Research on how acidification affects the coal's microscopic pore structure in the Guizhou mining region

By eliminating their minerals, acidification technology makes coal seams more permeable. Acidification technology were conducted in the Farr and Qinglong mining areas using a mixed acid solution to study the impact of acidification technology on increasing the permeability of primary structural coals in the Guizhou region, China. The microphysical and chemical structure of the coal samples before and after acidification transformation was compared using X‐ray diffraction, scanning electron microscope, mercury intrusion porosimetry, and Fourier transform infrared spectrometry experiments. The results show that after acidification, the volatile matter content is reduced by 4.2% and 16.25%, the carbon content is increased, the degree of coalification is deepened, the gas content of the coal seam is elevated, and the gas source is more abundant. The minerals on the surface are dissolved to different degrees, and the pores and cracks are dredged, so that the surface of the coal samples shows better connectivity, which is conducive to the diffusion and transport of coalbed methane. The volume and pore diameter of the macropore and mesopore in the Farr coal samples were improved after acidification, and the fractal structure was more prominent, the pore connectivity was good, and the pore diameter of the Qinglong coal samples was greatly increased. The content of oxygen‐containing functional groups increased by 11.2% and 2.1%, respectively, which is conducive to the desorption of coalbed methane after acidification, and with the increasing degree of metamorphism of the coal samples, the higher the content of oxygen‐containing functional groups was and the less pronounced the effect of acidification. Acidification technology can effectively remove the minerals in the coal body and has a good effect on penetration enhancement, and the results of the study can provide certain theoretical references for the penetration enhancement of coal seams in Guizhou mining areas.

that after acidification, the volatile matter content is reduced by 4.2% and 16.25%, the carbon content is increased, the degree of coalification is deepened, the gas content of the coal seam is elevated, and the gas source is more abundant.The minerals on the surface are dissolved to different degrees, and the pores and cracks are dredged, so that the surface of the coal samples shows better connectivity, which is conducive to the diffusion and transport of coalbed methane.The volume and pore diameter of the macropore and mesopore in the Farr coal samples were improved after acidification, and the fractal structure was more prominent, the pore connectivity was good, and the pore diameter of the Qinglong coal samples was greatly increased.The content of oxygen-containing functional groups increased by 11.2% and 2.1%, respectively, which is conducive to the desorption of coalbed methane after acidification, and with the increasing degree of metamorphism of the coal samples, the higher the content of oxygen-containing functional groups was and the less pronounced the effect of acidification.Acidification technology can effectively remove the minerals in the coal body and has a good effect on penetration enhancement, and the results of the study can provide certain theoretical references for the penetration enhancement of coal seams in Guizhou mining areas.

| INTRODUCTORY
Coalbed methane (CBM) is a hydrocarbon gas with methane as the main component.][7][8] Acidizing technology refers to the restoration or enhancement of the permeability of formation pores and cracks through the dissolving and erosioning action of acid on rock cement or blockages in formation pores and cracks, and so on.0][11] In the field of coal, gas pressure and ground stress increase with the increase in mining depth, the permeability of coal rock decreases, and gas is difficult to desorbed from the reservoir, leading to a significantly higher threat of gas dynamic hazards.3][14] By dissolving the minerals in the coal's pore cracks, acidification technology can increase the pore volume and porosity of the coal, as well as the connectivity of the pores, increasing the coal seam's permeability.Additionally, the coal's chemical structural parameters are altered by the acidizing process, which encourages methane desorption.The benefits of acidification technology are consistent with its traits of high resource richness and methane content but low penetration rates.Numerous minerals, including carbonate rock, silicate rock, and other minerals, are vulnerable to chemical interactions with acids in the cracks of the coal reservoir.These materials obstruct methane's ability to travel through and penetrate these fissures, which has a negative impact on the production of carbonate gas.The disruption of drilling liquid and solid well cement pulp during well completion is also likely to obstruct methane movement and infiltration in the shale reservoir, consequently slowing the rate of shale storage penetration. 15,16any professionals and academics have now studied acidity and transformation technologies.Different coal samples were acidified using a solution of hydrogen chloride (HCl), hydrogen fluoride (HF), acetic acid (HAc), and ammonium chloride (NH 4 Cl).8][19] The findings demonstrated that following acidification, the BET and pore volume rose, with the majority of this increase focused around 10 nm; however, this was not evident for high-rank coals, and the permeability of the specimens had a tendency to grow, stabilize, and decrease with time.The HF-soaked group, HCl-soaked group, and HF-HCl-mixed acid-soaked group were, respectively, set up to analyze the microstructures of their oxygen-containing functional groups and aliphatic chain contents by Fourier transform infrared (FTIR) and 13   CNMR experiments, and it was found that the mixed acid group was able to remove the ash effectively, the number of micropores and mesopores increased the most, and that the increase in the number of oxygencontaining functional groups and decrease in the aliphatic carbon content of the HF-soaked group were both most significant. 20Focusing on three typical inorganic acids, HF, HCl, and HNO 3 , the pore, microcrystalline and combustion properties of raw and acidified coal were analyzed using X-ray diffraction (XRD) and simultaneous thermal analysis (STA), and the fractal dimensionality theory, microcrystalline structure analysis theory, and thermal evaluation kinetic theory were utilized to analyze the pore, microcrystalline, and combustion properties of raw and acidified coal, respectively, and the results showed that the HF has the best enlarging effect on the volume of the pores and the pore size of the coal, followed by HNO 3 , and that inorganic acids can cause the layer spacing of the bituminous microcrystalline structure to collapse, and promote the transverse splicing and vertical stacking of the aryl structure, thus increasing the extent of the condensation of the aryl rings in the bituminous coal, as well as the degree of coal deterioration. 21The addition of sodium dodecyl sulfate to the acidification treatment intensified the substitution reaction of the benzene ring structure in coal, according to research employing FTIR on the impact of acidification on the chemical composition of coal. 6,19The effect of acid-thermal coupling on the destruction of coal pores and cracks was studied.At the ideal acidification experimental temperature of 50°C, the acid-thermal coupling effect not only altered the microscopic morphology and elemental composition of the coal samples, but it also encouraged the formation of the pore-fracture network within the coal and significantly increased connectivity, which was favorable for the diffusion and migration of coalbed methane. 22By calcining and HF acidifying the Anyang coal samples, the structural parameters of the coal were determined by XRD, scanning electron microscopy (SEM), Raman, and FTIR spectroscopy.It was discovered that this combination significantly weakened the fatty side chains at the edges, encouraged the delamination of the aromatic layers in the coal, and yielded ultrafine crystalline carbon materials such as graphite. 23,24Evaluated the impact of microwave heating-assisted acidification on the coal microstructure through an experimental investigation of coal samples from Pingdingshan in Henan.Experimental comparisons were also made between bituminous coal and anthracite degradation rates and pore structure indices under acidification in Shandong and Henan provinces. 25,26n this study, primary structural coal samples from various regions of Guizhou Province, China.These coal samples were soaked in a mixed acid solution of 3% HCl, 9% HF, and 3% HAc.Industrial and elemental analyses were then conducted on the coal samples before and after acidification to compare the composition and elemental changes of the samples and to explore changes in the internal microscopic composition of the samples as a result of acidification.SEM experiments were used to explore the causes of the creation of pores and cracks on the surfaces of the coal samples after the acidification treatment, and the impacts of acidification on the coal samples' tiny pores and cracks were seen.To address the impact of acidification on the types and contents of minerals in coal samples, XRD tests are used to analyze changes in the primary minerals of coal and the type of coal structure; study the mechanism of the impact of acidification on the quantity and complexity of pores within coal samples by using mercury intrusion into the pore space tests to analyze changes in the pore diameter and pore volume in combination with the fractal dimensions.Carry out the Fourier infrared spectrometry experiment to observe the intensity and number of characteristic peaks to determine the change of chemical functional groups, quantitatively analyze the change of oxygen-containing functional groups in the coal samples, and explore the effect of acidification on the desorption of coalbed methane.Through the results of the study, we hope to find out the suitable direction of gas extraction after acidification and permeability enhancement in low-permeability and high-gas coal seams, so as to improve the gas extraction rate, and to provide a certain theoretical basis for the gas extraction work in coal and gas-abnormalized mines as well as for the development of CBM.

| Material preparation
Coal samples were taken from fresh large primary coal from the 55301 working face of Farr coal mine in Shuicheng county, Guizhou province, and from the M18 coal seam of Qinglong coal mine in Bijie city, Guizhou province, and transported to the laboratory sealed with cling film.Small bits of raw coal are pulverized in a ball mill in accordance with GB/T 474-2008's specifications before being prepared through a standard sieve to create three mesh coal samples: (1) 60-80 mesh; (2) 80-100 mesh; and (3) 100-200 mesh coal powder.

| Acidification process
Using distilled water, hydrofluoric acid (40 wt.%), hydrochloric acid (37 wt.%), and acetic acid (100 wt.%), a mixed acid solution of 3% HCl + 9% HF + 3% HAc was created.Two portions of Farr and Qinglong coal powder were taken in accordance with the needs of the experiment.One portion was used as raw coal for drying and sealing, and the other portion was placed in an inert plastic bottle with a mixed acid solution.The bottle was then thoroughly shaken to ensure that the solids and liquids made complete contact with one another.After 12 h of reaction, the coal samples were cleaned and filtered until the pH level was neutral, and then the samples were put in a sealed bag for vacuum drying.The raw coal was numbered as FR-RAW and QL-RAW, and the samples treated with the mixed acid solution were numbered as FR-MIX and QL-MIX in that order.

| Experimental component
Six experiments of industrial analysis, elemental analysis, SEM, XRD, mercury intrusion porosimetry (MIP), and Fourier infrared spectroscopy were conducted on the samples before and after acidification to study the impact of acidification on the coalbed methane mining work in the coal reservoir in Guizhou Province.The experimental flow is shown in Figure 1, which explores the changes in the pores and the microstructures of the coal beds before and after acidification.analysis The chemical composition of coal is frequently ascertained in industry through industrial analysis and elemental analysis.Industrial analysis of coal divides coal into four components: moisture, ash, volatiles, and fixed carbon, which are used to preliminarily determine the characteristics and types of the coal.The element analysis of coal is used to determine the content of organic elements in coal, and the organic material in the coal mainly contains carbon, hydrogen, oxygen, nitrogen, sulfur, and other elements.To get a clear picture of the chemical composition of the samples before and after acidification, this study will compare the samples of Farr coal samples and Qinglong coal samples before and after acidification using industrial and elemental analytical methods to gain a thorough understanding of the chemical composition of the samples. 27,28

| Scanning electron microscope (SEM)
The working principle of a SEM is that, mainly through lens focusing and electric field acceleration, the electron beam is shot into the test sample, the surface of the sample is scanned point by point, and the physical signals are generated by the electron beam after contacting the test sample and are processed by the detector to present the characteristic images of the test sample, which is a widely used method to characterize the microstructure of the coal body.With the help of the SEM method, the surface structures of coal samples from Farr and Qinglong before and after acidification were compared to evaluate the impact of acidification on the pore structure characteristics of the coal surface.This experiment was carried out using a SU8010 emission SEM from Hitachi, Japan, with an accelerating voltage of 10 kV and a magnification of 500-10,000. 29,30

| XRD
A coal mineral, which is the collective term for all inorganic substances other than water, may be analyzed using an XRD graph.Coal minerals are primarily separated into four major categories: clay minerals, carbonate minerals, sulfur minerals, and oxide minerals.To test the types of minerals in the coal samples before and after acidification, XRD was used to scan the samples before and after acidification of the coal samples of Farr and Qinglong, and then the diffraction patterns were analyzed using the processing software MDI Jade 6.0.This experiment was carried out using an EDAX Orbis X-ray fluorescence spectrometer from the United States of America, and the X-ray light source was Cu-K rays (λ = 0.154 06 nm), with a tube voltage of 40 kV, a tube current of 40 mA, a scanning step of 0.013°, and a scanning from 5°to 40°carried out. 30

| MIP
Hg is frequently employed as an invasive fluid since it is a nonwetting medium.Due to its quick and straightforward analytical model, the MIP method is frequently used to test physical parameters in coal samples, including pore volume, pore size distribution, pore area, and pore structure.[33]

| FTIR spectroscopy
Infrared spectrum refers to the molecule's ability to selectively absorb certain wavelengths of infrared rays while causing the vibration energy level and rotational energy level to jump in molecules.By detecting infrared rays that are absorbed, we can get the infrared absorption spectrum of matter.5][36] A Nicolet 670 FTIR spectrometer was used to scan the samples to determine the alterations in the types and amounts of functional groups in the samples from Farr and Qinglong Coal Mines before and after acidification.OMNIC software was then used to process and evaluate the data.

| Industrial and elemental analysis
The findings of the industrial and elemental examinations of the coal samples from Farr and Qinglong before and after acidification are displayed in Table 1.The coal samples from the Farr and Qinglong mines that have not been acidified are represented by FR-RAW and QL-RAW, respectively, while the acidified coal samples are represented by FR-MIX and QL-MIX.The primary coal of the Farr coal sample is classified as poor coal, and the primary coal of the Qinglong coal sample is classified as semianthracite, as per the Chinese classification of coals, GB/T 5751-2009.
The two samples before and after acidification had relatively low amounts of moisture, ash, and volatile matter, which suggests that the samples were dry and had a high degree of metamorphism.After acidification, the volatile component content of Farr coal and Qinglong coal decreased by 4.2% and 16.25%, respectively, making the fixed carbon content increase, the degree of carbonization deepen, and the gas content of the coal layer increase.The volatile component content of Qinglong coal is much lower than that of Farr coal because, with the deepening of the degree of metamorphic coal, the esters in the coal will become oil or gas and disperse, so that the amount of carbon content increases and the volatility decreases. 1After acidification, the content of the C and O elements is increased, and the S, N, and H elements are reduced, which indicates that acidification is conducive to the desulfurization and denitrogenation of coal, and the effect of acidification on the Qinglong coal sample is better than that of the Farr coal sample.
Figure 2 below demonstrates how the changes in the components and elements after acidification have led to a different degree of decrease in the moisture, ash, and volatile components, increasing the fixed carbon content, increasing the C element content corresponding to it, deepening the degree of carbonization, and increasing the gas content of the coal layer.As the acid liquid dissolved the minerals in the cracks of the coal cavities and the crater canal was evacuated, the moisture and ash content of the two samples decreased.However, after acidification, Farr coal's ash reduction was greater than that of Qinglong coal's, even though the total amount was less.Because esters and aromatic hydrocarbons in coal will transform into oil or gas and disperse when coal's degree of degradation increases, the volatility of Qinglong coal is significantly lower than that of Farr

| SEM
Fractures and matrix pores make up coal. 37CBM is primarily stored in the pore-fracture structure of coal reservoirs, and the fractures are the main channels for CBM transportation and seepage.Enlarging the size of the pore-fractures and increasing the width and number of fractures can effectively increase the connectivity of the fracture system and the flow rate of gas transportation, which can improve the permeability of the coal beds and facilitate the discharge of the gas. 30EM can be used to determine the microscopic pore structure of the coal samples before and after acidification.The microstructural characteristics of the coal samples before and after acidification are shown in Figure 3, with a magnification of 5000.The Farr coal sample surface has a large amount of minerals and some pores and cracks in its primary coal, as can be seen from the figures.After acidification, the minerals on the coal sample's surface were clearly reduced, and new pores and cracks were also dissolved.Even though acidification disintegrated some pores and cracks, the Qinglong coal sample still contains a significant number of blocked minerals in the pores and cracks nearby, causing them to be blocked.The minerals were dissolved to varying degrees after acidification, according to a comparison of the two coal samples' microstructures before and after acidification, and the number and size of the pores and cracks increased significantly, and the connectivity of the fracture system was improved.
Whether it is cracks or pores, the acidification and penetration enhancement effect of the primary coal of Farr is better than that of the primary coal of Qinglong, probably because the coal sample of Qinglong belongs to semianthracite coal, which is more metamorphosed than the coal sample of Farr, and the coal sample has a high degree of polymerization of the aromatic ring and fewer side chains of the molecule.
It is difficult to determine precisely the elements of coal surface erosion through SEM alone.As seen in Figure 4, scanning telescopes typically include an energy dispersive spectrometer that can conduct both microstructure analysis and viewing at the same time. 22The most abundant elements in the two coal samples were carbon and oxygen, as well as traces of silicon and calcium, suggesting the presence of quartz, kaolinite, and calcite.Among them, the acidified coal contains almost nonexistent Si and Ca because HCl and HF fully react with minerals, and Si and Ca are dissolved in the liquid in the form of ions, resulting in a large loss of Si and Ca on the surface of the coal sample. 38,39The proportion of C elements on the surface of the coal sample increases by 15.5% and 6.7%, respectively, after acidification.Because when the content of other elements decreases, the relative content of C increases, and in an acid environment, the crystalline minerals buried in the coal body migrate to the surface, and these minerals contain C, which is conducive to the formation of corrosive pore networks inside the coal body.

| XRD
The porous fracture structure of the coal reservoir is the channel and place for material exchange with the outside world, and its degree of development and connectivity directly determine the permeability of the coal reservoir, which in turn affects the ease of coalbed methane desorption.After a long period of geological changes, many fissures in coal reservoirs are blocked by many minerals, such as quartz, kaolinite, calcite, hematite, chlorite, and so on.The principle diagram of coal reservoir acidizing and penetration enhancement is shown in Figure 5. 40 A chemical process known as coal reservoir acidizing is used to increase the permeability of coal reservoirs.It involves injecting acid into the coal reservoir and using the acid and the minerals there to dissolve minerals or blockages within the coal reservoir's pore and crack structure.This increases the coal reservoir's pore and crack structure's fluency, which promotes the seepage and desorption of coalbed methane, and the main chemical reaction equations of the coal seam with this mixed acid are as follows: As indicated in Figure 6, process experimental XRD data using MDI Jade6 software, create XRD figures, and determine the weight percentage of the primary mineral included in the coal sample.
The clay mineral quartz and the sulfide mineral pyrite make up the majority of the primary coal in the Farr coal sample.Pyrite's diffraction peak virtually vanished after acidification, and its weight percentage dropped by 75.56%.Quartz's diffraction peak also shrank, but its weight percentage rose by 41.77%.Large amounts of the clay-like minerals kaolinite and chlorite, as well as a negligible amount of the silicate mineral nacrite, are present in the primary coal of the Qinglong sample.
Chlorite's and albite's diffraction peaks essentially vanished after acidification, and their weight percentages fell by 55.56% and 40.5%, respectively.Although the kaolinite diffraction peak was lost, the weight ratio rose by 2.1%.After acidification, the proportion of clay minerals with higher content in coal samples, such as quartz and kaolinite, increased.This may be because there were fewer minerals overall, and because the acid's dissolution effect on pyrite and albite was better, their proportion decreased, which in turn caused an increase in the proportion of minerals with little change in diffraction peaks, such as quartz and kaolinite.
The two coal samples have various mineral kinds and contents, which causes the diffraction peaks of the coal samples to differ both before and after acidification.The mineral contents also exhibit varying degrees of reduction as a result of acidification.According to the partially diminished diffraction peaks of kaolinite and quartz, acidification partially dissolved the clay minerals kaolinite and quartz in the coal samples.Pyrite, chlorite, and sodium feldspar diffraction peaks essentially vanished, proving that acidity removed the majority of these sulfide, clay, and silicate minerals from the coal samples.The mineral content of coal is reduced to varying degrees after acidification, which shows that acidification has an obvious dredging effect on the pore cracks of coal reservoirs that is conducive to the diffusion and transport of coalbed methane.The dissolution effect of acidification on sulfide minerals and silicate minerals is better than that of clay minerals.

| MIP
Mercury fills large fractures and pores in the MIP experiments before continuing to enter smaller cracks and pores as the pressure rises.The hysteresis phenomenon in the pores is the cause of the extrusion curve's lack of overlap with the intrusion curve and the cumulative volume of the intrusion.Mercury can characterize the total volume of pores inside the coal body. 41,42n the high-pressure mercury injection experiments, mercury enters the coal samples' macropores when the pressure is less than 0.1 MPa, all of the macropores and some of the mesopores when the pressure is between 0.1 and 10 MPa, and the micropore when the pressure is greater than 10 MPa through capillary action. 43Figure 7 depicts the mercury intrusion and extrusion curves for the coal samples both before and after acidification.It can be seen from the Hg pressure curve of Farr primary coal that the mercury intrusion curve rises rapidly at the stage of <0.1 MPa and then slows down as the pressure increases, and the coal samples contain many macropores, with a cumulative invasion amount of 0.331 cm 3 /g; the mercury intrusion still rises within the pressure range of 0.1-10 MPa, and the rate of rise tends to flatten after >1 MPa, with the overall rate significantly smaller than that of the macroporous stage, and the cumulative intrusion volume was 0.428 cm 3 /g; at >10 MPa, the amount of mercury intrusion almost ceased to change, and the cumulative mercury intrusion volume was 0.436 cm 3 /g.Thus, the cumulative mercury that was injected into the macropores, mesopores, and micropores of the Farr primary coal can be expressed as 0.331, 0.097, and 0.008 cm 3 /g, respectively, with the mesopore accounting for the majority of the volume, the macropore coming in second, and the micropore accounting for the least amount.After acidification, the rate of increase of | 2331 mercury intrusion into the primary coal of Farr was faster than it was before acidification; it increased rapidly at a pressure of 0.1 MPa, slowed down at 0.1-1 MPa, and nearly stabilized after >10 MPa.The trend of mercury intrusion at each stage was similar to that which existed before acidification.The cumulative mercury intrusion in the three stages was 0.546, 0.7, and 0.713 cm 3 /g.The volumes of the macropore, mesopore, and micropore after acidification can be expressed as 0.546, 0.154, and 0.013 cm 3 /g, respectively.Acidification increased the cumulative intrusion volume of the macropore, mesopore, and micropore by 64.65%, 58.76%, and 62.5%, respectively.
The amount of Hg intrusion in the primary coal of the Qinglong coal sample at 0.1 MPa increased rapidly; the rising rate slowed down in the stage of 0.1-10 MPa and almost stopped changing after >10 MPa.The cumulative amount of Hg intrusion in the three stages mentioned above was 0.358, 0.567, and 0.578 cm 3 /g, and so the intrusion volumes of macropores, mesopores, and micropores were 0.358, 0209, and 0.011 cm 3 /g, respectively.Most of the coal body is composed of macropores and mesopores.The trend of the three pressure stages after acidification changed compared with that before acidification: the <0.1 MPa stage increased rapidly and then leveled off; the 0.1-10 MPa stage was almost unchanged; the >10 MP showed a slow increasing trend; and the cumulative Hg intrusion volumes were 0.175, 0.179, and 0.207 cm 3 /g, so that the microporous, mesoporous, and microporous volumes were, respectively, 0.175, 0.004, and 0.028 cm 3 /g.Acidification reduced the volume of microporous and mesoporous materials by 51.12% and 97.76%, respectively, and the volume of micropores increased by 154.55%.The reduction of microporous and mesoporous may be probably due to the oxidation of acetic acid that made the coal show the phenomenon of corrosion and thinning of the pore walls and pore closure during the pressurization process.
After acidification, the hysteresis loop area decreased by 71.22% and 49.91%, respectively, because some of the pores became less connected and more complicated as a result of acetic acid oxidation. 44Traditional theories, however, struggle to adequately capture the intricacy of coal's pore structure; therefore, fractal theory was introduced to mathematically represent this complexity.
The Frenkel-Halsey-Hill (FHH) Model 45,46 can be used to calculate the fractal dimension of the coal pore structure before and after acidification, and this equation is as follows: Here, p is the mercury intrusion pressure, MPa; dvp is the relative pressure dp of mercury increment, cm 3 /g; K is the slope of the curve of ( ) ln dvp dp and p ln( ); d is the fractal dimension obtained from the mercury intrusion experiment.The relationship betweend "K" and "d" is d = K − 4. The FHH model was fitted to the Hg intrusion experiments, and the results are shown in Figure 8, where d 1 , d 2 , and d 3 correspond to the fractal dimensions of macropore, mesopore, and micropore.
Fractal dimension, which is a measurement of the irregularity of the complex shape, represents the effectiveness of the space that the complex form occupies.The range of the fractal dimension is typically between 2 and 3. When the fractal dimension is small, it denotes a relatively simple development of the pore space, concentrated pore diameter and volume distribution, weak nonhomogeneity, and good pore connectivity.When the fractal dimension is large, however, it denotes a complex and variable development of the pore space, dispersed pore diameter and volume distribution, strong nonhomogeneity, and poor pore connectivity.When the fractal dimension is greater than three or less than two, it typically means that there is little to no fractal rule in this range of pore sizes.As shown in Figure 8, the fractal dimension of macropores after acidifying coal samples from Farr ranges from 2.846 to 3.062, that of mesopores ranges from 2.023 to 2.498, and that of micropores ranges from 2.794 to 3.163.The fits of macropores and mesopores are better than those of micropores, and the fractal features of macropores and micropores are not as prominent as those of mesopores.After acidification, the overall fractal dimension of the pore increases and the complexity increases, indicating that the mixed acid can effectively dissolve the minerals in the coal body, resulting in the coal body presenting more pore structure.However, after immersion in highpressure mercury to destroy the microporous structure, the fractal characteristics of the micropores are less obvious than those of the macropore and mesopore.In samples of Qinglong coal subjected to acidification, macropores have a fractal dimension between 3.367 and 2.147, mesopores between 1.572 and 3.202, and micropores between 3.323 and 3.755.has a good match between the macropore and the mesopore.The macropores' fractal dimension increases with acidification, and the fractal features become more obvious.Acidification plays a part in increasing pore connectivity, as shown by the fractal structure of mesopores going through three stages: not prominent, prominent, and not prominent.However, under high pressure, the complexity tends to increase, and the fractal characteristics of micropores are not readily apparent.
Therefore, acidification has a better fractal effect on both macropores and mesopores, which can reduce pore complexity and increase pore connectivity.But a poorer fractal effect on micropores is expected, given that the method of mercury compression results in the destruction of the pore structure under high pressure, so that the fractal structural characteristics of pores are no longer obvious.
Studying the pore size distribution of coal samples before and after acidification, the effect of acidification on the pores of each pore size can be clearly observed, and the results are shown in Figure 9.
According to pore size, the pores were classified as microporous (pores smaller than 100 nm), mesoporous (pores between 100 and 1000 nm), and microporous (pores larger than 1000 nm).The increment of mercury in the microporous and mesoporous stages after acidification was not much different from that before acidification, but the increment of mercury in the transition stages of mesopore and macropore rapidly jumped from 0.005 to 0.0766 cm 3 /g, and the increment of mercury in the stage of macropore was up to 0.0434 cm 3 /g, and the volume of the macropore was enlarged by 76.5%.While that in the stage of extralarge pore did not change obviously, which may be due to the fact that the dissolving effect of acidification on extralarge pore is not obvious, and the extra-large pore are prone to fracture under high pressure, which disintegrates the extra-large pore, resulting in a decrease in the number of extra-large pore.Therefore, acidifying can effectively dissolve the minerals attached to the mesopore, causing them to be transformed into the macropore, resulting in | 2333 an increase in the volume of the macropore.The pore diameter of Qinglong coal samples after acidification has a transition from mesopore and macropore to macropore, which increases the pore volume of macropores.The proportion of mesopore and macropore was the highest before acidification, and the volume of macropore with diameters around 10,000 nm was the largest, about 0.07 cm 3 /g.The pore volume of the remaining pore diameters showed a slight trend of change.The incremental amount of mercury in the extra-large pore with diameters of more than 100,000 nm increased rapidly to 0.115 cm 3 /g after acidification, and the acidification effect caused some of the macropores to transition to the extralarge pore, which resulted in an increasing trend of the macropores in both volume and diameter.The diameter and volume of the macropores of the Qinglong coal sample showed a trend of becoming larger after acidification, and only the volume of the Farr coal sample showed a rising trend, indicating that the destructive effect of the acid on the pore and fissure structure in the Farr coal body was more prominent than that of the Qinglong coal sample.

| FTIR spectroscopy
The infrared spectra of the Qinglong and Farr coal samples are depicted in Figure 10, and it can be seen that the peak positions and trends of the two coal samples after acidification are comparable to those before acidification.The infrared spectra of the coals can also reveal the kinds of functional groups that are present in the structure of the coals, and acidification has little impact on the kinds of functional groups.According to the wave numbers, the infrared spectra of coal may be separated into four categories: 700-900, 1000-1800, 2700-3000, and 3000-3600 cm −1 , among which the wave numbers from 1000 to 1800 are primarily distributed by the functional groups containing oxygen.The hydrophilicity and lipophilicity of coal can be influenced by oxygen-containing functional groups.Coal with superior hydrophilicity interacts with water more strongly, which lessens methane adsorption in the coal's pores.
The fitting results for the infrared spectral lines between 1000 and 1800 cm −1 are presented in Figure 11.The acidification process can increase the stability of the coal structure, and the kind of functional group is essentially the same because the strength of the absorption peak is reduced and the peak position is nearly the same as before acidification.Carboxyls, carbonyl groups, and ether-oxygen linkages are the most frequent functional groups in coal that contain oxygen.According to the diffraction peak position of the spectrum in the range of 1000-1800 cm −1 , the infrared absorption peak can be explained by the tensile vibration of C-O in phenol, alcohol, ether, and ester, the symmetrical deformation vibration of -CH 3 , the shear vibration of -CH 2 , the tensile vibration of C═C in aromatic rings, and the tensile vibration of carbonyl groups in aldehydes, ketones, and acids. 47The infrared spectral fitting parameters of oxygen-containing functional groups in coal samples before and after acidification are shown in Table 2.
Table 2 shows that the amounts of C-O, -CH 2 , and C═C functional groups increased by 10.01%, 78.11%, and 77.96% in the Farr coal samples, respectively, while the amounts of -CH 3 functional groups dropped by 81.3%.In Qinglong coal samples, the concentrations of the functional groups C-O, -CH 2 , and C═C increased by 10.95%, 2.28%, and 32.05%, respectively, whereas the contents of the functional group -CH 3 declined by 56.68%.After acidification, coal samples include more C-O functional groups; hence, acid oxidation can encourage the development of oxygen-containing functional groups, boosting coal's wettability. 48,49 functional groups to dissolve, forming -CH 2 , which increased the functional group content of -CH 2 .
To quantify the change of oxygen-containing functional groups in coal before and after acidification, the following equation 20 will be used: Here, I is the ratio of oxygen containing functional groups and aromatic carbon skeleton in the coal structure, indicating the degree of oxygen enrichment of the coal structure; A denotes the fitted peak area corresponding to a specific wave number; A v OH (− ) is the sum of alcohol hydroxyl group, phenol hydroxyl group, hydroxyl bond and free hydroxyl group in the range of 3300-3600 cm −1 ; A v (C−O) is the sum of C-O on the phenyl ether in the range of 1120-1160 cm −1 ; A v (C=C) is the sum of C═C backbone vibrations of aromatic compounds in the range of 1620-1680 cm −1 .The calculated oxygenated functional group contents of the four samples are listed in Table 3.
T A B L E 2 Infrared spectral fitting parameters of oxygen-containing functional groups in coal samples.

Sample
Center/cm The oxygenated functional group contents of the coal samples from Farr and Qinglong increased by 11.2% and 2.1%, respectively, indicating that the coal was easily oxidized in the acidified environment.In addition, when combined with the acetic acid oxidizing reaction, the oxygenated functional group contents of the coals showed a different degree of increase; there was less methane adsorption in the coal body, and acidity helped CBM desorb from the coal body.Although the increment of oxygencontaining functional groups in the Farr coal sample was higher than that in the Qinglong coal sample, the Qinglong coal sample had an advantage in the total amount because the internal development of the primary structural coal in the Qinglong coal sample was better, the degree of oxidation was higher than that of the Farr coal sample, and the oxidizing reaction was not obvious after the acid immersion, which led to a lower increment of oxygen-containing functional groups than that in the Farr coal sample.

| DISCUSSION
The poorly connected perforated structures within the coal body are caused by the drilling liquid and solid well cement residues remaining inside the perforation, the blocked micropores and mesopores cavities, the lack of connective cavities, and the increased number of closed cavities.The goal of increased production is achieved after the acidification technology reacts with the minerals in the coal storage to clear blockages in the spread and penetration channels.Significant amounts of carbonate and silicate rock are dissolved in the pore, which significantly reduces the blockage of the channel.
The acidification transformation mechanism is shown in Figure 12.HF, HCl, and HAc all have a clear dissolution effect on inorganic minerals like carbonates in coal, with HF having the greatest impact on the quality change of coal samples and being able to dissolve almost all inorganic minerals and HCl having the least impact.HAc also extracts small-molecule organic compounds like esters and ethers from coal or undergoes organic chemistry. 21,50,51s a result, when coal samples from two mines in Guizhou, China, are exposed to acid, the mineral content is dramatically reduced and the internal structure of the coal body is made more obvious.In the mining region of Guizhou, China, acidification technology has a significant impact on the transport and production of coalbed methane, but in actual applications, it is necessary to choose acids with various mass fractions based on the various geological features of each region and then conduct dissolution experiments on sample immersion to measure the dissolution rate to choose the best acid-liquid ratio.

| CONCLUSIONS
The main findings of this study, which examined the penetration enhancement effect of mixed acid on CBM in coal reservoirs in terms of changes in the internal composition, pore characteristics, microscopic mineral T A B L E 3 The degree of oxygen enrichment of oxygen-containing functional groups.| 2337 types, structures, surface pores, and cracks of the acidized coal samples, are as follows: (1) Following acidification, the volatile matter content of coal samples declines, the carbon content rises, the coalification process deepens, and the volume of coalbed methane increases.Acidification promoted the deterioration degree of Qinglong coal samples better than that of Farr coal samples and had some desulfurization and denitrogenation effects.Both coal samples were dissolved to varying degrees after acidification, and it was discovered that after acidification, the proportion of C elements on the surface of the Faar and Qinglong coal samples rose by 15.5% and 6.7%, respectively.And their surfaces displayed improved connectedness, which was advantageous for coalbed methane diffusion.
Because the coal samples from Qinglong belonged to semianthracite coal with a high degree of metamorphism, a high degree of polymerization of aromatic rings in the coal samples, and few molecular side chains, they had poor erosion efficacy.The blocked cracks and pores in the Farr coal samples were opened to a certain extent after acidification, and the effect of acidification and permeability enhancement was better than that of the Qinglong coal samples.(2) Acidification dissolves the minerals in the coal samples to different degrees, and sulfide and silicate minerals dissolve more readily than clay minerals.The proportion of clay minerals quartz and kaolinite with higher content in the coal samples increased by 41.77% and 2.1%, respectively, and the total mineral content in the coal samples showed a decreasing trend.Additionally, the covered or blocked pores and fissures were unblocked, and the acidification's dissolving impact was helpful for the transit of coalbed methane.(3) The Farr coal sample's macropore, mesopore, and micropore volumes increased after acidification by 64.65%, 58.76%, and 62.5%, respectively, whereas the Qinglong coal sample's macropore and mesopore volumes decreased by 51.12% and 97.76%, respectively, and the volume of micropores increased by 154.55%.When combined with the fractal dimension equation, acidification enhanced the macropore and mesopores' fractal structure and strengthened pore connections, which was advantageous for CBM seepage.The pore diameters of the macropore and mesopore in the coal body were enlarged and concentrated by acidification, showing the trend from mesopore to macropore.
(4) Although the functional group contents of the coal samples changed significantly, acidification had a minimal impact on the functional group types.Acidification technology can promote the fracture formation of the -CH 3 functional group to form the -CH 2 functional group, and the C-O functional group of the two coal samples in the range of 1000-1800 cm −1 increased by 10.01% and 10.95%, respectively.After acidification, the oxygencontaining functional group content of the Farr and Qinglong coal samples increased by 11.2% and 2.1%, respectively, showing that acidification is advantageous for the desorption of CBM.With the increasing degree of metamorphism of the coal samples, the oxygen-containing functional group content increased, and the acidification effect became less obvious.
coal.As a result, Qinglong coal has a higher carbon content than Farr coal.Due to HAc's reaction with coal to produce oxidation and the oxide layer, which increases the content of the O element after acidification.The desulfurization and denitrification effects of coal are achieved by the reaction between the N and S components in the acid and coal, which results in scattered nitrogen oxides and sulfur oxides.

F I G U R E 2
Changes in the content of components and elements in coal.(A) FR Sample, and (B) QL sample.HE ET AL.| 2327

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I G U R E 4 EDS analysis of variations in element content.F I G U R E 5 Schematic diagram of acidification and permeability enhancement of coal reservoir.

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I G U R E 6 Changes in mineral types and content of coal samples.(A) FR-RAW, (B) FR-MIX, (C) QL-RAW, and (D) QL-MIX.

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I G U R E 8 Fractal dimension for each stage of pores.(A) FR-RAW, (B) FR-MIX, (C) QL-RAW, and (D) QL-MIX.HE ET AL.

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I G U R E 9 Pore size distribution of full pores.(A) FR sample and (B) QL sample.F I G U R E 10 Infrared spectra of the coal samples.
It's possible that acidification caused the chemical bonds in the -CH 3 F I G U R E 11 Peak-fitting curve in the range of 1000-1800 cm −1 .(A) FR-RAW, (B) FR-MIX, (C) QL-RAW, and (D) QL-MIX.