55 Å). The high intensity of this peak indicates that the fibrous palygorskite is a major, and likely dominant, clay mineral constituent in this sample. The non-clay mineral suite is consistent with the P15 material, with prominent peaks corresponding to quartz, lizardite, plagioclase, and dolomite. In contrast, the diffractograms for the commercial Zenith and AquaGel samples reaffirm their composition as high-purity smectite clays. They are dominated by their strong primary smectite reflections at 7.11° 2θ (d = 12.43 Å) and 7.20° 2θ (d = 12.28 Å), respectively. The most critical distinction is the complete absence of the characteristic palygorskite peak in both commercial products. This fundamental difference in clay mineralogy, which is the presence of a significant fibrous clay component in KMG1 versus the purely platy structure of the commercial bentonites, is a key factor expected to govern the differences in their rheological behavior, particularly in terms of viscosity development and thermal stability.

To activate the samples, commercial technical-grade additives were used to formulate the final clay products for testing. Additives include sodium carbonate (Na₂CO₃) for activation, xanthan gum (XG) as an organic viscosifier, sodium hexametaphosphate (SHMP) as an inorganic dispersant, and sodium polyacrylate (PAA) as an organic polymer dispersant. All drilling fluid suspensions were prepared using deionized water.

3.2. Samples Preparation

The raw P15 and KM clays, with a natural moisture content of 40–42 wt%, were first mechanically downsized to a particle size of <2 cm. They were then activated with 3 wt% sodium carbonate (calculated on a dry weight basis). After thorough mixing, the activated clays were stored and allowed to age for a period of 30 days. Following the aging period, the samples were dried in an oven at 60 °C and subsequently pulverized to a particle size of <120 µm. The final moisture content of the pulverized powders ranged from 12 to 14 wt%. These activated base clays were then used to create three final products via dry mixing.

The experimental procedure followed is designed to evaluate the properties of the colloidal suspension formed after dispersing the prepared clay powders in water. It is important to clarify that while the initial dry particle size was pulverized to <120 µm, this represents the size of clay aggregates (tactoids). The desired colloidal properties are attributed to smectite platelets after hydration and disagglomeration into particles of true colloidal dimensions (typically <2 µm in lateral dimension and nanometers in thickness) [1,7]. The initial treatment of the clay is a standard preparatory step, consistent with industry practices outlined in API Specification 13A, which requires commercial bentonite to be a fine powder. This process increases the available surface area, facilitating the rapid and effective dispersion necessary to form the stable colloidal system.

Table 1. Summary of the composition and additives used to formulate the three experimental clay products from the Ventzia Basin materials.

Product Code	Base Clay	Additive 1 (Viscosifier)	Additive 2 (Dispersant)
P15B8	Activated P15 (Mg-Fe-Smectite)	1.0 wt% xanthan Gum	0.4 wt% SHMP
P15D4	Activated P15 (Mg-Fe-Smectite)	1.0 wt% xanthan Gum	0.6 wt% PAA
KMG1	Activated KM (Mixed Smectite-Palygorskite)	1.0 wt% xanthan Gum	0.4 wt% PAA

where SHMP is the sodium hexametaphosphate and PAA is the sodium polyacrylate.

presents the summarized composition and additives that were used to formulate the three experimental clay products from the Ventzia Basin.

3.3. Experimental Procedures

Drilling fluid suspensions with the clays from the Ventzia Basin and the commercial clays were prepared by dispersing 22.5 g of each modified clay product (P15B8, P15D4, and KMG1) and the commercial bentonites into 350 mL of deionized water. The suspensions were mixed for 20 min in a metal container using a Hamilton Beach 400 HD multimixer (Hamilton Beach Brands, Inc., Glen Allen, VA, USA).

Dynamic thermal aging, commonly known as hot rolling, is a standard laboratory procedure essential for evaluating the performance of drilling fluids under simulated downhole conditions. This method addresses the limitations of static testing by subjecting a fluid sample to simultaneous high temperature, elevated pressure, and continuous mechanical agitation. Foundational texts in drilling fluid engineering, such as those by [7], establish this test as a critical step in fluid design and quality control. The primary objective is to predict the stability of the fluid’s rheological and filtration properties, which are fundamental to its functions of cuttings transport, wellbore stability, and pressure control. By replicating the synergistic effects of heat and shear, dynamic aging provides a more realistic assessment of how a fluid system, and its chemical additives will behave during circulation in a wellbore. According to the procedure, following initial preparation, the fluid suspensions were immediately transferred to stainless steel aging cells, sealed, and subjected to dynamic thermal aging in a roller oven for a period exceeding 16 h. The aging was conducted at progressively increasing temperatures: 25 °C, 45 °C, 65 °C, 85 °C, 105 °C, and 125 °C, as per the testing protocol adapted from [42].

The rheological properties of the drilling fluids were characterized using a direct-indicating Couette rotational viscometer (Fann Model 35A), following the procedures outlined in [42]. For each measurement, the fluid sample was placed in the viscometer’s thermostatically controlled cup and subjected to a series of defined shear rates. Dial readings, which are proportional to shear stress, were recorded at standard rotational speeds of 600, 300, 200, 100, 6, and 3 revolutions per minute (rpm). From these readings, key rheological parameters were calculated using standard API formulas: Plastic Viscosity (PV) was determined as the difference between the 600 and 300 rpm readings [1,7,42]:

$$\mathrm{PV}={\mathrm{\theta }}_{600}-{\mathrm{\theta }}_{300}$$

(1)

while the Yield Point (YP) was calculated as the 300 rpm reading minus the Plastic Viscosity [1,7]:

$$\mathrm{YP}={\mathrm{\theta }}_{300}-\mathrm{PV}$$

(2)

In addition to these flow properties, the thixotropic behavior of the fluid was assessed by measuring gel strengths, which were recorded as the maximum dial deflection after the sample remained static for periods of 10 s and 10 min. These measurements provide a comprehensive rheological profile, enabling the analysis of the fluid’s carrying capacity and stability under dynamic conditions. All rheological and filtration properties were measured for each thermal aging step, the cells were cooled to ambient temperature, and the fluids were re-mixed under low speed for a few min before testing for workability. The dial readings of the Couette rotational viscometer were recorded at the standard rotational speeds.

The thixotropic properties of the drilling fluids, which describe their ability to form a reversible gel structure under static conditions, were quantified by measuring gel strength at 10-s and 10-min intervals. These two values provide insight into the fluid’s ability to suspend solids during both short and extended periods of static conditions. The procedure that was conducted using the same rotational viscometer immediately following the standard rheological measurements, in accordance with [42]. To ensure a consistent initial state, the fluid was first sheared at a rate of 600 rpm for approximately 10–15 s to break down any existing gel structure. The viscometer motor was then stopped, and the fluid was allowed to remain static for a period of 10 s. After this interval, the motor was turned on at the lowest standard speed (3 rpm), and the maximum deflection of the dial was immediately recorded. This peak value, reported in units of l b/100 ft², represents the 10-s gel strength. To determine the 10-min gel strength, the fluid was re-stirred at 600 rpm to break the gel, and the viscometer was then stopped for a period of 10 min. The measurement was repeated by turning the motor on at 3 rpm and recording the maximum dial reading.

The static filtration properties of each drilling fluid sample were determined using a standard Low-Pressure, Low-Temperature (LPLT) filter press, in strict accordance with [1,7,42]. This test quantifies the fluid’s ability to form a low-permeability seal on a porous medium. The procedure involved placing the fluid sample into a sealed test cell containing a sheet of filter paper. A constant differential pressure of 100 ± 5 psi or 690 ± 35 kPa was then applied to the top of the fluid for a duration of 30 min. The total volume of liquid (filtrate) that passed through the filter paper during this period was collected and measured in a graduated cylinder, with the final volume (in mL) reported as the API fluid loss. After depressurization, the filter cake formed on the paper was carefully removed, and its thickness was measured to evaluate the sealing characteristics of the solids in the fluid.

The results presented for each thermal aging condition are from a single, carefully executed experimental run, which was performed consistently for all five formulations across the entire temperature gradient. Our confidence in this approach is based on two key factors. First, prior to the thermal aging series, we conducted multiple preliminary experiments on the unaged (25 °C) formulations. These initial tests demonstrated a high degree of consistency and low variability in the measurements, validating the robustness of our sample preparation and testing protocols. Second, the primary strength and internal validation of our data come from the clear, systematic, and chemically logical trends observed across the wide temperature range for all five distinct fluid systems. The consistent behavior of each material as a function of temperature provides strong evidence for the reliability of the findings.

4. Results and Discussion

This section presents the experimental findings from the comprehensive evaluation of the three modified Ventzia clay drilling fluids (P15B8, P15D4, and KMG1) and the two commercial benchmarks (AquaGel and Zenith Bentonite). The primary objective of this analysis is to systematically assess the performance of these fluids, with a particular focus on their thermal stability. To achieve this, all key rheological and filtration properties were measured both at ambient temperature (25 °C) and after dynamic thermal aging at progressively higher temperatures of 55 °C, 85 °C, 105 °C, and 125 °C. The results are presented in a manner that directly compares the performance of the novel, locally sourced clays against the established industry standards. These commercial products serve as the most relevant high-performance benchmarks, allowing for a clear evaluation of the potential of the Ventzia-based formulations as viable alternatives for demanding drilling applications.

The analysis is structured to first provide a complete overview of the drilling fluid suspension rheological behavior, followed by a detailed examination of specific performance metrics. The section begins by presenting the full rheological curves (shear stress vs. shear rate) for each fluid at every temperature step, which offers a holistic view of their flow characteristics. Following this, the key rheological parameters derived from these curves, like the Plastic Viscosity (PV), the Yield Point (YP), and the 10-s/10-min gel strengths, are presented in bar charts to quantify and compare the trends in viscosity and thixotropic behavior. Subsequently, the focus shifts to filtration performance, with data on API fluid loss and filter cake thickness presented as a function of temperature. Throughout this section, the observed results are discussed and interpreted in the context of the unique mineralogy of the Ventzia clays, linking their performance directly to their material composition.

The complete rheological profiles of the five fluid systems are shown in Figure 6. At the baseline temperature of 25 °C, the commercial bentonites, AquaGel and Zenith, establish a high-performance benchmark, displaying robust, shear-thinning profiles with high shear stress values across all shear rates. In contrast, the experimental fluids formulated with Ventzia clays (P15B8, P15D4, and KMG1) exhibit more moderate initial rheological profiles, indicating a lower initial viscosity compared to the premium commercial products.

By examining the constitutive behavior, the drilling fluid suspensions presented in Figure 6a–e, a critical divergence in performance becomes evident as the aging temperature increases. The rheological curves for both commercial bentonites, AquaGel and Zenith, show a progressive and severe downward shift with increasing temperature. This trend signifies a substantial loss of viscosity and indicates significant thermal degradation, a common challenge for conventional water-based muds at elevated temperatures [17]. By 125 °C, the flow curves for both AquaGel and Zenith have nearly collapsed, demonstrating a near-total loss of their initial rheological properties and, consequently, their capacity to suspend and transport drill cuttings. This degree of thermal degradation is consistent with the well-documented limitations and aging effects of conventional smectite-based fluids [18,19]. For instance, the base mud formulation reported in [18] showed a similar, significant decline in viscosity after thermal aging at 121 °C (250 °F), confirming that our benchmark materials behaved as expected under severe thermal stress.

In stark contrast, the three modified Ventzia clay formulations demonstrate markedly superior thermal stability. The P15B8 and P15D4 fluids, based on the Mg-Fe-smectite, largely maintain the magnitude and shape of their rheological profiles up to 105 °C, with only a minor decrease in shear stress observed at 125 °C which is expected due to the extremity of the value with the physical meaning to push the drilling fluid suspension to its limits rather than obtaining a clear result at this temperature level. The KMG1 fluid, formulated with the mixed smectite–palygorskite clay, displays the most remarkable and desirable behavior. Its rheological profile not only remains stable but actually improves with heat, showing a clear increase in shear stress values as the temperature rises from 25 °C to 105 °C. Even after aging at 125 °C, the KMG1 fluid retains an adequate rheological profile quite close to its own initial state, comparable with the commercials and slightly better than the degraded drilling fluid suspensions with bentonites P15B8 and P15D4. This thermal enhancement is characteristic of palygorskite-based systems, where thermal energy can improve the interaction of the clay’s fibrous network, thereby increasing viscosity and gelation [30,31]. These initial findings strongly suggest that the unique mineralogy of the Ventzia clays, particularly the palygorskite-rich KMG1, imparts exceptional thermal resilience that can be comparable to the high-purity smectite commercial products.

To quantitatively assess the trends observed in the rheological curves, the key rheological parameters that are relevant for the drilling industry are the plastic viscosity (PV), yield point (YP), and the 10 sec and 10 min gel strengths of the Bingham-Plastic model. These parameters are presented in Figure 7. The figure consists of a series of bar charts that systematically compare the performance of the five drilling fluid suspensions (P15B8, P15D4, KMG1, ZB and AquaGel). Each chart is dedicated to a specific rheological parameter, with data grouped by material. Within each group, distinct bars represent the parameter’s value measured after dynamic thermal aging at progressively increasing temperatures of 25 °C, 45 °C, 65 °C, 85 °C, 105 °C, and 125 °C. This visual format allows for a direct comparison of each fluid’s initial properties and its subsequent stability under thermal stress.

By examining Figure 7a the plastic viscosity (PV) which reflects the mechanical friction between solid particles, reveals complex behaviors with thermal aging. At 25 °C, the commercial AquaGel suspension exhibits the highest PV, consistent with its composition as a high-grade sodium bentonite that achieves excellent initial dispersion. The other fluids show lower initial PVs. As the aging temperature increases to 85 °C, the Zenith bentonite and the Ventzia clays (P15B8 and KMG1) show a notable increase in PV, with KMG1 peaking at the highest value among all samples. This suggests that moderate thermal energy may enhance particle dispersion in these specific systems. However, above 85 °C, these fluids experience a significant drop in PV, indicating the onset of thermal degradation and particle aggregation. AquaGel shows a more gradual decline across the temperature range, while P15D4 displays a more erratic but generally low PV profile. The data indicates that while initial PV is high for some commercial products, thermal stability is inconsistent across all samples, with most showing a clear point of thermal failure above 85 °C. After 105 °C thermal degradation and particle aggregation takes place rendering the fluids inappropriate for drilling.

Figure 7b compares the yield point (YP), which is the critical measure of the drilling fluid’s cuttings suspension capacity, and also provides an indicator of performance differences. The commercial Zenith bentonite begins with the highest YP, but its performance is surpassed by the Ventzia clays as temperature increases. The most significant finding is the exceptional thermal response of the Ventzia clay formulations. The YP of P15B8, P15D4, and KMG1 progressively increases with temperature, peaking at 105 °C. The KMG1 fluid, which contains palygorskite, demonstrates a remarkable enhancement, with its YP rising from 9.6 Pa at 25 °C to an outstanding 33.0 Pa at 105 °C. Similarly, P15D4 reaches a YP of 27.3 Pa at the same temperature. This behavior signifies a thermally activated improvement in the fluid’s electrostatic network and, therefore, its carrying capacity. In stark contrast, the commercial bentonites exhibit instability. AquaGel’s YP fluctuates unpredictably, while the Zenith bentonite, despite maintaining a high YP up to 105 °C, suffers a collapse at 125 °C, where its value plummets by nearly 80%. This demonstrates that the unique mineralogy of the Ventzia clays provides a more reliable and ultimately superior suspension capability at elevated temperatures. This behavior is consistent with prior works on mixed-clay systems, which has repeatedly shown that combining fibrous palygorskite with platy smectite leads to complex and synergistic rheological effects. Specifically, studies of [30,31,43] investigated palygorskite–bentonite and sepiolite–bentonite mixed clay suspensions and have all have reported that the rheological profiles of such mixtures are superior to a simple average of the individual components. These works confirm that the interaction between the two clay types enhances the suspension’s structure. Therefore, the increase in Yield Point and Gel Strengths observed for our KMG1 fluid is not an anomaly but rather an expected and characteristic outcome of its mineralogy. Furthermore, ref. [44] demonstrated that a blend of bentonite and sepiolite (a fibrous clay similar to palygorskite) exhibited a significantly higher Yield Point than either clay individually, confirming that such blends can produce superior rheological properties [44].

Finally, Figure 7c,d present the thermal stability performance of the Ventzia clays with the thixotropic properties, measured as 10-s and 10-min gel strengths. These parameters quantify the drilling fluid’s ability to form a gel structure and suspend solids during static periods while drilling. The commercial Zenith bentonite displays a very high initial 10-min gel strength that degrades significantly with heat before a final collapse at 125 °C. The P15D4 formulation, however, maintains exceptionally high and stable gel strengths across the temperature range up to 105 °C. The other Ventzia clays, P15B8 and KMG1, also demonstrate a stable or progressively increasing gel structure up to 105 °C, reinforcing the observation that their performance is enhanced by thermal energy. After aging at 125 °C, all fluids show a substantial loss of gel strength, which is an expected outcome at this extreme testing temperature. However, the consistent and enhanced performance of the Ventzia-based fluids up to 105 °C presents a clear advantage over the commercial products, whose thixotropic properties are less stable under thermal load. Collectively, the rheological data indicates that the Ventzia clays, particularly the palygorskite-bearing KMG1 and the P15D4 formulation, offer exceptional thermal resilience, making them highly suitable candidates for high-temperature drilling fluid applications. The observed thermal degradation of the commercial bentonite suspensions is consistent with the known effects of high-temperature aging on Na-activated clays [19]. For instance, the base bentonite mud formulation reported by [18] showed a significant decline in viscosity and gel strength after thermal aging at 121 °C, a finding that aligns directly with the performance collapse we observed for both the AquaGel and Zenith bentonites under similar thermal stress [18]. In stark contrast, the superior thermal stability of our P15D4 formulation highlights the critical role of thermally stable polymeric additives. The same study by [18] demonstrated that the addition of a functionalized anionic polymer prevented this thermal degradation, allowing the fluid to maintain stable rheological properties. This directly supports our interpretation that the robust performance of P15D4 is attributed to the protective and dispersive action of the PAA polymer, a key factor in successful high-temperature drilling fluid design.

Further to the rheology, the ability of drilling fluids to control filtrate loss into the formation is a critical performance metric, directly impacting wellbore stability and preventing formation damage. This property was evaluated using the standard API LPLT filter press test, with the results for fluid loss and the corresponding filter cake thickness (mm) presented as a function of thermal aging temperature. Figure 8a displays the volume of filtrate (in mL) collected over 30 min, which quantifies the sealing efficiency of each fluid. The dashed line shows the 15 mL threshold that API stipulated so that the fluid will qualify for drilling fluid usage while at the same time minimizing filtrate losses. The second Figure 8b illustrates the thickness of the resulting filter cake (in mm), providing a physical measure of the solids’ deposition on the porous medium. An ideal drilling fluid will exhibit a low and stable fluid loss value across the operational temperature range, producing a thin, tough, and impermeable filter cake.

A critical evaluation of the filtration data of Figure 8, reveals distinct performance tiers, with the P15D4 formulation emerging as the clear top-performing fluid. This fluid, composed of Mg-Fe-smectite enhanced with xanthan gum and the organic dispersant PAA, begins with the lowest initial fluid loss of 10.8 mL. It demonstrates exceptional thermal stability, with the value only increasing marginally to 13.4 mL after aging at 105 °C. Even after being subjected to the extreme temperature of 125 °C, its fluid loss of 28 mL remains lower than or competitive with all other formulations except AquaGel. This superior performance is consistent with the known mechanism of its specific formulation. The organic polymer dispersant PAA is well-documented to effectively coat clay platelets, preventing severe electrostatic aggregation and allowing them to form a compact and low-permeability seal that remains robust even under significant thermal stress. As described by [45], the long-chain anionic macromolecules of PAA adsorb onto the clay surfaces, creating a protective barrier that physically prevents the platelets from undergoing electrostatic aggregation. This mechanism allows the particles to remain well-dispersed and form a more ordered, compact and low-permeability seal that remains robust even under significant thermal stress [45]. This outcome aligns with the work of [18], who demonstrated that another class of functionalized anionic polymer provided similar robust filtration control under HPHT conditions by creating a more resilient and less permeable filter cake.

In contrast, the commercial bentonites show divergent and ultimately less favorable behaviors. The AquaGel fluid serves as a benchmark for stability, maintaining a remarkably consistent fluid loss between 13.4 mL and 15.2 mL across the entire temperature range up to 125 °C. This indicates that its formulation provides reliable, albeit not superior (>15 mL), filtration control. The Zenith Bentonite, however, performs poorly. It starts with a moderate fluid loss of 15.2 mL that steadily worsens with temperature before experiencing catastrophic failure at 125 °C, where the fluid loss increases to 40 mL. An even more dramatic failure is observed with the P15B8 fluid, which uses the same base clay as P15D4 but is treated with the inorganic dispersant SHMP. Despite showing good initial control (12.4 mL), its performance degrades significantly above 85 °C, culminating in an extremely high fluid loss of 55.2 mL at 125 °C. This stark difference between P15D4 and P15B8 highlights the critical finding that the choice of dispersant is as important as the base clay. The SHMP, while effective for certain rheological properties, showed poor thermal stability for filtration control. This result is consistent with the known tendency of polyphosphates to undergo thermal hydrolysis at higher temperatures, which would compromise its ability to keep clay particles dispersed [7,14].

The palygorskite-rich KMG1 fluid exhibits an intermediate but ultimately undesirable filtration performance. Its fluid loss begins at a moderate 15.2 mL and progressively increases with temperature, reaching 20.4 mL at 105 °C and 34 mL at 125 °C. This behavior is consistent with its mineralogy. The fibrous, needle-like structure of palygorskite, while excellent for building a mechanically entangled viscosity network, does not form the compact, overlapping, platy structure required for an efficient, low-permeability seal, a fundamental distinction from smectitic clays [7,31]. Thermal energy appears to disrupt what little sealing capacity it has, leading to progressively higher fluid loss.

The analysis of Figure 8b, which is concerned with the filter cake thickness, provides complementary and corroborating evidence for these filtrate loss conclusions. The P15D4 fluid once again demonstrates its superiority by forming the most consistently thin and stable filter cake, maintaining a thickness between 1.19 mm and 1.59 mm up to 105 °C. A thin cake is highly desirable, as it minimizes the risk of downhole operational problems such as differential pipe sticking. The AquaGel fluid forms a slightly thicker but similarly stable cake, consistent with its stable fluid loss profile. The fluids that exhibited catastrophic fluid loss, with ZB and P15B8 bentonite, correspondingly produced the thickest filter cakes at 125 °C, both measuring a substantial 4.76 mm. This thick, porous deposition is a direct consequence of their inability to maintain a dispersed system at high temperatures, allowing a large volume of filtrate and solids to invade the permeable medium. The filter cake from the KMG1 fluid also thickened progressively with temperature, reinforcing the interpretation that its fibrous network is not effective at creating a compact seal.

Figure 9 illustrates the pH behavior of the five drilling fluid suspensions as a function of dynamic thermal aging temperature. The plot presents five distinct behaviors with markers, each representing a different fluid, tracking its pH value from an initial measurement at 25 °C through subsequent aging steps up to 125 °C. A general downward trend is observable for all formulations, which is consistent with the natural decrease in the pH of aqueous systems at higher temperatures due to the increased autoionization of water. This phenomenon is a recognized challenge in high-temperature water-based fluids, as the drop in pH can negatively impact both fluid properties and additive performance [46,47]. The pH of a water-based drilling fluid is a fundamental parameter that controls its rheological properties by dictating the electrochemical interactions between clay particles, a principle well-established in clay colloid chemistry [14]. Smectite clay platelets have permanently negative faces and pH-dependent edges. In acidic or neutral conditions, the edges become positively charged, promoting edge-to-face attraction.

This leads to an aggregated structure that increases the fluid’s Yield Point (YP), gel strengths, and Plastic Viscosity (PV). Raising the pH to an alkaline state (typically 9.0–10.5) neutralizes the edge charge, causing electrostatic repulsion between particles. This forces the system into a dispersed state, which significantly lowers the YP, gel strengths, and PV. For this reason, pH measurements at elevated temperatures were conducted, where trends were observed based on fluid composition and thermal stability of the additives. A natural decrease in pH with rising temperature would have been expected for all aqueous systems. However, the choice of dispersant showed to cause interesting behaviors. The formulation containing the inorganic dispersant sodium hexametaphosphate (P15B8) presents poor pH stability due to thermal hydrolysis of the polyphosphate above 85 °C, leading to an accelerated pH drop. This chemical instability aligns with its observed failure in filtration control. In contrast, the formulations using the more thermally robust organic dispersant sodium polyacrylate (P15D4 and KMG1) seem to maintain a more stable pH at high temperatures. This superior chemical stability is consistent with their enhanced performance, underscoring the importance of selecting thermally stable additives for high-temperature applications.

5. Conclusions

This research work investigated successfully the suitability of the Ventzia basin clays, when enhanced with appropriate additives, to serve as a high-performance low-density additive for thermally stable water-based drilling fluids. The experimental results revealed a clear matching between the Ventzia clay formulations with commercial bentonites under dynamic thermal aging. The unique mineralogy of the Mg-Fe-smectite and palygorskite clays contributed to a desirable increase in Yield Point and gel strengths at temperatures up to 105 °C, a direct contrast to the significant thermal degradation observed in the commercial products. Critically, this study highlighted the primary importance of the additive package in dictating overall performance. The findings therefore establish that high-performance, sustainable drilling fluids can be developed from local Greek resources, provided their inherent thermal stability is complemented by a thermally robust chemical additive system. Based on the experimental investigation presented in this research, the primary conclusions are as described below:

• The unique Mg-Fe-smectite and smectite–palygorskite clays from the Ventzia basin are viable and effective base materials for formulating high-temperature water-based drilling fluids. When properly enhanced, they demonstrate performance that can meet and, in some aspects, exceed that of commercial API-grade bentonites subjected to thermal stress.
• The modified Ventzia clay formulations exhibited excellent rheological stability compared to the commercial benchmarks of AquaGel and ZB. Their yield point (YP) and gel strengths showed a profound increase with temperature up to 105 °C, indicating enhanced suspension capacity under heat. In contrast, both the commercial bentonites and Ventzia Basin bentonites experienced significant thermal degradation and loss of rheological properties at temperature 125 °C.
• The choice of dispersant was identified as a critical factor for fluid performance at high temperatures. The organic polymer dispersant (PAA) provided excellent thermal stability, resulting in excellent filtration control. The inorganic dispersant (SHMP) proved to be less thermally stable, leading to failure in filtration properties at high temperatures.
• A direct link was established between the chemical stability of the dispersant and the physical performance of the fluid. The pH reduction in the SHMP-treated fluid, which is inferred to result from thermal hydrolysis, correlated directly with its failure in filtration control.
• The study confirmed that fluid properties are a function of specific clay mineralogy. While the fibrous palygorskite in the KMG1 fluid provided exceptional rheological enhancement at high temperatures, its structure was inherently less effective at forming a low-permeability filter cake, resulting in poor filtration control. This highlights a performance trade-off between rheological and filtration properties based on the dominant clay type.

This research contributes significantly to the principles of sustainable development within the energy sector by demonstrating the viability of locally sourced industrial minerals as high-performance substitutes for imported materials. By validating the unique smectite–palygorskite clays from the Ventzia basin for demanding drilling fluid applications, this work provides a pathway to reduce the economic costs and carbon footprint associated with the long-distance transportation of commercial bentonites. The development of these local resources promotes regional economic resilience and supports a circular economy model where indigenous materials are optimized for local industrial needs. Furthermore, the formulation of more thermally stable and efficient water-based drilling fluids enhances operational safety and minimizes environmental risk, aligning with the industry’s continuous efforts to improve its sustainability profile. This approach of leveraging local geology to solve regional engineering challenges serves as a model for sustainable resource management, fostering both economic and environmental benefits. Our future work is oriented towards a more comprehensive performance evaluation under diverse and challenging conditions. This will include testing these formulations in saline and high-hardness saline water to fully assess their viability in complex ionic environments, which is a critical step for real-world drilling applications. Furthermore, we will investigate the inclusion of advanced organic polymers to further improve viscosity and filtration control at high temperatures.