*Article* **Study on the Mineralogical and Geochemical Characteristics of Arsenic in Permian Coals: Focusing on the Coalfields of Shanxi Formation in Northern China**

**Liqun Zhang 1,2, Liugen Zheng 1,2,\* and Meng Liu <sup>1</sup>**


**Abstract:** The Huainan Coalfield is a typical multi-coal seam coalfield. In order to systematically investigate the distribution, occurrence, and integration of arsenic (As) in Shanxi coal, 26 coal samples and three rock samples were collected in the No. 1 coal seam of Huainan coalfield. The minerals, major element oxides, and As were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), polarized light microscopy, X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS). The results indicated that the coals of Shanxi Formation were characterized by very low ash yields and low total sulfur contents. The identified minerals by XRD in the studied coals are dominated by kaolinite, quartz, calcite, and a lesser amount of pyrite. The As content ranges from 10.33 mg/kg to 95.03 mg/kg, with an average of 44.74 mg/kg. Compared with world coals, the studied coals have higher contents of As, which are characterized by enrichment. Based on statistical analyses, As shows an affinity to ash yield and possible association with silicate minerals. The contents of As in all occurrence fractions were ranked from high to low as follows: residual > Fe-Mn oxides > organic > exchangeable > carbonate. Using B, *w*(Sr)/*w*(Ba) and *w*(B)/*w*(Ga) geochemical parameter results to invert the depositional environment of the Huainan Shanxi Formation, a suitable coal-forming environment can cause relatively enriched As in coal.

**Keywords:** coalfield; arsenic; enrichment; depositional environment

#### **1. Introduction**

China is the world's largest energy consumer, with coal as the main fuel for its energy consumption [1]. Coal contains potentially harmful trace elements, of which As is receiving increasing attention due to its volatility, toxicity, and carcinogenicity [2,3]. As one of the volatile elements in coal, As will be released in the process of coal processing and combustion, which may seriously affect the soil and water quality around the mining area, interfere with the normal function of the immune system, and pose a threat to human health. Therefore, As concentrations and its mode of occurrence in coal have been studied by several researchers during the last three decades [4–7]. These studies show that As enrichments in coal could be controlled by several parameters, such as presence of Asbearing sulfide minerals, clastic influx and/or influence of seawater into paleomires, redox conditions within paleomires, or influence of hydrothermal solutions during coalification.

The previous studies show that Chinese coals display variable ash yields, and some studies reported up to 10 mg/kg As concentrations; however, the As content is significantly different in different coal ages, regions and coal types [8]. In China, high-As coal is widely distributed in point-like distribution, mainly located in the three northeastern provinces of Henan Yima, Shanxi Datong, Guangxi Nanning, Gansu and parts of Yunnan [9]. In addition, the occurrence modes and enrichment origins of As in different regions and different coal

**Citation:** Zhang, L.; Zheng, L.; Liu, M. Study on the Mineralogical and Geochemical Characteristics of Arsenic in Permian Coals: Focusing on the Coalfields of Shanxi Formation in Northern China. *Energies* **2022**, *15*, 3185. https:// doi.org/10.3390/en15093185

Academic Editors: Jing Li, Yidong Cai, Lei Zhao and Rajender Gupta

Received: 11 March 2022 Accepted: 21 April 2022 Published: 27 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

types in China are different. Some scholars have found that the most important geological sources of As in the Santanghu coalfield are related to penetration of fissure-hydrothermal solutions and groundwater into coal seams [10]. It is found that the accumulation of As in the peat mire environment of the Guizhou No. 6, 7, 23, and 27 coal seams is mainly controlled by the marine influence during and/or after peat accumulation [11]. China is rich in coal reserves, and the regions involved in many studies are relatively scattered. Specific to the Huainan Coalfield, previous studies have mainly focused on the Permian Upper Shihezi and Lower Shihezi formations [12–15]. However, with the increase in demand for coal resources, the mining of the Huainan Coalfield has gradually shifted from shallow coal to the deep Shanxi Formation. The influence of As on the environment, and the difficulty and approach to utilize or remove specific trace elements, are mainly dependent on the occurrence of elements [16]. Different occurrence states have a great impact on the migration, transformation, and bioavailability of As in the natural environment. Therefore, it is necessary for us to conduct a systematic study on the As in Shanxi Formation coal.

Some studies have inferred the depositional environment of the Permian coal-bearing strata in the Huainan Coalfield by using the characteristics of geochemistry, mineralogy, paleontology, sedimentary structure, lithology, and coalbeds [17–19]. The Shanxi Formation was an important peat accumulation period in the Huainan Coalfield, and it is distributed in the foreland fold-thrust belt and its frontal area of the Dabie-Sulu orogenic belt. In the early stages of coalification and post-generation rock formation, it experienced multiple stages of strong regional tectonic movements and more frequent seaward-regressive events. A systematic investigation was conducted from the No. 1 seam in the Huainan Coalfield to provide basic data on the characteristics of the coal quality and the geochemical composition. The purpose of this study is to: (1) investigate the chemical characteristics and mineral distribution of Shanxi coal, (2) analyze the occurrence characteristics of As in the coal seam and explore its depositional environment, and (3) discuss the origin of As enrichment. Collectively, the results of this study could provide a theoretical basis for the processing and utilization of associated resources and potential evaluation in the Huainan coalfield.

#### **2. Geological Background**

The Huainan coalfield is an important coal production area in east China. It is located in the north-central part of Anhui Province. It extends into the Chuxian area in the east and extends to the vicinity of Fuyang in the west. The coal mining area is 180 km in length, 15–20 km in width and covers an area of 3200 km2 (Figure 1). The coal field is a complex syncline structure. The structural features of the main body of the complex syncline in Huainan are distributed in the east-west direction due to the squeezing action of the compressive stress in the north and south. A series of compression-torsional inverse faults, thrust faults and large nappe bodies are developed on the north and south wings of the complex syncline, and the imbricate structure of the two wings of the complex syncline is formed, which makes some strata in the south wing reverse upright.

The Carboniferous-Permian period was an important peat-forming period in the study area [20]. As one of the five major coal fields in China, the Huainan Coalfield is a typical Permian multi-peat forming environment; in turn, up to 21 coal seams are located in the Permian sequences (Figure 2). The coal-bearing strata in the Huainan Coalfield include the Benxi Formation of the Late Carboniferous, the Taiyuan Formation, the Shanxi Formation and the Lower Shihezi Formation of the Early Permian, and the Upper Shihezi Formation of the Late Permian. The Upper Shihezi Formation, the Lower Shihezi Formation, and the Shanxi Formation constitute the main mineable coal-bearing sequences in Huainan, which are a complete deltaic system developed on the offshore bay [2]. Among them, the Shanxi Formation was integrated into the Taiyuan Formation and was in contact with the Lower Shihezi Formation, and the mineable coal seams in this formation are the No. 1 and No. 3 coal seams. The Shanxi Formation shows a set of detrital coal-bearing sequences dominated by deltaic sediments, with a complete cyclonic structure of pre-triangle, delta front, and delta plain deposits. The lower part is a prodelta facies deposit, and its lithology

is mainly siltstone-silty mudstone and dark mudstone. The No. 1 and No. 3 coal seams in the main mineable coal seams are developed in this sedimentary system.

**Figure 1.** Location map of Huainan Coalfield, China.

**Figure 2.** Generalized stratigraphic column and specific sampling points in the study area.

#### **3. Methodology**

#### *3.1. Sampling*

During the coal exploration studies of the Shanxi Formation coals, a total of twentynine samples (including coal, roof, floor, and parting samples) were collected from the Zhangji mine and Xinjier mine by using channel sampling, with a sample taken every 10 cm interval. Among them, the numbers of coal samples are ZJ-1–ZJ-9 and XJ-1–XJ-17, and the roof, the parting, and the floor samples are numbered XJ-T, XJ-P, and XJ-B respectively (Figure 2). All samples were immediately stored in polyethylene bags to prevent contamination, oxidation, and loss of moisture. They were brought back to the laboratory and let dry naturally, then pass through a 200-mesh sieve after grinding.

#### *3.2. Analysis*

According to the Chinese National Standard GB/T211-2008, the moisture (M), ash yield (Ad) and volatile matter (V) in coal were measured by automatic industrial analyzer (SDTGA5000a, Sundy, Changsha, China). The sulfate sulfur (Ss), pyritic sulfur (Sp), and organic sulfur (So) were determined following GB/T215-2003, and the total sulfur (St) was determined following GB/T 214-2007.

Phase-mineral composition of coal was determined by XRD (SmartLab 9, Rigaku Industrial Corporation, Osaka, Japan), acceleration voltage ≤ 45 kV, tube flow ≤ 200 mA, power ≤ 9 kW, scanning range was 2◦~160◦ (2θ), 2θ angle indication error was 0.017◦, resolution was 27%, and diffraction intensity stability was 1.1%. The fine structure observation was analyzed by SEM (S-4800, Hitachi Corporation, Tokyo, Japan); the secondary electron resolution was 1.0 nm (15 kV), magnification was 20~800, acceleration voltage was 0.1~30 kV, and beam was 1 pA~2 nA. The microscopic morphology of minerals was observed by polarized light microscope (BX53, Olympus, Tokyo, Japan); the microscope condition was manual focusing, the lifting range was 50 mm, and the visual magnification was 40×–500×.

The major oxides were determined by X-ray fluorescence spectrometry (ZSX Primus II type, Rigaku Industrial Corporation, Tokyo, Japan). The 0.1 g sample was accurately weighed in an acidic mixture and digested into a transparent solution on a hot plate at 110 ◦C. Then, each solution was filtered through a 0.45 μm membrane and made up to 25 mL with deionized water with 5% HNO3. The trace elements (As, B, Sr, Ba, Ga) in the coal were determined by ICP-MS (Agilent 7500cx, Agilent, Palo Alto, CA, USA). The working parameters of the suppressor were: RF power 1500 W, auxiliary gas (Ar) flow 0.90 L/min, atomizer (Ar) flow 0.25 L/min, and the error analysis was −1.775 ± 2.745. The interference of ArCl to element As is eliminated by collision cell, and the flow rate of collision gas (He) is 0.7 mL/min. The chemical forms of As were analyzed by a sequential chemical extraction procedure (Table 1), and the recovery was 97.2~101.7%. The accuracy of As was determined by standards reference material GBW11116.


**Table 1.** Sequential chemical extraction procedure used for arsenic speciation. Adapted with permission from Elsevier, 2022 [2].

#### **4. Results**

*4.1. Standard Coal Characteristics*

The moisture (M) content of coal in the Zhangji Mine is 1.79~2.17%, with an average of 1.98%. The ash yield (Ad) of coals in the Zhangji Mine is 5.94~13.30%, with an average of 8.38%. The content of volatile matter (V) in Zhangji Mine coal is 29.97~38.04%, with an average of 33.72%. The total sulfur (St) content is 0.11~0.37%, with an average of 0.16%, and the pyritic sulfur (Sp), sulfate sulfur (Ss), and organic sulfur (So) accounts for 64%, 12%, and 24% of total sulfur, respectively (Table 2).

**Table 2.** Main coal quality parameter values of coal in the Shanxi Formation in the Huainan Coalfield (%).


M, moisture; Ad, ash yield; V, volatile matter St, total sulfur; Sp, pyritic sulfur; Ss, sulfate sulfur; So, organic sulfur.

The moisture (M) content of coal in Xinjier Mine is 1.26~2.16%, with an average of 1.81%. The ash yields (Ad) of coals in Xinjier Mine is 4.83~14.69%, with an average of 9.11%. The content of volatile matter (V) in Xinjier Mine coal is 24.89~34.61%, with an average of 29.61%. The total sulfur (St) content is 0.18~2.45%, with an average of 0.69%, and the pyritic sulfur (Sp), sulfate sulfur (Ss), and organic sulfur (So) accounts for 52.17%, 5.80%, and 42.03% of total sulfur, respectively. Among them, the total sulfur content in the XJ-1–XJ-4 areas are higher (Table 2). The coal samples from both mines could be classified as ultra-low moisture, low ash yield, medium-high volatile, and low-sulfur according to Chinese National Standards MT/850-2000 and GB/T 1522.4-2010.

#### *4.2. Mineralogical Compositions*

According to the XRD analyses results (Figure 3), the identified minerals in the Shanxi Formation raw coal samples are mainly kaolinite (Al4[Si4O10](OH)8), calcite (CaCO3), quartz (SiO2), and a small amount of pyrite (FeS2). The microscopic morphology of pyrite is the aggregation of spherical, nodular, granular, framboidal, and fine-grained pyrite (Figure 4b,c). Banded (Figure 4d) and cell-filled kaolinite (Figure 4e) closely co-existed with pyrite (Figure 4f), formed at the same time as pyrite, and belong to the syngenetic minerals in the early diagenetic stage. Calcite is distributed in veins (Figure 4g) or filled with organic cell cavities (Figure 4h), which indicates epigenetic origin. Quartz is identified as filling the cell cavities of kinetoplastids (Figure 4i), indicating precipitation during peatification or early diagenetic stages.

**Figure 3.** XRD analysis of the coal in the Shanxi Formation of Huainan (Kln—kaolinite; Qtz—quartz; Cal—calcite; Py—pyrite).

**Figure 4.** SEM (**a**,**b**) and optical images (**c**–**i**) of coal in Shanxi Formation. (Kln—kaolinite; Qtz—quartz; Py—pyrite) ((**a**,**b**): SEM image; (**c**): spherulitic pyrite; (**d**): banded kaolinite; (**e**): kaolinite filling the cell cavity; (**f**): co-existed with pyrite; (**g**): veined calcite; (**h**): calcite filling the cell cavity; (**i**): quartz filling the cell cavity).

#### *4.3. Major Oxides*

The average of major oxides of coal in the Zhangji mine is Al2O3 > SiO2 > Fe2O3 > CaO > MgO > TiO2 > Na2O>P2O5, and the content ranges are Al2O3 (%) 1.81~6.21 (3.65), SiO2 (%) 0.31~5.5 (2.89), Fe2O3 (%) 1.01~3.22 (1.68), CaO (%) 0.01~0.88 (0.35), MgO (%) 0.03~0.86 (0.24), TiO2 (%) 0.05~0.51 (0.22), Na2O (%) 0.05~0.06 (0.05), P2O5 (%) 0.01~0.08 (0.03). The [*w*(CaO) + *w*(MgO) + *w*(Fe2O3)]/[*w*(SiO2) + *w*(Al2O3)] ratio (*C*) of coal in the Zhangji mine ranges from 0.14~0.87 (0.41). The average of major oxides of coal in the Xinjier mine is SiO2 > Al2O3 > CaO > Fe2O3 > MgO > TiO2 > P2O5 > Na2O, and the content ranges are SiO2 (%) 2.13~24.13 (2.89), Al2O3 (%) 1.48~10.65 (3.65), CaO (%) 0.58~4.98 (0.35), Fe2O3 (%) 0.4~1.71 (1.68), MgO (%) 0.03~1.26 (0.24), TiO2 (%) 0.11~0.48 (0.22), P2O5 (%) 0.02~0.17 (0.03), Na2O (%) 0.01~0.11 (0.05). The C of coal in the Xinjier mine ranges from 0.19~0.41 (0.28) (Table 3). The main element oxides in Shanxi Formation coal are SiO2 and Al2O3, and the ash yield belongs to SiO2-Al2O3-Fe2O3-CaO.

**Table 3.** Content range of major oxides in the coal of Shanxi Formation in Huainan (%).


*C* = [*w*(CaO) + *w*(MgO) + *w*(Fe2O3)]/[*w*(SiO2) + *w*(Al2O3)].

#### *4.4. Content and Vertical Distribution of As*

The As content in the Zhangji mine ranges from 12.51~95.03 mg/kg, with an average of 46.64 mg/kg, and the enrichment coefficient of As in Zhangji coal [21,22] (CC = content of trace elements/world average of elements in coal) is 5.62. The As content in the Xinjier mine ranges from 10.33~76.10 mg/kg, with an average of 43.73 mg/kg (Table 4), and the CC of As in Xinjier coal is 5.27. According to the Chinese coal industry standard (MT/T803-1999), the Shanxi Formation coal of Huainan Coalfield belongs to high As coal. In order to better understand the enrichment of As in coal, the As content in the coal of the Zhangji mine and Xinjier mine was compared with the Upper Shihezi and Lower Shihezi formation. The As of the Upper Shihezi formation is 6.27 mg/kg, while As in the Lower Shihezi formation is 4.81 mg/kg (Table 4). It can be seen that As shows obvious changes in the three mines. The As content in Shanxi formation coal was significantly higher than that in the Upper Shihezi and Lower Shihezi formations. However, there was a small difference in As content between the Upper Shihezi and the Lower Shihezi formations.



<sup>a</sup> From Chen et al. [14]. <sup>b</sup> From Tian et al. [8]. <sup>c</sup> From Ketris and Yudovich [21]. <sup>d</sup> From Dai et al. [22]. <sup>e</sup> From Finkelman et al. [23].

The vertical distribution characteristics of As content in coal are shown in Figure 5. The content of As changed significantly, among which XJ-4 (10.33 mg/kg) had the lowest content and ZJ-2 (95.03 mg/kg) had the highest content. The As content in the roof (XJ-T), parting (XJ-P), and floor (XJ-B) of the Shanxi formation is relatively high, and respectively are 244.65 mg/kg, 107.97 mg/kg, and 124.65 mg/kg (Table 4).

**Figure 5.** Vertical distribution of As, Ba, B, Sr, Ga in coal.

#### **5. Discussion**

#### *5.1. Depositional Environment of Coal*

The presence of high sulfur content is attributed to regional volcanism, peat environments, and depositional environments with strong sulfide mineralization [24,25]. Shanxi Formation coal is low-sulfur coal, but the total sulfur content in the XJ-1–XJ-4 area is relatively high (Table 2). Nevertheless, some previous studies show that the depositional environment affected by seawater may lead to the phenomenon of high sulfur in local coal seams [26]. The ash yield belongs to SiO2-Al2O3-Fe2O3-CaO, indicating that more detritus minerals were transported to the study area and deposited on the coastal delta plain where it was open to clastic influx. The high content of SiO2 and Al2O3 indicates that the minerals in the raw coal are composed of clay minerals (such as kaolinite and illite) and quartz [27]. In this study, the average ash yield of coal seams in the Shanxi Formation were considerably lower than coals from the Upper Shihezi Formation (20.12%) and Lower Shihezi Formation (21.27%) [28]. The change in ash content in the Huainan Coalfield is called "increasing strati graphically upward" [14,29].

The [*w*(CaO) + *w*(MgO) + *w*(Fe2O3)]/[*w*(SiO2) + *w*(Al2O3)] ratio (C) of coal can indicate the depositional environment of the peat accumulation stage. Coal with C ≤ 0.22 could be accumulated within terrestrial environments (e.g., freshwater lake-shore), whereas C > 0.22 could imply transition areas between terrestrial to shallow marine (e.g., back-mangrove conditions or delta front) [30,31]. The C in the coal of the Zhangji and Xinjier Mines is 0.41 and 0.28, respectively (Table 3), indicating that the sedimentary facies present sea-land alternate facies, where marine influence into paleomires could be common. However, in practice, the conclusion of C is often affected by other sedimentary environment indicators. For instance, the presence of gastropods, ostracod fauna, and charophyta remains obtained from coal seams in northern Turkey points to the predominance of freshwater conditions. Peat is deposited in sloughs with water from karst aquifers rich in sulfate and calcium; in this case, freshwater coal exhibits characteristics similar to saltwater or ocean-influenced peat/coal [32]. Furthermore, the presence of clastic Mg-bearing silicates or clay minerals

(e.g., chlorite or smectite) inputs into freshwater lakes, which increases the supply of dissolved Mg ions; in turn, the C values could be elevated [33]. Therefore, paleontological study of Shanxi Formation coal seams should be undertaken in the future for better understanding of the marine influence on peat-forming environments. Previous studies have suggested that Al2O3/TiO2 is the most effective indicator for the properties of sedimentary parent rock. When the ratio of Al2O3/TiO2 is 3:8, 8:21, or 21:70, it means that the sediments are formed by mafic, intermediate, or felsic igneous rocks, respectively [34,35]. The values of Al2O3/TiO2 in the Huainan Shanxi Formation were widely distributed, ranging from 7.89 to 72.25, with an average of 24.19, indicating that the clastic materials in the coal mainly come from felsic rocks [36].

Even though B enrichments could be controlled by several parameters, the mass fraction of B has a good linear relationship with the paleo-salinity [23,26,37–39]. Generally, 50 mg/kg and 110 mg/kg are divided into fresh water/mildly brackish water and mildly brackish water/brackish water [40]. The highest content of B in Shanxi Formation coal is 354.60 mg/kg, with an average of 162.00 mg/kg (Table 5). Among them, the content range of B in the five coal samples of ZJ-1, XJ-1, XJ-2, XJ-4, and XJ-10 is between 50 mg/kg and 110 mg/kg, and other samples are all more than 110 mg/kg. This showed that Huainan Shanxi Formation is in the stage of mildly brackish-brackish water deposition (Figure 6a). In addition, the *w*(Sr)/*w*(Ba) is a geochemical indicator that distinguishes between terrestrial and marine sedimentary environments in terrigenous clastic sediments [41]. *w*(Sr)/*w*(Ba) < 0.6 indicated terrestrial freshwater deposition, *w*(Sr)/*w*(Ba) between 0.6~1 suggested mildly brackish water deposition, and *w*(Sr)/*w*(Ba) > 1 indicated brackish water deposition [41,42]. The *w*(Sr)/*w*(Ba) ratio of the 26 samples in the Huainan Shanxi Formation was greater than 1 (Figure 6b), suggesting that the depositional environment of the Shanxi Formation was brackish water deposition. However, the usability of this indicator (*w*(Sr)/*w*(Ba)) is not widely acknowledged. Therefore, to verify this theory and accurately determine the differences between the sedimentary geochemical behaviors of Sr and Ba under different salinity conditions, it is recommended that in future studies, selective extraction of the Sr and Ba concentrations of different salinities is used to discriminate between marine and terrestrial sedimentary environments in terrigenous clastic sediments. An alternative geochemical indicator of the depositional environments is *w*(B)/*w*(Ga) ratio in coals, that is, fresh water (<3), brackish water influences (3~5) and brackish water influences (>5) [43]. The values of *w*(B)/*w*(Ga) in XJ-2 and XJ-10 in Shanxi Formation coal in the study area are slightly lower than the mildly brackish water/brackish water boundary value of 5, and other samples *w*(B)/*w*(Ga) are greater than 5 (Figure 6c). In conclusion, the depositional environment of Shanxi Formation in Huainan is mildly brackish-brackish water deposition.

**Table 5.** The content range of B, Sr, Ba and Ga in coal.


**Figure 6.** Judging the depositional environment of the Huainan Shanxi Formation. (**a**): B; (**b**): *w*(Sr)/*w*(Ba); (**c**): *w*(B)/*w*(Ga).

#### *5.2. Geochemistry of As*

The occurrence of As in coal is complicated, and it participates in the formation of inorganic or organic bound states in the structure of coal. For coal samples with a narrow ash yield range, correlation coefficients can help to infer how trace elements are present in coal in depositional environments [44]. In this study, we analyzed the correlation between As and ash, sulfur, and coal ash components, and combined with sequential chemical extraction experiment to explore the occurrence of As in the coals of Shanxi formation. Generally, As has a strong affinity for sulfur, and the relationship between As and sulfur increases and decreases simultaneously [45]. In this study, As and S contents are negatively correlated (r = −0.41), suggesting that less sulfur-arsenic minerals were present in the studied coal samples. However, the lack of As in SEM-EDX spectra does not mean that either pyrite or clay minerals do not contain As. Since the As measurement capacity is low, measurable amounts of As could not be seen in SEM-EDX spectra. Of course, the lack of As within framboidal pyrite grains are expectable, since these grains could have lower As concentrations [6,46]. On the other hand, the correlation coefficient between As and ash content was 0.53, showing a positive correlation (Figure 7a), indicating that the main carrier of As in coal might be affilated with aluminosilicate minerals in the studied samples (e.g., clay minerals). In agreement with this correlation, As is positively correlated with Al2O3 and TiO2, indicating that As could be mainly affilated with clay minerals in coal samples [47,48]. According to the result of sequential chemical extraction experiment (Figure 7b), the order of As content in each speciation is residual > Fe-Mn oxides > organic > exchangeable > carbonate. The main speciation of As is residual with 81.36%. The residual is difficult to dissolve by weak acid and general solvent, and its chemical activity is weak in the environment.

**Figure 7.** Correlation analysis (**a**) and speciation distribution of As (**b**) in coal.

The As distribution in the study area varies widely, which is mainly related to the peat-forming environment and mineralogical compositions of coal seams. This study shows that the Shanxi formation was deposited in a brackish environment, with decreasing marine influence along stratigraphy upward, increasing input of terrigenous detrital materials and significantly reduced content of As. Influenced by the depositional environment of Shanxi formation, there are clastic influxes of different origins in the roof (XJ-T), parting (XJ-P), and floor (XJ-B), which causes Shanxi Formation coal to have a special rock roof and good waterproof roof, parting and floor. The roof is a thick sandstone mainly composed of feldspar, and the floor is made up of aquifer-bearing Carboniferous, Ordovician, and Cambrian formations. The parting is mostly mudstone or carbonaceous mudstone, which will lead to high trace elements in the roof, parting, and floor. In the early and end stages of peat mires, the higher concentration of mineral solution seems to penetrate into paleomires, and the change of environmental conditions is not conducive to the normal growth of plants; trace elements are precipitated due to the obstructed circulation process, which leads to the higher content of trace elements in the top and bottom layers [49]. Due to the influence of depositional environment, a thin layer of mudstone and carbonaceous mudstone with high trace elements in gangue inclusion is formed.

#### *5.3. Controlling Factors on As Enrichment*

There are generally one or more particular geological factors that may influence the enrichment of trace elements in coals for different coal basins and coal forming periods [50,51]. The As content in Shanxi Formation coal in the Huainan Coalfield is obviously higher than that of the Upper Shihezi and Lower Shihezi formations, which indicates that As may have local enrichment characteristics and is closely related to the depositional environment of coal seam.

The contents of B and As in the Huainan Permian Upper Shihezi Formation (No. 11 and 13 coal seams), Lower Shihezi Formation (No. 4, 5, 6, 7, 8, and 9 coal seams), and Shanxi Formation No. 1 coal seam are shown in Figure 8 [14]. The high B contents in the No. 1 coal seam indicates that the peat marsh formed in the Shanxi Formation was greatly affected by sea water. Boron content in each coal seam of the Shihezi Formation indicates that the upper and lower Shihezi Formation are generally transitional phase or brackish water deposition environments, and the B content in No. 5 coal seam and No. 11 coal seam is lower than the fresh water/mildly brackish water boundary value 50 mg/kg, which is characterized by continental deposition.

**Figure 8.** Content changes of B and As in different coal seams in Huainan.

During the Permian, several transgressions and regressions took place in the study area. The change trend of B contents in the longitudinal direction of the No. 1 coal seam in the Shanxi Formation to the No. 4–9 coal seams in the Lower Shihezi Formation to the No. 11 and 13 coal seams in the Upper Shihezi Formation is the same as that of As. The No. 1 coal seam of the Shanxi Formation was more affected by seawater than the coal seams of the Shihezi Formation. The S content in the coal has the overall order of Shanxi Formation > Lower Shihezi Formation > Upper Shihezi Formation. In this study, As occurs less in sulfides and more in silico-aluminate minerals. This shows that As in Shanxi Formation coal is mainly derived from terrestrial detrital sediments, which were brought into coal-forming mires by water and adsorbed into coal by humic acid, leading to enrichment of As in coal. Appropriate depositional environments can lead to relatively enriched As in coal.

#### **6. Conclusions**

The coal in the Shanxi Formation belongs to the ultra-low ash, ultra-low total moisture, medium-high volatile, low-sulfur coal category. The major minerals include quartz, kaolinite, calcite, and a small amount of pyrite. The coal in the Huainan Shanxi Fomation was mainly affected by seawater, and the detrital material in the coal mainly comes from felsic rock. The enrichment coefficient CC of the coal is 5.39, which is characterized by enrichment. Among the samples, there are clastic materials from different sources in the roof, floor, and parting, and the As content is significantly higher than that in the coal. The residual state is the main form of As, and As is mainly found in clay minerals such as aluminosilicate. In addition, the recognition results of paleo-salinity characteristics indicate that the environment as a whole is brackish-saltwater sedimentation. A suitable depositional environment can lead to the relative enrichment of As in coal. The main source of As is terrigenous detritus, which is carried into the coal-marsh by water and adsorbed into coal by humic acid, thus leading to the enrichment of As in coal.

**Author Contributions:** Writing—original draft, L.Z. (Liqun Zhang); Writing—review and editing, L.Z. (Liqun Zhang); Methodology, L.Z. (Liqun Zhang) and M.L.; Investigation, L.Z. (Liqun Zhang); Software, L.Z. (Liqun Zhang); Conceptualization, L.Z. (Liugen Zheng); Resources, L.Z. (Liugen Zheng); Funding acquisition, L.Z. (Liugen Zheng); Supervision, L.Z. (Liugen Zheng); Data curation, M.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the University Synergy Innovation Program of Anhui Province (GXXT-2021-017) and the National Natural Science Foundation of China (42072201). We acknowledge the editors and reviewers for polishing the language and for in-depth discussion.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data generated or used during the study appear in the submitted article.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


### *Article* **The Influence of Fracturing Fluid Volume on the Productivity of Coalbed Methane Wells in the Southern Qinshui Basin**

**Wenwen Chen 1, Xiaoming Wang 1,\*, Mingkai Tu 1, Fengjiao Qu 2, Weiwei Chao 3, Wei Chen <sup>4</sup> and Shihui Hou <sup>5</sup>**


**Abstract:** Hydraulic fracturing is the main technical means for the reservoir stimulation of coalbed methane (CBM) vertical wells. The design of fracturing fluid volume (FFV) is mainly through numerical simulation, and the numerical simulation method does not fully consider the water block damage caused by the leakage of fracturing fluid into the reservoir. In this work, the variance analysis method was used to analyze the production data of 1238 CBM vertical wells in the Fanzhuang block and Zhengzhuang block of the Qinshui Basin, to clarify the relationship between the FFV and the peak gas production (PGP) under the different ratios of critical desorption pressure to reservoir pressure (Rc/r), and to reveal the controlling mechanism of fracturing fluid on CBM migration. The results show that both the FFV and Rc/r have a significant impact on gas production. When Rc/r < 0.5, the PGP decreases with the increase of the FFV, and the FFV that is beneficial to gas production is 200–500 m3. When Rc/r > 0.5, the PGP increases first and then decreases with the increase of FFV. Specifically, the FFV that is favorable for gas production is 500–700 m3. Excessive FFV does not significantly increase the length of fractures due to leaks in the coal reservoir. Instead, it is more likely to invade and stay in smaller pores, causing water block damage and reducing gas production. Reservoirs with high Rc/r have larger displacement pressure, which can effectively overcome the resistance of liquid migration in pores, thereby reducing the damage of the water block. Therefore, different reservoir conditions need to match the appropriate fracturing scale. This study can provide guidance for the optimal design of hydraulic fracturing parameters for CBM wells.

**Keywords:** coalbed methane; hydraulic fracturing; fracturing fluid volume; analysis of variance; water block

#### **1. Introduction**

Hydraulic fracturing is one of the important reservoir stimulations of coalbed methane (CBM) wells, and its main purpose is to form efficient conductivity fractures and improve coal seam permeability [1,2]. However, while hydraulic fracturing improves the permeability of coal seams, tons of fracturing fluid are injected into the coal seam [3,4], and a large amount of fracturing fluid is leaked into the coal reservoir, which also brings many adverse effects to the development of coalbed methane, such as water block damage [5–8], clay swelling [9,10], and consequently increased difficulty in methane desorption, etc. [11,12].

In view of the adverse effects of fracturing fluid leakage, a large number of scientific research and experiments have been carried out on the influencing factors of the leakage. Yuan et al. [4] and Chang et al. [10] studied the self-absorption process and microscopic migration mechanism of coal reservoirs after hydraulic fracturing and evaluated the effect of this process on permeability. Wang et al. [13] expounded the influence of the wettability of coal on the irreducible water content from a microscopic point of view, and discussed

**Citation:** Chen, W.; Wang, X.; Tu, M.; Qu, F.; Chao, W.; Chen, W.; Hou, S. The Influence of Fracturing Fluid Volume on the Productivity of Coalbed Methane Wells in the Southern Qinshui Basin. *Energies* **2022**, *15*, 7673. https://doi.org/ 10.3390/en15207673

Academic Editor: Mohammad Sarmadivaleh

Received: 22 September 2022 Accepted: 14 October 2022 Published: 18 October 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the effective flowback of fracturing fluid after hydraulic fracturing. Meng et al. [14] studied the effects of effective stress, porosity, permeability, fracturing fluid viscosity, and reservoir pressure on the fracturing fluid leakage factor, and proposed a model that takes into account stress-sensitivity effects to estimate the overall filter leakage factor. Through numerical simulation, Guo et al. [15] showed that fracture geometry, reservoir characteristics, pressure conditions, and temperature have significant effects on fracturing fluid leakage. Guo et al. [16] observed that natural fractures play a dominant role in the leak of fracturing fluid, and the wider the opening of natural fractures, the greater the leak of fracturing fluid. From the perspective of fracturing engineering, Wu et al. [17] showed through experimental results that increasing the injection pressure will increase the fracturing fluid loss; true triaxial hydraulic fracturing experiments showed that high injection rates can cause a large amount of fracturing fluid to leak along the bedding direction [18]. However, the unreasonable fracturing scale design may also be the root cause for fracturing fluid leakage, which has rarely attracted the attention of scholars.

The design of FFV needs to consider different reservoir conditions. The appropriate fracturing fluid scale can not only achieve the maximum fracture-forming effect, and improve the permeability, but also reduce the reservoir damage caused by the leakage. At present, the optimal design of hydraulic fracturing parameters mainly uses numerical simulation methods to adjust the number, length, spacing, and conductivity of fractures, so as to determine the amount of fracturing fluid [18–21]. Usually, the numerical simulation method does not fully consider the flowback of fracturing fluid after hydraulic fracturing. When the flowback effect is not good, the fluid leakage will cause damage to the reservoir, especially related to the impact of fluid migration in the reservoir. Specially, in the process of CBM development in the southern Qinshui Basin by PetroChina Huabei Oilfield Company, the scale of hydraulic fracturing experienced a change from small to large, and the amount of fracturing fluid increased from 300 m3 to 1000 m3, as shown in Table 1. However, it was found that the fracture length and stimulation effect did not increase with the increase of fracturing scale, but large-scale fracturing resulted in more fracturing fluid leaking. During the development process of the Zhengzhuang (ZZ) block, the differences in geological parameters within the block were not considered, and all vertical wells have adopted the same hydraulic fracturing scale, resulting in large differences in gas production of single wells in the block [22].


**Table 1.** Fracturing fluid volume development stage.

In this paper, we aim to better clarify whether different FFVs have an impact on gas production, and design the amount of fracturing fluid that matches the geological conditions. By analyzing the production data of 1238 wells in the Fanzhuang (FZ) block and ZZ block, the influence of different FFV on the gas production was studied, the FFV for optimal gas production under the different ratios of critical desorption pressure to the reservoir pressure (Rc/r) was discussed, and the effect of water block damage caused by fluid leakage on gas production was clarified. The research results can provide some insights into the optimal design of hydraulic fracturing parameters for CBM reservoirs.

#### **2. Geological Setting**

The Qinshui Basin, located in southeastern Shanxi Province (Figure 1a), is a typical example of the successful development of high-rank coal in China [23]. The FZ block and ZZ block are located in the southern Qinshui Basin (Figure 1b). The study area consists of the Pennsylvanian Benxi (C2b) and Taiyuan (C2t) Formations, the Permian Shanxi (P1s), Xiashihezi (P1x), Shangshihezi (P2s) and Shiqianfeng (P2sh) Formations, and Triassic deposits. The main coal-bearing strata are the Shanxi and Taiyuan Formations, and the No.3 coal seam of the Shanxi Formation is stably distributed in the whole area and is the main layer for CBM development in the study area [24–26].

**Figure 1.** (**a**) Location of the Qinshui Basin in China; (**b**) location of development block in the southern Qinshui Basin; (**c**) the buried depth of the No.3 coal seam.

The FZ block and ZZ block are bounded by the Sitou Fault. The ZZ block is situated west of the fault, and the FZ block is located east of the fault [27–29]. The stratigraphic structure of the study area is relatively complex. Local folds and faults are relatively developed, and the regional structural form is mainly distributed in the north-northeast, and the stratigraphic dip is 3◦ to 8◦. The thickness of the No. 3 coal seam ranges from 5 to 7 m, and its burial depth varies between 300 and 1200 m (Figure 1c). The vitrinite reflectance (*Ro,max*, %) varies between 3.1% and 3.9%, and the gas content is between 14 and 30 m3/t. The reservoir permeability is generally lower than 1 mD, with an average of 0.27 mD [30–32].

#### **3. Methods**

In order to analyze the influence of FFV on gas production in the southern Qinshui Basin, the production data of 1238 CBM wells in FZ block and ZZ block since 2006 were collected and analyzed. These wells all use conventional hydraulic fracturing fluid. The fracturing fluid is potassium chloride solution with a concentration of 1%, and the FFV is between 200–1000 m3. After the fracturing operation, the well is shut in, and the fracturing fluid is almost completely leaked into the reservoir. The basic data collected include FFV, critical desorption pressure, reservoir pressure, Rc/r, and peak gas production (PGP). Among them, the critical desorption pressure is the bottom hole flow pressure at the initial gas desorption during the CBM drainage process; the Rc/r is the ratio of the critical desorption pressure to the reservoir pressure; the PGP is the maximum daily production after the first hydraulic fracturing stimulation.

In this work, one-way analysis of variance (ANOVA) was used to analyze the influence of FFV and Rc/r on gas production. First, as many as possible FFV and Rc/r are grouped, and then the least-significant difference (LSD) method is used to test the significant differences between the different groups, and the adjacent and insignificant groups are merged [33]. The grouping level of FFV and Rc/r is actually obtained, which is convenient for multiway ANOVA. Then, the multiway ANOVA on the significant effect of gas production is carried out using the Rc/r and FFV grouping level determined after one-way ANOVA, and then the optimal combination that beneficial to gas production is found [34].

In this work, SPSS software is used for ANOVA, the system default significance level α is set at 0.05 and compared with *p*-value of the test statistic. If the *p*-value < 0.05, it is considered that the different grouping levels of the independent variable have a significant impact on the dependent variable. F ratio is the test statistic, the F ratio is the between-group variance divided by the within-group variance in a data set. If F > 1, there are statistically significant differences between groups, a high F ratio indicates the greater likelihood of statistically significant differences between groups [34].

#### **4. Results**

#### *4.1. One-Way ANOVA*

Table 2 shows the results of one-way ANOVA on the PGP when the FFV is divided into 8 groups (200–300 m3, 300–400 m3, 400–500 m3, 500–600 m3, 600–700 m3, 700–800 m3, 800–900 m3, and 900–1000 m3). The results show that the amount of fracturing fluid has a significant effect on PGP (F = 20.35, *p* = 0.000). Analysis of the pairwise comparison between different groups by the LSD method shows that the difference between the 300–400 m<sup>3</sup> and 400–500 m<sup>3</sup> groups is not significant, and they are combined into a group of 300–500 m3. Similarly, the 700–800 m3, 800–900 m3, and 900–1000 m3 were combined into a group of 700–1000 m3. The differences between other adjacent groups are significant, and the results show that the effects of different FFV groups on PGP are significantly different. The average PGP when the FFV is divided into 8 groups is shown in Figure 2a.



df: The degree of freedom (df) of the statistic.

**Figure 2.** (**a**) Average PGP when FFV is divided into 8 groups; (**b**) average PGP when FFV is divided into 5 groups; (**c**) average PGP when FFV is divided into 3 groups; (**d**) average PGP when Rc/r is divided into 4 groups.

Table 3 shows the results of one-way ANOVA on the PGP when the FFV is divided into 5 groups (200–300 m3, 300–500 m3, 500–600 m3, 600–700 m3, and 700–1000 m3). The results show that the amount of fracturing fluid has a significant effect on the PGP (F = 35.1, *p* = 0.000). Figure 2b shows that with the increase of FFV, the average PGP first increases and then decreases. When the FFV is 500–600 m3, the average PGP is the largest, and when the FFV exceeds 500–600 m3, the average PGP shows a rapid downward trend.



df: The degree of freedom (df) of the statistic.

Table 4 shows the results of one-way ANOVA on the PGP when the FFV is divided into 3 groups (200–500 m3, 500–700 m3, and 700–1000 m3). The results show that the amount of fracturing fluid has a significant effect on PGP (F = 54.6, *p* = 0.000). The LSD method analyzes the pairwise comparison between the three different grouping levels, and the results show that the differences between the groups are significant. It can be known from Figure 2c that when the FFV is 200–500 m3, the average PGP is 2136 m3; when the FFV is 500–700 m3, the average PGP is 2795 m3; and when the FFV is 700–1000 m3, the average PGP dropped rapidly to only 1121 m3.


**Table 4.** One-way ANOVA results of 3 groups of FFV.

df: The degree of freedom (df) of the statistic.

The one-way ANOVA process of Rc/r on gas production is shown in Table 5, which shows that Rc/r has a significant impact on gas production when Rc/r is divided into 10, 7, and 6 groups. Table 6 shows the results of one-way ANOVA on PGP when the Rc/r is divided into 4 groups (0–0.3, 0.3–0.5, 0.5–0.8 and 0.8–1). The results showed that the Rc/r had a significant impact on the PGP (F = 111.53, *p* = 0.000). The results of pairwise comparison between different groups by the LSD method showed that the differences in PGP between the four different grouping levels were significant. Figure 2d shows that the average PGP is positively correlated with the Rc/r.

**Table 5.** F and *p*-value of one-way ANOVA for different groups of Rc/r.


**Table 6.** One-way ANOVA results of 4 groups of Rc/r.


df: The degree of freedom (df) of the statistic.

One-way ANOVA shows that both the FFV and Rc/r have a significant impact on the gas production. Combined with the actual situation, the FFV is divided into 3 groups and the Rc/r is divided into 4 groups for multiway ANOVA, as shown in Table 7.

**Table 7.** Multiway ANOVA variable grouping level table.


df: The degree of freedom (df) of the statistic.

#### *4.2. Multiway ANOVA*

The Levene's test results [F (11, 1226) = 23.76, *p* = 0.000] of the multiway ANOVA of the Rc/r and FFV can show that the overall variance of the samples in each group is homogeneous, which meets the precondition of the variance test. Table 8 shows that the model used for the multiway ANOVA was statistically significant (F = 40.2, *p* = 0.000). The interaction between Rc/r and FFV had a very significant impact on PGP (F = 7.42, *p* = 0.000).


**Table 8.** Multiway ANOVA results of Rc/r and FFV.

df: The degree of freedom (df) of the statistic.

The multiway ANOVA shows that when the FFV is constant, the larger the Rc/r, the better the gas production, and the Rc/r have a significant contribution to the gas production. When the Rc/r is constant, the amount of fracturing fluid is different, and the gas production is also different. The specific performance is as follows: (a) when Rc/r < 0.5, the gas production is negatively correlated with the amount of fracturing fluid. when Rc/r is 0–0.3, the FFV is 700–1000 m<sup>3</sup> and the gas production decreases rapidly; when Rc/r is 0.3–0.5, the FFV is 500–700 m3 and the gas production decreases rapidly. (b) when Rc/r > 0.5, the gas production increases first and then decreases with the increase of the FFV, but there are differences. When Rc/r is 0.5–0.8, the gas production when the FFV is 700–1000 m3 is less than that when the FFV is 200–500 m3. When Rc/r is 0.8–1, the gas production when the FFV is 700–1000 m<sup>3</sup> is greater than that when the FFV is 200–500 m3. In short, the gas production decreases when the FFV is 700–1000 m3, indicating that excessive FFV is not conducive to the increase of gas production (Figure 3).

**Figure 3.** The average peak gas production when Rc/r interacts with FFV.

#### **5. Discussion**

*5.1. Intrusion and Retention of Fracturing Fluid*

The process of hydraulic fracturing of CBM wells is the process of intrusion of fracturing fluid into pores and fractures of coal seams. The schematic diagram of fluid distribution in pores and fractures before hydraulic fracturing is shown in Figure 4a. The water intrusion process is mainly affected by injection pressure (Pd), imbibition capillary force (Pc), viscous resistance (Pn), fluid resistance (Pf), and gas pressure (Pg) in the coal seam. It is generally considered that Pd and Pc are the main driving forces [35,36]. When the coal reservoir is saturated with gas and contains more free methane gas, Pg is also a non-negligible

resistance to prevent water migration in pores [37,38]. Therefore, for the fluid migrating in the pores and fractures of coal seams during the water invasion process, the pressure difference (ΔPi) across the pores is:

$$
\Delta \mathbf{P\_i} = \mathbf{P\_d} + \mathbf{P\_c} - \mathbf{P\_n} - \mathbf{P\_f} - \mathbf{P\_g} \tag{1}
$$

$$P\_{\mathbb{C}} = \frac{2\delta\cos\theta}{r} \tag{2}$$

where σ is the interfacial tension between the solution and the air, N/m; θ is the contact angle between the solution and coal, (◦); *r* is the radius of the pore, m.

**Figure 4.** (**a**) Fluid distribution in pores and fractures before hydraulic fracturing; (**b**) fluid distribution in pores and fractures after hydraulic fracturing; (**c**) fluid distribution in pores and fractures at the early stage of drainage; (**d**) fluid distribution in pores and fractures at the later stage of drainage.

When ΔPi is >0, water intrusion occurs. The water retained in the pores may fill the pores, or may form multi-level and intermittent water columns (Figure 4b).

The water intrusion experiments in coal pillars show that water intrusion occurs simultaneously in micropore-transition pores, mesopores, macropores and fractures, and the water intrusion rate decreases sequentially. The speed of water intrusion in the microporetransition pore is mainly determined by the capillary force of imbibition; the more complex the pore structure, the smaller the degree of water intrusion, and the more difficult it is to flow back after water intrusion. The water saturation of pores and fractures increased with the increase of injection time and inlet pressure during the water invasion process [39].

In the practice of hydraulic fracturing, with the increase of the fracturing scale, the injection rate and pressure need to be increased accordingly. In this way, the amount of fracturing fluid invading into the pores also increases, and the radius of the pores that can be invaded is smaller, and more fracturing fluid enters the complex pores and micropores. Therefore, if the scale of hydraulic fracturing is too large, the more fracturing fluid that is leaked and retained in the pores and fractures, and the residual fracturing fluid will affect the gas production [40]. As shown in Figure 3, when the amount of fracturing fluid is 700–1000 m3, the average PGP decreases compared with that when the amount of fracturing fluid is 500–700 m3, and more external liquid stays in the pores and fractures of the coal.

#### *5.2. Fluid Migration and Water Block Damage during Drainage*

At the early stage of CBM well drainage, there is saturated water in the pores and fissures. As the water in the fracture is drained first, the fluid pressure in the fracture will decrease and the gas begins to desorb (Figure 4c). After the water in the fracture is drained out, some of the water in the pore does not migrate with the water in the fracture, but stays in the pore (Figure 4d). According to the principle of gas-liquid two-phase fluid flow, it can be known that the fluid in the pore mainly considers the two-phase flow driven by the pressure difference [41]. The pressure difference for liquid column migration in the pore is [13,42]:

$$
\Delta \mathbf{P\_0} = \mathbf{P\_{\mathcal{G}}} - \mathbf{P\_{\mathcal{C}}} - \mathbf{P\_W} - \mathbf{P\_f} - \mathbf{G} \tag{3}
$$

where Pg is the gas pressure in the pore; Pw is the fluid pressure in the fracture; and G is the gravity of the liquid column in the pore.

Pg is the main driving force for the liquid column migration in the pore, and Pc and Pw are the main resistances. When the pore radius is small enough and the liquid column is short enough, Pf and G can be ignored. When ΔPo = 0, the fluid in the pore does not migrate (Figure 5a). As the fluid in the fracture migrates out, Pw will decrease, the pressure drop will be transferred to the pore, and part of the adsorbed gas will be desorbed from the pore, and Pg will increase. When ΔPo > 0, the liquid column migrates to the fracture. At this time, the fluid pressure balance in the pore is destroyed, and the gas at the bottom of the pore will push the liquid column at the bottom upward until a new balance is reached (Figure 5b). With the progress of drainage, the pressure drop is effectively transferred to the internal pores, and more gas is desorbed. When the flow resistance of the liquid column can be overcome, the gas will break through the constraints of the liquid and migrate out. At this time, the pores and fractures are fully connected (Figure 5c). In this process, the part of the liquid column that cannot overcome its flow resistance is bound in the pores, blocking the pores, affecting the migration of gas, and forming water block damage.

**Figure 5.** (**a**) Schematic diagram of the force when the fluid in the pores does not migrate; (**b**) schematic diagram of the fluid in the pores in a new balance state; (**c**) schematic diagram when pores and fractures are fully connected.

The gas flooding experiment also showed that the water block damage of macropores and fractures can be relieved, and the water block of mesopores can be partially relieved, while the water in micropores and transition pores was difficult to displace [39,43,44]. In actual production, a large amount of fracturing fluid invades into the pores, and there is no additional driving force during the drainage process, and it will be more difficult for the fracturing fluid to be completely discharged from the micropores and transition pores. Therefore, this part of water must rely on the driving force of the gas in the pores to be discharged.

#### *5.3. Mechanism of FFV Affecting Gas Production*

It can be seen from the above analysis that water block mainly comes in two ways: (1) during the fracturing process, a large amount of fracturing fluid intrudes into the pores and fractures, and is trapped by capillary force; (2) in the process of drainage, the driving force of the gas in the pore is not enough to overcome the resistance of its migration, and the fracturing fluid retained in the pore cannot be discharged back. According to the Hagen–Poiseuille law, the volume of the fracturing fluid discharged from the pores against the capillary resistance is [45]:

$$Q = \frac{\pi r^4 \Delta \mathcal{P}\_o}{8\mu L} \tag{4}$$

where *Q* is the volume of fracturing fluid discharged, m3; *r* is the radius of the pore, m; *μ* is the dynamic viscosity, Pa s; *L* is the length of the liquid column, m.

Take the derivative of Equation (4) as

$$\frac{dL}{dt} = \frac{r^2 \Delta \mathcal{P}\_0}{8\mu L} \tag{5}$$

By the integral of Equation (5), it can be obtained that the time (*t*) for the liquid column of length (*L*) to flow back from the pores is

$$\mathbf{t} = \frac{4\mu L^2}{\Delta P\_\mathrm{o} r^2} \tag{6}$$

Substitute Equation (3) into Equation (6):

$$t = \frac{4\mu L^2}{(P\_\text{g} - P\_\text{c} - P\_\text{w} - P\_\text{f} - G)r^2} \tag{7}$$

When the scale of hydraulic fracturing fluid increases, the amount of invading fluid in pores increases, the length of the liquid column (*L*) increases, and the radius (*r*) of the pores that can be invaded becomes smaller; at the same time, the P and G of the liquid column will have to be considered, increasing the resistance and time for the fluid to move out, making it easier to cause water block [37].

According to the analysis of the microseismic fracture monitoring report of CBM wells in the study area, the FFV has no significant effect on the length of the fracture, and the length of the fracture does not increase with the increase of the FFV (Figure 6). The correlation between gas production and fracture length is also not obvious (Figure 7). It can be seen that increasing the scale of fracturing has not always brought positive effects on gas production [22]. From the one-way ANOVA of FFV and PGP (Figure 2), it can be seen that with the increase of FFV, the PGP increases first and then decreases. Within a certain scale, increasing the amount of fracturing fluid has a positive effect on gas production. When it exceeds a certain scale, it will have a negative effect on gas production. This is because excess fracturing fluid does not play a role in creating fractures and increasing reservoir connectivity. The excess fracturing fluid is leaked into the coal reservoir or surrounding rock along the pore and fracture channels, and the positive effect is not as great as the negative effect of water block caused by excessive fracturing fluid staying in the reservoir [46]. Therefore, the negative impact of FFV on gas production is mainly reflected in the water block damage caused to the reservoir.

**Figure 6.** Diagram of FFV and hydraulic fracturing fracture length.

**Figure 7.** Diagram of hydraulic fracturing fracture length and PGP.

It can be seen from Equation (7) that the greater the ΔPo, the greater the fluid resistance that can be overcome. Therefore, under different reservoir conditions, the degree of difficulty in releasing the water block is also different. Lyu et al. [38] and Lu et al. [42] showed that water block did not occur when the differential pressure driving force was greater than the resistance. The larger the Rc/r value, the better the fluid drainage index of the reservoir, which was more conducive to the desorption of gas, resulting in high production [47]. The Rc/r value can represent the gas saturation to a certain extent. Higher gas saturation leads to higher desorption pressure and is beneficial for early gas production, leading to greater total gas production [48,49]. It can be seen that the larger the Rc/r value, the easier the gas desorption, the greater the pressure of gas in the pore, and the greater the driving force of liquid migration in the pore. Therefore, when the Rc/r increases, it becomes easier for the water in the pores to be displaced out.

As shown in Figure 3, when Rc/r > 0.5, the gas production with the FFV of 500–700 m<sup>3</sup> is better than that with the FFV of 200–500 m3, indicating that the positive effect of increasing the FFV on the reservoir is greater than the water block damage to the reservoir. When the amount of fracturing fluid increases to 700–1000 m3, the gas production decreases, indicating that the damage of the water block caused by increasing the volume of fracturing fluid is greater than the positive effect caused by increasing the volume of fracturing fluid. In particular, when Rc/r is 0.5–0.8, the gas production with the FFV of 700–1000 m<sup>3</sup> is smaller than that with the FFV of 200–500 m3, implying that the negative effect of excessively increasing the amount of fracturing fluid is greater, and more water blocks are not released. When Rc/r is 0.8–1, the gas production with the FFV of 700–1000 m3 is higher than that of the FFV of 200–500 m3, indicating that the positive effect brought by the large FFV is greater than the negative effect. The water block is easier to be eliminated when the

Rc/r is 0.8–1 than when the Rc/r is 0.5–0.8. When Rc/r < 0.5, the gas production will be less when the FFV is increased, which indicates that the fracturing fluid enters the pores and is difficult to be displaced, which is more likely to cause water block damage to the reservoir. Therefore, the smaller the Rc/r value, the larger the FFV, and the more serious the water block damage.

#### *5.4. The Uncertainty or Limitation of This Study*

There are many factors affecting the production of CBM [32,50–53]. This paper illustrates that the amount of fracturing fluid is also one of the factors affecting the production of gas through the ANOVA of production data. This paper selects the Rc/r and FFV for multivariate ANOVA, and proposes that the design of FFV can be based on the Rc/r in the study area. The combination of other geological factors and FFV may also affect gas production and multivariate ANOVA of other geological parameters, and FFV may be attempted later. In this paper, the coal samples in the study area are not used for the analysis of water intrusion, drainage, and water block in pores and fractures, and the experiment of Li et al. [40] is cited for illustration. The data in this paper are from the FZ block and ZZ block in Qinshui Basin, which is a high-rank coal, so the applicability of the optimal combination and the reference of FFV parameter design are only applicable to this block.

#### **6. Conclusions**


**Author Contributions:** Methodology, writing—original draft, W.C. (Wenwen Chen); data curation, investigation, F.Q. and W.C. (Weiwei Chao); data curation, resources, M.T. and W.C. (Wei Chen); revised and edited the manuscript, X.W. and S.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Science Foundation of China (No. 41972184, No. 42262022 and No. 41902177) and Jiangxi Provincial Natural Science Foundation (grant number 20212BAB214030).

**Acknowledgments:** The authors would like to give their sincere thanks to the teachers in the department for their comments on the revision of the article, and thanks to the former colleagues of PetroChina Huabei Oilfield Company for their valuable comments.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

