The Effect of Organic Acid Modification on the Pore Structure and Fractal Features of 1/3 Coking Coal

Abstract

The acidification modification of coal seams is a significant technical measure for transforming coalbed methane reservoirs and enhancing the permeability of coal seams, thereby improving the extractability of coalbed methane. However, the acids currently used in fracturing fluids are predominantly inorganic acids, which are highly corrosive and can contaminate groundwater reservoirs. In contrast, organic acids are not only significantly less corrosive than inorganic acids but also readily bind with the coal matrix. Some organic acids even exhibit complexing and flocculating effects, thus avoiding groundwater contamination. This study focuses on the 1/3 coking coal from the Guqiao Coal Mine of Huainan Mining Group Co., Ltd., in China. It systematically investigates the fractal characteristics and chemical structure of coal samples before and after pore modification using four organic acids (acetic acid, glycolic acid, oxalic acid, and citric acid) and compares their effects with those of hydrochloric acid solutions at the same concentration. Following treatment with organic acids, the coal samples exhibit an increase in surface fractal dimension, a reduction in spatial fractal dimension, a decline in micropore volume proportion, and a rise in the proportions of transitional and mesopore volumes, and the structure of the hydroxyl group and oxygen-containing functional group decreased. This indicates that treating coal samples with organic acids enhances their pore structure and chemical structure. A comparative analysis reveals that hydrochloric acid is more effective than acetic acid in modifying coal pores, while oxalic acid and citric acid outperform hydrochloric acid, and citric acid shows the best results. The findings provide essential theoretical support for organic acidification modification technology in coalbed methane reservoirs and hydraulic fracturing techniques for coalbed methane extraction.

1. Introduction

The extraction of coalbed methane (or gas) from coal seams is a primary technical approach in both coalbed methane engineering and gas disaster control engineering [1]. However, due to factors such as the depth of some coal seams, their low hardness, and low permeability, micropores and small pores constitute a large proportion of the pores within these coal seams, while medium and large pores are less prevalent. This makes it challenging to extract coalbed methane from the coal seams [2]. Therefore, to effectively extract coalbed methane, it is necessary to modify the coalbed methane reservoirs [3].

Hydraulic fracturing is one of the effective technical methods for coalbed methane reservoir modification. Through hydraulic fracturing, a large number of fractures are created in the coal seam, providing essential channels for coalbed methane extraction [4]. However, for coal seams that are deeply buried, soft, and have low permeability, the fractures generated by hydraulic fracturing are prone to closing, causing the permeability of the coal seam to revert to its original level [5]. To address this challenge, engineering solutions have been developed from two perspectives: on one hand, materials such as sand and ceramic particles are injected into the coal seam to support the fractures created by hydraulic fracturing, preventing them from closing [6]; on the other hand, fracturing fluids are used to modify the coal seam, transforming micropores and small pores into mesopores and macropores and converting closed and semi-closed pores into connected pores, thereby significantly improving the permeability of the coal seam and enhancing coalbed methane extraction efficiency [7].

Currently, research on fracturing fluids primarily focuses on two categories: surfactant-based fracturing fluids and acidic fracturing fluids. In the field of surfactants, many scholars have studied the effects of surfactant solutions on the wettability of coal and the desorption of coalbed methane. Wang et al. [8] and An et al. [9] suggest that adding surfactants to water can effectively improve the properties of the solution, reduce its surface tension, enhance the wettability of coal, and thereby improve the desorption capacity of methane. Jiang et al. [10] argue that surfactant solutions can significantly reduce methane desorption, acting as a sealing agent. Li et al. [11] and Wang et al. [12] concluded that adding surfactants to water inhibits methane desorption from coal. In the field of acidic fracturing fluids, Wang et al. [13] used experimental methods such as Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) to discover that fracturing fluids containing 3% hydrochloric acid effectively modified the pore characteristics of coal samples, enhancing the extractability of coalbed methane. Ni et al. [14] modified coal samples using nitric acid solutions and found through low-temperature nitrogen adsorption experiments that nitric acid solutions could effectively improve the pore characteristics of coal samples. Balucan et al. [15] treated coal samples with hydrochloric and hydrofluoric acids and observed through permeability experiments under uniaxial compression that both the compressive strength and permeability of the coal samples significantly increased.

However, although some scholars have conducted some research in the field of acidic fracturing fluids, most of the current acidic fracturing fluids are formulated with inorganic acids, which are corrosive and pollute groundwater. In addition, previous researchers have mostly focused on combining fractal theory with pore analysis, and few studies have deeply analyzed the correlation between such changes and gas permeability. Therefore, it is crucial to explore the effect of organic acid modification on coal structure.

In recent years, organic acids have shown broad application prospects in environmental management and green chemistry, with advantages such as biodegradability, environmental friendliness, and mild reaction conditions [16]. Compared with traditional inorganic acids, organic acids exhibit stronger affinity and chemical modification capabilities toward the coal matrix [17]. Organic acids not only enhance coal bed porosity and permeability but also reduce residual pollution via biodegradation, offering significant value for green unconventional natural gas mining and efficient coalbed methane extraction [18]. Therefore, this study focuses on the 1/3 coking coal from the Guqiao Coal Mine of Huainan Mining Group Co., Ltd., in China. The study systematically examines the fractal properties of coal samples both pre and post pore modification, employing four organic acids and contrasting their outcomes with hydrochloric acid solutions at identical concentrations. This research holds significant theoretical importance for the development of organic acidification modification technology and hydraulic fracturing techniques in coal seams.

2. Experimental Methods

2.1. Collection and Preparation of Coal Samples

(1)

Collection of Coal Samples

The coal samples were taken from 1/3 coking coal of A1 seam of Guqiao Coal Mine, Huainan Mining (Group) Co., Ltd., in Huainan, China, with a volatile fraction of 28~37% and bond index ≥ 65. The average depth of burial of the A1 seam was 780 m (±15 m SD), and the average Pratt’s hardness f was 0.5 ± 0.1. To prevent oxidation reactions caused by exposure to air, the freshly collected coal samples were sealed and transported back to the laboratory.

(2)

Preparation of Coal Samples

To meet experimental specifications, the coal samples were ground into particles sized between 60 and 80 mesh, corresponding to a diameter range of 0.180–0.250 mm, as illustrated in Figure 1. The coal particles were then dried in an oven at 35 °C (±0.5 °C), sealed in airtight bags, labeled, and stored for subsequent experiments.

2.2. Acidification and Modification of Coal Samples

(1)

Acid Solutions

In this study, two concentrations (3.00 ± 0.05% and 5.00 ± 0.05%) of acetic acid, glycolic acid, oxalic acid, and citric acid were selected for treatment, along with hydrochloric acid at identical concentrations [19,20,21]. Raw coal (untreated) and coal samples infiltrated with purified water served as controls. The specific experimental design is summarized in

Table 1. Formulation of multi-component acid.

Acid Solution	Solution Concentration (%)
	YM	SM	A	B	C	D	E	F	G	H	I	J
HA	/	0	3	5	0	0	0	0	0	0	0	0
AA	/	0	0	0	3	5	0	0	0	0	0	0
GA	/	0	0	0	0	0	3	5	0	0	0	0
OA	/	0	0	0	0	0	0	0	3	5	0	0
CA	/	0	0	0	0	0	0	0	0	0	3	5

YM, raw coal; SM, pure water-treated coal sample; hydrochloric acid, HA; acetic acid, AA; glycolic acid, GA; oxalic acid, OA; citric acid, CA.

.

This setup allows for a comprehensive comparison of the effects of different organic acids and their concentrations on the modification of coal samples, as well as the influence of untreated and water-treated samples.

(2)

Acidification Treatment of Coal Samples

(1)

Weighing Coal Powder: Using a precision balance, weigh approximately 30 g of coal sample and place it into a sealed container. Label the container for identification.

(2)

Preparation of Acid Solutions: Prepare acid solutions according to the concentration requirements listed in Table 1.

(3)

Mixing Acid Solution with Coal Powder: Pour 300 mL of the prepared acid solution into the corresponding sealed container. Thoroughly mix the acid solution with the coal sample and allow the mixture to react for 48 h.

(4)

Filtration and Drying: Filter the coal sample and rinse it thoroughly with clean water to remove any residual acidic substances. Place the sample in a drying oven set at 35 °C for drying. Once dried, store the sample in a sealed bag, label it, and preserve it for further use.

2.3. Experiment

2.3.1. Low-Temperature N₂ Adsorption

A Micromeritics ASAP 2460 BET analyzer, produced by Micromeritics Instrument Corporation in the Norcross, GA, USA, was utilized to examine the pore structure of the coal samples, as depicted in Figure 2. Among them, coal samples treated with different acid solutions were tested at least three times to strengthen the reliability of the results. The experiments aimed to measure the pore volume and structural characteristics of the coal samples before and after acidification modification. Using the obtained data, the fractal properties of the samples were determined to analyze how various organic acids influence the alteration of coal pore configurations.

2.3.2. FTIR Measurements

Functional group analysis was performed using a Bruker (Berlin, Germany) Fourier transform infrared (FTIR) spectrometer. For sample preparation, 1 mg of coal was mixed with 200 mg of spectroscopic-grade KBr and finely ground in an agate mortar for 120 s to ensure homogeneity. The mixture was then pressed into a transparent pellet under 10 MPa for 2 min. FTIR spectra were acquired in the 4000–400 cm⁻¹ range at a resolution of 4 cm⁻¹, with 32 scans per measurement to optimize signal clarity. To ensure reproducibility, triplicate measurements were conducted for each sample. Prior to analysis, all pellets were vacuum-dried to eliminate residual moisture interference.

2.3.3. SEM Experiment

This study employed a Flex SEM 1000 scanning electron microscope (manufactured by Hitachi High-Technologies Corporation, Tokyo, Japan) to comparatively analyze the surface pore morphology characteristics of coal samples before and after treatment with different acidic solutions. The specific testing conditions were as follows: high vacuum mode (resolution 4.0 nm), an adjustable accelerating voltage of 0.3–20.0 kV, and continuously adjustable magnification from 6× to 300,000×. Prior to testing, the coal samples were subjected to drying and dehydration treatment, followed by sputter-coating with a gold layer to enhance conductivity, ensuring optimal observation results and instrument protection.

3. Pore Structure

3.1. Characterization Methods for Coal Pores

As a porous material, coal typically retains gases in an adsorbed form within its pores. Investigating the pore structure properties of coal is crucial for the effective extraction of coalbed methane and mitigating gas-related hazards. Numerous researchers employ the low-temperature nitrogen adsorption technique to examine the pore configuration of coal. The purpose of this method is to precisely determine the specific surface area and pore size distribution of solids by studying the adsorption behavior of gases on the surface and within the internal structure of solids.

Under constant temperature conditions, the adsorption capacity of a solid changes with pressure. The relationship between the adsorption capacity of the solid and the pressure follows the BET (Brunauer–Emmett–Teller) theoretical model. Formula (1) is as follows [22]:

$${\displaystyle \frac{P/P_0}{V\left(1-P/P_0\right)}}={\displaystyle \frac{C-1}{V_{m}C}}\times P/P_0+{\displaystyle \frac{1}{V_{m}C}}$$

(1)

where $P/P_0$—relative pressure; $C$—BET constant; $V$—adsorbed gas volume (cm³/g); and $V_m$—monolayer adsorption capacity (cm³/g).

The pore size can be analyzed using the BJH (Barrett–Joyner–Halenda) model [23], and Formula (2) is as follows:

$$\mathrm{r}={\displaystyle \frac{2{\mathsf{\gamma }\mathrm{V}}_{\mathrm{m}}}{\mathrm{RT}\mathrm{In}\left(P/P_0\right)}}+0.354{\left({\displaystyle \frac{-5}{\mathrm{In}\left(P/P_0\right)}}\right)}^{1/3}$$

(2)

where γ—surface tension, N/m; ${\mathrm{V}}_{\mathrm{m}}$—molar volume of the liquid, L/mol; $\mathrm{R}$—gas constant, J/(mol·K); $\mathrm{T}$—absolute temperature, K; $P$—actual vapor pressure, MPa; and $P_0$—saturated vapor pressure, MPa.

For different values of $P/P_0$, the pore size at which capillary condensation occurs in the medium also varies. For each $P/P_0$, there is a critical pore size $r_K$ associated with it. Pores smaller than $r_K$ will be filled with nitrogen. The relationship between $r_K$ and $P/P_0$ satisfies Formula (3):

$$r_K=-0.414/\log (P/P_0)$$

(3)

3.2. Examination of Coal Sample Pore Configuration Characteristics

The specific surface area of coal refers to the total surface area per unit mass of the coal sample, with units of m²/g. The pore volume refers to the volume of pores per unit mass of the coal sample, with units of cm³/g. Based on the low-temperature nitrogen adsorption experimental data, the specific surface area of the coal samples can be calculated using the BET theoretical model Formula (1), while the average pore size and pore volume are calculated using the BJH theoretical model Formula (2). In this study, the Hodot pore classification method [24] is used to categorize coal pores into micropores (<10 nm), transitional pores (10–100 nm), mesopores (100–1000 nm), and macropores (>1000 nm). Since pores larger than 100 nm provide effective channels for coalbed methane extraction, this study focuses only on the changes in micropores, transitional pores, and mesopores. The detailed outcomes are presented in

Table 2. Pore structure parameters of coal sample.

Samples	Average Pore Size/nm	BET Specific Surface Area/(m2/g)	BJH Pore Volume /(104 cm3/g)	Proportion of Pore Volume in Different Pore Size Ranges/%
				Micropores	Transitional Pores	Mesopores
YM	14.3509	0.4415	16.5024	14.62	79.87	5.51
SM	15.0254	0.4045	20.8896	12.77	79.54	7.69
A	18.6597	0.3913	20.0790	10.67	81.10	8.23
B	19.6766	0.3858	21.9271	10.45	80.98	8.57
C	17.9554	0.4086	21.5613	12.02	80.13	7.86
D	17.0052	0.3991	23.8201	11.70	80.21	8.09
E	18.4808	0.3902	20.3019	10.71	80.76	8.53
F	19.7517	0.3854	19.4417	10.36	79.74	9.90
G	21.3328	0.2939	15.6756	8.83	80.21	10.96
H	24.2460	0.2327	15.9081	7.06	79.49	13.45
I	22.4552	0.2492	10.7666	6.89	81.99	11.12
J	23.7204	0.2654	13.6148	6.53	79.56	13.91

YM, raw coal; SM, pure water-treated coal sample; A, 3%HA; B, 5%HA; C, 3%AA; D, 5%AA; E, 3%GA; F, 5%GA; G, 3%OA; H, 5%OA; I, 3%CA; J, 5%CA.

.

From Table 2, it can be observed that compared to the raw coal sample (YM), the coal samples treated with acidification modification show significant changes in average pore size, specific surface area, and pore volume. The measured average pore size ranges from 18.6597 to 24.246 nm, and the pore volume ranges from 10.7666 × 10⁻⁴–23.8201 × 10⁻⁴ cm³/g. The specific surface area of coal samples treated with hydrochloric acid, acetic acid, glycolic acid, oxalic acid, and citric acid decreased by 3.26–4.62%, 0.82–1.34%, 3.53–4.72%, 27.34–42.47%, and 38.39–34.39%, respectively, compared to coal samples treated with water (SM). Notably, sample G experiences the most substantial decrease, with a drop of 47.29%.

The percentage of the microporous pore volume of coal sample YM (raw coal) was 14.62%, while the percentage of microporous pore volume of coal samples treated with acid solution were all significantly decreased, and the percentage of transition and mesopore was significantly increased. This is because organic acids can efficiently dissolve metal minerals (e.g., Fe, Al oxides) and some organometallic compounds associated with micropores in coal. These substances often act as the “supporting skeleton” of the micropores, and their dissolution leads to the collapse of the microporous walls, which merge into larger pores. Research by Jin K et al. [25] indicates that the microporous structure influences gas adsorption capacity significantly. Cheng Yuanping et al. [18] also confirmed that methane primarily exists in coal in the form of micropore filling, accounting for 99% of the total methane adsorption capacity. This suggests that the modified treatment of organic acid solution can reduce the gas adsorption capacity of gas coals.

The DFT model [26] is capable of analyzing pores approximately 2 nm in size and precisely forecasting capillary condensation and hysteresis effects without requiring adjustments. Based on this, this study utilizes Density Functional Theory (DFT) to analyze the pore size distribution of coal samples subjected to different acidification modifications, using low-temperature nitrogen adsorption experimental data. The pore size distribution curves and cumulative pore volume curves for coal samples under various acidification modification conditions were obtained, as shown in Figure 3.

From Figure 3, it can be observed that the width of the peaks reflects the number of pores within the corresponding size range. A wider peak indicates a greater number of pores and a more widespread distribution within that range, while a narrower peak suggests a more concentrated pore size distribution and a more uniform number of pores. Additionally, a higher vertical coordinate value for a specific pore size indicates a greater number of pores of that size. The peak value represents the maximum proportion of pores, indicating the highest density of pores at that particular pore size.

In terms of pore size distribution, the coal samples subjected to acidification modification exhibit a multi-peak distribution characteristic, with the most pronounced peak intensity in the 2–5 nm range, indicating a higher number of pores within this size range. As the pore size increases, the peak intensity shows a fluctuating decrease, suggesting a gradual reduction in the number of pores. Additionally, significant peaks and peak widths are observed around 18 nm and 40 nm, indicating that transitional pores contribute substantially to the total pore volume.

From the pore size–cumulative pore volume curves, it can be seen that the coal samples treated with organic acid decreased significantly in the microporous range, indicating that the pore volume of the micropores contributes less to the total pore volume. However, the cumulative pore volume of coal samples treated with organic acid increased because the minerals filled in the pores were dissolved by acid solution, which increased the number of pores. Among them, citric acid is the most influential on the pore volume, which is because citric acid is not only highly acidic, it is also a flocculant that can flocculate the fat-soluble components in the coal in a grease state into a solid state, thus further optimizing the pore structure.

4. Fractal Characteristics of Coal Sample Pores

Coal, as a porous medium, has a highly complex pore structure, which can be accurately characterized using fractal geometry. The fractal dimension is an important parameter for describing the complexity of pores. Researchers have developed multiple approaches, such as the fractal BET model, thermodynamic model, and Frenkel–Halsey–Hill (FHH) fractal model, to accurately determine the fractal dimension of coal. In this study, based on low-temperature nitrogen adsorption experimental data, the FHH fractal model is used to calculate the fractal dimension of coal. The formula is as follows, as shown in Formulas (4) and (5) [27]:

$$\ln V=C+A\left[\ln \left(\ln \left({\displaystyle \frac{P_0}{P}}\right)\right)\right]$$

(4)

$$A=-{\displaystyle \frac{3-D}{m}}$$

(5)

where $V$—gas adsorption volume at equilibrium pressure $P$, cm³/g; $C$—intercept of the fitted curve with the y-axis, a constant; $A$—parameters for calculating the fractal dimension; $P_0$—saturated vapor pressure, MPa; $P$—gas equilibrium pressure, MPa.

When van der Waals forces are dominant, $m$ = 3; when capillary cohesion is dominant, $m$ = 1. To more accurately reflect the connectivity of the meso- and macropores, $m$ = 1 was taken in this study to obtain Formula (6) [28,29,30]:

$$D=A+3$$

(6)

The adsorption quantities at different relative pressures ($P/P_0$) were measured by low-temperature nitrogen adsorption experiments, and a linear regression of $\ln V$ against $\mathrm{In}\left(P/P_0\right)$ was carried out to obtain the slope A. The fractal dimension $D$ was calculated by substituting the value of A according to Formula (6). Depending on the relative pressure, the FHH fractal curve can be divided into two stages: within the P/P₀ = 0–0.5 and P/P₀ = 0.5–1 stages. The surface fractal dimension $D$₁ can be calculated by $D$₁ = 3 + $A$₁ to indicate the roughness of the pore surface, and when $D$₁ is larger, this indicates that the pore surface is more irregular, which can enhance gas adsorption. The spatial fractal dimension $D$₂ can be calculated by $D$₂ = 3 + $A$₂ to indicate the complexity of the pore structure. A decrease in $D$₂ reflects a decrease in the homogeneity of the pore network, which usually forms dominant seepage channels and significantly enhances permeability. The specific fractal dimension is shown in

Table 3. Fractal characteristics of coal samples treated with different acid solutions.

Samples	$\mathit{P}/{\mathit{P}}_0$ = 0–0.5		$\mathit{P}/{\mathit{P}}_0$ = 0.5–1
	$\mathit{D}$1	R2	$\mathit{D}$2	R2
YM	2.4473	0.9961	2.6335	0.9526
SM	2.4466	0.9960	2.6296	0.9412
A	2.6697	0.9965	2.4460	0.9455
B	2.6593	0.9941	2.4426	0.9500
C	2.6277	0.9931	2.4381	0.9605
D	2.6698	0.9940	2.4096	0.9595
E	2.6546	0.9944	2.4415	0.9587
F	2.6688	0.9973	2.4177	0.9592
G	2.6885	0.9950	2.4410	0.9763
H	2.7915	0.9961	2.3459	0.9912
I	2.7656	0.9949	2.4271	0.9960
J	2.8031	0.9963	2.3712	0.9675

.

From Figure 4 and Table 3, it can be observed that the range of $D$₁ is 2.4465~2.8031, and the range of $D$₂ is 2.3458~2.6335. The curve fitting for $D$₁ and $D$₂ of all coal samples is effective, with R² values exceeding 0.94. Across the entire relative pressure range, the pore structures of the coal samples exhibit significant fractal characteristics, and the fractal dimensions of each coal sample fall between 2 and 3. This indicates that $D$₁ and $D$₂ can accurately describe the characteristics of the pore surfaces.

Compared to the coal samples treated with water (SM), $D$₁ of the coal samples treated with hydrochloric acid, acetic acid, glycolic acid, oxalic acid, and citric acid increased by 8.69–9.12%, 7.40–9.12%, 8.50–9.08%, 9.88–14.09%, and 13.03–14.57%, respectively. $D$₂ decreased by 6.98–7.11%, 7.28–8.37%, 7.15–8.06%, 7.11–10.79% and 7.70–9.83%, respectively. This is because metal ions are often present in coal as carbonates (CaCO₃), sulfates (CaSO₄), or oxides (Fe₂O₃) to fill pores or plug pore throats. In the case of citric acid, for example, the citric acid solution dissolves these minerals by a complexation reaction, e.g., Formula (7):

$$CaC{O}_3+2H^{+}\to {Ca}^{2+}+C{O}_2+H_{2}O$$

(7)

After mineral dissolution, the $D$₂ fractal dimension decreased from 2.6335 to 2.3712 and pore connectivity improved. The results of Qu et al. [31] showed that the fractal dimension has an important effect on the permeability of coal bodies. Zhang et al. [32] similarly confirmed that the pore structure and connectivity of coal are the main factors affecting the permeability of coal reservoirs. This suggests that modified treatment with organic acid solutions can increase gas permeability by dissolving minerals and enlarging pores.

5. FTIR Functional Group Analysis

As shown in Figure 5, the results of FTIR spectra showed that the positions of the main absorption peaks of the untreated and treated coals were the same, and the overall trends of the spectra were similar. There is a significant difference in the intensity of the characteristic peaks of different coal samples, which indicates that different acid solutions will affect the content of some functional groups, while the basic structure will not be changed [33].

Compared to the original coal, the absorbance peaks of each functional group changed significantly after treatment with different acid solutions. In the range of 3600~3000 cm⁻¹, the intensity of the hydroxyl group stretching vibration peaks was significantly weakened, indicating that some of the hydroxyl-containing small-molecule organic compounds in the coal were solubilized during the acid treatment, resulting in a reduction in free hydroxyl groups. The enhancement of the absorption peaks in the range of 3000~2800 cm⁻¹ may be related to the alkylation reaction of hydrocarbons in coal under acidic conditions, which contributes to the increase in the structure of fatty chains. In the range of 1800~1000 cm⁻¹, the vibration peaks of oxygen-containing functional groups were changed. The anions in the acid solution can form stable chelates with metal ions (e.g., Ca²⁺, Mg²⁺) in the coal, prompting the metal ions to dissociate from the oxygen-containing functional groups, thus increasing the proportion of free oxygen-containing groups. The enhancement of the C-H out-of-plane bending vibrational peaks of aromatic hydrocarbons in the range of 900 to 700 cm⁻¹ may be attributed to the substitution of H⁺ for metal ions bound to the aromatic ring, resulting in an increase in the number of aromatic hydrogens. Overall, the reduction in hydroxyl structure and oxygen-containing functional group structure of the coal samples under the effect of different acid solutions decreased the adsorption affinity of the coal, which made it easier for methane molecules to be desorbed from the surface of the coal and improved the desorption efficiency.

6. SEM Test Results and Analysis

In order to further investigate the effect of acidic fracturing fluid on the pore structure of coal, scanning electron microscopy was used to test the pore surface topographic features of coal samples before and after acid solution treatment. We take the raw coal and the samples after citric acid solution treatment as examples, as shown in Figure 6a,b.

As can be seen from the figure, there are many fine particles on the surface of the original coal particles, and the surface of the treated coal particles is smoother. The reason for this is that the citric acid solution dissolves the mineral components in the coal samples, greatly reducing the mineral particles and increasing the surface roughness of the coal. At the same time, the mineral dissolution contributed to the pore enlargement and the formation of new channels, which optimized the pore network structure and further enhanced the gas flow capacity, which coincided with the increase in $D$₁ and decrease in $D$₂ in the fractal characteristics.

7. Conclusions

This study focuses on the 1/3 coking coal from the Guqiao Coal Mine of Huainan Mining Group Co., Ltd., in China. It systematically investigates the effects of four organic acids—acetic acid, glycolic acid, oxalic acid, and citric acid—on the pore structure and chemical structure of coal samples. The results are compared to the effects of hydrochloric acid solution at the same concentration, leading to the following conclusions:

(1)

The modification treatment with organic acid solution changed the pore structure of the coal samples, resulting in a decrease in the specific surface area, a decrease in the percentage of microporous pore volume and an increase in the percentage of mesoporous pore volume. The pore distribution was optimized and the contribution of transition pores was enhanced, which significantly improved the extraction efficiency of coalbed methane.

(2)

The surface fractal dimension of the coal samples increased and the spatial fractal dimension decreased after the organic acidification modification treatment, indicating that the organic acidic liquid can simplify the pore structure of coal and improve pore connectivity, thus enhancing the transport capacity of coalbed methane.

(3)

The chemical action of the organic acid solution of the coal sample results in a reduction in hydroxyl and oxygen-containing functional groups, which reduces the number of adsorption sites on the surface of the coal, thus weakening the adsorption capacity of methane and making it more susceptible to desorption.

(4)

Through comparative analysis, the improvement effects of different organic acids on the pores of coal samples were significantly different. Among them, oxalic acid and citric acid have better modification effects than hydrochloric acid, while citric acid has the best performance, which can optimize the pore structure and enhance the efficiency of coalbed methane extraction more effectively. Therefore, it is recommended to give priority to citric acid, followed by oxalic acid, in the acidification modification of coal reservoirs in order to obtain a more significant effect of production increase.

The research findings provide essential theoretical support for organic acidification modification technology in coalbed methane reservoirs and coalbed methane extraction technology using hydraulic fracturing in coal seams.