Development and Characterization of High-Strength Coalbed Fracturing Proppant Based on Activated Carbon Skeleton

Abstract

To address the challenges of low permeability, high gas adsorption, and a fragile structure in coalbed methane reservoirs, this study developed a high-strength composite proppant with an activated carbon skeleton via nitric acid pretreatment, silica–alumina sol coating, and calcination. Orthogonal experiments optimized the preparation conditions: 30–40 mesh activated carbon, Si/Al molar ratio of 4:1, calcination at 650 °C for 2 h. The resulting proppant exhibited an excellent performance: a single-particle compressive strength of 55.5 N, porosity of 33.2%, crushing rate of only 2.3% under 50 MPa closure pressure, and permeability 48.5% higher than quartz sand. In simulated acidic coalbed environments (pH 3–5), its acid corrosion rate was <2.8%, and it enhanced methane desorption by 16.2% compared to pure coal. Additionally, the proppant showed a superior transport performance in fracturing fluids, with better distribution uniformity in fractures than ceramsite, and its hydrophobic surface (contact angle 115.32°) improved fracturing fluid flowback efficiency. This proppant integrates high strength, good conductivity, gas desorption promotion, and corrosion resistance, offering a novel material solution for efficient coalbed methane extraction.

1. Introduction

In the context of the global energy transition toward low-carbon sources, coalbed methane has emerged as a pivotal clean and efficient unconventional natural gas resource, playing a critical role in ensuring energy security and mitigating greenhouse gas emissions [1,2,3]. China, endowed with abundant coalbed methane reserves, boasts recoverable reserves reaching 36.8 trillion cubic meters, ranking among the world’s top reserves [4]. However, the commercial exploitation of coalbed methane faces formidable challenges due to the unique geological characteristics of most reservoirs [5,6]. These reservoirs are typically characterized by ultra-low permeability (<0.1 mD), attributed to the dense and poorly connected pore–fracture networks in coal matrices. A high gas adsorption capacity stems from the extensive microporous structure of coal, which tightly binds methane molecules [7,8]. Fragile coal rock is prone to collapse under the disturbance of fracturing operations [6,9]. These factors collectively result in low single-well production and hinder the large-scale development of coalbed methane resources [10,11].

Fracturing technology has long been recognized as the cornerstone for unlocking the potential of low-permeability coalbed methane reservoirs [12,13]. By injecting high-pressure fracturing fluids, artificial fractures are created in the coalbed, and proppants are introduced to prop open these fractures, maintaining their conductivity for sustained gas flow [14]. As such, proppants are indispensable components that directly determine the success of fracturing operations and the long-term productivity of coalbed methane wells. Traditional proppants, however, fail to meet the complex demands of coalbed environments. Quartz sand, despite its low cost and widespread use, exhibits insufficient compressive strength (typically <30 MPa), leaving it susceptible to crushing under the high-geostress conditions prevalent in deep coalbeds (often exceeding 30 MPa) [15]. This crushing leads to the blockage of fractures and a drastic decline in conductivity. Ceramsite, another commonly used proppant, offers higher strength but suffers from high density (>3.0 g/cm³), causing rapid sedimentation in fracturing fluids [16,17]. This uneven distribution within fractures results in inadequate propping of the fracture tips and excessive accumulation at the fracture mouths, compromising the overall fracturing effect. Traditional proppants such as quartz sand and ceramsite, in their unmodified state, lack active interactions with coal matrices to effectively promote gas desorption. While some studies have explored surface modification of these proppants to partially enhance gas desorption, their intrinsic inert surface properties still limit such promotion compared to carbon-based materials with intrinsic affinity for coal matrices [18,19,20].

Activated carbon, a porous carbon material, has garnered significant attention as a potential base material for advanced proppants due to its distinctive properties. Its large specific surface area (usually >1000 m²/g) and well-developed pore structure provide ample space for gas diffusion and adsorption [3,9], while its lightweight nature (1.3–1.5 g/cm³) helps slow sedimentation rates and improve suspendability in fracturing fluids [21]. Moreover, the abundant surface active groups (such as hydroxyl and carboxyl groups) on activated carbon enable chemical modification, facilitating the formation of stable composite structures with other materials [22,23]. These characteristics make activated carbon an ideal skeleton for developing proppants that can simultaneously address mechanical strength, conductivity, and gas desorption challenges [24,25].

While quartz sand and ceramsite suffer from limitations like low porosity and water retention, previous attempts to apply carbon-based materials as proppants have been reported. Early studies exploited their high porosity for gas diffusion, but their practical application was hindered by critical drawbacks: insufficient mechanical strength (easily crushed under reservoir pressure), poor wettability control (prone to water locking), and weak compatibility with fracturing fluids [26,27]. These unresolved issues left a gap in developing robust carbon-based proppants suitable for coalbed methane reservoirs—motivating our work to address these limitations through silica–alumina coating modification.

Building on these advantages, this study proposes a novel approach to fabricate high-strength composite proppants by integrating activated carbon with silica–alumina oxides via the sol–gel method. The preparation process involves nitric acid pretreatment to clean the activated carbon surface, remove impurities, and enhance its surface reactivity, coating with silica–alumina sol to form a uniform and dense protective layer, and calcination to strengthen the bonding between the activated carbon skeleton and the coating. By systematically investigating the effects of key parameters such as activated carbon particle size, silica–alumina molar ratio, calcination temperature, and holding time on the proppant’s performance [9,24,28], this research aims to optimize the preparation conditions and achieve a proppant with superior mechanical properties, high conductivity, and excellent coalbed adaptability. The comprehensive evaluation of the proppant’s microstructure, mechanical strength, corrosion resistance, and gas desorption promotion ability will provide valuable insights into its application potential, ultimately contributing to the technical advancement of efficient coalbed methane extraction [3,29,30,31,32,33].

2. Experimental Design

2.1. Experimental Materials

Activated carbon particles (20–40 mesh, with a specific surface area of 1030 m²/g and a density of 1.34 g/cm³, purity 95%) were procured from Aladdin. Nitric acid (30%), sodium silicate, and sodium aluminate (all of analytical grade) were purchased from Sinopharm. Deionized water was prepared in-house. The main raw materials (activated carbon, sodium silicate, sodium aluminate, etc.) used in this study have a laboratory-scale unit cost of approximately USD 2.5–3.0 per kg, slightly higher than quartz sand but comparable to ceramsite, with the potential for a 30–40% cost reduction in large-scale production.

2.2. Experimental Instruments

A Fourier transform infrared spectrometer (FTIR, Nicolet iS50, Thermo Fisher Scientific, Madison, WI, USA) was used to analyze surface functional groups. An automatic specific surface area and porosity analyzer (Autosorb-IQ, Quantachrome, Tallahassee, FL, USA) was utilized to measure the specific surface area and pore size distribution. A universal testing machine (WDW-100, Jinan, China) was used to test the single-particle compressive strength. Scanning electron microscopy (SEM, SU8020, Hitachi, Tokyo, Japan) was employed to observe the surface morphology and pore structure of the proppant. A thermogravimeter (TGA Q500, TA Instruments, New Castle, NSW, USA) was used to assess the thermal stability of the sample under N₂ atmosphere. A high-temperature and high-pressure conductivity meter (FCES-100, Haian Petroleum Scientific Research Instruments, Haian, China) was employed for measuring the conductivity of the proppant packing under simulated coalbed conditions. A gas adsorption and desorption analyzer (WY-98A, Chongqing, China) was used to test the methane desorption rate. Volumetric adsorption apparatus (Micromeritics ASAP 2020, Norcross, GA, USA) and a contact angle goniometer (Model: DSA100, Krüss, Hamburg, Germany) were utilized.

2.3. Proppant Preparation Process

The pretreatment of activated carbon involved immersing the particles in 30% nitric acid solution, then subjecting them to a 60 min treatment in a 60 °C water bath, followed by thorough washing with distilled water until the pH reached neutrality. Finally, they were dried at 110 °C for 6 h to remove surface impurities and activate the pores (Figure 1).

Silica–alumina sol was selected as the coating material for the subsequent process due to its intrinsic properties, which synergistically enhance proppant performance from multiple dimensions: the silica–alumina network it forms (composed of Si-O-Si and Al-O-Si bonds) exhibits excellent chemical stability in the typical acidic coalbed environment (pH 3–5), effectively isolating the activated carbon skeleton from corrosive media and ensuring long-term structural integrity. Meanwhile, it possesses outstanding thermal resistance, capable of withstanding calcination temperatures up to 650 °C in this study without decomposition, forming a dense and firmly bonded coating with the activated carbon skeleton during calcination and preventing cracking or detachment caused by thermal stress. Furthermore, the reactive silanol groups (Si-OH) and aluminate groups in the sol can chemically react with oxygen-containing functional groups (e.g., -OH, -COOH) on the surface of nitric acid-pretreated activated carbon, significantly enhancing coating adhesion and preventing delamination under high closure pressure. Moreover, its colloidal particles are small and uniformly dispersed, resulting in a thin and permeable coating that covers the activated carbon surface without excessively blocking its intrinsic pores. This maintains gas diffusion channels while enhancing mechanical properties, ensuring the proppant retains a high porosity of 33.2% and good permeability.

For the preparation of the composite slurry, sodium silicate and sodium aluminate were dissolved in deionized water according to different molar ratios of silicon to aluminum (3:1, 4:1, 5:1). The solutions were diluted to a concentration of 10% and magnetically stirred for 30 min to form a stable sol (Figure 2).

In the coating and forming stage, the pretreated activated carbon was submerged in the prepared sol and ultrasonicated for 30 min (at a power of 300 W) to ensure a uniform coating. Excess slurry was removed by vacuum filtration, and the samples were pre-dried at 80 °C for 2 h.

The calcination treatment entailed placing the pre-dried samples in a muffle furnace, heating them to temperatures of 600 °C, 650 °C, and 700 °C at a rate of 5 °C/min, holding them at these temperatures for 2 h, and then allowing them to cool naturally to room temperature.

2.4. Optimization of Preparation Conditions

Based on the sol–gel reaction mechanism, an L9(3⁴) orthogonal experiment was designed, considering factors such as activated carbon particle size (20–30 mesh, 30–40 mesh), silicon–aluminum molar ratio (3:1, 4:1, 5:1), temperature for calcination (600 °C, 650 °C, 700 °C), and holding time (1 h, 2 h, 3 h). The single-particle compressive strength (target > 50 N) and permeability improvement rate (target > 40%) were selected as evaluation indicators to optimize the preparation conditions [34,35,36,37,38,39,40,41,42,43].

The permeability improvement rate (η) is calculated as follows:

η = (K_CP-AC − K_{Quartz Sand})/K_{Quartz Sand} × 100%

where

• K_CP-AC: Conductivity of CP-AC proppant pack (mD·cm), tested by FCES-100 conductivity meter (Section 2.2) under 30 °C and 50 MPa;
• K_{Quartz Sand}: Conductivity of quartz sand proppant pack (10.44 mD·cm) under the same testing.

2.5. Characterization Methods

For microstructural analysis, FTIR analysis was carried out with a scanning range of 4000–500 cm⁻¹, and 32 scans were performed to identify surface functional groups and chemical bonding. The Brunauer–Emmett–Teller (BET) method was utilized to quantify the specific surface area and pore size distribution via nitrogen adsorption–desorption isotherms.

Mechanical properties were evaluated using a universal testing machine with a loading rate of 0.5 mm/min to measure the single-particle compressive strength, and the crushing rate was determined at a closure pressure of 50 MPa.

Conductivity was measured using a high-temperature and high-pressure conductivity meter to assess the conductivity of the proppant packing under simulated coalbed conditions (temperature 30 °C, pressure 20–60 MPa).

The gas desorption promotion ability was evaluated using a gas adsorption and desorption analyzer to test the methane desorption rate of a mixed system consisting of a coal sample (200 g) and the proppant (50 g), with the results compared to those of a pure coal sample. Gas adsorption/desorption tests were performed under conditions simulating in situ CBM reservoirs: temperature was controlled at 32 ± 2 °C (matching the average formation temperature of the study coal seam) and pressure was regulated from 0.1 to 6 MPa (covering the typical pressure range of the target coalbed methane reservoir), using 99.99% pure methane as the test gas.

Corrosion resistance was determined by immersing the proppant in a hydrochloric acid solution with a pH of 3, soaking it at 60 °C for 72 h, and then calculating the acid corrosion rate. Acid resistance was evaluated using HCl (pH = 3) because the formation water in the target coal seam is dominated by chloride-type brine (Cl⁻ concentration ~1200 mg/L, as measured in field samples), making HCl more representative of the in situ acidic environment than H₂SO₄.

To investigate the influence of the flow rate on transport efficiency, comparative experiments were conducted using the composite proppant (CP-AC) and conventional ceramsite under two flow rates (5 L/h and 10 L/h) in a simulated fracture model (length 140 cm, width 2 cm, height 20 cm). The vertical accumulation height of proppants at distances of 20 cm, 40 cm, 60 cm, 80 cm, 100 cm, 120 cm, and 140 cm from the wellbore was measured to evaluate their transport stability. The 140 cm fracture model is a simplified laboratory setup designed to assess proppant transport under controlled conditions (e.g., constant flow velocity, uniform fracture aperture). While it does not fully replicate the complexity of in situ fractures (such as irregular morphology, heterogeneous coal matrix, and dynamic in situ stress), it effectively characterizes key transport behaviors (e.g., proppant migration efficiency and distribution uniformity) that are critical for guiding field applications.

3. Results and Discussion

3.1. Structural Characterization

3.1.1. FTIR Analysis

Figure 3 depicts the FTIR spectrum of the proppant (Figure 3). The broad absorption band spanning from approximately 3400 to 3600 cm⁻¹ can be ascribed to the stretching vibration of -OH groups, which are present on both the activated carbon surface and the silica–alumina coating. A prominent peak emerges at around 1000–1050 cm⁻¹, corresponding to the asymmetric stretching vibration of Si-O-Si bonds. The intensity of this peak varies with the silicon–aluminum molar ratio (ranging from 3:1 to 5:1), indicating different contents of silicate in the coating. Moreover, a relatively weak peak is observed at approximately 700–750 cm⁻¹, which is assigned to Al-O-Si bonds, verifying the formation of a cross-linked silica–alumina network within the coating. At a calcination temperature of 650 °C, the peak intensity of Si-O-Si bonds reaches its maximum, implying an optimal condensation of the silica–alumina sol and an enhanced bonding between the coating and the activated carbon skeleton. After nitric acid pretreatment, a distinct small peak emerges in the range of 1650–1700 cm⁻¹, corresponding to the stretching vibration of C=O bonds in carboxyl groups. This observation confirms an increase in surface oxygen-containing functional groups on the activated carbon, which in turn enhances its ability to form chemical bonds with the silica–alumina coating (

Table 1. Summary of functional groups in the FTIR spectra of the proppant.

Functional Group	Characteristic Wavenumber Range (cm−1)	Change in Spectral Intensity
(-OH)	3400–3600	Broad absorption band
(Si-O-Si)	1000–1050	Varies with silicon–aluminum molar ratio (3:1 to 5:1); reaches maximum at 650 °C calcination
(Al-O-Si)	700–750	Relatively weak
(C=O)	1650–1700	Distinct small peak emerges after nitric acid pretreatment (intensity increases)

).

3.1.2. BET Analysis

The BET results indicated that the specific surface area of the original activated carbon (AC) was 1030 m²/g. After the pretreatment and coating process (CP-AC), the specific surface area decreased to 590 m²/g. This was due to the filling of some pores by the coating material. However, the pore volume increased slightly, and the average pore size shifted towards a larger value, which was beneficial for gas flow and proppant–coal interaction.

To better investigate the changes in the internal pore structure of activated carbon and the type of adsorption toward polymers, we employed the N₂ adsorption–desorption method to measure the adsorption isotherms of the two activated carbon samples. As shown in Figure 4, according to the IUPAC classification [30], the adsorption isotherm of the original activated carbon (AC) presents a typical Type I isotherm, with an adsorption hysteresis loop appearing at relative pressures of 0.4 < P/P₀ < 0.8. This confirms that AC has a rich pore structure and is a microporous adsorbent with a uniform pore size distribution.

The activated carbon after pretreatment and coating (CP-AC) exhibits an adsorption isotherm that differs from AC. Its isotherm shows an enlarged hysteresis loop compared to AC, spanning a wider range of relative pressures (0.5 < P/P₀ < 0.9), indicating a mixed adsorption behavior involving both micropores and mesopores. Additionally, the isotherm of CP-AC tends to rise more sharply at higher relative pressures (P/P₀ > 0.8), which may be attributed to the increase in average pore size. This phenomenon suggests the presence of more mesopores and larger pores in CP-AC, leading to enhanced capillary condensation and multi-molecular layer adsorption as the relative pressure increases [31], which aligns with the earlier BET results of an increased pore volume and larger average pore size.

3.1.3. SEM Analysis and Particle Size Distribution

Scanning electron microscopy was used to observe the surface morphology and microstructure of the composite proppant (CP-AC). A single SEM image of CP-AC revealed that after nitric acid pretreatment, silica–alumina sol coating, and calcination at 650 °C, the composite proppant exhibited a relatively regular particle morphology, with a uniform and dense silica–alumina coating covering the surface. The coating was tightly bonded to the activated carbon skeleton without obvious cracks or detachment, further confirming the formation of stable chemical bonds between the silica–alumina network and the activated carbon surface, as indicated by FTIR analysis. Under a high magnification, it was observed that the coating retained some micron-scale pores, which were interconnected with the pore structure of the activated carbon itself. These pores contributed significantly to the proppant’s porosity of 33.2% and provided channels for gas flow.

Particle size statistical analyses were performed on 100 randomly selected CP-AC particles in the SEM image using image analysis software (Image-Pro Plus 6.0). The results showed that the particle size distribution range of CP-AC was 340–590 μm, with an average particle size of 470 ± 30 μm (Figure 5). This narrow particle size distribution benefited from the selection of 30–40-mesh activated carbon raw materials and the uniform coating process, which helped the proppant form a stable packing structure in fractures and reduced fluctuations in conductivity caused by uneven particle gradation under high pressure. Size distribution analysis was performed on 100 particles randomly selected from three independent SEM images (captured in different regions of the sample) to ensure representativeness. Consistent size distribution patterns (peak at 2–5 μm) were observed across all images, indicating the uniformity of the coating morphology. This multi-region sampling strategy minimizes bias from local inhomogeneity, supporting the reliability of the observed size characteristics.

3.1.4. Thermogravimetric Analysis (TGA)

The thermal stability of the composite proppant (CP-AC) and the original activated carbon (AC) was evaluated using a thermogravimeter (TGA Q500, TA Instruments, New Castle, NSW, USA) under an N₂ atmosphere. The samples were heated from room temperature to 800 °C at a heating rate of 10 °C/min, with a gas flow rate of 50 mL/min to ensure complete oxidation.

The TGA curve of AC (Figure 6a) showed a two-stage weight loss process. The first stage (25–180 °C) exhibited a weight loss of approximately 4.95%, which was attributed to the evaporation of adsorbed water and volatile impurities. The second stage (180–680 °C) involved a significant weight loss of 74.59%, corresponding to the oxidation and combustion of the carbon skeleton. In contrast, the TGA curve of CP-AC (Figure 6b) demonstrated enhanced thermal stability. A slight weight loss of 3.85% occurred in the low-temperature range (25–200 °C), indicating reduced moisture adsorption due to the silica–alumina coating. The main weight loss stage shifted to 200–700 °C, with a total weight loss of 45.65%, which was significantly lower than that of AC. This delay in oxidation temperature and reduction in weight loss confirmed that the silica–alumina coating effectively protected the activated carbon skeleton from early oxidation, thereby improving the overall thermal stability of the proppant.

3.1.5. Wettability Analysis

Wettability, a critical parameter reflecting the interaction between proppants and coalbed fluids, directly affects fracturing fluid flowback efficiency and gas migration behavior. In this study, the wettability of activated carbon (AC) and the composite proppant (CP-AC) was evaluated using a contact angle goniometer (Model: DSA100, Krüss, Hamburg, Germany). Static contact angles of deionized water droplets on the sample surfaces were measured at room temperature (25 °C) via the sessile drop method, with five replicate measurements for each sample to ensure accuracy (Figure 7).

The results indicated a significant shift in wettability after the composite modification. The original activated carbon exhibited a contact angle of 28.88°, demonstrating strong hydrophilicity. This characteristic is attributed to the abundant oxygen-containing functional groups (e.g., -OH and -COOH) on the AC surface, which form hydrogen bonds with water molecules and enhance surface affinity for water. In contrast, the composite proppant (CP-AC) showed a substantially increased contact angle of 115.32°, indicating distinct hydrophobicity. This transformation is primarily due to the formation of a nonpolar silica–alumina network (composed of Si-O-Si and Al-O-Si bonds) in the coating, which covers part of the polar functional groups on the activated carbon surface and reduces the interaction energy with water molecules. Additionally, the micron-scale pores retained in the coating induce a “Cassie–Baxter” effect, further reinforcing the hydrophobicity.

The transition in proppant wettability holds practical significance for coalbed methane extraction. The hydrophobic surface of CP-AC can reduce the retention of fracturing fluids on the proppant surface, accelerating fluid flowback and mitigating permeability damage caused by water blocking. Meanwhile, the contact angle of 115.32° enhances the affinity between the proppant and nonpolar methane gas, facilitating gas diffusion and desorption in pore channels. This hydrophobic property enables the composite proppant to adapt better to high-water-content or water-blocking-prone coalbed environments. Notably, contact angle testing serves as a quantitative basis for evaluating such wettability: the measured 115.32° far exceeds the 90° threshold for hydrophobicity, providing objective evidence for the strong water-repellent capability. This quantifiable characteristic, compared to hydrophilic quartz sand (contact angle typically <60°), directly links laboratory measurements to field performance, confirming the proppant’s potential to alleviate water blocking and improve gas production efficiency in practical applications.

3.2. Mechanical Properties

3.2.1. Single-Particle Compressive Strength

The single-particle compressive strength of the proppants was significantly affected by the preparation conditions. The optimal conditions for achieving high compressive strength were identified through orthogonal experiments: an activated carbon particle size of 30–40 mesh, a silicon–aluminum molar ratio of 4:1, a calcination temperature of 650 °C, and a holding time of 2 h. Under these conditions, the single-particle compressive strength reached 55.5 N, which was 20% higher than that of proppants prepared under non-optimal conditions (

Table 2. L9 (3⁴) Orthogonal experiment results for single-particle compressive strength of proppants.

Test No.	Activated Carbon Particle Size (Mesh)	Si/Al Molar Ratio	Calcination Temperature (°C)	Holding Time (h)	Single-Particle Compressive Strength (N)
1	20-30 (A1)	3:1 (B1)	600 (C1)	1 (D1)	46.25 (minimum value)
2	20-30 (A1)	4:1 (B2)	650 (C2)	2 (D2)	48.5
3	20-30 (A1)	5:1 (B3)	700 (C3)	3 (D3)	47.0
4	30-40 (A2)	3:1 (B1)	650 (C2)	3 (D3)	50.0
5	30-40 (A2)	4:1 (B2)	700 (C3)	1 (D1)	52.0
6	30-40 (A2)	5:1 (B3)	600 (C1)	2 (D2)	49.0
7	20-30 (A1)	3:1 (B1)	700 (C3)	2 (D2)	47.5
8	20-30 (A1)	4:1 (B2)	600 (C1)	3 (D3)	48.0
9	30-40 (A2)	5:1 (B3)	650 (C2)	1 (D1)	51.0
Optimal Condition	30-40 (A2)	4:1 (B2)	650 (C2)	2 (D2)	55.5 (maximum value)

).

The proppants exhibited a crushing rate of 3.2% under 52 MPa closure pressure, which is well below the 10% maximum allowed by API RP 56 (for medium-depth reservoirs) and meets the stricter ISO 13503-2 requirement (<5% for high-performance proppants) [44]. This confirms its compliance with industry standards and superior structural stability compared to conventional proppants.

Figure 8 is a schematic diagram illustrating the mechanical strength enhancement mechanism of the silica–alumina-coated activated carbon proppant (CP-AC). On the left, uncoated activated carbon (AC) undergoes severe fragmentation under applied pressure (indicated by orange “Pressure” arrows)—its inherently porous and relatively loose structure lead to stress concentration, causing extensive crack propagation and subsequent breakage into small fragments. In contrast, on the right, CP-AC is enveloped by a dense silica–alumina coating, and its internal structure is reinforced by cross-linked networks formed via **-Si-O-Si-** and **-Al-O-Si-** chemical bonds. When pressure is exerted, the silica–alumina coating not only fills the original microcracks of AC to reduce stress concentration points but also enables uniform stress distribution through the rigid Si-O-Si/Al-O-Si cross-linked architecture, thus maintaining the structural integrity of CP-AC without breakage. This diagram clearly demonstrates the core mechanism: the silica–alumina coating enhances the mechanical strength of the activated carbon proppant via chemical bonding and structural support.

3.2.2. Crushing Rate

The crushing rate of the proppants under a closure pressure of 50 MPa was also measured. Tests were conducted using 50 g of proppant particles immersed in distilled water (simulating in situ formation water), with pressure maintained for 30 min before sieving through a 40-mesh screen to quantify crushed fragments. The proppants prepared under the optimal conditions had a crushing rate of only 2.3%, much lower than the industry standard of 10%. The enhanced mechanical strength (55.5 N compressive strength, 2.3% crushing rate at 50 MPa) arises from synergistic effects of the silica–alumina coating: its cross-linked network (Si-O-Si/Al-O-Si bonds, confirmed by FTIR) forms a rigid layer to disperse stress; chemical bonding with pretreated activated carbon’s oxygen-containing groups strengthens interfacial adhesion; and the dense coating reduces skeleton defects—avoiding brittle failure like quartz sand, thus boosting robustness. This low crushing rate indicated that the proppants had good mechanical stability and could maintain their integrity under high pressure in the coalbed.

3.3. Conductivity

To assess the flow capacity of the proppant pack, conductivity measurements were conducted under simulated coalbed environments, with results presented in Figure 9. Proppants fabricated under the optimized preparation conditions exhibited a conductivity of 15.5 mD·cm, representing a 48.5% improvement over conventional quartz sand. This enhanced conductivity is linked to the activated carbon skeleton’s well-developed porous structure and the uniformity of its coating, both of which promote efficient gas migration within fractures. The synergistic enhancement of conductivity stems from the coordinated effects of the activated carbon skeleton and silica–alumina coating: The microporous–mesoporous structure of activated carbon, retained after pretreatment (confirmed by BET analysis), provides a primary channel for gas diffusion. Even after coating, it maintains 33.2% interconnected porosity, reducing flow resistance and increasing gas transport channels compared to dense, non-porous quartz sand. The silica–alumina coating, formed by uniform sol particles, avoids blocking the activated carbon’s pores; instead, it preserves micron-scale pores (observed via SEM) that connect with the skeleton’s pores, forming a continuous gas flow network—unlike quartz sand, whose irregular particle stacking often causes narrow or blocked channels. Additionally, the coating enhances compressive strength (55.5 N), limiting the crushing rate to 2.3% under 50 MPa and reducing fine particle blockage, which plagues quartz sand due to its low strength and resultant permeability decline. Moreover, the hydrophobic coating (contact angle 115.32°) reduces fracturing fluid retention and avoids water blockage, unlike quartz sand’s hydrophilicity, which traps water and impairs conductivity. Together, the activated carbon provides a porous foundation, while the coating ensures structural stability and controlled porosity, overcoming quartz sand’s limitations to achieve a 48.5% conductivity improvement.

3.4. Gas Adsorption–Desorption Behavior and Mechanism Analysis

3.4.1. Methane Desorption Promotion Effect

The gas desorption promotion ability of the proppants was evaluated by comparing the methane desorption rate of the CP-AC with that of the AC sample. The results demonstrated that the proppants could enhance the gas desorption rate by 16.2%. The large specific surface area and porous structure of the activated carbon skeleton provided more adsorption sites for gas molecules, promoting the desorption of methane from the coal surface. The results are presented in Figure 10.

To further elucidate the underlying mechanism of gas desorption promotion by the composite proppant, molecular simulations were performed to investigate the interactions between the proppant surface and methane molecules, as well as between the coal matrix and methane molecules. The simulation models were constructed based on the actual structures obtained from experimental characterizations. The activated carbon skeleton surface with oxygen-containing functional groups (carboxyl and hydroxyl groups identified by FTIR) and a representative coal matrix model (composed of polycyclic aromatic hydrocarbons) were built. Methane molecules were introduced into the simulation system, and the interactions were analyzed using the COMPASS force field. The results showed that the interaction energy between methane molecules and the composite proppant surface was lower than that between methane and the coal matrix. Specifically, the van der Waals force between methane and the proppant was weakened by approximately 18.3% compared to that with the coal matrix, while the hydrogen bonding between the proppant’s carboxyl groups and water molecules in the coalbed further reduced the adsorption affinity of methane to the coal surface (

Table 3. Interaction energy analysis of different systems.

System	Van der Waals Energy (kcal/mol)	Hydrogen Bond Energy (kcal/mol)	Total Adsorption Energy (kcal/mol)
Methane–Coal Matrix	−28.6 ± 1.2	−3.2 ± 0.5	−31.8 ± 1.5
Methane-CP-AC	−23.4 ± 0.9	−5.7 ± 0.3	−29.1 ± 1.1

). Moreover, the porous structure of the activated carbon skeleton provided channels for methane diffusion, with a calculated diffusion coefficient of 2.3 × 10⁻⁹ m²/s, which was 32.6% higher than that in the pure coal matrix model. The results are presented in

Table 4. Methane diffusion behavior in different matrices.

Parameter	Coal Matrix	CP-AC Porous Structure	Enhancement Ratio
Diffusion Coefficient (m2/s)	1.73 × 10−9 ± 0.08 × 10−9	2.30 × 10−9 ± 0.11 × 10−9	32.9%
Mean Square Displacement (MSD) at 500 ps	65.2 ± 3.1 Å2	86.6 ± 4.2 Å2	32.8%

. These molecular simulation results directly confirmed that the composite proppant promotes methane desorption by reducing the adsorption energy barrier and enhancing molecular diffusion, consistent with the experimental observation of a 16.2% increase in gas desorption rate. Figure 11 illustrates the comparison of methane (CH₄) desorption behavior between the silica–alumina-coated activated carbon proppant (CP-AC, left) and uncoated activated carbon (AC, right). The optimized pore structure of the silica–alumina coating in CP-AC facilitates the diffusion and desorption of CH₄ molecules, as schematically shown in the center. In contrast, uncoated AC (with a more irregular and less favorable pore configuration) exhibits less efficient CH₄ desorption. Notably, CP-AC achieves a 16.2% increase in the desorption rate compared to AC, attributed to the coating’s tailored pore structure, which enhances the accessibility and release of adsorbed CH₄.

3.4.2. Nitrogen Adsorption Behavior and Competitive Interaction with Methane

To investigate the adsorption behavior of N₂ in coalbed methane, a series of experiments were carried out. Activated carbon (AC) was selected as the adsorbent due to its widespread use in gas separation processes. The experiments were conducted using a volumetric adsorption apparatus (Micromeritics ASAP 2020), which allowed for accurate measurement of gas adsorption under controlled conditions.

The AC samples were first pretreated at 300 °C under vacuum for 3 h to remove any pre-adsorbed impurities. Then, pure N₂ gas (purity > 99.99%) was introduced into the adsorption chamber. The temperature was maintained at 298 K (simulating in situ coalbed temperature), and the pressure range was varied from 0 to 100 kPa. At each pressure point, the system was equilibrated for 2 h to ensure that adsorption equilibrium was reached. The amount of N₂ adsorbed was calculated based on the change in pressure and volume in the system, according to the ideal gas law.

In addition to the single-component N₂ adsorption experiments, binary-component adsorption experiments with N₂ and CH₄ (simulating coalbed methane composition) were also performed. The gas mixture with a fixed ratio of N₂ and CH₄ (e.g., 70% N₂ and 30% CH₄ by volume) was introduced into the adsorption chamber, and the adsorption isotherms were measured under the same temperature and pressure conditions as the single-component experiments.

The adsorption of N₂ on activated carbon can be divided into three main stages, as illustrated in Figure 12. At low pressures, N₂ molecules are adsorbed onto the surface of the activated carbon via van der Waals forces, forming a monolayer. The activated carbon has a complex pore structure, including micropores (pore diameter < 2 nm) and mesopores (2–50 nm). As shown in the pore size distribution curves in Figure 12a, before adsorption (represented by the red dashed line), the AC has a certain pore volume distribution in both micropore and mesopore regions. After monolayer adsorption (blue solid line), the pore volume in the mesopore region shows a slight decrease, while the micropore region remains relatively unchanged. This indicates that monolayer adsorption mainly occurs on the surfaces of mesopores and at the entrances of some micropores. The surface area of the activated carbon, especially the outer surface of mesopores, provides active sites for N₂ molecules to attach.

As the pressure increases, N₂ molecules start to penetrate into the micropores of the activated carbon. The small size of N₂ molecules allows them to fit into the narrow micropores. The micropores in activated carbon have a high surface-to-volume ratio, providing strong adsorption forces for N₂. As can be seen from the pore size distribution curves in Figure 12b, after micropore filling, the pore volume in the micropore region decreases significantly, and there is also a further reduction in the mesopore region. This is because some N₂ molecules that were initially adsorbed on the mesopore surface may migrate into the micropores. The filling of micropores is crucial for the overall adsorption capacity of activated carbon for N₂, as micropores contribute a large proportion of the total surface area of the adsorbent.

At higher pressures, close to the saturation vapor pressure of N₂ at the experimental temperature, multilayer adsorption occurs on the surface of the activated carbon, and capillary condensation takes place in the mesopores and larger pores. In Figure 12c, the pore size distribution curves show a significant decrease in the pore volume of both mesopore and macropore (pore diameter > 50 nm) regions after this stage. Multilayer adsorption is due to the continuous attraction of N₂ molecules to the already-adsorbed N₂ layers on the carbon surface. Capillary condensation occurs when the pressure is high enough for N₂ to form a liquid-like phase in the pores. This is because the curved liquid–vapor interface in the pores reduces the vapor pressure required for condensation (Kelvin effect). The presence of moisture in the coalbed environment can also affect this stage, as water molecules may compete with N₂ for adsorption sites, especially in the polar regions of the activated carbon surface.

In the binary-component adsorption system of N₂ and CH₄, competitive adsorption occurs. CH₄ has a larger molecular size and a higher polarizability compared to N₂. The activated carbon has a certain selectivity for CH₄ over N₂ due to differences in adsorption energy. However, the adsorption capacity for both gases is affected by their partial pressures in the mixture. At low partial pressures of CH₄, N₂ can still be adsorbed to a significant extent. As the partial pressure of CH₄ increases, it competes more effectively with N₂ for adsorption sites, reducing the amount of N₂ adsorbed.

In conclusion, the adsorption of N₂ in coalbed methane on activated carbon is a complex process influenced by factors such as pore structure, gas pressure, temperature, and the presence of other gases. Understanding these mechanisms is crucial for optimizing the separation and purification of coalbed methane.

3.5. Corrosion Resistance

In the simulated acidic coalbed environment (pH = 3–5), the acid corrosion rate of the proppants was less than 2.8% after 72 h soaking, which was significantly lower than that of conventional ceramsite (e.g., 11.3% under the same conditions) and comparable to high-purity quartz sand (e.g., 3.4% under the same conditions). The silica–alumina coating on the surface of the proppants played a critical protective role, effectively preventing the activated carbon skeleton from acid corrosion. This favorable corrosion resistance, combined with its comprehensive performance advantages over traditional proppants, ensures the long-term stability of the proppants in the coalbed. Figure 13 and Figure 14 illustrates the acid resistance performance of uncoated activated carbon (AC, left) and silica–alumina-coated activated carbon proppant (CP-AC, right) after being exposed to acid for 72 h. On the left, uncoated AC shows severe fragmentation—acid droplets penetrate its porous structure, causing extensive cracking and breakage over the 72 h period. In contrast, CP-AC on the right maintains its structural integrity because the silica–alumina coating acts as a protective barrier, preventing the acid from corroding the inner activated carbon skeleton.

3.6. Proppant Transport Performance

The results showed significant differences in transport performance between CP-AC and ceramsite under both flow rates (Figure 15). At a flow rate of 5 L/h, CP-AC maintained a higher accumulation height than ceramsite across all measured distances. Near the wellbore (20 cm), CP-AC reached 18.6 cm, slightly higher than ceramsite (16.5 cm). As the distance increased, the height of both proppants decreased, but CP-AC showed slower and more irregular decay. For instance, at 60 cm, CP-AC retained 16.5 cm, while ceramsite dropped sharply to 9.8 cm (a 26.9% reduction compared to CP-AC’s 4.0% drop). At the fracture tip (140 cm), CP-AC still maintained a height of 7.8 cm, which was five times that of ceramsite (1.6 cm), demonstrating its superior ability to reach distal fractures.

At an increased flow rate of 10 L/h, both proppants showed an improved transport efficiency due to an enhanced fluid carrying capacity, but CP-AC remained advantageous. Near the wellbore (20 cm), CP-AC reached 19.0 cm, marginally higher than ceramsite (17.2 cm). At 100 cm, CP-AC’s height (15.2 cm) was 2.3 times that of ceramsite (6.5 cm). A notable observation was the irregular decay of CP-AC: its height decreased moderately from 100 cm (15.2 cm) to 120 cm (12.0 cm) but dropped more sharply to 5.7 cm at 140 cm, simulating the natural weakening of flow at fracture tips. Even so, CP-AC’s tip height (5.7 cm) was still 2.5 times that of ceramsite (2.3 cm), confirming its superior retention under high flow conditions.

The superior transport performance of CP-AC is attributed to its low density (1.45 g/cm³) compared to ceramsite (3.1 g/cm³), which reduces the settlement tendency, and its porous structure, which enhances interaction with fracturing fluids to form a “viscous buffer layer” that slows settlement. The irregular decay trends reflect real-world fracture heterogeneities, such as varying flow velocities and local turbulence, further validating CP-AC’s adaptability to complex coalbed environments.

4. Conclusions

In this study, a high-performance coalbed fracturing proppant with an activated carbon (AC) skeleton was successfully synthesized through a systematic process involving nitric acid pretreatment, silica–alumina sol coating, and calcination. Orthogonal experimental optimization confirmed that the optimal preparation conditions—30–40-mesh AC particles, a Si/Al molar ratio of 4:1, and calcination at 650 °C for 2 h—yielded a proppant with exceptional comprehensive properties. Mechanically, the proppant exhibited a single-particle compressive strength of 55.5 N and a crushing rate of only 2.3% under 50 MPa closure pressure, outperforming traditional quartz sand and meeting the demands of high-geostress coalbed environments. Its permeability improvement rate, at 48.5% higher than quartz sand, combined with a porosity of 33.2%, ensured efficient gas flow through fractures, supported by a well-developed porous structure and stable silica–alumina coating. In terms of coalbed adaptability, the proppant demonstrated strong acid corrosion resistance (<2.8% corrosion rate in pH 3–5 environments after 72 h) and a 16.2% enhancement in methane desorption efficiency. Molecular simulations and experimental data confirmed that this desorption promotion stemmed from reduced methane–surface interaction energy and improved diffusion dynamics. Additionally, its low density (1.45 g/cm³) enhanced transportability in fracturing fluids, with distal fracture accumulation five times higher than ceramsite, while its hydrophobic surface (contact angle 115.32°) facilitated fracturing fluid flowback.

This novel proppant integrates high mechanical strength, superior conductivity, gas desorption promotion, and environmental stability, addressing key limitations of conventional proppants in coalbed methane extraction. However, it is important to acknowledge certain limitations: the proppant is currently suitable for medium–shallow coalbeds (≤2000 m depth, ≤50 MPa closure pressure, single acidic environments) but not for deep seams (>2000 m, >50 MPa) or multi-ion extreme conditions. It remains at the lab scale (<100 g per batch) without pilot validation, and assumptions like using regular simulated fractures may differ from actual irregular coal seams. Uncertainties include an untested long-term performance (only 72 h tests) and unconfirmed long-term compatibility with coal matrices. Future work should focus on scaling up production to reduce costs and validating the performance under field conditions with complex geological variations, further solidifying its potential as a transformative material for efficient CBM recovery.