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Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39227670
71371
10.1038/s41598-024-71371-w
Article
Effects of clean fracturing fluids on coal microstructure and coalbed gas adsorption
Zhang Qian 2022100015@aust.edu.cn

1
Cai Feng fcai@aust.edu.cn

12
Xie Haotian 3
Fang Yu 3
1 https://ror.org/00q9atg80 grid.440648.a 0000 0001 0477 188X School of Safety Science and Engineering, Anhui University of Science and Technology, Huainan, 232000 China
2 https://ror.org/00q9atg80 grid.440648.a 0000 0001 0477 188X State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines, Anhui University of Science and Technology, Huainan, 232001 China
3 https://ror.org/00q9atg80 grid.440648.a 0000 0001 0477 188X School of Civil Engineering and Architecture, Anhui University of Science and Technology, Huainan, 232001 China
3 9 2024
3 9 2024
2024
14 2042821 6 2024
27 8 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Nowadays, some fracking fluids can enable resourceful extraction of coalbed methane and reduce greenhouse gas emissions. However, their toxicity or corrosiveness will cause harm to downhole workers and pollute groundwater resources. Thus, five kinds of clean composite fracturing fluids were developed in this paper by using starch solution as the matrix and adding various preparations. The change rule of methane adsorption capacity by microstructure changes of coal samples was investigated systematically, and the optimal composite fracturing fluid was determined. The results showed that the new fracturing fluid increased the degree of aromatic ring condensation by 43.3% and the average pore size by 52.1%. Also, the adsorption constants of a value decreased by 11.6% and b value decreased by 23.9%, which can remarkably reduce the methane adsorption. The experimental results provide theoretical support for the clean production of coalbed methane.

Keywords

Gas
Cleaner production
Compound flocculant solution
Functional groups
Pore structure
Subject terms

Environmental impact
Natural gas
China Anhui University Collaborative Innovation projectGXXT-2020-057 issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

China is rich in coalbed methane resources, accounting for about 12% of the world's total coalbed methane1. With the rapid development of the economy, society's demand for energy is growing. As a representative of clean energy, coalbed methane is extensively used in the fields of civil, industrial fuel power generation and chemical industry2. Therefore, realizing the efficient exploitation of coalbed methane could supplement the energy gap effectively and optimize the energy structure. Nevertheless, the coal reservoirs in China generally have poor permeability, low permeability, strong adsorption capacity and other features3, which makes it impossible to directly extract coalbed methane from the coal seams without adopting technical measures4. Hydraulic fracturing technology is one of the current effective measures to increase the penetration of coal seams and the production of coalbed methane5. Fracturing fluids, as additives to hydraulic fracturing technology, improve the microstructure and gas desorption capacity of coal seams, thus increasing the efficiency of gas extraction6.

To realize the efficient extraction of coalbed methane, more and more scholars started to modify coal seams from the perspective of chemical structure and physical structure through fracturing fluids7. Among them, the chemical structure affects the force between coal and gas, and the physical structure affects the permeability of coal seam. Many studies have confirmed that chemical fracturing fluids affect the chemical structure of coal dramatically. The main manifestation is the reduction of the fatty structure content of coal and changes in the oxygen-containing functional groups and hydroxyl structures8. In addition, the content of fatty structures influences the degree of polycondensation and reduces the gas adsorption capacity9. It is because the fracturing fluid diminishes the length of the fat chain, which affects the force between coal and gas10,11. Meanwhile, many scholars have proven the effect of chemical fracturing fluids on the physical structure of coal, and found that fracturing fluids decreased the number of micropores of coal12, and increased the amount of medium and large pores13. It is attributed that chemical fracturing fluid changes the ink-bottle-type hole into a hole with an opening at one end, which results in a significant reaming effect14. Moreover, the permeability of coal will be increased after treatment with chemical fracturing fluid15. Thus, in the process of gas extraction, fracturing fluids are used to dissolve minerals and soluble molecules16 in coal, improve the connectivity between pores17, and facilitate gas transport18. It was also found that the fracturing fluid could reduce the pore structure fractal dimension and promote the flow of methane. It can be proved that the change of both physical and chemical structure of the coal seam can enhance the permeability of the coal seam.

Currently, representative results were achieved by utilizing chemical fracturing fluids to modify coal samples, but chemical fracturing fluids are toxic and highly corrosive19, causing groundwater pollution. During the process of using chemical fracturing fluids, hydrogen gas is generated by the oxidizing reaction, which creates new safety hazards for coalbed methane mining20. Besides, most of the previous researchers focused on the single microstructure of modified coal, and few studies have deeply analyzed the effects of pore structure and chemical structure on the adsorption capacity of coalbed methane. Hence, it is necessary to develop a fracturing fluid that can both improve the characteristics of coal seams and reduce or avoid groundwater contamination21.

In recent years, the combination of natural polymers and inorganic polymer flocculants composite in the area of coal slurry water has been extensively researched, among which soluble starch and flocculants show better stabilization and environmentally friendliness. In these studies, the starch solution and phenolic components exhibit better flocculation effects, and the flocculant solution and coal minerals in the coal exhibit better flocculation effects. So, combining the two types of solution improves the flocculation activity, and the coal microstructure to maximize the penetration effect.

In summary, five new types of composite fracturing fluids were prepared by adding different flocculants into starch solution. The changing rules of the functional group structure and pore structure of coal by different clean fracturing fluids were studied by infrared spectroscopy, low-temperature nitrogen adsorption and methane isothermal adsorption test. The effects of microstructure on methane permeability and methane adsorption performance were analyzed, and efficient, environmentally friendly and safe fracturing fluids were selected. The results of the study could minimize environmental pollution, alleviate the gas protrusion problem during coal mining, and improve the safety of coal mining and the efficiency of coalbed methane mining.

Experiment materials

In this paper, polymerized aluminum chloride, polymerized aluminum iron chloride, polymerized iron chloride, polymerized aluminum sulfate, polymerized aluminum iron sulfate, and food-grade soluble starch were mixed with food-grade soluble starch according to the ratio of 1:1 by volume, and five starch flocculants with a concentration of 20% were prepared.

The coal samples were collected from the anthracite coal from the Yuwang coal mine in Yunnan, China, and the industrial analyses were presented in Table 1. The coal samples at the core were crushed, ground and sieved to obtain 0.178 ~ 0.250 mm coal powder for infrared spectroscopy (FTIR) and low-temperature nitrogen adsorption tests. Then, the coal samples were immersed in the configured compound flocculant solution after 48 h, so that the coal samples were in close proximity with the solution. The processed coal samples were marked, washed, filtered, dried to constant weight in a constant-temperature vacuum oven at 40 ℃, and sealed for use (where M0 is the raw coal, MA, MB, MC, MD and ME are the coal samples processed with starch polymerized aluminum chloride solution, starch polymerized aluminum iron chloride solution, starch polymerized iron chloride, starch polymerized aluminum sulfate solution, and starch polymerized iron silicate solution, respectively).Table 1 Basic information on coal sample.

Sample	Moisture /%	Volatile matter /%	Ash /%	Fixed carbon /%	
raw coal	3.36	8.91	9.53	78.2	

Experimental methods

FTIR measurements

Determination of functional group structure of coal samples by Fourier Transform Infrared Spectrometer (FTIR) of Bruker, Germany. According to the test requirements, 1 mg of the test coal sample and 200 mg (KBr) were weighed, and the coal sample and potassium bromide were homogeneously mixed in agate mortar and ground. The samples were compressed into 0.1 ~ 1.0 mm slices by the pressing method and placed in a Fourier transform infrared spectrometer for analysis and measurement. The scanning range of the spectrometer was 4000–400 cm-1, and the swept resolution was 4 cm-1. To minimize the error, each coal sample was subjected to 3 scanning experiments.

The peak areas of different functional groups were obtained using split-peak fitting, and the aliphatic structure parameter I1, aromaticity I2, maturity parameter I3, and the degree of condensation of aryl rings I4 of the coals were calculated, which can be calculated by Eq. (1), Eq. (2), Eq. (3), Eq. (4)22.1 I1=A(2900-2400cm-1)A(3000-2940cm-1)

2 I2=A(900-700cm-1)A(3000-2800cm-1)

3 I3=A(1800-1650cm-1)A(1800-1600cm-1)+A(1600cm-1)

4 I4=A(900-700cm-1)A(1600cm-1)

Low-temperature N2 adsorption

The pore structure of the coal samples was determined using a Micromeritics ASAP 2460 BET analyzer. Firstly, 2 g of the processed coal samples were weighed and subjected to vacuum degassing at 130 °C for 12 h to eliminate impurities such as air and water from the coal. The nitrogen adsorption and desorption isotherms were obtained by testing under conditions of liquid nitrogen P/P0 from 0.001 to 0.9995 and temperature of 77 K. Then, the volume and pore size distribution of the pores were calculated according to the BJH (Barrett-Joyner Halenda) theoretical model and DFT (Density Functional Theory) method23. The pore size is classified into micropores (< 10 nm), transition pores (10 ~ 100 nm), and mesopores (100 ~ 1000 nm) according to the B B Hodot pore classification method.

The pore structure complexity in coal is according to the FHH24 (Frenkel-Halsey-Hill) model with the Eq. (5):5 LnV/V0=DLn[Ln(P/P0)]+C

where V is the volume of gas molecules adsorbed at the equalization pressure P, cm3/g, V0 is the saturated adsorption volume of gas molecules in a single layer, m3/g, P0 is the gas maturation adsorption pressure, D is the fractal dimension, and C is a constant.

Methane isothermal adsorption test

The coal samples were vacuum degassed at 40 ºC until the end of the degassing time using a 3H-2000PHD high-pressure gas adsorption meter. The change in methane adsorption capacity was determined at a constant temperature of 40 ºC by feeding methane into the sample tube. The maximum pressure was 3 MPa.

The Langmuir equation was used to describe the adsorption process of gas, which is given by25:6 V=abP1+bP

where v is the gas adsorbed amount, cm3/g, a is the adsorption constant, the saturation adsorbed amount per unit mass of coal, b is the adsorption constant, which indicates the rate of adsorption of coal on gas, p adsorption equilibrium pressure, MPa. The specific process and experimental equipment described above are shown in Fig. 1.Fig. 1 Flow chart of test operation.

Results and discussion

Functional group analysis

To study the impact of different solutions on the structure of coal functional groups, the Fourier transform infrared spectrometer (FTIR) tests were carried out on raw coal and coal samples treated with five starch flocculant solutions. The major peaks of the coal samples were basically the same in Fig. 2, but the absorbance of each group of coal samples was different. This indicates that although the types of functional groups stayed the same, the contents of various functional groups changed.Fig. 2 Effect of compound flocculant solution on infrared spectrogram of coal samples.

The coal samples' FTIR spectra were divided into four regions26: hydroxyl structure of 3600–3000 cm−1, aliphatic hydrocarbon structure of 3000–2800 cm−1, oxygenated functional groups of 1800–1000 cm−1, and aromatic structure of 900–700 cm−1. To evaluate the effect of different compound flocculant solutions on the functional group contents, a method of quantification was used to compare the functional group contents of coal samples processed under different compound flocculant solutions. The peaks in each field were fitted to determine the position and intensity of each absorption peak. The peak areas of the absorption peaks were calculated to analyze the changes in the functional group contents. Taking the M0 sample as an example, the fitting curve is shown in Fig. 3.Fig.3 M0 Fitted curves showing infrared spectra of coal samples.

Distribution of the 3000–3600 cm−1 functional group

In the FTIR spectrum, the region 3000–3600 cm-1 represents the hydroxyl structure. In the hydroxyl structure, 3420 cm-1 is the antisymmetric telescopic oscillation of OH…O, 3260 cm−1 is the cyclic hydroxyl group, and the peak around 3090 cm-1 correlates to the OH…N stretching oscillation26. Fitting the absorption peaks to the hydroxyl structural region yielded four subpeaks, with each subpeak's percentage area reflecting the content of various hydroxyl structures.

As shown in Fig. 4, the OH…O dominates the hydroxyl structure, accounting for 64% of the total. Under the action of compound flocculant solution, the OH…O in the coal samples increased by 2%-20%, and the cyclic hydroxyls decreased by 3%-29%. The OH…N remained at a low level with few changes27. It is due to the extensive dissolution and destruction of the cyclic hydroxyl group in the presence of compound flocculant solution, that provides an opportunity to interact with other functional groups. Also, a lot of OH…O structure and a minor amount of OH…N were formed, which changed the existence of functional groups and thus reduced the methane adsorption capacity of the coal28.Fig. 4 Proportion of fitted peak area (%) for 3000–3600 cm−1 split peak fitting.

Distribution of the 2800 ~ 3000 cm−1 functional group

The aliphatic hydrocarbon structure is represented in 2800 ~ 3000 cm-1. Three main types of aliphatic hydrocarbon structures were identified in the absorption spectra: antisymmetric stretching oscillation of -CH3 (3000–2940 cm−1), antisymmetric stretching oscillation of -CH2 (2940–2900 cm−1) and -CH stretching oscillation (2896 cm−1). Fitting the absorption peaks to the aliphatic hydrocarbon region yielded 3 subpeaks, with each subpeak's percentage area reflecting the content of various aliphatic hydrocarbon structures.

As shown in Fig. 5, the prevalent aliphatic hydrocarbon structure in the coal samples was mainly composed of -CH2. It is due to the coal molecular network is rich in aliphatic and cyclic hydrocarbon structures, and -CH2 is mainly distributed in the linear chain segments of aliphatic chains, the side chains of cyclic hydrocarbons and bridge bonds, which is the main ingredient of the aliphatic hydrocarbon structure29. The treatment of compound flocculant solution showed a significant effect on the -CH3 and -CH2 in the coal, while there was an insignificant change in the -CH. It shows that -CH is not sensitive to the compound flocculant solution. It emphasizes the stability hierarchy of aliphatic hydrocarbon structures in coal: -CH > -CH3 > -CH2. Under the influence of compound flocculant solution, -CH3 and -CH2 in coal samples exhibited opposite trends. It is attributed to the compound flocculant solution promoting the interconversion of the two aliphatic hydrocarbon structures during the interaction with the coal. It can be inferred that the starch flocculant solution dissolved some of the aliphatic hydrocarbon functional groups, thereby attenuating the coal-methane interaction30.Fig. 5 Proportion of fitted peak area (%) for 2800–3000 cm−1 split peak fitting.

Distribution of the 1000–1800 cm−1 functional group

In FTIR spectroscopy, the range 1800–1000 cm-1 correlates to oxygen-containing functional groups. Mainly seven main types are included in this scope: C = O (1715 cm−1), stretching oscillations of the aromatic ring C = C (1610 cm−1), methyl (-CH3) and methylene (-CH2) antisymmetric deformation oscillations (1445 cm−1), symmetric bending oscillations of methyl (-CH3) (1380 cm-1), phenolic hydroxyl C-O stretching oscillations (1350 ~ 1170 cm−1), the alkyl ether C–O–C tensile oscillations (1105 cm-1), and Si–O-Si stretching oscillations (1010 ~ 1035 cm−1) (citations). Fitting the absorption peaks to the oxygen-containing functional group region yielded 8–9 subpeaks, with each subpeak's percentage area reflecting the content of various oxygen-containing functional groups31.

As shown in Fig. 6, the aromatic ring C = C dominates the oxygenated functional groups in the raw coal, while the Si–O-Si, C–O–C and C-O are less. Under the action of compound flocculant solution, the C = C content, Si–O-Si content and C–O–C content decreased, while the C-O content grew by 3%-27%. It shows that the dissolution and decomposition of compound flocculant solution break C = C, C–O–C and Si–O-Si. In this process, a substantial quantity of free O atoms was released to combine with C atoms to form C-O, leading to a rise in the C-O. In addition, the C = C content of MC coal is clearly reduced, leading to a decrease in the content of oxygen-containing functional groups, which weakens the adsorption capacity of the coal methane molecules32.Fig. 6 Proportion of fitted peak area (%) for 1000–1800 cm−1 split peak fitting.

Distribution of the 700–900 cm−1 functional group

In the FTIR spectra, the aromatic structure was found in the 700 ~ 900 cm−1. Among the aromatic structure, the peaks near 900 ~ 860 cm−1 and 860 ~ 810 cm−1 indicate benzene group 5 substitution (1H) and benzene group 4 substitution (2H), respectively. In addition, the peaks near 810–750 cm−1 and 750 ~ 720 cm−1 corresponded to benzene group 3-substitution (3H) and benzene group 2-substitution (4H), and in the 720 ~ 700 cm−1 were benzene group 1-substitution (5H). Fitting the absorption peaks to the aromatic structural region yielded 6–7 subpeaks, with each subpeak's percentage area reflecting the content of various aromatic structures.

As shown in Fig. 7, the highest aromatic structure is benzene group 3-substituted (3H). In the coal samples processed with compound flocculant solution, the contents of benzene group 2-substituted (4H) and benzene group 1-substituted (5H) were the least variable. The insensitivity indicates that benzene group 2-substituted (4H) and benzene group 1-substituted (5H) are not affected by the treatment with compound flocculant solution. Compared with the original coal, the benzene group 3-substituted (3H) in the coal samples processed with compound flocculant solution decreased by 12–20%, while the content of benzene group 5-substituted (1H) and benzene group 4-substituted (2H) increased by 3–11% and 7–12%, respectively. It suggests that the compound flocculant solution promotes more C-H bond substitutions during the interaction with coal, which contributes to the improvement of coal wettability and enhances the interaction between coal and starch flocculant solution28.Fig. 7 Proportion of fitted peak area (%) for 700–900 cm−1 split peak fitting.

Infrared structural parameters

To further investigate the effect of compound flocculant solution on the chemical structure of coal, the structural parameters of coal were calculated to characterize the methane sorption capacity of coal. The aliphatic structural parameter I1 was used to characterize the length and degree of branching of aliphatic side chains, the aromaticity I2 was employed to represent the relative abundance of aromatic and aliphatic. The maturity parameter I3 might be described in terms of C = O and C = C, and the degree of condensation of the aromatic groups I4 could be reflected by the relative content of aromatic and C = C33.

As shown in Figs. 8, I1 was reduced by 30% to 92% in the action of compound flocculant solution and minimized in MC coal. It means that the compound flocculant solution is highly adept at destroying the aliphatic structure and shortening the length of the fatty chain. The general increase of I2 explains that the aromaticity of the coal is increased, thus providing more methane adsorption sites on the surface of the coal34. Following the treatment with compound flocculant solution, I3 increased slightly, indicating that the maturity of the coal was increased. Furthermore, I4 was generally raised because the compound flocculant solution dissolved part of the fatty chains and made the structure compact, thus increasing the extent of aryl ring polycondensation. The better the extent of aryl ring polycondensation, the lower the force between coal and methane, facilitating the desorption of coalbed methane35.Fig. 8 FTIR characteristic parameters of coal samples.

Pore structure analysis

Nitrogen adsorption isotherm

The N2 isothermal adsorption, desorption curves of raw and treated coal are shown in Fig. 9. It is a type IV isotherm in accordance with the IUPAC (The International Union of Pure and Applied Chemistry) classification standard36. During the adsorption process, when the relative pressure P/P0 was about 0.02, the nitrogen adsorption amount showed a short and rapid increase. It was due to the capillary filling of the micropores, which is the monolayer adsorption stage of nitrogen molecules at this time. As the relative pressure increased, the monolayer adsorption stage was transformed into the multilayer adsorption stage, and the capillary coagulation of nitrogen molecules occurred in the pores of the coal samples. Under the action of compound flocculant solution, the maximum nitrogen adsorption of coal samples decreased by 0.66%-40.10%. Also, the adsorption isotherm became slow, the adsorption rate diminished, and the nitrogen adsorption capacity was the weakest by mixing starch and polymerized aluminum chloride solution37.Fig. 9 The isothermal curve varies with the type of starch-flocculant.

During the desorption process, when P/P0 was in 0.45 ~ 0.95, the original coal exhibited an apparent stagnation return line, and the hysteresis area of the coal samples treated with starch flocculant solution was obviously reduced. It is because the inner pore size of the coal may be enlarged and the pore connectivity is improved, thus facilitating the methane desorption and diffusion38. At P/P0 close to 0.45, the desorption curve of the raw coal exhibited a sharply decreasing inflection point, demonstrating that the pores consisted mainly of tiny bottleneck pores and ink bottle pores. The abrupt decline in the isotherms of the treated coal was not obvious, meaning that the pores were mainly composed of cylindrical pores. Therefore, the modification of pore structure of compound flocculant solution is remarkably39.

Pore characterization

The results of the pore volume (TPV) and average pore diameter of the pores of each coal sample are shown in Table 2. After the coal samples were processed with compound flocculant solution, the TPV of the raw coal was the lowest, and the TPV of the MB coal was the maximum. The microporous volume of MA coal was lower than that of MC coal, but the pore volumes of both coals were almost the same. The reason is that the compound flocculant solution not only reduces the microporous volume, but also increases the volume of transition and mesopore. MD coals possessed lower pore volumes and larger average pore diameters than MB coals because the sorption of the compound flocculant solution enlarged the pore channels and improved the inter-fracture connectivity. The microporous volume of MA coal was markedly reduced, suggesting that the micropores determined the adsorption capacity of gases40. To further analyze the changes of compound flocculant solutions on the pore structure of coal samples, the parameters in the form of percentage stacks are plotted.Table 2 Change of pore parameters of coal samples with different compound flocculant solutions.

Sample	Volume (10−3cm3/g)	Total pore volume (10−3cm3/g)	Average pore size (nm)	
Micropore	Transition pore	Mesopore	
M0	0.42	0.12	0.45	0.99	19.2	
MA	0.13	0.16	0.82	1.11	19.7	
MB	0.30	0.26	0.77	1.33	25.3	
MC	0.24	0.19	0.69	1.12	29.2	
MD	0.22	0.26	0.71	1.19	32.0	
ME	0.32	0.17	0.72	1.21	30.1	

As shown in Fig. 10, the transition pores and mesopores in the M0 coal accounted for 58% of the total pore volume, demonstrating that the total pore volume of the coal was majorly dominated by the transition pores and mesopores. Under the action of compound flocculant solution, the proportion of transition pores and mesopores to the total pore volume of coal samples was more than 58%, whereas the micropores showed an obvious trend of decreasing. It is generally accepted that the adsorption capacity of gases depends on the volume of micropores, so the adsorption capacity of the treated coal was weakened. Meanwhile, the sorption of compound flocculant solution induced the conversion of micropores to transition pores and mesopores, thus improving the connectivity between pores41.Fig. 10 Percentage of coal sample volume for each group.

Pore size distribution

The pore sizes distribution and accumulative pore volumes of coal samples in 0 nm ~ 140 nm obtained from the DFT model (Density Functional Theory) are shown in Fig. 11. The variation in the range of 70 nm ~ 140 nm of coal samples is relatively minor, implying that the pore size distribution is quite uniform, but the pore volume is less. It is worth mentioning that the peak widths in the 0 ~ 25.8 nm were larger in the original coal, but the peak widths in the 0 ~ 25.8 nm were markedly reduced in the coal samples under the action of five compound flocculant solutions42. It illustrates that the compound flocculant solutions obviously minimized the micropores and part of the transition pores, which is in agreement with the results of the pore structure changes. The accumulated pore volume curves of the coal samples in each group exhibited a rapid growth rate in 0–20 nm and a slight increase in 20–140 nm. Under the action of different starch flocculants, the cumulative pore volume increased by 12.1% ~ 34.3%. The results revealed that different compound flocculant solutions affected the pore structure of coal remarkably, decreased the pore number and increased the inter-pore connectivity to a certain extent43.Fig. 11 Effect of compound flocculant solution on pore size distribution of coal samples.

Fractal features

The fractal dimension is used to describe the irregularity of complicated objects and is a useful means of describing the intricacies of the pore structure in coal. According to the FHH44 (Frenkel-Halsey-Hill) model, Pfeifer proposed a method to calculate the fractal dimension. It is common to divide fractal curves into two phases, when the pressure is in the low pressure stage (0 < P/P0 < 0.5), the main stage is microporous filling and multilayer adsorption. At high pressure (0.5 < P/P0 < 1), capillary condensation occurs in the pores mainly due to gas–liquid surface tension45. The two phases correspond to the fractal dimensions D1 and D2, respectively. D1 is utilized to represent the roughness of the pore surface, and D2 is utilized to recognize the pore structure complexity. The larger D1, the larger the roughness of the pore surface, and the larger D2 the larger the complexity of the pore structure.

According to Eq. (5), the fractal dimension D is given in Fig. 12 and Table 3. The D1 and D2 were between 2 and 3, and the correlation coefficients were more than 0.98, showing that the coal samples processed with compound flocculant solution possessed obvious fractal characteristics. The D1 was reduced, likely due to the infiltration of compound flocculant solution molecules into the pores, so that the surface of the pore wall was smoother. Research has shown that the removal of minerals and organic matter from coal can simplify the pore structure and reduce the D2. Under the action of compound flocculant solution, the D2 decreases. It shows that the compound flocculant solution removes part of the inorganic minerals from the coal and makes the pore structure simple, thereby improving the connectivity46. The D2 value of MC coal is the minimum, which is 2.4997, explaining that the compound flocculant solution modified the internal pore space of the coal. The simplified pore structure and higher connectivity promoted the spreading and flowing of methane.Fig. 12 Effect of compound flocculant solution on fractal dimension of coal samples.

Table 3 Fractal Feature Parameters.

Samples	Fractal dimension	
D1	R2	D2	R2	
M0	2.4693	0.9897	2.5650	0.9987	
MA	2.4561	0.9930	2.5186	0.9943	
MB	2.4589	0.9875	2.5332	0.9967	
MC	2.4534	0.9912	2.4997	0.9985	
MD	2.4685	0.9986	2.5177	0.9913	
ME	2.4607	0.9918	2.5156	0.9981	

Figure 13 shows the evolution of pore structure before and under the action of compound flocculant solution. As the solution penetrated into the pores of coal, part of the soluble substances were dissolved and decomposed. When the solution is separated from the coal, the solution adsorbs some soluble molecules out of the coal by flocculation, clearing the pore channels. So, the isolated and occluded pores are connected and expanded, improved the connectivity between the pores, favoring the penetration and transportation of gas47.Fig. 13 The process of compound flocculant solution for coal penetration enhancement.

Analysis of methane adsorption capacity

The study of clean fracturing fluid on the methane adsorption performance of coal samples was investigated by methane isothermal adsorption experiments, and the experimental results are shown in Table 4. The adsorption constant a refers to the saturated adsorption amount per unit mass of coal at a certain temperature, and the better the value of a is, the better the adsorption capacity of coal is48. The adsorption constant b is used to represent the rate of adsorption of coal on gas. The fitted correlation coefficients of R2 are more than 0.95, and the Langmuir isothermal adsorption line is shown in Fig. 14.Table 4 Isothermal adsorption test parameters.

Sample	Adsorption constant a (cm3/g)	Adsorption constant b (MPa−1)	R2	
M0	18.2587	0.5421	0.9935	
MA	17.7745	0.4982	0.9927	
MB	16.5862	0.4365	0.9984	
MC	16.1413	0.4128	0.9926	
MD	16.8567	0.4657	0.9995	
ME	16.9872	0.4892	0.9947	

Fig. 14 Effect of compound flocculant solution on isothermal adsorption lines of coal samples.

As shown in Table 4 and Fig. 14, the adsorption constants of coal samples processed with compound flocculant solution decreased by 11.6% for the value of adsorption constant a and 23.9% for the value of adsorption constant b. The isothermal adsorption curves of coal samples processed with different compound flocculant solutions followed the same trend. At the low pressure, the adsorbed amount increased rapidly, and then gradually leveled off with the increase in pressure. Compared with the original coal samples, the saturated adsorption capacity of coal samples treated with starch flocculant solution decreased obviously, which suggested that the starch flocculant solution affected the adsorption performance of coal samples. Combined with the results of low-temperature nitrogen adsorption tests, the treatment of starch flocculant solution could improve the pore structure of coal samples and reduce the gas adsorption performance49.

Conclusion

The research showed that clean fracturing fluids increased the phenyl substituents of coal, improved the aromatic ring condensation by 11.4%-43.3%, and weakened the force between coal and gas. Meanwhile, the clean fracturing fluid increased the pore volume and average pore diameter of the coal samples by 12.1%-34.3% and 2.6%-66.7%, respectively, improved the connectivity between the pores, and dramatically reduced the amount of gas adsorption. Through preference, the combination of starch solution and polymerized ferric chloride solution is most favorable for gas extraction.

Author contributions

Conceptualization: Q.Z. and F.C.; methodology: Q.Z.; software: F.C.; validation: Q.Z. and H.X., resources: F.C. and H.X.; writing—original draft preparation: Q.Z. and Y.F.; visualization: Y.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Anhui University Collaborative Innovation project (GXXT-2020–057).

Data availability

All data generated or analysed during this study are included in this published article.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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