Mitigation of Fracturing Fluid Leak-Off and Subsequent Formation Damage Caused by Coal Fine Invasion in Fractures: An Experimental Study

Abstract

During the hydraulic fracturing process of coalbed methane (CBM) reservoirs, significant amounts of secondary coal fines are generated due to proppant grinding and crack propagation, which migrate with the fracturing fluid into surrounding fracture systems. To investigate whether coal fines can form plugs to reduce fluid leak-off during the hydraulic fracturing stage, we conducted physical simulation experiments on coal seam plugging and unplugging to demonstrate that coal fines indeed contribute to reducing fluid leak-off during hydraulic fracturing. We also explored the plugging mechanisms of coal fines under different concentrations and particle sizes in fracturing fluids, and revealed the damage law of coal fines of temporary plugging on reservoir permeability. Research results indicate the leak-off volume of fracturing fluids containing coal fines is lower than an order without coal fines, demonstrating a significant effect of coal fines in decreasing fluid leak-off. The temporary plugging rate of coal fines increases with higher concentrations and decreases with larger particle sizes, achieving rates exceeding 90%. The high temporary plugging effect of coal fines results from the superposition of internal and external filter cakes. Under conditions of small particle size and high concentration, the damage to fractures during the fine return process is minimized. Considering the potential damage of coal fines to propping fractures and wellbore, the concentration of coal fines in fracturing fluids should be kept relatively low while ensuring a high temporary plugging effect. Overall, these findings provide crucial insights into optimizing the temporary plugging performance of coal fines during the hydraulic fracturing stage and controlling their behavior during the fracturing fluid flow-back stage, thereby enhancing reservoir fracturing effectiveness and improving CBM production rates.

1. Introduction

Coalbed methane (CBM) is a low-carbon fossil energy primarily composed of CH4. Efficient development and utilization help alleviate the burden of human energy consumption, reduce mine gas hazards, and mitigate greenhouse gas emissions [1,2,3,4]. CBM generally exists in an adsorbed state (over 90%) within low-permeability coal seams [5,6]. To obtain substantial industrial gas flow, reservoir stimulation through fracturing is typically required, creating a complex network of interconnected fractures in the coal seams to provide favorable artificial pathways for CBM production [7,8]. Hydraulic fracturing is a common reservoir enhancement technique in the CBM development process, where high-pressure pumps inject fracturing fluid into the reservoir via wells, creating artificial fractures that connect with natural fissures [9,10,11]. However, the connectivity between natural fissures and artificial fractures has dual implications: during production, it enhances reservoir permeability, thereby increasing CBM production rates [12,13], yet during fracturing, it may create favorable pathways for fracturing fluid leak-off, leading to difficulties in initiating fractures or even failure of the fracturing process [14,15].

During hydraulic fracturing, the propagation of fractures and the grinding of proppants against fracture walls cause damage and fragmentation of surrounding coal, resulting in the generation of significant amounts of coal fines, which impairs the permeability of fracture networks during production [16,17,18]. In the single-phase water flow stage, coal fines require a certain incipient motion velocity; these fines are detached and migrate at sufficiently high flow velocities [19]. Some of the migrated coal fines are expelled from the fractures, which decreases the amount of coal fines present within the fractures and thereby reduces the likelihood of coal fines causing blockages in the fractures, while others become trapped in narrow openings within fractures, gradually narrowing throats and ultimately plugging them, impairing reservoir permeability [20,21]. The migration–retention/output of coal fines is a dynamic process influenced mainly by flow rates, fracture width, fracturing fluid properties, characteristics of fines and fractures themselves, and other factors [22,23,24,25,26]. During the gas–water two-phase flow stage in CBM reservoirs, unlike the single-phase water flow stage, the flow pattern of gas and water becomes a critical factor controlling the migration–retention/output of coal fines [27,28]. With lower gas content, bubbles can capture coal fines and alter their migration trajectories, with larger bubbles having a greater ability to carry coal fines than smaller ones [29]. The output of coal fines in the gas–water two-phase flow stage is higher than that in the single-phase water flow stage; therefore, utilizing these characteristics can promote the output of coal fines and reduce their detrimental impact on the conductivity of reservoir fractures [30,31].

Therefore, fracturing fluid leak-off and coal fine generation are the two major challenges in hydraulic fracturing of coal seams (Figure 1), which have long constrained the effectiveness of hydraulic fracturing in CBM reservoirs.

Based on the above analysis, it can also be seen that the current situation of coal fines causing clogging within fractures has been extensively studied by many scholars, primarily focusing on the damage to reservoir permeability during the production phase. However, the issues associated with coal fines during the hydraulic fracturing stage are rarely reported. The hydraulic fracturing stage precedes the production phase where fine issues arise; significant amounts of reservoir coal fines are already generated during fracturing. Additionally, the single-phase water flow stage is a period characterized by frequent occurrences of coal fine damage issues during the CBM production phase [11,20]. Focusing on these issues, with the research approach of turning coal fines from “harmful to beneficial”, it is hoped that coal fines in the fracturing fluid can help reduce fluid leak-off, effectively alleviating the simultaneous hazards of coal fines and fracturing fluid leak-off during the hydraulic fracturing stage. Additionally, a systematic study can be established on the issue of coal fines during both the hydraulic fracturing and production phases, exploring the mechanism of temporary plugging by coal fines and their impact on reservoir permeability during the production phase. This will provide a reference for the comprehensive control of coal fines from the hydraulic fracturing to production stages.

Given the insufficient research on the mechanism of fines during the hydraulic fracturing and fracturing fluid flow-back stages, an experimental setup for simulating temporary plugging and unplugging of coal seam fractures was independently constructed. Indoor physical simulation experiments were conducted to demonstrate the positive role of coal fines in temporary plugging and reducing fracturing fluid leak-off during hydraulic fracturing. Using a controlled variable method, this study investigated the effects of different concentrations and particle sizes of coal fines in fracturing fluid on temporary plugging efficiency in fractures. It also analyzed the damage law of coal fines of temporary plugging on reservoir permeability under various concentrations and particle sizes, identifying fundamental parameter ranges for the role of coal fines in both stages. The objective was to enhance the effective utilization of coal fines to reduce fracturing fluid leak-off during fracturing, thereby minimizing the damage to reservoir permeability during the fracturing fluid flow-back stage and providing guidance for achieving high and stable production rates of CBM.

2. Experimental Methodology

2.1. Sample Preparation

The experimental coal samples were obtained from the No. 3 coal seam in the Lu’an mining area, China. Cores with a diameter of 25 mm and a length of 50 mm were drilled from the mined coal blocks. The Brazilian splitting method was used to split the coal samples along their long axis into two halves, and the surface particles prone to detachment were cleaned to reduce experimental errors caused by coal fine detachment. The fractured coal samples were reassembled to simulate a coal seam unit containing a single fracture (Figure 2).

Table 1. Basic properties of coal samples in the experiments.

Test Name	Test Content		Test Result
Analysis of maceral composition in coal sample	Demineralized bases (%)	Vitrinite	78.75
		Inertinite	21.25
		Exinite	-
	Mineral containing bases (%)	Total organic content	29.41
		Clay	5.80
		Carbonate	1.79
	Average random reflectance (%)		1.78
	Standard deviation (%)		0.068
Quantitative analysis of whole-rock X-ray diffraction	Calcite (%)		30.5
	Dolomite (%)		9.3
	TCCM (%)		60.2
Industrial and elemental analysis of coal samples	Cinder feature (1–8)		4
	Proximate analysis (%)	Mad	1.18
		Aad	12.58
		Vad	14.80
		FCad	74.48
	Ultimate analysis (%)	St,d	0.30
		Cdaf	90.13
		Hdaf	4.09
		Odaf	3.96
		Ndaf	1.48

presents the basic properties of the coal samples, including microscopic composition analysis, whole-rock X-ray diffraction quantitative analysis, and industrial and elemental analysis results, providing essential foundational data support for subsequent experiments. The vitrinite primarily consists of matrix vitrinite, the inertinite group mainly comprises semi-fusinite bodies, and no the exinite was observed. The clay impregnates the matrix vitrinite, or fills the cell cavities, with occasional calcite veins filling the cracks, and siderite appearing as nodules. TCCM is the total amount of clay minerals. The char residue characteristics (CRCs) refer to the shapes of the residual materials after coal pyrolysis, which are categorized into eight distinct shapes, each assigned a specific characteristic code. M_ad, A_ad, V_ad, and FC_ad denote the contents of ash, moisture, volatile matter, and fixed carbon, respectively, under air-dried conditions. S_t,d is the total content of the S element in coal samples under anhydrous conditions. C_daf, H_daf, O_daf, and N_daf represent the content of C, H, O, and N elements in the coal sample after removing moisture and ash.

After crushing the remaining coal samples obtained from the cores, three groups of coal fines were sieved using standard screens to obtain particle sizes of 80–100 mesh, 120–150 mesh, and >180 mesh. The particle sizes of the three groups of coal fines were measured (Figure 3), showing that their respective D50 values were 195.86 μm, 112.32 μm, and 29.19 μm; their respective D90 values were 342.29 μm, 157.18 μm, and 65.18 μm. These coal fines were used to prepare coal fine suspension fluids. Deionized water and solid KCl were used to prepare a 2% KCl solution, which was employed to inhibit the swelling and dispersion of the clay. Measured quantities of coal fines were added to the 2% KCl solution to create coal fine suspension fluids with varying concentrations and particle sizes (specific preparation parameters are detailed in

Table 2. Summary of the conditions for the temporary plugging and unplugging experiments.

Test No.	Coal Fine Parameters		Temporary Plugging	Unplugging
	Concentration (g/L)	Particle Size (Mesh)
1	No coal fines	No coal fines	Confining pressure: 3 MPa Plugging pressure: 2 MPa Plugging time: 60 min	Confining pressure: 3 MPa Unplugging velocity: 1 mL/min Unplugging time: 30 min
2	3	>180
3	5	>180
4	7	>180
5	10	>180
6	5	120–150
7	5	80–100

). This was performed to simulate fracturing fluids containing coal fines of different concentrations and particle sizes, in preparation for subsequent simulated experiments on coal seam plugging and unplugging.

2.2. Experimental Apparatus

The experiment utilized a self-built apparatus for temporary coal seam plugging and unplugging (Figure 4), comprising a core holder, ISCO pumps (including pumps A and B), a gas cylinder, an intermediate container, a high-precision electronic balance, a fraction collector, and a data acquisition system. Pump A operated in constant flow mode, primarily for delivering unplugging fluids, while pump B was primarily used to provide confining pressure to the core holder. The gas cylinder provided driving force for liquid output from the intermediate container. The intermediate container (HY-2, Jiangsu Tuochuang Scientific Research Instrument, Nantong, China), equipped with a piston and automatic stirring function at 1350 rpm, effectively prevented sedimentation and blockages in the coal fine suspension during the experiments. The fraction collector (SBS-160, Shanghai Jiapeng Technology, Shanghai, China) was used to gather the output liquid containing coal fines during the unplugging experiments, with collection parameters set at 5 min per tube. The high-precision electronic balance was primarily utilized for monitoring the cumulative leakage volume of fracturing fluid during the temporary plugging experiment. Additionally, the experiment also required the use of a turbidimeter (TL2300, HACH, Loveland, CO, USA) to measure the turbidity of the output liquid during unplugging processes.

2.3. Experimental Conditions and Procedures

The specific experimental conditions are detailed in Table 2. The temporary plugging and unplugging experiments under conditions without coal fines (Test No. 1) served as controls against experiments conducted with coal fines.

The specific experimental steps are as follows:

(1)

Preparation: After vacuuming and saturating the prepared coal sample with a 2% KCl solution for 48 h, load it into the core holder. Start pump B and set the confining pressure to 3 MPa. Load the prepared coal fine suspension into the intermediate container and continue stirring to ensure uniform dispersion in the 2% KCl solution.

(2)

Initial permeability measurement: Open valve B and start pump A. Set the pump A liquid injection rate to 1 mL/min of 2% KCl solution into the core holder. Monitor the pressure difference across the core holder in real time. Once the pressure difference stabilizes, measure the initial permeability of the coal sample. The permeability calculation follows Darcy’s law:

$$K={\displaystyle \frac{q\mu L}{A(p_1-p_a)}}$$

(1)

where K is the permeability of the coal sample, m²; p₁ and p_a are the inlet and outlet pressures of the core holder, Pa; q is the liquid flow rate, m³/s; μ is the liquid viscosity, Pa·s; A is the cross-sectional area of the coal sample, m²; and L is the length of the coal sample, m.

(3)

Temporary plugging experiment: Close valve B and pump A, then sequentially open valve A and the gas cylinder. Set the output pressure of the gas cylinder to 2 MPa and inject the liquid from the intermediate container into the core holder at constant pressure. Continuously monitor the leak-off of the fracturing fluid and calculate the temporary plugging rate of coal fines in the fracturing fluid:

$${\tau }_P={\displaystyle \frac{K_0{-K}_P}{K_0}}\times 100\%$$

(2)

where τ_p is the temporary plugging rate, %; K₀ is the initial permeability of the coal sample, m²; and K_p is the temporary plugging permeability of the coal sample, m².

(4)

Unplugging experiment: Close the gas cylinder and valve A in sequence, then open valve B and start pump A. Inject 2% KCl solution into the core holder at 1 mL/min. Monitor the pressure difference across the core holder in real time and collect the output liquid during the unplugging process. Calculate the damage rate of coal fines to the permeability of the coal sample after unplugging:

$${\eta }_d={\displaystyle \frac{K_0{-K}_d}{K_0}}\times 100\%$$

(3)

where η_d is the permeability damage rate due to coal fines, %; and K_d is the permeability of the coal sample after the unplugging pressure stabilizes, m².

(5)

Post-experimental processing: Release the pressure from the experimental system, remove the coal sample, clean the coal sample and equipment, and restore the initial operational settings of the apparatus. Varying the coal fine parameters for Tests No. 1–7, repeat steps 1–5. Analyze the impact of coal fines of different concentrations and particle sizes in the fracturing fluid on the temporary plugging rate and the permeability damage rate.

3. Results and Discussion

3.1. Characteristics of Temporary Plugging of Coal Fines

The change in fracturing fluid leak-off volume is the most direct evidence of the temporary plugging effect of coal fines. By continuously monitoring the cumulative leakage volume of fracturing fluids under different conditions of coal fine concentrations and particle size, the influence of different coal fine concentrations and particle sizes on the temporary plugging effect of coal fines can be analyzed. As shown in Figure 5a, with the increase in coal fine concentration in the fracturing fluid, the cumulative leakage volume of the fracturing fluid decreases. When the coal fine concentration increases from 3 g/L to 10 g/L, the cumulative leakage volume of the fracturing fluid decreases from 149.28 mL to 85.60 mL, a reduction of 1.85 times. As shown in Figure 5b, with the increase in coal fine particle size in the fracturing fluid, the cumulative leakage volume of the fracturing fluid increases. When the coal fine particle size increases from >180 mesh to 80–100 mesh, the cumulative leakage volume of the fracturing fluid increases from 118.89 mL to 158.28 mL, an increase of 1.33 times. Although it can be seen from the fracturing fluid leak-off results that large-particle-size and low-concentration coal fines have a poorer plugging effect, compared with the final cumulative leakage volume of fracturing fluids under conditions without coal fines, those containing coal fines is still an order of magnitude lower. Therefore, coal fines do have a significant temporary plugging effect, which can reduce the leak-off of fracturing fluids in the process of coalbed hydraulic fracturing. They are a natural “temporary plugging agent”.

Collecting fracturing fluids of the cumulative leak-off under different concentrations and particle sizes of coal fines and testing their respective turbidity reveals that the turbidity of the cumulative leak-off fracturing fluids closely matches that of the 2% KCl solution (NTU = 0.55), with almost no coal fines present in the cumulative leak-off fracturing fluids. This suggests that most coal fines are filtered out at the injection port of the fracturing fluid, achieving a temporary plugging effect that reduces fracturing fluid leak-off. By monitoring the leakage volume of the fracturing fluid, the flow rate of the liquid during the temporary plugging process can be determined. Then, using Formula (1), the permeability of the coal sample under various coal fine parameters is calculated (

Table 3. Final permeability after temporary plugging under different coal fine parameters.

Test No.	Coal Fine Parameters		Permeability (mD)
	Concentration (g/L)	Particle Size (Mesh)
1	No coal fines	No coal fines	62.64
2	3	>180	3.21
3	5	>180	2.38
4	7	>180	1.34
5	10	>180	0.89
6	5	120–150	4.87
7	5	80–100	6.44

). Subsequently, Formula (2) is employed to determine the temporary plugging rate for different concentrations and particle sizes of coal fines, as shown in Figure 6, indicating that the temporary plugging efficiency increases with higher coal fine concentration, specifically by 94.88%, 96.20%, 97.86%, and 98.58%. Conversely, the temporary plugging efficiency decreases with increasing coal fine particle size, namely by 96.20%, 92.22%, and 89.72%, respectively.

3.2. Strength of Plugging and Mechanisms of Unplugging

The monitoring of unplugging pressure is primarily used to analyze the plugging strength of coal fines and the migration process of temporary coal fine blockages, while monitoring the turbidity of the produced liquid mainly serves to assist in explaining the reasons for changes in unplugging pressure. The curve of unplugging pressure over time under different coal fine concentrations is shown in Figure 7a. For any concentration of coal fines, during the initial stage of unplugging, the unplugging pressure sharply rises to form a pressure peak. With increasing coal fine concentration, the pressure peak also increases, indicating greater plugging strength. For instance, as the coal fine concentration increases from 3 g/L to 10 g/L, the pressure peak rises from 0.11 MPa to 0.48 MPa, marking a 4.4-fold increase in plugging strength. The decrease in unplugging pressure from peak to trough signifies the breakthrough of the coal fines forming the plug, followed by a gradual increase, primarily due to the reverse migration of residual coal fines within the fracture. Moreover, with higher coal fine concentrations, the unplugging pressure values after rebound tend to decrease. This is mainly because the sudden release of high pressure results in a brief high-speed flow, facilitating easier removal of plugging of coal fines in fractures.

The variation in coal fine concentration in the output liquid, as depicted in Figure 7b, corroborates these observations. Due to the release of peak pressure within the first 5 min of unplugging, the turbidity of the output liquid is extremely high during this time. However, even though some coal fines continue to be produced after 5 min, the pressure does not decrease, indicating that low flow rates struggle to carry away stagnant coal fines within the fracture; most of the coal fines in the output liquid result from coal fines dislodged from outside the fracture. It is noteworthy that at a coal fine concentration of 3 g/L, the low concentration fails to form a complete plugging zone. Consequently, after the peak pressure is released, it is difficult to generate high-velocity flow to scour the fractures, leading to a higher final pressure value compared to the conditions with 7 g/L and 10 g/L coal fine concentrations. Furthermore, as the number of coal fines involved in the temporary plugging at 3 g/L is relatively small, the likelihood of coal fines causing damage to the fractures in the unplugging process is reduced, resulting in a lower final pressure value compared to the conditions with a 5 g/L coal fine concentration.

The variation over time in unplugging pressure under different coal fine particle size conditions is depicted in Figure 8a. As coal fine particle size increases, the unplugging pressure curve becomes smoother. This is primarily because larger particle sizes of coal fines create larger gaps between them, resulting in relatively poorer compactness of the coal fine blockage. Therefore, under different coal fine particle size conditions, it becomes challenging to use peak pressure to judge the plugging strength of the coal fines. The slope of the curve from the rise in unplugging pressure to its peak clearly shows that smaller coal fine particle sizes lead to a faster increase in unplugging pressure and higher plugging strength. For coal fine particle sizes of 80–100 mesh and 120–150 mesh, the insufficient plugging strength causes premature release of unplugging pressure, making it difficult to generate high-speed flow to clear coal fines from inside the fracture. Combined with Figure 8b, the variation in turbidity of the output liquid illustrates that when the coal fine particle size is 80–100 mesh, a significant number of temporary coal fine blockages are eroded by the fluid within the first 5 min of the unplugging flow, and the subsequent pressure increase is due to the initial reverse migration and plugging by coal fines within the fracture. The subsequent rebound in unplugging pressure after reaching a trough is mainly caused by residual coal fines within the fracture undergoing reverse plugging.

After the unplugging experiment, coal samples were taken out as shown in Figure 9, and the external characteristics of the temporary coal fine blockages on the coal sample surface were analyzed. It is not difficult to observe that the high temporary plugging effect of coal fines on the coal seam fracture occurs in two steps: firstly, coal fines fill the fracture of the coal seam, forming an internal filter cake based on the size and shape of the fracture, thus completing initial plugging; secondly, an external filter cake forms on the outer surfaces of the fracture to further plug it. However, even when coal fines are relatively thoroughly removed from inside the fracture, some coal fines still remain trapped within the fracture. Considering the changes in unplugging pressure and concentration described above, these trapped coal fines are very likely to migrate in reverse and cause reverse plugging within the fracture. Through the analysis above, it is evident that coal fines with small particle sizes and high concentrations in fracturing fluid significantly enhance plugging effectiveness, which corroborates the conclusion regarding temporary plugging rates. The main reason for this lies in how concentration and particle size affect the compactness of the coal fine filter cake: under the same coal fine concentration, larger particle sizes result in larger gaps between coal fines, weakening the temporary plugging effect of coal fines, whereas under the same particle size of coal fines, higher concentrations lead to more coal fines participating in plugging, making it easier to form a thick filter cake that reduces fracturing fluid leak-off and enhances temporary plugging effectiveness in coal fines.

3.3. Formation Damage Caused by Temporary Plugging with Coal Fines

Based on the monitoring of unplugging pressure under different concentrations and particle sizes of coal fines, the final permeability of the coal sample after unplugging was calculated using Formula (1), as detailed in

Table 4. Final permeability after unplugging under different coal fine parameters.

Test No.	Coal Fine Parameters		Permeability (mD)
	Concentration (g/L)	Particle Size (Mesh)
1	No coal fines	No coal fines	62.64
2	3	>180	32.12
3	5	>180	19.27
4	7	>180	46.84
5	10	>180	55.07
6	5	120–150	9.96
7	5	80–100	13.19

. Subsequently, the permeability damage rate caused by coal fines after unplugging was determined using Formula (3). As shown in Figure 10, with the increase in coal fine concentration, the permeability damage rates are 48.72%, 69.23%, 25.23%, and 12.09%, respectively. As the coal fine particle size increases, the permeability damage rates are 69.23%, 84.10%, and 78.95%, respectively. These differences in permeability damage rates are mainly due to variations in the plugging strength of coal fines at different concentrations and particle sizes. Combined with the characteristics of temporary plugging and pressure changes during fracturing fluid flow-back at different concentrations and particle sizes, it can be analyzed that under the conditions of large particle size and low concentration, the plugging effect of coal fines is poor, leading to early pressure relief during fracturing fluid flow-back and making it difficult to temporarily flush out the retained coal fines in the fractures at high speed. This results in greater damage to reservoir permeability by coal fines during the fracturing fluid flow-back process. Therefore, maintaining a small particle size while appropriately increasing the coal fine concentration can effectively reduce the damage to reservoir permeability caused by temporary plugging.

However, during the CBM production process, the concentration of temporary coal fine blockages affects the degree of damage caused by coal fines to the propping fractures and the wellbore (Figure 11). To prevent coal fines from obstructing propped fractures and the wellbore, a lower concentration of coal fines reduces the likelihood of plugging due to coal fine retention, minimizing damage to the propping fractures’ conductivity and reducing the risk of incidents like pump burial and pump sticking during the production process [32,33,34,35]. Therefore, by combining the rate of temporary plugging by coal fines in coal seams with their damage rate, it can be analyzed that, while maintaining a high rate of temporary plugging, the concentration of coal fines in fracturing fluid should be maintained at a lower level to help reduce the overall damage to the CBM production channel system during the production process. This also helps to achieve the function of temporarily plugging coal fines to control fluid leak-off during the hydraulic fracturing stage.

4. Conclusions

During the development of coalbed methane (CBM), the issue of damaging reservoirs due to coal fines is a significant concern. However, coal fines are already notably present during the hydraulic fracturing stage. This study focuses on the temporary plugging effect of coal fines in fracturing fluids during this stage. The primary objective is to utilize the positive effect of coal fines in reducing the leak-off of fracturing fluids to mitigate their negative impacts during production. Additionally, this study explores the effects of damage patterns of coal fines on coal seam permeability during the flow-back stage of fracturing fluid, analyzing the influence of coal fine concentration and particle size on temporary plugging and permeability damage effects. This helps determine the effective ranges of basic parameters for coal fines to increase temporary blocking rates and reduce damage rates, thereby enhancing hydraulic fracturing effectiveness and CBM recovery rates. Key insights from this study are as follows:

(1)

Coal fines act as a natural “temporary plugging agent” to reduce fracturing fluid leak-off, and temporary plugging rates increase with higher concentrations and decrease with larger particle sizes.

(2)

The high temporary plugging effect of coal fines occurs through the combination of internal and external filter cakes. Increased coal fine concentration enhances plugging strength, while larger particle sizes reduce it. The permeability damage rate caused by temporary plugging coal fines in fractures initially increases and then decreases with increasing concentration and particle size. However, relatively lower damage rates occur under the conditions of smaller particle sizes and higher concentrations.

(3)

In balancing the temporary plugging rate and damage rate of coal fines on coal seam fractures, it is beneficial to maintain a lower concentration of coal fines in the fracturing fluid while ensuring a high temporary plugging rate. This approach helps mitigate the damage to the CBM production channel system during extraction. Simultaneously, during the hydraulic fracturing stage, temporary plugging by coal fines can compensate for fracturing fluid leak-off, thereby enhancing the effectiveness of reservoir fracturing and production enhancement.

It is crucial to acknowledge that the findings of this study are limited to laboratory-scale experiments. Further investigation is required to evaluate the behavior of coal fines with respect to fluid leak-off reduction and reservoir damage under larger-scale and more complex operational conditions, which will enhance our understanding of the subject and improve its applicability in field settings.