Ceramsite-Based Graphite Composite Thermally Conductive Proppant: Preparation, Characterization, and Performance Regulation

Abstract

Coalbed methane (CBM) reservoirs are characterized by low permeability and poor methane desorption, which limit recovery rates. To address this, a novel graphite composite thermally conductive proppant is proposed, offering enhanced thermal conductivity and mechanical performance. The composite consists of porous ceramsite as a mechanical scaffold, epoxy resin as an interfacial binder, and graphite as a thermally conductive reinforcement. The effects of graphite content and resin dosage on the composite’s structure, thermal conductivity, suspension stability, surface wettability, and interfacial adhesion are systematically investigated. The results show that an optimized formulation with 20 wt% graphite and 1.0 g epoxy resin achieves a thermal conductivity of 3.8 W/(m·K)—6.3 times that of pure ceramsite—along with an improved thermal response under simulated stimulation, good suspension stability (suspension ratio of 0.53 in 0.2 wt% guar gum solution), a hydrophobic surface (contact angle 74.9°) to mitigate water lockup, and strong interfacial adhesion (125 nN under 2500 nN load) for durable proppant performance. Microscopic analysis confirms the formation of a continuous “resin–graphite–ceramsite” three-phase interface and a percolative thermal conductive network. This study provides a feasible design strategy for high-performance thermally conductive proppants and demonstrates their potential for application in the hydraulic fracturing of unconventional oil and gas reservoirs.

1. Introduction

As the world moves toward low-carbon energy systems, coalbed methane (CBM) has become an attractive unconventional natural gas resource due to its abundance and clean burning properties. CBM extraction needs to be efficient to guarantee energy security and help achieve carbon neutrality. But CBM reservoirs generally have low permeability, low reservoir pressure, and strong methane adsorption by coal matrix micropores. These inherent features mean that normal depressurization methods do not work well, so the total amount we can get back is not good enough [1,2].

Hydraulic fracturing is still the primary reservoir stimulation method; proppants such as quartz sand and ceramsite are injected into the wellbore to maintain the conductivity of artificial fractures in an open state [3]. Many studies have been done on improving the performance of proppants through surface improvements, controlling shapes, or applying composite resins [4]. Even though we have made so much progress, traditional proppants are just passive mechanical supports, and they do not solve the core issue: how to get the methane which has been adsorbed to come off quickly and go into the cracks [5].

Reservoir temperature increases are known to reduce methane adsorption capacity and accelerate desorption rates [6]. Thermal stimulation techniques such as steam injection, hot water circulation, and in situ electric heating have, thus, gained more attention [7]. But these methods have their own limitations: heat transfer mainly relies on fluid convection or radiation, and this method is not effective in coal rocks due to their extremely low thermal conductivity (0.2–0.5 W/(m·K)) [8]. It causes uneven heating, high energy usage, and inadequate stimulation outcomes in deep reservoirs or low-permeability matrix regions [8]. Some recent exploratory studies have attempted to improve the heat transfer near the wellbore by adding thermally conductive particles (e.g., metal powders, carbon materials) to the fracturing fluids or cement slurries [9]. But these ways face difficulties such as particles settling down, not fitting well with the rock formation, and being unable to go through deep cracks [10]. So, we need a proppant material that can be carried far away to the fracture and take part in the whole reservoir heating plan [11].

To fill this gap, this paper proposes a “material-enabling” approach: creating a multifunctional proppant with built-in high thermal conductivity. During fracturing, this proppant—alone or combined with conventional proppants—is pumped into the fractures to form what is known as an “embedded fracture thermal conduction network,” which has excellent fluid flow characteristics and efficient heat transfer [12]. With downhole thermal stimulation added such as electric or electromagnetic heating, heat will be rapidly and evenly spread out on the surface of the fracture and nearby coal matrixes via the proppant network, enabling local and precise heating [13]. This activates three kinds of enhancements simultaneously: (1) heating-induced desorption that raises coal’s temperature directly, reducing methane adsorption and speeding up desorption [14]; (2) thermal expansion that improves micropermeability due to different thermal expansion coefficients between coal and pore water; when heated unevenly, it generates new small cracks [8]; and (3) possible catalytic pyrolysis that occurs if graphite and other carbon materials formed from coal might serve as catalysts for breaking down coal’s organic compounds at elevated temperatures, releasing additional in-place methane [15].

Graphite was selected as the optimal thermal conductor filler due to its outstanding inherent thermal conductivity, excellent heat resistance at high temperatures, light weight, and potential catalytic role [16]. To effectively bind the graphite to the ceramsite substrate and form a durable, functional composite, an epoxy resin system was selected as the polymeric matrix and interfacial agent. This choice was based on epoxy resin’s exceptional adhesive strength to diverse surfaces, its excellent chemical and thermal stability in downhole conditions, its ability to be processed into a thin, conformal coating on irregular particles, and its tunable cross-linking density which allows for balancing between mechanical rigidity and fracture toughness. But making top-notch graphite composite proppants comes with its own set of difficulties: ensuring that graphite and the inorganic ceramsite matrix bond tightly and lastingly even under great pressure and fluid surroundings [17]; maintaining essential proppant characteristics (floating, not crumbling easily, permitting electrical passage) even after incorporating graphite [18]; and altering the blend and minor structure so that the heat web links up best and makes surfaces wettable for various liquids flowing via coal [19]. To solve these problems, a three-step preparation method of “matrix activation–interfacial construction–functional composite” was designed. Porous ceramsite was used as the main frame because it is strong, anti-corrosive and cheap [20]. First, the surfaces of ceramsites were treated with epoxy resins to clean, activate and create a tough polymer transitional layer. This layer could also serve as a binder to immobilize graphite, change the shape of the particles, and maybe go through a process of carbonization under the temperature field of the wellbore to enhance the interfacial bonding [21]. Next, the graphite flakes were evenly and firmly attached to the surface of the modified ceramsite by carefully controlling the ratio of epoxy resin to graphite, thus creating a “ceramsite–resin–graphite” three-phase composite which can be adjusted [22].

We hypothesize that the epoxy resin coating is critical for constructing an effective interface to anchor graphite and facilitate the formation of a percolative thermal network. To test this, the present work focuses on two key variables: graphite content (for conductivity) and resin dosage (for interfacial bonding and stability). The primary objectives are to (1) systematically evaluate their individual and combined effects on the composite’s properties and (2) identify the optimal composition that maximizes thermal performance while maintaining proppant functionality. The selection of graphite and epoxy resin is based on their complementary roles as a conductive filler and a versatile polymeric binder/matrix, respectively. This work aims to provide a novel technical pathway for the development of high-performance thermally conductive proppants, as well as theoretical and practical insights for the rational design of multifunctional composites [23].

2. Experimental Section

2.1. Materials and Chemical Reagents

Industrial-grade porous ceramsite (purchased from a Shandong-based building materials company) served as the composite matrix, with Al₂O₃ and SiO₂ accounting for >85% of its chemical composition. Graphite powder (D50 = 20.5 μm, fixed carbon content > 99%) was supplied by Qingdao Huatai Graphite Co., Ltd. (Qingdao, China) as the thermally conductive filler. Bisphenol A epoxy resin (E-44, epoxy value 0.44 eq/100 g; Yueyang Petrochemical) (Yueyang, China) and polyamide curing agent (type 650, amine value 200 ± 20 mg KOH/g) were used as surface modifiers and binders. Analytical-grade anhydrous ethanol (Sinopharm Group) (Tianjin, China) and laboratory-synthesized deionized water (resistivity ≥ 18.2 MΩ·cm) were employed for cleaning. Industrial-grade guar gum powder (viscosity ≥ 5500 mPa·s) was used to prepare suspension test solutions.

2.2. Preparation of Composite Thermally Conductive Proppant

2.2.1. Pretreatment of Ceramsite Matrix

Raw ceramsite was initially crushed using a jaw crusher and ball-milled in a planetary ball mill at 300 rpm for 2 h. The milled powder was sieved to collect 50–100 mesh particles (150–300 μm). These particles were ultrasonically cleaned in anhydrous ethanol and deionized water for 30 min each to remove surface contaminants, then dried in a blast oven at 80 °C for 12 h. The pretreated ceramsite was designated as TL.

2.2.2. Preparation of Epoxy Resin-Modified Ceramsite (TL-1)

Epoxy resin coating was performed to improve interfacial compatibility and bonding. Briefly, 2.0 g of epoxy resin (E-44) was thoroughly mixed with 0.66 g of the polyamide curing agent (a stoichiometric mass ratio of 3:1) in a 100 mL glass beaker. The mixing was conducted using a digital overhead stirrer (IKA RW 20) at 500 rpm for 5 min at room temperature (25 °C) to obtain a homogeneous mixture. Subsequently, 6.0 g of pretreated TL particles (resulting in an epoxy resin-to-ceramsite mass ratio of 1:3) was added to the resin mixture. The slurry was stirred at the same speed (500 rpm) for 15 min to ensure a uniform and complete coating of each particle. After mixing, the slurry was transferred to a Teflon-coated tray and cured under ambient laboratory conditions (25 °C, relative humidity 50 ± 10%) for 24 h. After curing, the product consisted of lightly agglomerated coated particles, designated as TL-1.

2.2.3. Preparation of Graphite/Ceramsite Composite Proppant

A two-variable experimental design was adopted, varying the epoxy resin dosage (Y = 1.0 g, 2.0 g) and graphite loading (0.6, 1.2, 1.8, 2.4 g). For each batch, the epoxy resin and curing agent were first mixed in the designated mass ratio (3:1) following the procedure in Section 2.2.2. Then, a fixed mass of 6.0 g of TL particles was introduced into the uncured resin mixture to form a pre-coated slurry. Immediately afterwards, a precisely calculated mass of graphite powder (e.g., 1.2 g for a 20 wt% target in the TL-20%C-1 composite) was added gradually to the slurry. To achieve uniform dispersion of graphite within the viscous resin layer and prevent particle agglomeration, the entire mixture was vigorously stirred at 800 rpm for 20 min using the aforementioned overhead stirrer. Special attention was paid to scrape the beaker walls intermittently to ensure homogeneous incorporation of all components. The final slurry was then spread on a Teflon tray and cured under the same conditions as in Section 2.2.2 (25 °C, 24 h) (Figure 1). It was designated as TL-X%C-Y (where X represents the graphite mass fraction and Y the epoxy resin mass in grams per 6.0 g of TL). For instance, TL-20%C-1 denotes the composite containing 20 wt% graphite (1.2 g) and 1.0 g of epoxy resin.

2.3. Characterization and Performance Testing Methods

Field-emission scanning electron microscopy (FE-SEM, Hitachi SU8220) (Tokyo, Japan) equipped with an energy-dispersive spectrometer (EDS, X-MaxN 80) (Oxfordshire, UK) was used to characterize microstructure and elemental distribution; the acceleration voltage was set to 10 kV during observation. Fourier transform infrared spectroscopy (FT-IR, Thermo Nicolet iS50) (Waltham, MA, USA) in attenuated total reflection (ATR) mode (4000–500 cm⁻¹, 32 scans, 4 cm⁻¹ resolution) was employed for chemical structure analysis [24]. Thermal conductivity was evaluated using a custom steady-state heat flow platform: a precision hotplate (IKA C-MAG HS 7) (Staufen, Germany) provided a 200 °C heat source, and a K-type thermocouple coupled with a data logger recorded surface temperature every 5 s over 30 min [25]. Suspension stability was assessed by measuring the mass ratio of suspended particles in guar gum solutions of varying concentrations (0.1, 0.2, 0.3 wt%); the test was conducted at 25 °C, and each sample was tested in triplicate [15]. Surface wettability was determined by means of the sessile drop method using a contact angle goniometer (DataPhysics OCA 20), with deionized water serving as the test liquid and a droplet volume of 5 μL; five test points were measured on each sample, and the average value was adopted [19]. AFM (Bruker Dimension Icon) (Stanford, CA, USA) PeakForce QNM mode was applied to determine the interfacial adhesion at different loads (500–3000 nN) [26]. The crystal structure of the materials was analyzed using an X-ray diffractometer (XRD-7000, Shimadzu, Kyoto, Japan).

3. Results and Discussion

3.1. Characterization Analysis

3.1.1. From TL to TL-1

Raw ceramsite (TL) has a very rough surface with lots of little bumpy holes and grooves when looked at under a microscope (see Figure 2a). The porous and grooved structure leads to a low density and large specific surface area [20]. From the EDS mapping in Figure 2b, it can be seen that Al, Si and O are the main elements. Epoxy resin modification (TL-1) significantly alters the surface morphology (Figure 2c): large pores and sharp edges are partially covered by a smooth, continuous polymer film, but the macroscopic roughness is retained. This film makes the surface smoother, and it also adds some organic functional groups such as epoxy, hydroxyl, and amino, which can give a chemical bond to the graphite and make it better to mix with organic materials [21,27]. FT-IR spectroscopy (Figure 2d) shows these changes: TL-1 still has the characteristic Si-O-Si stretching vibration (~1028 cm⁻¹) and O-H broad peak (~3430 cm⁻¹) of ceramsite, and there are new peaks due to epoxy resin (aliphatic C-H stretching at ~2925 and 2850 cm⁻¹, N-H bending of primary amines near ~1600 cm⁻¹) [21]. Physical coating plus chemical bonding forms a good three-phase composite [22].

3.1.2. Graphite Composite Formation and Three-Phase Interface Analysis

Graphite is added to give the composite different surface features. In TL-20%C-1 (Figure 3a), there is an evident “embedded” structure: lamellar graphite particles (5–15 μm) are embedded within the epoxy resin film, and their edges are distinct, with the interfaces between them being closely adhered. It forms a “sandwich” structure—ceramsite core, epoxy resin layer, and graphite functional shell—showing that graphite is not just mixed into the ceramsite matrix, but chemically bonded to it [17], having extensive contact and overlap, forming a graphite network on the surface necessary for heat conduction [28].

EDS mapping (Figure 3b) indicates that C (graphite/resin) and Al/Si (ceramsite) are complementarily distributed in TL-20%C-1. There is a high C signal on the graphite part and a middle C signal on the exposed resin area [20]. FT-IR Spectroscopy (Figure 3c) shows the formation of composites: all graphite-containing samples have ceramsite and epoxy resin peaks, along with shoulder peaks around 1617 cm⁻¹ and 1507 cm⁻¹ corresponding to C=C in-plane stretching of graphite’s carbon ring skeleton [24]. They become stronger when there is more graphite; therefore, we can tell from the graphs how much graphite has been added [24].

Figure 4 shows the XRD patterns of TL, TL-1, and TL-20%C-1. As can be seen from the figure, TL exhibits distinct characteristic XRD diffraction peaks, with multiple characteristic peaks observable near 25.9°, 35.5°, 43.6°, and 57.8°. After coating with epoxy resin (sample TL-1), the intensities of these characteristic diffraction peaks of TL significantly decrease or almost disappear, indicating effective encapsulation of TL by the surface polymer. Upon further addition of graphite (sample TL-20%C-1), a characteristic diffraction peak belonging to the graphite (002) crystal plane appears near 26.8° in the pattern, confirming the successful introduction of graphite.

3.2. Thermal Conductivity

3.2.1. Initial Effect of the Epoxy Resin Modification Layer

The time–temperature curve (Figure 5a) shows that TL and TL-1 have different thermal responses. At 200 °C heating (the external heat source), TL has a faster initial heating rate and a slightly higher quasi-steady-state temperature than TL-1, which is always 2–3 °C lower. This difference is due to the “thermal barrier” effect of the epoxy resin layer (intrinsic thermal conductivity < 0.2 W/(m·K)), which increases the interfacial thermal resistance and hinders heat conduction [8]. This result confirms the continuity and integrity of the resin coating on TL-1 [27].

3.2.2. Synergistic Effect of Graphite Content and Thermal Conductive Network

The thermal conductivity of the pristine graphite used in this study is 107.2 W/(m·K). Graphite incorporation fundamentally enhances the thermal conductivity of the composites (Figure 5b,c), with consistent trends observed for both resin dosages. At 10 wt% graphite (e.g., TL-10%C-1, TL-10%C-2), thermal conductivity is marginally improved (1.2 W/(m·K) for TL-10%C-1 compared to TL-1 (0.8 W/(m·K)), but the heating curve plateaus at a low temperature. This indicates that low graphite loading fails to form a continuous thermal network, with heat transfer still dominated by the low-conductivity resin matrix and ceramsite–resin interface [28].

At 20 wt% graphite and above, thermal conductivity undergoes a qualitative improvement: heating curves rise rapidly and linearly, reaching a plateau at 190–194 °C. Notably, TL-20%C-1, TL-30%C-1, and TL-40%C-1 exhibit nearly identical final temperatures (Figure 3b) and thermal conductivity values (3.8–4.0 W/(m·K)), as do their 2.0 g resin counterparts (3.2–3.4 W/(m·K), Figure 3c). This confirms a graphite “percolation threshold” of ~20 wt%, above which graphite flakes form a three-dimensional connected thermal network [28]. However, the composite’s thermal conductivity (~4.0 W/(m·K)) remains orders of magnitude lower than that of pristine graphite (107.2 W/(m·K)), primarily due to the significant interfacial thermal resistance at the boundaries between contacting graphite flakes and between graphite and the resin. Heat transfer is then dominated by the low-resistance graphite network, bypassing the resin matrix and resulting in stable, high thermal conductivity [8]. Further increases in graphite content yield minimal improvements, as network connectivity approaches saturation [28].

3.2.3. Optimization of Epoxy Resin Dosage

Above the percolation threshold, more resin means worse thermal conductivity. From Figure 5d, we can see that TL-20%C-1 (1.0 g resin) has a better performance than TL-20%C-2 (2.0 g resin). It heats up faster and reaches a higher steady-state temperature by 3 °C, with thermal conductivities of 3.8 W/(m·K) and 3.2 W/(m·K) respectively. The bonding strength vs. interfacial thermal resistance trade-off is significant: epoxy resin functions as both a binder for the graphite–ceramsite interface and a thermal barrier that increases the interfacial resistance [20]. To cut back the resin quantity from 2.0 g to 1.0 g, yet maintain the network linked together (verified via SEM), raise the proportion of graphite above resin and slim down the resin bonding layer’s thickness, thus lowering the interfacial thermal resistance. This supports the idea of using fillers to their fullest extent with the least amount of binder [22].

3.3. Suspension Stability

3.3.1. Suspension Enhancement Mechanism of Epoxy Resin Modification

Raw ceramsite (TL) has a bad suspension stability (<0.01 suspension ratio) in guar gum solution (Figure 6a) because it is too heavy (density > 2.0 g/cm³) and has a wet surface which causes it to sink quickly [15]. Epoxy resin modification (TL-1) greatly enhances suspension performance, achieving a suspension ratio of 0.38 with 0.2 wt% guar gum. Improvements occur due to reduced visible density (epoxy resin density ≈ 1.2 g/cm³) and altered surface characteristics. A polymer layer changes Zeta potential and hydrophilicity, increasing steric hindrance and interactions between guar gum molecules for better dispersion stability.

3.3.2. Suspension Behavior of Graphite Composites and Trade-Off with Thermal Conductivity

Graphite composites have more complicated suspensions (Figure 6b,c). All composites outperform TL, yet their suspension ratios are marginally less or identical to those of TL-1. For each resin dosage group, as the amount of graphite increases, so does the suspension ratio (e.g., from 0.54 for TL-10%C-1 to 0.52 for TL-40%C-1 with 0.2 wt% guar gum). This means that resins make something less dense, but graphite, which is pretty heavy (around 2.2 g per cubic centimeter), can help a little bit, along with small changes in how things fit together, since graphite does not like water much (Section 3.4) [19].

Most importantly, TL-20%C-1′s suspension ratio is 0.53, just 1.9% less than TL-10%C-1, yet its thermal conductivity is much better at 3.8 W/(m·K), with a steady-state temperature 20 °C higher. So, this is a good balance of thermal conduction, and enough suspension stability makes it perfect for use underground [18]. TL-20%C-2 has a slightly higher suspension ratio (0.55) but worse thermal performance (3.2 W/(m·K)). So, TL-20%C-1 is the best compromise between these two important engineering properties [15].

3.4. Surface Wettability and Interfacial Adhesion Behavior

3.4.1. Transition from Hydrophilic to Hydrophobic

Contact Angle Measurement (Figure 7) shows a great difference in surface energy. Raw ceramsite (TL) and epoxy-modified ceramsite (TL-1) are hydrophilic, with contact angles of about 45.6° and 52.3° respectively, which is due to the presence of polar -OH and Si-O groups on their surfaces [20]. Graphite inclusion causes hydrophobic transformation: TL-20%C-1 has a contact angle of 74.9° because graphite has a low surface energy and a nonpolar aromatic carbon ring structure [19]. This hydrophobicity reduces the formation of water retention in the fractures (water locking) and enhances the gas phase flow and permeability in CBM reservoirs [16].

3.4.2. Nanoscale Interfacial Adhesion

AFM measurements (Figure 8) measure nanoscale interfacial adhesion. In Figure 8a, the adhesion force increases linearly with the probe load force due to a stronger van der Waals interaction caused by a closer contact and a larger actual contact area [26]. At each load force, adhesion is as follows: graphite composite > TL-1 > TL. For example, at a load force of 2500 nN, TL-20%C-1 has an adhesion force of 125 nN, which is 23% higher than TL (102 nN) and 15% higher than TL-1 (108 nN).

This adhesion also improves through both morphological and chemical improvements: the epoxy resin layer gives a viscoelastic surface which deforms when the probe is applied, increasing the contact area [11]; graphite flake adds a complex surface shape and more points of contact, making the van der Waals force stronger, so the silica AFM probe can fit into the space between graphite flakes better [26]. Figure 8b shows that the adhesion force does not change much within the first 3 s after contact, so we know the adhesion comes from the surface itself rather than how it changes over time as a soft, stretchy material [26]. This high and steady nanoscale sticking power means that big pieces will fit into the cracks just right, so water cannot wash them away, and they will stay tough under pressure for a long time [18].

4. Conclusions

This study reports the development and systematic characterization of a ceramsite-based graphite composite thermally conductive proppant, with an emphasis on preparation optimization, microstructure regulation, and performance evaluation. Changing the amount of graphite added and the amount of resin used will give us composites that have different structures and properties, and they are related to each other.

Our key findings are as follows. (1) By adjustment, a “ceramsite–resin–graphite” three-phase structure can be formed with a critical graphite percolation threshold of about 20 wt%, forming an effective thermal conductive network. (2) The optimized composite (20 wt% graphite, 1.0 g resin) has a higher thermal conductivity of 3.8 W/(m·K), which is 6.3 times that of pure ceramsite. It also has better suspension stability, with a suspension ratio of 0.53 in 0.2 wt% guar gum solution, a hydrophobic surface with a contact angle of 86.1°, and improved interfacial adhesion with a value of 125 nN at 2500 nN load, addressing some of the main needs for proppant use in hydraulic fracturing. (3) An epoxy resin interlayer improves the interface compatibility and fixes graphite flake; a graphite network controls the heat conduction of the composite. In addition to producing materials, this research provides a method for creating high-performance thermally conductive proppants: using hard porous ceramsite as a framework, enhancing the epoxy resin layer to fix useful fillers, and achieving the desired level of thermal conductivity improvement through the formation of a graphite percolation network. The resulting composite material could be used for the hydraulic fracturing of unconventional reservoirs such as CBM. The application context of the developed proppant is critical: it is intended for downhole placement in coal seams, where long-term stability under high-pressure, fluid-saturated, and confined conditions is the primary concern, differing from the weatherability requirements of proppants used in open-air environments. In the future, we will focus on the long-term stability under simulated reservoir conditions and the proppant–coal rock interaction mechanism so that this technology can be applied in practice.