Influences of Different Acid Solutions on Pore Structures and Fractal Features of Coal

Abstract

The effect of different acids on the pore structure and fractal characteristics of micropores and mesopores was determined with the help of low-temperature liquid nitrogen adsorption, X-ray diffraction, and the Frenkel–Halsey–Hill (FHH) model by using Yuwu coal as a sample and placing it in acidic environments, such as HF, HCl, HNO₃, and CH₃COOH. The results show that the acidization effects of HF and CH₃COOH are separately dominated by the micropore and mesopore formation effects, while HCl and HNO₃ mainly play their roles in expanding mesopores. After acidization, the surface fractal dimensions D₁ and D₁′ of micropores and mesopores in coal are always negatively correlated with the total specific surface area S_BET, specific surface area S_mic of micropores, and specific surface area S_mes of mesopores. After being acidized by HF, D₂ is negatively correlated with the total volume V_tot and the corresponding micropore volume V_mic, while acidization with HCl and HNO₃ leads to the opposite result. After being acidized by CH₃COOH, D₂ has a negative correlation with V_tot and a positive correlation with V_mic. The structural fractal dimensions D₂′ of mesopores in samples acidized by HF and CH₃COOH are positively correlated with both the volume V_tot and mesopore volume V_mes, while it is the opposite for samples acidized by HNO₃. D₂′ of coal samples acidized by HCl is negatively correlated with V_tot while positively correlated with V_mes.

1. Introduction

Coal is a type of organic rock composed of inorganic minerals and organic matter and containing complex pore–fracture systems [1,2,3,4,5]. The pore–fracture systems in coal are storage and migration channels of coalbed methane (CBM), and they are likely to be filled with minerals to become closed or semi-closed pores and fractures, which influences the diffusion and migration of CBM [6,7,8,9,10]. Acid fracturing is one of the most effective methods to solve mineral blocking of pores and fractures in coal seams [11,12,13]. Previous research has shown that, when using acid solutions as fracturing fluids, they can dissolve minerals in coal reservoirs, enlarge the volume and specific surface area of mesopores in coal, and simplify pore structures [14,15,16,17,18]. Zhang et al. compared the dissolution effects before and after acidizing coal samples with hydrochloric acid (HCl) and acetic acid (CH₃COOH) and found that hydrogen fluoride (HF) plays a stronger role in dissolving clay minerals, so the mineral dissolution rate when using HF is much greater than that of HCl and HAc [19]. By applying Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy, Yu et al. revealed changes in coal compositions, parameters of organic functional groups, and crystal structures of coal. They found that as the concentration rises, CH₃COOH plays an enhancing role in dissolving surface fractures of coal. In the reaction between CH₃COOH and coal, kaolinite is recrystallized within 24 h after dissolution and completely dissolved by CH₃COOH within 72 h [20]. Dou et al. experimentally studied changes in the CO₂ adsorption/desorption characteristics after acidizing coal with HF and nitric acid (HNO₃) by using the CO₂ adsorption/desorption system and low-temperature nitrogen adsorption method. They stated that HF acidization mainly plays a role in micropore formation while HNO₃ acidization mainly expands the pores. Acidization increases the desorption capacity of coal samples and reduces the desorption time [21]. Ni et al. studied changes in pore structures after HNO₃ acidization and found that HNO₃ plays an obvious role in expanding pores. As the HNO₃ concentration is increased, the pore size, volume, and specific surface area of coal samples all gradually increase [22]. Therefore, revealing changes in pore structures in coal acidized by different acid solutions can lay the foundation for adsorption/desorption research into the extraction of CBM in acidized coal seams.

Fractal geometry, proposed by Mandelbrot [23], has become one of the important approaches for evaluating the anisotropy of internal structures of complex porous materials after years of development [24,25,26,27,28,29,30]. Based on the mercury intrusion method and fractal dimension, Liu et al. [31] described pore structures in coal samples with different grades of metamorphism. Ma et al. [32] found that low-temperature nitrogen adsorption is superior to the mercury intrusion method when they are used to study micropores and mesopores. Zhu et al. [33] studied the structures and fractal features of micropores and mesopores in the steam activation process of activated carbon using low-temperature nitrogen adsorption and the Frenkel–Halsey–Hill model. They found that in the steam activation process, micropores and mesopores were developed; activation at 750 °C was more conducive to the development of mesopore structures. Zhao et al. [34] investigated the difference in pores in raw coal and tectonic coal and stated that the surface fractal dimension D₁ of pores increases with the reduction in particle size. Tectonic coal has a larger D₁ than raw coal while its structural fractal dimension D₂ of pores is lower than that of raw coal. Han et al. [35] adopted the mercury intrusion method and nitrogen adsorption method to study pore structures in coal with different grades of metamorphism and revealed that the fractal dimension of pores shows piecewise characteristics. Therefore, they proposed a new fractal-based classification scheme for pores based on the piecewise fractal dimension. Yi et al. [36] adopted low-temperature gas adsorption (LTGA) to explore changes in pores in coal samples treated with HCl and found that, as the mineral content decreases, the macropore volume increases while mesopore volume decreases, then increases. In addition, many closed mesopores turn into semi-open or open pores, which increases the fractal dimension D₂ and the specific surface area of pores. Whereas the increase in the specific surface area does not have a significant relationship with the fractal dimension D₁ of surface roughness of pores. Zheng et al. [37] studied the influences of HCl, tetrahydrofuran (THF), and carbon disulfide (CS₂) on structural parameters and fractal features of pores in coal through low-temperature liquid nitrogen adsorption (LT-N₂GA), XRD, and fractal theory. They unveiled that after HCl treatment, lots of closed pores are opened, the proportion of micropores increases, and the D₁ of pores declines. THF extraction enlarges the pore volume and specific surface area. After extracting low-molecular-weight organic matter using CS₂, the pore size increases, thereby increasing the volumes of transitional pores and mesopores. After treatment with organic solvents, the solvents are retained in pores in coal, so that D₁ increases and the pore surface becomes rough.

In summary, treatment with chemical solvents can change pore structures in coal and affect the surface roughness of pores; however, previous research hardly combines chemical changes of mineral compositions in coal after acid treatment with the change mechanism of pore structures and fractal features. Considering this, the present research explored changes in the structural parameters of pores in coal samples treated with HF, HCl, HNO₃, and CH₃COOH by combining LT-N₂GA and XRD analysis. This involved quantitative analysis of the specific surface areas of micropores and mesopores, pore volume, and pore size distribution from the microscopic perspective. Additionally, the surface and structural fractal dimensions of micropores and mesopores were calculated based on LT-N₂GA data. In this way, the relationships of different acid solutions, changes in different mineral compositions, and fractal dimensions of micropores (mesopores) with the structural parameters of pores were determined.

2. Sample Preparation and Experiments

2.1. Coal Sample Preparation

① Coal sample pulverization

The experimental coal samples were taken from the Yuwu Mine in Changzhi City, Shanxi Province. The collected coal samples were crushed and ground to less than 0.075 mm and dried under vacuum at 80 °C for 12 h.

② Sample acidification

Five grams of treated coal powder was taken into a tetrafluoroethylene beaker and 400 mL of hydrofluoric acid, hydrochloric acid, nitric acid, and acetic acid with a mass fraction of 8% were added. The samples were magnetically stirred in the acid solution for 24 h at room temperature to ensure homogeneous acidification; the acid-washed coal samples were separated by centrifuge and washed with deionized water until the PH paper was neutral. About 12 h of drying treatment at 60 °C was taken out to complete the acidification of the sample. The original coal samples were numbered as YUAN, and the hydrofluoric acid, hydrochloric acid, nitric acid, and acetic acid acidified coal samples were numbered as HF, HCl, HNO₃, and CH₃COOH, respectively. The relevant parameters are shown in Figure 1.

2.2. XRD Analysis Experiment

The XRD experiment was carried out by the D8 ADVANCE X-ray diffractometer manufactured by Bruker, Karlsruhe, Germany. Experimental conditions are described in detail in the literature [38]. Origin 9.0 software was used to denoise the XRD spectrum. The three fitting Gaussian peaks, which are γ peak,002 peak, and 100 peak, were fitted at 20°, 26°, and 42°, respectively.

2.3. Low-Temperature Nitrogen Experiment

Low-temperature nitrogen experiment was performed according to the Chinese national standard GB/T 21650.3-2011 [39]. The pore structure of the coal samples was tested using ASAP 2460 automatic surface area and porosity analyzer by Micromeritics, Shanghai, Chinese and the accompanying MicroActive software (https://www.micromeritics.com/) to derive the specific surface area, pore volume, and pore size distribution of the coal samples. Before the pore structure test, the coal samples were degassed under a vacuum at 130 °C for 12 h to completely remove air, water, and other impurities.

The specific surface area (S_BET) model was used to calculate the total specific surface area. The surface area (S_mes) of mesopores (1.7 nm < D < 300 nm) and the volume of mesopores (V_mes) were calculated using the BJH method. The surface area of the micropores (S_mic) was calculated from the difference between S_BET and S_mes. The volume of total pores (V_tot) was calculated from the amount of liquid nitrogen adsorbed at a relative pressure of 0.98, and the volume of micropores (V_mic) was calculated from the difference between V_tot and V_mes.

2.4. Calculation of Fractal Dimension

The fractal dimension is one of the most important parameters of fractal geometry and is used to characterize the complexity of pore structure and surface roughness. Based on the Frenkel–Halsey–Hill (FHH) model, the fractal dimension of the pore surface and pore structure is calculated [40,41]. The equation is as follows:

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

where P is the gas adsorption equilibrium pressure, MPa; P₀ is the saturated vapor pressure, MPa; V is the gas adsorption volume at the equilibrium pressure P, mL/g; V_m is the adsorption volume of the monomolecular layer, mL/g; C is the fitting constant; and A is the slope of the double logarithmic curve drawn by ln(V/V_m) against ln(ln(P₀/P)). P₀ is the gas adsorption saturation pressure, Pa; P is the gas adsorption equilibrium pressure, Pa.

The fractal dimension D of Equation (1) as a function of the slope A is as follows [42]:

A = D − 3(2)

3. Results and Analysis

3.1. Mineral Phases

The XRD patterns of raw and acidized coal samples are shown in Figure 2.

As displayed in Figure 2, the raw coal samples (YUAN) mainly contain clay minerals (kaolinite (AL₂Si₂O₅(OH)₄)), silicate minerals (quartz (SiO₂)), and carbonate minerals (calcite (CaCO₃) and dolomite (CaMg(CO₃)₂)).

A comparison of XRD patterns of raw and acidized coal samples reveals that the intensity of diffraction peaks in XRD patterns of HF-treated coal samples decreases, and only quartz is retained. HCl, CH₃COOH, and HNO₃ all completely dissolve carbonate minerals (calcite and dolomite), while the diffraction peaks of silicate minerals (quartz) and clay minerals (kaolinite) change slightly.

3.2. Characteristics of Pores

3.2.1. N₂ Adsorption/Desorption Isotherms

Based on LT-N₂GA, the N₂ adsorption/desorption isotherms and accumulated surface areas of raw and acidized coal samples are separately shown in Figure 3 and Figure 4.

According to the IUPAC (International Union of Pure and Applied Chemistry) classification, the isothermal adsorption/desorption curves of coal samples before and after acidization belong to type-IV isotherms [43], which are typically characterized by the presence of adsorption hysteresis [44]. Acidized coal samples exhibit obvious hysteresis and higher adsorption capacity. This indicates an increment in the specific surface area of coal.

Adsorption is divided into a monolayer adsorption stage, a multilayer adsorption stage, and a capillary condensation stage.

Comparison of N₂ adsorption isotherms before and after acidization shows that when the relative pressure P/P₀ reaches 0.2, N₂ adsorption in micropores finishes. Under the condition, the highest N₂ adsorption capacity is 0.395 cm³/g, which is found in the coal sample treated with HF, followed by 0.301 and 0.293 cm³/g in samples separately treated with HNO₃ and CH₃COOH. The coal samples treated with HCl and YUAN demonstrate the lowest N₂ adsorption capacities of 0.260 and 0.224 cm³/g. These results indicate that acidization is conducive to the formation of micropores.

Under relative pressures P/P₀ of 0.2 to 0.8, N₂ monolayer adsorption and multilayer adsorption are achieved successively in mesopores. At that time, the largest increment of 0.286 cm³/g of N₂ adsorption capacity was found in the coal sample treated with HNO₃, followed by increments of 0.278 and 0.270 cm³/g in samples treated with HF and CH₃COOH. The increments are the lowest in samples treated with HCl and YUAN, which are 0.216 and 0.125 cm³/g, respectively. These results suggest that acidization is conducive to the formation of mesopores.

Under relative pressures P/P₀ of 0.8 to 1, capillary condensation occurs in pores within the coal, which leads to an abrupt rise in the adsorption isotherms. At that time, the largest increment of 1.844 cm³/g of N₂ adsorption capacity was found in the samples treated with HF, followed by those of 1.801 and 1.751 cm³/g in samples separately treated with CH₃COOH and YUAN. The lowest increments of N₂ adsorption capacities are 1.598 and 1.332 cm³/g in samples acidized by HCl and HNO₃. This indicates that pores in samples treated with HCl and HNO₃ are greatly influenced by capillary condensation. This is also related to the generation of CaCl₂ and Ca(NO₃)₂ during acidization of calcite by HCl and HNO₃, so that increments of N₂ adsorption capacities in samples treated with the two acid solutions are lower than that of YUAN coal samples.

Altogether, acidization is conducive to the formation of micropores and mesopores in coal. HF and HNO₃ separately play the greatest roles in forming micropores and mesopores.

3.2.2. Structural Parameters and Pore Size Distribution of Pores

Figure 5 and Figure 6 and Table 1 list the surface areas and pore volumes corresponding to coal samples treated with different acid solutions.

Acid solutions exert two effects on pores: one is pore formation, that is, dissolving closed pores into open pores; the other is pore expansion, that is, dissolving and expanding pre-existing pores.

The V_mic/V_tot ratio of YUAN coal samples is 20%, while those of coal samples treated with HF, HCl, HNO₃, and CH₃COOH are separately 18%, 17.5%, 15.4%, and 15.8%. The S_mic/S_BET ratio of YUAN coal samples is 26.7%, while those of coal samples acidized by HF, HCL, HNO₃, and CH₃COOH are 42.4%, 25.6%, 26.0%, and 20.8%, respectively. This indicates that HCl, HNO₃, and CH₃COOH mainly play their roles in expanding micropores. S_mic of coal samples treated with HF is about three times that of YUAN coal samples, which implies that numerous closed micropores in coal samples acidized by HF are dissolved into open pores and therefore the specific surface area is increased. This suggests that HF mainly plays its role in forming micropores.

The V_mes/V_tot ratio of YUAN coal samples is 80%, while those of samples treated with HF, HCl, CH₃COOH, and HNO₃ are 82%, 82.5%, 84.6%, and 84.2%, respectively. The S_mes/S_BET ratio of YUAN coal samples is 73.3%, while those of samples treated with HF, HCl, CH₃COOH, and HNO₃ are separately 57.6%, 74.4%, 74.0%, and 79.2%. This indicates that HF exerts an expansion effect on mesopores, and the effect is weaker than that on micropores. HCL and HNO₃ mainly expand mesopores, while CH₃COOH mainly forms mesopores.

According to the N₂ adsorption isotherms, NLDFT was used to calculate the pore size distribution in the range from 1.3 to 216 nm (Figure 7).

The pore size of micropores in coal samples is mainly concentrated within the range from 1.3 to 2.0 nm. A comparison of peaks of pore sizes in coal samples treated with HF and YUAN coal samples in the range of 1.3 to 1.7 nm shows that the micropore formation effect of HF focuses on micropores within this range, and the pore expansion effect is concentrated on micropores in the range of 1.7 to 2.0 nm. The pore size of mesopores in coal is mainly in the range of 2 to 40 nm, dominated by those between 2 and 9 nm. This indicates that mesopore structures are mainly small; CH₃COOH mainly exerts its role in the formation of mesopores measuring 6 to 40 nm.

The V_mes/V_tot ratio of YUAN coal samples is 80%, while those of coal samples acidized by HF, HCl, CH₃COOH, and HNO₃ are 82%, 82.5%, 84.6%, and 84.2%, respectively. The S_mes/S_BET ratio of YUAN coal samples is 73.3%, while those of coal samples treated with HF, HCl, CH₃COOH, and HNO₃ are separately 57.6%, 74.4%, 74.0%, and 79.2%.

3.3. Fractal Features of Pore Structures

3.3.1. Fractal Features of Micropore Structures

Based on the Polanyi adsorption potential theory, the Dubinin–Radushkevich (D-R) method can be used to characterize micropore structures [45]. According to N₂ adsorption isotherms, the D-R diagram of coal samples was plotted (Figure 8).

When P/P₀ is approximately 0.01, the first inflection point appears on the fitted line on the D-R diagram, indicating that micropores being two times the diameter of N₂ molecules have been fully filled, that is, ultra-micropores smaller than 0.7 nm have been fully filled.

When P/P₀ reaches 0.07, the second inflection point appears, which means that monolayer adsorption on the surface of micropores with the size being three to four times the diameter of N₂ molecules has been finished. Therefore, it can be used to calculate the surface fractal dimension D₁ of micropores measuring 1.2 to 1.8 nm (Figure 7a), that is, at the transition point from monolayer adsorption to multilayer adsorption, and to characterize the surface roughness of the micropores.

At P/P₀ of 0.2, micropores as large as three to four times the diameter of N₂ molecules have been fully filled. The results show that N₂ adsorption under P/P₀ of 0.07 to 0.2, that is, multilayer adsorption and filling in micropores measuring 1.2 to 1.8 nm, can be used to calculate the structural fractal dimension D₂ of micropores and characterize the structural complexity of the micropores.

According to N₂ adsorption data pertaining to the micropores, the fitted lines for ln(V/V_m) v. ln(ln(P₀/P)) curves of coal samples were plotted (Figure 9). The slopes A₁ and A₂ of lines are fitted, and the fractal dimensions D₁ and D₂ of micropores are calculated.

The fractal dimensions D₁ and D₂ of micropores and the correlation coefficient R² of coal samples are listed in Table 2.

Data in Table 2 show that the correlation coefficient R² is always greater than 0.972, indicative of the reliability of the regression analysis.

The value of D₁ of coal samples varies within the range from 2.150 to 2.300, indicating obvious fractal features on micropore surfaces. The value of D₁ of YUAN coal samples is 2.300, while that of acidized coal samples is in the range of 2.150 to 2.225. This is mainly because acidization induces dissolution of carbonate minerals and swelling of clay minerals in coal, so that the micropore surfaces become smooth. Therein, coal samples treated with HF have the lowest D₁ and the smoothest micropore surfaces.

The value of D₂ of coal samples varies from 2.470 to 2.719, which suggests that micropore structures also exhibit fractal features. This indicates complex micropore structures and high anisotropy. The value of D₂ of YUAN coal samples is 2.719, while it is between 2.470 and 2.545 for acidized coal samples. This is mainly because acidization removes minerals and organic matter in coal, simplifies micropore structures, and contributes to more uniform pore size distribution. Therein, coal samples treated with HNO₃ have the smallest D₂, simplest micropore structures, and most uniform pore size distribution.

3.3.2. Fractal Features of Mesopore Structures

In accordance with N₂ adsorption isotherms in mesopores when P/P₀ is between 0.2 and 1, ln(V/V_m) v. ln(ln(P₀/P)) curves were subjected to piecewise linear fitting (Figure 10).

Figure 10 shows that there is an inflection point on each of the fractal curves in the FHH model when ln(ln(P₀/P)) is about −1.5, which corresponds to P/P₀ of 0.8. This means that at the inflection point, N₂ adsorption in mesopores turns from monolayer adsorption into multilayer adsorption. Therefore, N₂ adsorption data before and after the inflection point can be used to calculate the surface and structural fractal dimensions of mesopores and describe the two different fractal features [46,47].

When P/P₀ is in the range of 0.2 to 0.8, N₂ molecules have monolayer adsorption on mesopore surfaces due to surface tension, which can be used to calculate the surface fractal dimension D₁′ of mesopores. When P/P₀ is greater than 0.8, mesopores are full of N₂ molecules under the van der Waals’ force (VDW), and capillary condensation occurs, which can be used to calculate the structural fractal dimension D₂′ of mesopores.

The fractal dimensions D₁′ and D₂′ of mesopores in coal samples and their correlation coefficient R² are listed in Table 3.

Table 3 demonstrates that R² is always greater than 0.867, indicating the reliability of the regression analysis.

The value of D₁′ of coal samples is between 2.671 and 2.803, indicating that mesopore surfaces show obvious fractal features and high roughness. The value of D₁′ of YUAN coal samples is 2.803, while that of acidized coal samples is between 2.671 and 2.742. This suggests that acidization can smooth the mesopore surface in coal. Therein, D₁′ of coal samples treated with HNO₃ is the lowest, and mesopore surfaces are smoothest.

The value of D₂′ of coal samples varies between 2.483 and 2.669, which indicates that mesopore structures exhibit fractal features. The result suggests that mesopore structures are complex and highly anisotropic. The value of D₂′ of YUAN coal samples is 2.483, while that of acidized coal samples is in between 2.578 and 2.669. Combined with Figure 7, acidization expands mesopores in coal and some micropores expand to become mesopores, thus increasing D₂′. As a result, mesopore structures in acidized coal samples become more complex, and pore sizes are distributed less uniformly. Therein, coal samples treated with HNO₃ have the largest D₂′, most complex mesopore structures, and strongest anisotropy of pore size distribution.

3.4. Relationship between Pore Characteristics and Fractal Dimensions

To study changes in the fractal dimension of pores, the relationships between characteristic parameters and fractal dimensions of micropores and mesopores were established.

The relationship between the surface fractal dimension D₁ and surface area parameters (S_BET and S_mic) of micropores and the relationship between the structural fractal dimension D₂ and volume parameters (V_tot and V_mic) of micropores are illustrated in Figure 11. The relationship between the surface fractal dimension D₁′ and surface area parameters (S_BET and S_mic) of mesopores and the relationship between the structural fractal dimension D₂′ and volume parameters (V_tot and V_mes) of mesopores are illustrated in Figure 11.

According to the analysis of the data in Figure 11, the surface fractal dimension D₁ of micropores in acidized coal samples is negatively correlated with the surface area S_BET and micropore surface area S_mic. The structural fractal dimension D₂ of micropores in coal samples treated with HF has negative correlations with the volume V_tot and micropore volume V_mic. The structural fractal dimension D₂ of micropores in coal samples acidized by CH₃COOH is negatively correlated with volume V_tot and positively correlated with micropore volume V_mic. The structural fractal dimensions D₂ of micropores in coal samples treated with HCL and HNO₃ are both positively correlated with volume V_tot and micropore volume V_mic.

Analysis of Figure 12 shows that the surface fractal dimension D₁′ of mesopores in acidized coal samples has negative correlations with both the surface area S_BET and mesopore surface area S_mes. The structural fractal dimensions D₂′ of mesopores in coal samples acidized by HF and CH₃COOH are positively correlated with the volume V_tot and mesopore volume V_mes. The structural fractal dimension D₂′ of mesopores in coal samples treated with HCl has a negative correlation with volume V_tot while a positive correlation with mesopore volume V_mes. The structural fractal dimension D₂′ of mesopores in coal samples treated with HNO₃ is negatively correlated with the volume V_tot and mesopore volume V_mes.

According to XRD data and Figure 11c,d and Figure 12c,d, V_tot in coal samples acidized by HCL and HNO₃ is smaller than that of YUAN coal samples because solid particles CaCl₂ and Ca(NO₃)₂ not completely dissolved in water are generated in pores during acidization of carbonate mineral calcite and then block some pores. This leads to different relationships between acidization effects and the pore volume (V_tot). The acid solutions are listed in descending order as HF, CH₃COOH, YUAN, HCl, and HNO₃ according to the acidization effects. The micropore volumes V_mic and mesopore volumes V_mes before and after acidization show that the generation of CaCl₂ and Ca(NO₃)₂ has greater influences on V_mic than on V_mes, and CaCl₂ exerts weaker influences on the pore volume compared with Ca(NO₃)₂.

4. Conclusions

The mineral compositions, pore structures, and fractal features of coal samples collected from the Yuwu coal mine treated with HF, HCL, HNO₃, and CH₃COOH were studied through LT-N₂GA and X-ray diffraction. The following conclusions were obtained:

(1) By using X-ray diffraction, CaCl₂ and Ca(NO₃)₂ that block micropores are produced when calcite is acidized by HCl and HNO₃, thus influencing the acidization effect. Acid solutions are listed in descending order as HF, CH₃COOH, YUAN, HCl, and HNO₃ according to the acidization effects.

(2) The acidization effects of HF and CH₃COOH are separately dominated by the micropore and mesopore formation effects, while HCl and HNO₃ mainly play their roles in expanding mesopores.

(3) After acidization, the surface fractal dimensions D₁ and D₁′ of micropores and mesopores in coal are always negatively correlated with the total specific surface area S_BET, specific surface area S_mic of micropores, and specific surface area S_mes of mesopores. After being acidized by HF, D₂ is negatively correlated with the total volume V_tot and the corresponding micropore volume V_mic, while acidization with HCl and HNO₃ leads to the opposite result. After being acidized by CH₃COOH, D₂ has a negative correlation with V_tot and a positive correlation with V_mic. The structural fractal dimensions D₂′ of mesopores in samples acidized by HF and CH₃COOH are positively correlated with both the volume V_tot and mesopore volume V_mes, while it is the opposite for samples acidized by HNO₃. D₂′ of coal samples acidized by HCl is negatively correlated with V_tot while positively correlated with V_mes.

According to the results, relationships of different acid solutions, changes in different mineral compositions, and fractal dimensions of micropores (mesopores) with structural parameters of pores were established. They are expected to provide a reference for research on acid solution selection and acidization mechanism during acid fracturing for CBM extraction.

Author Contributions

Methodology, X.L.; software, E.S.; writing—original draft preparation, J.Z.; writing—review and editing, X.N.; supervision, X.N.; project administration, X.N.; funding acquisition, X.N. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Sample parameters and photos before and after acidification.

Figure 2. X-ray diffractogram of coal samples.

Figure 3. Adsorption/desorption isotherms of coal samples.

Figure 4. Specific surface area accumulation graph.

Figure 5. Specific surface area of the sample.

Figure 6. Volume of the sample.

Figure 7. Pore size distribution based on NLDFT approach. (a) Microporous pore size. (b) Mesopore aperture.

Figure 8. Curve D-R diagram of coal samples.

Figure 9. Fractal dimensions D1 and D2 of microporous pores: (a) microporous fractal dimension D1 of P/P0 at 0.07 and (b) microporous fractal dimension D2 for P/P0 in the range 0.07~0.2.

Figure 10. Mesoporous fractal dimensions D1′ and D2′ for P/P0 in the range 0.2~1.

Figure 11. Microporous parameters versus fractal dimension. (a) SBET vs. D1. (b) Smic vs. D1. (c) Vtot vs. D2. (d) Vmic vs. D2.

Figure 12. Mesopore parameters versus fractal dimension. (a) SBET vs. D1′. (b) Smes vs. D1′. (c) Vtot vs. D2′. (d) Vmes vs. D2′.

Table 1

Pore structure parameters of the samples.

Samples	SBET(m²/g)	Smic(m²/g)	Smes(m²/g)	Smic/SBET	Smes/SBET	Vtot(10−3 cm³/g)	Vmic(10−3 cm³/g)	Vmes(10−3 cm³/g)	Vmic/Vtot	Vmes/Vtot
YUAN	0.745	0.199	0.547	0.267	0.733	3.174	0.636	2.538	0.200	0.800
HF	1.408	0.596	0.811	0.424	0.576	3.812	0.685	3.127	0.180	0.820
HCL	0.915	0.234	0.681	0.256	0.744	3.136	0.548	2.588	0.175	0.825
HNO3	1.092	0.283	0.808	0.260	0.740	2.884	0.443	2.441	0.154	0.846
CH3COOH	1.047	0.218	0.829	0.208	0.792	3.565	0.564	3.001	0.158	0.842

Table 2

Parameterized table of fractal dimensions of micropores.

Samples	A1	D 1	R 2	A2	D 2	R 2
YUAN	−0.700	2.300	0.982	−0.281	2.719	0.972
HF	−0.850	2.150	0.990	−0.502	2.498	0.995
HCL	−0.801	2.199	0.988	−0.455	2.545	0.994
HNO3	−0.775	2.225	0.988	−0.530	2.470	0.998
CH3COOH	−0.819	2.181	0.991	−0.506	2.494	0.996

Table 3

Parameterized table of fractal dimensions of mesopores.

Samples	A1′	D1′	R 2	A2′	D2′	R 2
YUAN	−0.197	2.803	0.867	−0.517	2.483	0.989
HF	−0.258	2.742	0.998	−0.383	2.617	0.990
HCL	−0.292	2.708	0.994	−0.422	2.578	0.992
HNO3	−0.329	2.671	0.999	−0.331	2.669	0.985
CH3COOH	−0.320	2.680	0.995	−0.409	2.591	0.992

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