The rising energy demands worldwide and difficulty in developing novel clean energy sources have greatly stimulated the exploitation of coalbed methane. Clay minerals are common fractions of coal; thus, understanding their CH4 adsorption capacities and pore structures is vital. In this study, coal, parting, roof, and floor samples were collected from the Yangquan Mining District. The mineral compositions, CH4 adsorption capacities, and pore structures of the samples were analyzed using X-ray diffraction, the CH4 isothermal adsorption method, and the low-temperature N2 adsorption method, respectively. The results indicated that organic matter had a much higher CH4 adsorption capacity (33.80 cm3/g, 35°C) than that of clay minerals. The CH4 adsorption capacities of various clay minerals are significantly different, with smectite (18.01 cm3/g), kaolinite (5.81 cm3/g), mixed-layer illite-smectite (4.47 cm3/g), and illite (2.08 cm3/g) present in decreasing order. The pore sizes of the samples consisted of sizes <6 nm, and six pore size groups (Groups 1–6) were identified in the PSD patterns. These pore size groups were associated with different clay minerals. We propose that the CH4 adsorption capacities of clay minerals are mainly influenced by their pore structures, which are in turn associated with their species and formation processes. Furthermore, the conversion of kaolinite to illite, and the illitization of mixed-layer smectite-illite, exerted a negative effect on their CH4 adsorption capacities.
Coalbed methane (CBM), an important unconventional energy source, has become the focus of resource exploration and exploitation, particularly in China [1,2,3]. CBM is most commonly found in the adsorbed state (>85%), followed by the free state (approximately 10%) and dissolved state (<5%) [1,4,5]. Thus, the CH4 adsorption capacity is an important indicator in coal reservoir evaluation. Coal is mainly composed of organic matter, and, to a lesser extent, inorganic minerals [6,7]. Numerous studies have indicated that the CH4 adsorption capacity of coal reservoirs is primarily attributed to its organic matter . The minerals in coal are mainly composed of clay minerals (>30%) such as kaolinite, illite, and mixed-layer illite-smectite [7,8]. Moreover, the minerals in the anthracite of the Qinshui Coalfield are even all clay minerals . Previous studies have reported that clay minerals possess the capacity to adsorb CH4, but they noted that this capacity is lower than that of organic matter [10,11,12]. Thus, the CH4 adsorption capacity of clay minerals in coal, and their influence on coal reservoirs, should not be neglected.
Numerous studies have reported the CH4 adsorption capacities of various clay minerals. Tang and Fan  and Fan et al.  investigated the CH4 adsorption capacities of illite, kaolinite, smectite, mixed-layer illite-smectite, and chlorite purchased from the Source Clay Repository of the Clay Mineral Society, which collected these samples from Wallisville, Texas, USA; Warren County, Georgia, USA; Crook County, Wyoming, USA; Slovakia; and Flagstaff Hill, El Dorado County, California, USA, respectively. The results indicated that smectite had the highest CH4 adsorption capacity, followed by illite, kaolinite, mixed-layer illite-smectite, and chlorite. According to previous studies [12,14,15], the factors that influence the CH4 adsorption capacities of clay minerals include the clay mineral species, surface area, pore structure, formation process, moisture, temperature, and pressure. Ca-smectite not only has an external surface area but also an internal surface area, resulting in a higher CH4 adsorption capacity than that of other clay minerals [10,12,13]. Clay minerals of sedimentary origin generally have higher CH4 adsorption capacities than those of metamorphic origin . While moisture and temperature have negative effects on the CH4 adsorption capacity, pressure has a positive effect [16,17,18]. Furthermore, the same impact trend of the three aforementioned parameters is also seen in shale .
Clay minerals in coal have different CH4 adsorption capacities and pore structures, and their impacts on the entire coal reservoir are also different. Our works in this study were exhibited as follows: (1) to investigate the mineral compositions, CH4 adsorption capacities, surface areas, and pore structures of the coal reservoirs; (2) to calculate the CH4 adsorption capacities of different clay minerals; and (3) to ascertain the pore structures of different clay minerals in coal reservoirs and their diagenetic influences.
2 Geological setting
The Yangquan Mining District is located in the northern region of the Qinshui Coalfield and covers an area of 1,400 km2 (Figure 1) [20,21]. Several large-scale coal mines are in this mining district, including Guoyang First Coal Mine, Xinjing Coal Mine, Guoyang Second Coal Mine, Guoyang Fifth Coal Mine, and Sijiazhuang Coal Mine. The Yangquan Mining District is a notable area for CBM exploration and exploitation in China because it has abundant amounts of CBM, with a resource reserve of 6.448 × 1011 m3 . Some experimental well drilling in the coal-bearing strata in the Sijiazhuang Block have produced a high-yielding commercial gas flow . The main coal-bearing strata include a diachronous sequence, Carboniferous-Permian Taiyuan Formation, and Permian Shanxi Formation. The coal is mainly composed of low-volatile bituminous coal and anthracite [21,24]. The No. 15 coal seam, located at the lowermost part of the Taiyuan Formation, has a thickness of 5.0–8.7 m. The coal seam is minable in the entire mining district, followed by the No. 3, 6, 8, and 12 coal seams, which are locally or regionally minable [20,21]. The No. 15 coal seam is the major gas-producing coal reservoir in the Yangquan Mining District, with an average gas content of 16.0 m3/t .
3 Samples and methods
3.1 Sample collection and preparation
One floor sample (XJ-15-di) and two parting samples (XJ-15-g1 and XJ-15-lvshi) were collected from the No. 15 coal seam of the Xinjing Coal Mine at a depth of 530 m. One coal sample (YQ-15-c) and one parting sample (YQ1-15-g2) were collected from the No. 15 coal seam of the Guoyang First Coal Mine at a depth of 495 m. Additionally, one roof sample (YQ5-15-ding) was collected from the No. 15 coal seam of the Guoyang Fifth Coal Mine at a depth of 510 m. The mineralogical compositions of the parting, roof, and floor samples were expected to be similar to those of the minerals in the coal sample, but the proportions of different minerals in the parting, roof, and floor samples covered a wider range than those of the coal sample. All samples were used in the calculation of the CH4 adsorption capacities of different minerals in the coal reservoir, rather than using the same species of pure minerals collected from other strata, as reported by Tang and Fan , and Fan et al. . All samples were collected from the fresh working face of the coal mines. Approximately 2 kg of each sample was collected and placed in plastic bags immediately after collection to avoid possible contamination and oxidation. According to the Chinese Standard Method GB/T 19560-2008 , all the samples were crushed and ground to <2 mm and then sieved using standard sieves to obtain powder samples with particle sizes ranging from 0.18 mm (80 mesh) to 0.25 mm (60 mesh) for subsequent analysis.
3.2 X-ray diffraction analysis
The minerals in the parting, roof, floor, and coal samples were analyzed using X-ray diffraction (XRD). Non-oriented powder XRD patterns were obtained using an X-ray powder diffractometer (D8 ADVANCE, Germany). The operating conditions for which were as follows: power − 2.2 kW; scanning time − 4 min; step size −0.019651°; fixed div slit − 0.6 mm; fixed det slit − 10.5 mm; fixed anti slit − 6.76 mm, and 2θ interval – 7–45°. The XRD pattern of each sample was examined using Quan software, developed by Lin , to quantitatively calculate the proportion of each mineral phase identified via XRD. This was performed according to the methods outlined by Chung [27,28,29] and the Chinese Petroleum and Gas Industry Standard Method SY/T 5163-2010 .
3.3 Analysis of organic matter content
The XRD results indicate that carbonate minerals were absent in all the samples (Section 4.1). The organic carbon contents of the roof and floor samples were determined according to the Chinese Geology and Mineral Resource Standard Method DZ/T 0279.27-2016 . The organic carbon contents of the coal and parting samples were determined according to the Chinese Standard Method GB/T 476-2008 . Yu  reported that the organic carbon portion of the total organic matter in the Chinese low-volatile bituminous coal and anthracite is approximately 90%. Therefore, the organic matter contents of the coal, roof, floor, and parting samples in the present study were calculated as follows: Organic matter = Organic carbon/90%.
3.4 CH4 isothermal adsorption experiment
Methane isothermal adsorption analysis is a classical method for determining the adsorbed methane content under the conditions of gradually increasing pressure and constant temperature. In this study, the experiments were conducted under dry conditions at a pressure of <9 MPa and a temperature of 35°C. The calibration gas was helium (He; 99.999 mass%), and the carrier gas was nitrogen (N2; 99.999 mass%). Before the experiments were conducted, the coal, parting, roof, and floor samples (approximately 20 g) were degassed, at approximately 100°C, under a vacuum for approximately 8 h. The CH4 isothermal adsorption experiments were conducted using a high-pressure high-temperature adsorption analyzer (H-Sorb 2600, China) made by the Gold APP Instruments Corporation in China. According to the Chinese Standard Method GB/T 19560-2008 , the Langmuir equation, which is a model describing the monolayer adsorption state of CH4 on porous materials, was used to calculate the CH4 adsorption content of the samples and is expressed as follows:
The Langmuir volume of the samples (V L) represents the saturated CH4 adsorption amount, p represents the experimental pressure, V represents the CH4 adsorption content corresponding to p, and b represents the adsorption constant. The V Ls reflects the CH4 adsorption capacities of the samples.
3.5 Low-temperature N2 adsorption experiment
The low-temperature N2 adsorption and desorption analysis (77.35 K at 101.3 kPa) is a common method used to characterize porous materials [34,35]. The N2 adsorption and desorption curves of the coal, parting, roof, and floor samples were obtained using a specific surface area and pore size analyzer (V-Sorb 2008P, China) made by the Gold APP Instruments Corporation in China. Before the experiment was carried out, all samples (1–2 g for each sample) were degassed in a vacuum, at approximately 200°C, for approximately 8 h to remove adsorbed moisture and volatile matter. Porous materials can be classified into three categories according to their size: micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). The specific surface areas of some micropores (1–2 nm), mesopores (2–50 nm), and macropores (50–200 nm) were calculated using the Brunauer–Emmett–Teller (BET) method . The BET equation is as follows:
where p o represents the saturated steam pressure of N2 at a temperature of 77.35 K; p represents the absolute pressure of N2, p/p o represents the relative pressure, and c represents the BET equation comprehensive constant. The fractions of the specific surface areas contributed by the micropores (1–2 nm) were calculated using the t-plot method . The fractions of the specific surface areas contributed by the meso- and macropores (50–200 nm) were calculated by subtracting the micropore surface area from the BET surface area.
The pore volumes of the micropores, with a pore size of 1–2 nm, were calculated using N2 adsorption data according to the density functional theory (DFT) method . Furthermore, the pore volumes of the meso- and macropores, with a 50–200 nm pore size, were calculated using the Barrett–Joyner–Halenda (BJH) method [36,39]. The BJH method is more suitable for calculating the pore volumes of meso- and macropores, based on the Kelvin equation, as follows:
where γ represents the surface tension of liquid nitrogen, V m represents the molar volume of liquid nitrogen, r represents the droplet radius of liquid nitrogen, R represents the gas constant, and T represents the temperature in Kelvin. The DFT method is more suitable for calculating the pore volumes of micropores. The total pore volumes of the coal, parting, roof, and floor samples represent the sum of the volumes of the 1–2 nm micropores, mesopores, and 50–200 nm macropores. The pore size distributions (PSDs) of the samples were obtained using the DFT model [35,38].
4.1 Sample compositions
The sample compositions, including the contents of organic matter and inorganic mineral matter, are shown in Table 1. The parting samples had a much higher content of organic matter (32.7% on average) than those of the roof (0.5%) and floor (2.9%) samples. The minerals in the parting, roof, and floor samples were significantly different. The parting samples mainly contained illite (33.2% on average), followed by kaolinite (18.0%), and quartz (16.1%). The minerals in the roof and floor samples were mainly kaolinite (72.7%), followed by quartz (18.7%), and mixed-layer illite-smectite (6.9%). The coal sample had an organic matter content of 88.6%, and the XRD results indicated that the minerals in the coal sample were mainly illite. This result is consistent with that of previous studies on the mineral compositions of coal, parting, roof, and floor samples in the North China Coal Basin [9,40].
The smectite proportion (S%) of the mixed-layer illite-smectite is 15%.
Qz, quartz; Kao, kaolinite; Ilt, illite; Ilt/Sme, mixed-layer illite-smectite.
4.2 CH4 isothermal adsorption
The CH4 isothermal adsorption results for the coal, parting, roof, and floor samples are shown in Figure S1, Table 2, and Table S1. The CH4 adsorption capacities of the samples varied significantly. The coal sample had the highest V L of 29.57 cm3/g, which was followed by the parting samples (10.50 cm3/g on average), and roof and floor samples (1.64 cm3/g on average). The organic matter in the samples was strongly correlated with V L (Figure S2, r = 0.99), indicating that organic matter was the main adsorbent of CH4. Although the adsorption capacities of the samples showed a decreasing trend with various minerals (r = −0.99), this does not mean that the minerals had no adsorption capacity. It has been reported that the adsorption capacity of clay minerals is lower than that of organic matter [10,41,42].
|Sample||V L (cm3/g)||P L (MPa)|
4.3 Low-temperature N2 adsorption
4.3.1 N2 adsorption curves
The low-temperature N2 adsorption and desorption curves for the coal, parting, roof, and floor samples are shown in Figure 2. According to the classification proposed by Brunauer et al. , all the samples have Ⅱ-type adsorption curves characterized by an inverse-S shape, indicating the existence of both mesopores and macropores . Notably, there were differences between these adsorption curves. The adsorption content of N2 at a very low relative pressure (p/p o <0.01) increased significantly as organic matter decreased and inorganic mineral matter, especially clay minerals, increased (Figure S3). This indicates that clay minerals have more micropores in the range of 1–2 nm than organic matter, which results in a significant increase in the adsorption content of N2 at a very low relative pressure (p/p o <0.01). This indicates the presence of different pore structures between the organic matter and inorganic minerals. The pore structures of both the organic matter and clay minerals vary widely. The pore structure of organic matter is mainly dependent on factors such as its type and maturity, and those of clay minerals are mainly associated with factors such as their species and formation processes [12,45,46,47]. At medium relative pressure (p/p o = 0.3–0.8), the N2 adsorption contents of all the adsorption curves increased slowly, but stably, owing to the multilayer adsorption of N2 in the meso- and macropores. The N2 adsorption contents increased sharply at high relative pressure (p/p o = 0.9–1.0), close to the saturated vapor pressure, which is mainly attributed to N2 capillary condensation (Figure 2 ). Capillary condensation during adsorption, and evaporation during desorption in some meso- and macropores can cause hysteresis loops to appear between the adsorption and desorption curves, the shapes of which can reflect the pore types of porous media (Figure 2 [49,50]). The desorption curve was forced to coincide with the adsorption for all samples at a p/p o of 0.45–0.5, indicating the collapse of the hemispherical meniscus of condensed nitrogen in the mesopores of <4 nm [14,47,51]. According to their shapes, the hysteresis loops are divided into four types by the IUPAC, which represent different types of pores . All the samples have a hysteresis loop characterized by the H3 type, indicating the dominance of slit-shaped and wedge-shaped pores.
4.3.2 Specific surface area
The specific surface areas of the coal, parting, roof, and floor samples calculated using the t-plot and BET methods are listed in Table 3. According to the results, XJ-15-di had the highest 1–2 nm micropore surface area. It also had meso- and macropore surface areas of 3.46 and 8.85 m2/g, respectively, which are the lowest for the coal sample. Li et al. investigated the pore structure of coal-measure shale in the Yangquan Mining District using low-temperature N2 and CO2 adsorption methods  (Table 3), which had a similar diagenesis and mineral composition to the parting, roof, and floor samples in the present study. The parting, roof, and floor samples had meso- and macropore surface areas comparable to those of the Yangquan coal-measure shale, but the 1–2 nm micropore surface areas were significantly lower than the micropore surface area of the Yangquan coal-measure shale. This indicates that a considerable amount of the <1 nm micropores present in the parting, roof, and floor samples were not detected in this study. Based on the Yangquan coal-measure shale, the <1 nm micropore surface areas of the parting, roof, and floor samples were estimated and shown in Table 3. These surface areas varied from 9.27 to 13.63 m2/g. Li also investigated the pore structure of the primary Yangquan coal using the low-temperature N2 and CO2 adsorption methods  and reported that the <1 nm micropore surface area of the Yangquan coal was 272 m2/g, as shown in Table 3. The total specific surface area of the coal sample was much higher than that of the parting, roof, and floor samples. This is compatible with the higher CH4 adsorption capacity of the coal sample than that of the parting, roof, and floor samples. In addition, the CH4 adsorption capacity is dependent on not only the specific surface area but also on the surface functional groups in coal that adsorb CH4 via hydrogen bonds [41,54,55], which are absent in clay minerals [56,57].
|Sample||Specific surface area (m2/g)||Citation|
|2–200 nm||1–2 nm||<1 nm||Total|
4.3.3 Pore volume and pore size distribution
The pore volumes of the coal, parting, roof, and floor samples, calculated using the DFT and BJH methods, are shown in Table 4. XJ-15-di had the highest meso- and macropore volumes of 0.0289 cm3/g, and XJ-15-lvshi had the highest 1–2 nm micropore volume of 0.0010 cm3/g. The mesopore, macropore, and 1–2 nm micropore volumes of the coal sample were the lowest. The micropore volume of the Yangquan coal-measure shale was much higher than the 1–2 nm micropore volume of the parting, roof, and floor samples . Based on the Yangquan coal-measure shale, the <1 nm micropore volumes of the parting, roof, and floor samples, which varied from 0.0033 to 0.0064 cm3/g, are shown in Table 4. The <1 nm micropore volume of the Yangquan coal was 0.072 cm3/g that was cited from Reference , as shown in Table 4. The <1 nm micropore volume was much higher than the 1–2 and 2–200 nm micropore volumes, indicating a dominant pore size of <1 nm in the Yangquan coals . The total pore volumes of the parting, roof, and floor samples were strongly correlated with the total specific surface area (r = 0.88), which supports the calculations of the specific surface areas and pore volumes. Although the total specific surface area of the coal sample was much higher than those of the parting, roof, and floor samples, the total pore volume of the former had the same order of magnitude with the latter samples. This was mainly attributed to their different pore structures, and the fact that the coal sample had more micropores.
|Sample||Pore volume (cm3/g)||Citation|
|2–200 nm||1–2 nm||<1 nm||Total|
The PSDs of the samples, calculated using the DFT model, are shown in Figure 3. The PSD patterns contained different peaks representing different pore size groups, and some peaks overlapped with each other. To quantify the pore size groups of the samples, a deconvolution method was applied to determine the positions (mean size), areas (pore volume), and widths (range) of the peaks in the PSD patterns of the samples, using a commercially available data-processing program [14,58] (OriginPro software). A normal distribution was used to describe the PSD of each pore size group. The pore sizes of the samples were mainly distributed in the range of <6 nm, and three to five peaks were identified in each PSD pattern (Figure 3 and Table 5).
|Sample||Group 1 (Ilt/Sme)||Group 2 (Ilt)||Group 3 (Kao)|
|Position||Half width||Area||Position||Half width||Area||Position||Half width||Area|
|Sample||Group 4||Group 5||Group 6|
|Position||Half width||Area||Position||Half width||Area||Position||Half width||Area|
Position represents mean pore size (nm) and area represents pore volume (cm3/g).
5.1 Comparison between methane adsorption capacities and compositions
The CH4 adsorption capacities of the coal, parting, roof, and floor samples reflected the sum of the CH4 adsorption capacities of the various components. Organic matter has a much higher CH4 adsorption capacity than inorganic mineral matter, clay minerals have higher CH4 adsorption capacities than other non-clay minerals, and the CH4 adsorption capacities of various clay minerals are also significantly different [41,42]. Of all the clay minerals, Ca-smectite has the highest CH4 methane adsorption capacity, followed by illite, kaolinite, and Na-smectite. Meanwhile, chlorite has the lowest CH4 adsorption capacity [12,13,42]. In the present study, an overdetermined equation was established as follows:
where represents the coefficient matrix of the sample compositions, represents the unknown vector of the V L values of various components in the samples, and represents the vector of the V L values of the samples. The least-squares method was used to solve this equation, which may be expressed as follows:
The calculation results (significant, P < 0.05) indicate that the organic matter had the highest V L value of 33.80 cm3/g. However, the V L values of the minerals were much lower, and quartz (−3.50 cm3/g) and illite (−1.40 cm3/g) even had negative values. The CH4 adsorption capacity of the organic matter was much higher than that of the inorganic clay minerals. The inorganic clay minerals showed a blocking effect on the organic matter in the coal, parting, roof, and floor samples, which are the mixtures of organic matter and inorganic clay minerals. Therefore, the CH4 adsorption capacity of each mineral calculated using equation (5) can be considered the sum of the net CH4 adsorption capacity and the blocking effect (negative effect) on organic matter. It has been reported that the CH4 adsorption capacity of quartz is very limited, close to 0, and the negative value (−3.50 cm3/g) calculated using equation (5) is mainly attributed to the blocking effect of quartz on organic matter. If the blocking effects of the clay minerals are similar to those of quartz, the net CH4 adsorption capacities of the clay minerals can be calculated by subtracting the blocking effect from the results of equation (5). The net methane adsorption capacities of mixed-layer illite-smectite, kaolinite, and illite were 4.47, 5.81, and 2.08 cm3/g, respectively (Table 6). Compared with other studies on the CH4 adsorption capacities of various clay minerals [10,12,13,41], the kaolinite in the present study had a higher V L value than those of other studies, whereas the CH4 adsorption capacities of illite were similar. Kaolinites with different formation processes have different pore structures [12,59]. The kaolinite in the parting, roof, and floor of the Yangquan coal seam is a detrital mineral with a terrigenous origin . Erosion during mechanical transport, from the sediment source region (the Yinshan Oldland) to the peat mire, resulted in kaolinite with a relatively well-developed pore structure. It has been reported that detrital kaolinite has a higher CH4 adsorption capacity than kaolinite of a syngenetic origin [9,12]. The mixed-layer illite-smectite in the present study had an adsorption capacity between smectite and illite, depending on the ratio of the layers of smectite to illite. The mixed-layer illite-smectite in the roof and floor samples had a smectite layer proportion (S%) of 15%. Moreover, if the illite layers have a similar CH4 adsorption capacity to that of the illite in the parting samples, the V L of the smectite layers can be calculated to be approximately 18.01 cm3/g, which is higher than that of the other clay minerals and that of the smectite reported in other studies [12,13,42] (Table 6). The Ca-smectite has a much higher CH4 adsorption capacity than that of Na-smectite, kaolinite, illite, and chlorite. This is because the interlayer distance of Ca-smectite is approximately 0.51 nm higher than the molecular diameter of CH4 (0.38 nm), whereas the interlayer distances of the other clay minerals (0.01 nm kaolinite, 0.04 nm K-illite, 0.07 nm NH4-illite, 0.30 nm Na-smectite, close to 0 nm for chlorite) are lower than the molecular diameter of CH4 [21,41,42]. This results in the surface area of the smectite including not only external surface area (50 m2/g ) but also including internal area (750 m2/g ) different from other clay minerals. The existence of an internal surface area contributed to the higher CH4 adsorption capacity of the Ca-smectite. The relatively higher CH4 adsorption capacity of the smectite layers in the mixed-layer smectite-illite in the present study is inferred to be associated with the Ca-smectite.
|Particle size (mm)||Temperature (°C)||Source||V L of various clay minerals (cm3/g)|
|None||60||Tang et al. (2014)||3.01||3.48||3.46||4.02||0.88|
|<0.053||50||Ji et al. (2012)||9.18||2.69||1.79||10.75||0.22|
|None||60||Liu et al. (2013)||3.88||2.22||6.01|
Kao, kaolinite; Ilt, illite; Ilt/Sme, mixed-layer illite-smectite; Chl, chlorite.
Because of the significantly higher specific surface area, the organic matter in the samples had a much higher CH4 adsorption capacity than the inorganic clay minerals. For the coal sample, the organic matter contributed to the CH4 adsorption capacity, and its contribution to the total CH4 adsorption capacity was as high as 99.3%. For the parting, roof, and floor samples, such as XJ-15-lvshi, both the organic matter (49.3%) and clay minerals (50.7%) contributed to the total CH4 adsorption capacity.
5.2 Comparison between PSDs and compositions
The PSD patterns of the coal, parting, roof, and floor samples contained different peaks representing different pore size groups, which corresponded to different types of porous materials. For example, the pore size group with a mean size of 1.03–1.08 nm was identified in the PSD patterns of XJ-15-di and YQ5-15-ding but was absent in the PSD patterns of the other samples. Correspondingly, mixed-layer illite-smectite was present in XJ-15-di and YQ5-15-ding but was absent in the other samples. It was inferred that the mixed-layer illite-smectite contributed to the pore size group with 1.03–1.08 nm as the mean size range. According to the comparison between the PSDs and the compositions of the samples, it can be seen that some pore size groups and some clay minerals were present and absent simultaneously (Figure 3 and Table 5). It is suggested that these clay minerals corresponded to different pore size groups. The pore size group with 1.34–1.40 nm as the mean size range (referred as Group 3) was mainly associated with the kaolinite, and its pore volume (peak area) was positively correlated with the kaolinite content, although there were only three points (Figure 4 and Table 5). The pore size group with 1.20 nm to 1.36 nm as the mean size range (referred as Group 2) was mainly associated with illite, and its volume was positively correlated with the illite content, as expected (Figure 4 and Table 5). The mixed-layer illite-smectite had a pore size group with a mean size range of 1.03–1.08 nm (referred as Group 1; Figure 4 and Table 5). The three pore size groups with mean size ranges of 1.62–1.78, 2.51–3.04, and 4.62–4.95 nm (referred as Groups 4, 5, and 6, respectively) were present in all the PSD patterns. Notably, the pore volume of Group 4 first increased and then decreased with increasing OM content (Figure 5). According to the report of Feng et al.  on the pore structure of organic matter in the terrestrial shale of North China, this pore size group may occur in the organic matter that developed along the interface between inorganic minerals and organic matter. They also determined that this pore size group is mainly associated with the shrinkage of organic matter during maturation. The pore volumes of Groups 5 and 6 showed no significant variation with organic matter content (Figure 5), indicating that these two pore size groups were developed in both organic matter and inorganic clay minerals. In the PSD patterns of XJ-15-di and YQ5-15-ding, which contained mixed-layer illite-smectite, the curves showed an increasing trend as the pore size decreased when the pore diameter was <1 nm (Figure 3). It is inferred that for XJ-15-di and YQ5-15-ding, there is a peak centered at approximately 0.51 nm, which represents the interlayer spaces of the smectite layers.
5.3 Influence of diagenetic conversion on the CH4 adsorption capacity
The CH4 adsorption capacities of various clay minerals are mainly influenced by their pore structures, which in turn are further influenced by their species and formation processes. In the present study, the pores with a diameter of <6 nm were the major contributors to the CH4 adsorption capacities of the clay minerals. The <6 nm pore volumes of the clay minerals were the sum of the 1–6 nm pore volumes, which included Groups 1, 2, 3, 4, 5, and 6, and the <1 nm pore volume. Each pore volume was calculated as follows: the pore volumes of Groups 1, 2, and 3 in the mixed-layer illite-smectite, illite, and kaolinite, respectively, were calculated based on the regression equations in Figure 4 when the content of each clay mineral was 100%; the pore volume of Group 4 was 0 when organic matter was absent; the pore volumes of Groups 5 and 6 were calculated as the averages of the parting, roof, and floor samples; and the < 1 nm pore volume was calculated similarly to the average of the parting, roof, and floor samples. The < 6 nm pore volume of the smectite layers in the mixed-layer illite-smectite was calculated according to the < 6 nm pore volume of illite and proportion of the smectite layer. The results for the <6 nm pore volumes of kaolinite, illite, mixed-layer illite-smectite, and smectite layers were 0.00531, 0.00519, 0.00525, and 0.00559 cm3/g, respectively. A comparison between the CH4 adsorption capacities and < 6 nm pore volumes of the clay minerals showed that the CH4 adsorption capacity increased significantly as the < 6 nm pore volume increased (Figure 6).
The diagenetic conversion had a significant influence on the pore structures of the clay minerals in this study. The clay minerals in the Yangquan coal reservoir mainly undergo two conversion processes during coalification: the conversion of kaolinite to illite and the illitization of mixed-layer illite-smectite . The action of both processes resulted in a decrease in the CH4 adsorption capacity (Figure 7).
The illite in the intra-seam partings was mainly converted from detrital kaolinite at approximately 150°C during coalification [9,21]. Importantly, illite is a 2:1 type clay mineral, whereas kaolinite is a 1:1 type. The conversion of kaolinite to illite was mainly attributed to the incorporation of an Si–O tetrahedron . This caused the blocking and filling of the illitic pores inherited from kaolinite. Due to this blocking and filling effect, Group 3 of the kaolinite, which had a pore size of 1.34 nm to 1.40 nm, was reduced to Group 2 of the illite (1.20–1.36 nm). Therefore, the illite had a lower CH4 adsorption capacity, lower <6 nm pore volume, and smaller pore size than kaolinite (Figure 6 and Table 4). In addition, the interlayer cations of the illite in the present study were mainly composed of NH4 + rather than K+ , and NH4 + and CH4 are isoelectronic species with similar physical properties. In light of this, it would be beneficial to study whether the presence of NH4 + influences the CH4 adsorption capacity of illite.
Compared with illite, smectite has crystal lattice defects, which result in the presence of a greater internal surface area and micropore volume . The mixed-layer smectite-illite in the intra-seam partings was gradually converted to illite during coalification. Meanwhile, the crystal lattice defects of the smectite layers in the mixed-layer smectite-illite gradually decreased. It is reasonable to believe that the illitization of the mixed-layer smectite-illite results in a decrease in the internal surface and micropore volume, as well as further decreases in the CH4 adsorption capacity.
The organic matter in the Yangquan coal reservoir had a CH4 adsorption capacity of 33.80 cm3/g (35°C), which was significantly higher than that of various clay minerals. The clay minerals in the Yangquan coal reservoir comprised mainly of kaolinite, mixed-layer illite-smectite, and illite, with CH4 adsorption capacities of 5.81, 4.47, and 2.08 cm3/g, respectively (35°C). The CH4 adsorption capacity of the smectite layers in the mixed-layer illite-smectite was calculated to be 18.01 cm3/g, which was notably higher than that of the other clay minerals.
The CH4 adsorption capacities of the clay minerals were largely influenced by their pore structures, which were mainly slit-shaped and wedge-shaped. The pore sizes of the samples were mainly distributed in the range of < 6 nm, and three to five pore size groups were identified in each PSD pattern. Groups 1 (1.03–1.08 nm pore diameter), 2 (1.20–1.36 nm), and 3 (1.34–1.40 nm) corresponded to mixed-layer smectite-illite, illite, and kaolinite, respectively. Group 4 (1.62–1.78 nm) mainly developed along the interface of organic matter and inorganic mineral matter, and Groups 5 (2.51–3.04 nm) and 6 (4.62–4.95 nm) developed in both organic and inorganic mineral matter. The <6 nm pore volumes of kaolinite, illite, mixed-layer illite-smectite, and smectite layers were 0.00531, 0.00519, 0.00525, and 0.00559 cm3/g, respectively. Finally, the conversion of kaolinite to illite and the illitization of mixed layer smectite-illite had negative effects on their pore volumes and CH4 adsorption capacities.
This research was supported by the National Natural Science Foundation of China (No. 41502154), the Scientific and Technological Project of Henan Province (Nos. 202102310335 and 192102310271), the Technological Key Research Program of the Education Department of Henan Province (No. 20A170007), and the Doctor Foundation of the Henan Institute of Engineering (No. D2106014).
Conflict of interest: Authors state no conflict of interest.
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