Microscopic Pore Structure Characteristics and Methane Adsorption of Vitrain and Durain

During reservoir evaluation, the microscopic pore structure of low-rank coal is mainly characterized in order to study the coalbed methane diffuse and migration mechanisms and control. The low-rank coals are very different in pore type and size, so it is necessary to use various techniques to describe their pore structure. For vitrain and durain of the Coal Member of the Yan’an Formation from Huanglong Coalfield, their chemical composition and microscopic pore structure characteristics were studied, and the factors of influencing the pore size distribution (PSD) were explored. Obviously, vitrain and durain are different in chemical composition. Vitrain has higher moisture content, volatile yield, and vitrinite group content than durain. Vitrain and durain mainly contain vitrinite and inertinite, respectively. The pore structure characteristics (e.g., pore types and PSD) of vitrain and durain were systematically by mercury intrusion porosimetry (MIP), low-temperature nitrogen adsorption, and carbon dioxide (CO2) adsorption. The vitrain and durain samples with a micropore size of <2 nm were mainly tested on their specific surface area (SSA) and pore volume (PV). The results show that microporous vitrain has larger SSA and PV than microporous durain, while mesoporous and macroporous vitrain has smaller SSA and PV than mesoporous and macroporous durain. SSA is very positively correlated with PV. The ash content is negatively correlated with SSA and PV. The ash content influences microporous vitrain more greatly than microporous durain, but mesoporous and macroporous durain more greatly than mesoporous and macroporous vitrain. SSA is positively correlated with the vitrinite content of durain and negatively correlated with the inertinite and exinite contents of durain. However, SSA is negatively correlated with the vitrinite and exinite contents of vitrain and positively correlated with the inertinite content of vitrain. Vitrain has higher methane adsorption capacity, desorption rate, and recovery ratio than durain. There are parameters that are obviously affected by the micropore characteristics.


Introduction
Coalbed methane (CBM) resource is abundant in China. For the coalbed methane with a buried depth of less than 2000 m, its geological resource and recoverable resources are 30 × 10 12 m 3 and 12:5 × 10 12 m 3 , respectively. The low-rank coalbed methane resource accounts for 47% of total resources. Two coalbed methane industrial bases, which are located in the Qinshui Basin and the eastern margin of Ordos Basin, have been successfully operated. In spite of a break-through in low-rank coalbed methane development, lowrank coalbed methane has not been industrially produced yet. The microscopic pore structures of coals not only control the adsorption/desorption seepage and diffusion processes of coalbed methane but also affect the extraction efficiency of coalbed methane.
The microscopic pore structure of coal significantly influences the adsorption/desorption, seepage, and diffusion processes of coalbed methane. In this paper, the microscopic pore structure characteristics of coal, including pore cavity and throat size distribution and geometry, pore volume aperture and SSA, pore mineral filling and distribution, and coal seam skeleton particle size and distribution, were mainly studied by image observation and fluid injection. The pore fractures were observed and quantified by scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), high-resolution field emission scanning electron microscopy (FE-SEM), and focused ion beam scanning electron microscopy (FIB-SEM) [1][2][3]. Fluid injection methods include mercury injection, gas adsorption (N 2 and CO 2 ), low field nuclear magnetic resonance (NMR), and small-angle scattering (SAS). Mercury injection method has unique advantages in the measurement of large and medium pores. The radius of the hole can be measured in the range of 3.75-750 nm. According to the characteristics of mercury inlet and mercury ejection curves, the coal pore structure can be determined, but the information of nanoscale micropores and closed pores cannot be reflected, and the sample is damaged seriously [4]. In the gas adsorption method, the measuring range of pore size depends on the molecule size of the injected gas. The liquid nitrogen adsorption method and the CO 2 adsorption method have a pore diameter measuring range of 0.4~2.0 nm and 0.35 nm, respectively [5]. The pore distribution volume and SSA were calculated by different interpretation models [6]. A relatively complete capillary pressure curve and PSD were obtained by mercury injection, liquid nitrogen adsorption, and determination of microscopic pore structure [7][8][9]. The PSD connectivity and the proportion of movable fluid were obtained by NMR at different scales, with measuring accuracy from 8 nm to 80 μm, so the NMR method is feasible and effective for the medium-large pore structure of coal [10,11]. The pore structure distribution at a diameter of 1 nm~20 μm was obtained by a small-angle X-ray scattering method [12]. It is different in measurement principle and very different in pore SSA from the gas adsorption method [13]. A lot of researchers have studied how to characterize the pore structures of coal, but to the best knowledge of the authors, few researchers have studied the pore structure characteristics of the coals with different lithotypes in detail [14][15][16].
According to the pore classification standard of IUPAC, macropore, mesopore, and micropore are defined as a pore with a size above 50 nm, between 2-50 nm, and below 2 nm, respectively. For the samples from Yan'an Formation in Huanglong Jurassic Coalfield, their PSD was studied in this paper. The vitrain and durain samples were collected and separated by hand in the mine before their chemical compositions were studied by industrial analysis and maceral identification. Their pore structure characteristics were studied by mercury injection, liquid nitrogen adsorption, and carbon            Geofluids dioxide adsorption. The differences in pore structure between vitrain and durain, the influencing factors, and the effect of pore structure on methane desorption were discussed.

Sample and Experiment
2.1. Sample Description and Preparation. Six samples from three mines in the Yan'an Formation of the Huanglong Jurassic Coalfield were used to study their microstructures. The sampling sites in the study area are shown in Figure 1. Sample 1#, which was collected from No. 2 main coal seam of the Guojiahe Mine in the Yonglong Mine Field, mainly contains semidark coal and semibright coal with macrolithotype.
Sample 2#, which was collected from No. 4 main coal seam of Dafosi Mine in the Binchang Mine Field, contains a lot of dull coal, semidark coal, and a small amount of semibright coal with macrolithotype.
Sample 3#, which was collected from No. 2 main coal seam of Huangling 2 Mine in Huangling Mine Field, mainly contains semidark coal and minorly semibright coal with macrolithotype.
The aforementioned coal samples were separated by hand to obtain six samples, including three vitrain samples and three durain samples, for this study.

Experimental
Method. The six samples were studied by proximate analysis, maceral analysis, vitrinite reflectance (R O ), scanning electron microscopy (SEM) analysis, mercury intrusion analysis, low-temperature N 2 , and CO 2 gas adsorption (Table S1) in the Key Lab of Coal Resources Exploration and Comprehensive Utilization, Ministry of Natural Resources, China.

Geochemical Composition.
The results of the proximate analysis and maceral analysis are shown in Table S2. The three vitrain samples have lower ash content (A d ) and higher volatile content (V ad ) than the three durain samples.

Mercury Intrusion Porosimetry.
Mercury Intrusion Porosimetry (MIP) is more favorable for the measurement of macropores (>50 nm). The pore size was calculated according to Equation (1) proposed by Washburn (1921).
The SSA was calculated according to Equation (2) proposed by Rootare and Prenzlow (1967).
where d is the pore diameter (nm); P is the applied pressure, MPa; γ is the surface tension of mercury; θ is the contact angle between mercury and the solid surface; S is specific surface area, m 2 /g; V is the amount of mercury input, cm 3 /g. The values of γ and θ are 0.480 N/m and 140°, respectively [17]. The parameters of pore structure measured by mercury injection are shown in Table S3. Both vitrain and durain have small displacement pressure (P d ), indicating that they have good pore permeability. Vitrain has a larger average throat radius (R) than durain. Both vitrain and durain have the skewness (S k ) of greater than 1, indicating the thicker pore throat. Vitrain has a much bigger structural coefficient (φ) than durain, indicating the intense circumfluence of pore bending. Both vitrain and durain have a small sorting coefficient of throats (S p ), reflecting the uniform pore distribution. Durain has bigger mercury saturation (S Hg ) than vitrain and exhibits a large lag space in the mercury curve. It is concluded that durain has better pore connectivity than vitrain. Durain mainly has open pores, while vitrain may have semiclosed pores. Vitrain has smaller SSA and pore volume than durain.
The mercury injection curves of the samples are shown in Figure 2. The difference in mercury saturation indicates that the coals mainly have open pores. The coal samples have low mercury removal efficiency, indicating that the coals have a large number of fine bottleneck pores.
The PSD values of samples obtained by mercury intrusion test are shown in Figure 3 consisting of dV/d (logD) curves and dA/d (logD) curves. The dV/d(logD) and dA/d (logD) are calculated from the measured cumulative pore volume and SSA. The curves of dV/d(logD), dA/d(logD) versus pore width perfectly reflect PSD [18]. The PSD values of the samples used in the MIP are mainly >50 nm. The PSD values of more than 1 μm also were identified in all samples. Durain has higher dV/d(logD) and dA/d(logD) value than vitrain, suggesting that durain has bigger PVs and SSAs bigger than vitrain.

Liquid N 2 Physisorption.
It is believed that the nitrogen adsorption method is suitable for mesopores (2-50 nm). The pore size and SSA were mainly calculated according to    [6]. The PV and SSA results are shown in Table S3. Durain has a larger pore size, SSA, and PV than vitrain, indicating that durain has a higher adsorption capacity than vitrain (Table S3). The PV mainly distributes in mesopores. For the samples 1# and 3#, SSA mainly distributes in mesopores. For the sample 2#, SSA mainly distributes in micropores. The nitrogen adsorption-desorption curve of coal can be used to analyze the pore structure characteristics [19]. N 2 adsorption curves of coal samples are shown in Figure 4. The recommended physisorption isotherms of IUPAC in 2015 were divided into eight types. In this paper, the vitrain and durain samples have the adsorption behaviors closer to Types I and IV(a) adsorption curves; the adsorption content sharply increases at a low relative range (P/P0 = 0 − 0:1) because of micropore filling effect, indicating that the rocks contain micropores (Type I); and the curve exhibits a hysteresis loop at high relative pressures (P/Po > 0:45), indicating the multilayer range is associated with capillary condensation in mesopores (Type IV) [20] and then behaves like multilayer sorption and capillary condensation at higher pressure (P/P0 = 0:8 − 1), suggesting the existence of both mesopores and macropores in the samples [6,21,22]. The adsorptiondesorption curves of all the samples are not completely closed in the low-pressure hysteresis loop ( Figure 4) due to filling or adsorption in micropores, as considered by many researchers [23]. These samples have an inflection point, at which the desorption curve sharply drops at the relative pressure (P/P 0 ) of 0.5, indicating the existence of slit-shaped pores or ink-bottle-shaped pores [20,24]. The relative pressure at the inflection point calculated according to the Kelvin equation corresponds to a pore size of about 3 nm, indicating that the pores with the pore size < 3 nm have one closed end.
The PSD values of the samples obtained from the Liquid N 2 adsorption test are shown in Figure 5 consisting of dV/d (logW) curves and dA/d (logW) curves. Figures 5(a)-5(c) show that SSA mainly increases in the micropores (<2 nm), and durain has higher increment of SSA than vitrain. Figures 5(d)-5(f) consisting of the curves of dV/d(logW) vs. pore width show the PV increment distribution characteristics. The samples 1# and 3# have consistent PV distribution curves and have mesopores and macropores. For the sample 2#, the PV mainly distributes in micropores. Durain has higher dV/d (logW) and dA/d (logW) values than vitrain, suggesting that durain has bigger PVs and SSAs than vitrain. This result is consistent with that from the mercury injection test.
3.4. CO 2 Physisorption. Low-pressure adsorption (LPA) of CO 2 is a powerful tool to calculate PSD, SSA, and PV of the pores with a size of 0.3-1.5 nm [25]. Table S4 lists the micropore size of 0.6330-0.6619 nm, PV of 0.01846-0.02609 cm 3 /g, and SSA of 102.898-195.631 m 2 /g obtained from the CO 2 adsorption test by DFT model. The above results show that vitrain has a bigger micropore size, PV, and SSA than durain.
The CO 2 adsorption isotherm and PSD of micropores are shown in Figures 6 and 7, respectively. The CO 2 adsorption isotherms of the durain and vitrain samples are similar to the Langmuir curve ( Figure 6). The curves go upward at low pressure (P/P0 < 0:01) and tend to be linear at high The samples 3#-A and 3 #-J have significantly different isotherm curves, suggesting that they have significantly different pore characteristics. Ref. [25] believes that the DFT model is more suitable to reflect the adsorption and filling of gas in micropores, so it is used to analyze micropores. The CO 2 dV/d(logW) and dA/d(logW) curves of the six samples (Figure 7) suggest that the samples have a multimodal PSD in the micropores and PSD of 0.4-0.9 nm.
3.5. CH 4 Isothermal Adsorption. Langmuir monolayer adsorption model is widely used for adsorption of gas in coal [26,27]. GAI-100 isothermal adsorption analyzer was used to simulate the original temperature of coal seams (Figure 8).  Although the methane adsorption of coal conforms to the Langmuir Equation, the desorption process is delayed and not completely reversible [28]. The relation between adsorption capacity of coal and gas pressure can be displayed by the Langmuir Equation: where V represents the adsorption capacity at the pressure P, cm 3 ·g; P represents adsorption pressure, MPa; V L is the Langmuir volume, representing the maximum volume that can be adsorbed at infinite pressure, cm 3 /g; and P L is Langmuir pressure, at which the adsorbed volume is half the Langmuir volume, MPa. Weibull model was used to fit the desorption curve. However, it is found that Equation (4) has a better fitting effect, and the parameters initially have physical significance [29]. In this paper, Equation (4) was used for desorption data fitting: Therefore, the theoretical desorption rate (R 1 ) and recovery rate (R 2 ) can be calculated according to Equations (5) and (6), respectively: where C is residual adsorption at scarcity pressure, which is fitted by Equation (4), and C des is residual adsorbance at abandonment pressure. According to the methane production experiences from the United States, the abandonment pressure is around 0.7 MPa, and it is impossible to produce the coalbed methane in the pressure drop stage [30]. The isothermal adsorption and desorption experiments of vitrains and durains were conducted at 25°, respectively. The adsorption and desorption curves are shown in Figure 8, and the relevant calculation parameters are shown in Table S5. The results show that vitrain has greater V L, R 1 , and R 2 than durain (Figure 9), and vitrain and durain have almost identical P L in the adsorption process. 11 Geofluids restriction; low-pressure N 2 adsorption is effective for mesopores (2-50 nm), and CO 2 adsorption is precise at the micropore level (0.35-2 nm) [25,31]. The experimental method combining MIP, N 2 adsorption, and CO 2 adsorption indicates the distribution characteristics of different pore sizes. More comprehensive information on pore size is obtained by integrating the advantages of various experimental methods. In summary, the curve splicing method was used to connect the three analysis results at 2 nm and 50 nm, respectively, to describe the change characteristics of SSA and PV within the PSD of the sample.

Effect of Ash Content on Pore Structure Characteristics.
The effects of ash content on SSA and PV are rather complicated [15,32,33]. The curves of ash content (A d ) versus micropore, mesopore, and macropore are shown in Figure 12. For vitrain and durain, the ash content (A d ) has a negative linear correlation with SSA of micropore. Durain has higher correlation coefficient (R 2 = 0:8326) than vitrain (R 2 = 0:2159). In spite of no correlation between ash content and SSA (R 2 = 0:0301) of mesopore and SSA (R 2 = 0:0027) of macropore in durain, the ash content has a positive linear correlation with SSA (R 2 = 0:8553) of mesopore and SSA (R 2 = 0:7281) of macropore in vitrain. Ref. [15] argued that ash content affected both SSA and PV, and the increase of ash content was obviously related to the SSA of the micropores. Ref. [34] believed that the ash content was not significantly correlated with PV (R 2 = 0:34) of mesopore, but positively correlated with SSA (R 2 = 0:68) of mesopore. Ref. [32] found that the micropore characteristics were closely associated with the ash content, and a negative correlation between ash content and micropore characteristics was observed. For vitrain and durain, Ref. [33] also confirmed that the ash content had a distinct correlation with PV and SSA. Their results showed that the ash content influenced SSA of micropores in vitrain more greatly than mesoporous and macroporous coals and SSA and PV of mesopore and macropore in durain more greatly than microporous coal. The results accord with the previous understanding and extend the effects of the ash content on the pore structure of the coals with different lithotypes.

Effect of Maceral Composition on Pore Structure
Characteristics. The effects of maceral composition on SSA and PV are complicated [15,32,35,36]. The effects of maceral composition on PV and SSA of vitrain and durain were studied by the curves of their respective properties vs. vitrinite, exinite, and inertinite contents (Figures 13-15). Figure 13 shows the correlation between vitrinite group and SSA. For durain, the vitrinite group is positively correlated with SSA, and its content is positively correlated with SSA of micropore, not mesopore and macropore. The correlation coefficient (R 2 ) between vitrinite group content and SSA of micropore is 0.9644, mesopore 0.0094, and macropore 0.0637. For vitrain, the vitrinite group content is negatively correlated with SSA, and the correlation coefficient (R 2 = 0:6882) between vitrinite group content and SSA of 13 Geofluids micropore is significantly higher than that of mesopore (R 2 = 0:1147) and macropore (R 2 = 0:0487).
The correlation between the inertinite group and SSA is shown in Figure 14. For vitrain, there is a positive correlation between inertinite content and SSA. For durain, there is a negative correlation between inertinite content and SSA. For durain, the correlation between the inertinite group content and the SSA of micropore (R 2 = 0:9346) is significantly greater than that of mesopore (R 2 = 0:0272) and macropore (R 2 = 0:1013), indicating that the inertinite group content has a significant effect on the SSA of micropore. For vitrain, the correlation coefficient between inertinite group content and SSA is not greater than 0.3938, meaning a weak correlation.
For vitrain and durain, the exinite group content is negatively correlated with SSA ( Figure 15). For micropore, durain has a stronger correlation between exinite content and SSA than vitrain. For micropore, the correlation coefficient between exinite group content and SSA in durain is 0.9874 and 0.0599, respectively (Figure 15(a)), implying that the exinite influences the SSA in durain more greatly than the SSA in vitrain. For mesopore and macropore, the correlation between exinite group content and SSA in vitrain is 0.775 and 0.6651, respectively, much higher than that in durain (Figures 15(b) and 15(c)), suggesting that the exinite influences the SSA in vitrain more greatly than the SSA in durain.
The correlation between maceral composition and SSA and PV in coal is different. Ref. [35] studied the SSA and PV of carboniferous bituminous coal in Zonguldak Basin and concluded that there was no obvious correlation between the maceral composition and SSA and PV. According to the research of Ref. [15], the effects of maceral composition on SSA and PV are too complicated to be quantified unless pure macerals are available for analysis. Ref. [32] used the ratio of vitrinite to inertinite (V/I) to represent the maceral composition, which showed a negative correlation with the mesopore characteristics and insignificant correlation with the micropore characteristics. The research result of Ref. [36] shows that there is a strong correlation between SSA, PV, and maceral composition, i.e., vitrinite, liptinite, and inertinite have strongly positive, weakly positive, and strong negative correlation with SSA and PV, respectively. In this study, the correlation between maceral composition and SSA and PV in vitrain and durain was compared. The results show that the maceral composition of durain has a negative correlation with SSA and greatly influences the SSA of micropore. For vitrain, there is an insignificantly positive correlation between vitrinite content and SSA and a negative correlation between exinite and inertinite content and SSA, and the SSA of mesopore and macropore is significantly affected by the exinite and inertinite content. 14 Geofluids

Analysis of the Factors Controlling the Adsorption
Capacity, Desorption Rate, and Recovery Ratio. Researchers used different methods to study the factors affecting the adsorption of methane from coal. The methane adsorption capacity of coal mainly depends on the chemical composition, physical and chemical structure, deterioration degree, and moisture of coal and is also affected by external factors, such as temperature and pressure [5,14,37,38]. There are also many studies on the methane-from-coal desorption law and influencing factors. In this paper, the microscopic pore structure of vitrain and durain which are different in lithotype was analyzed, and the methane adsorption and desorption characteristics were studied by isothermal adsorption and desorption experiments. The experimental and calculated data are shown in Table S5. The results show that vitrain has greater methane adsorption capacity, desorption rate, and recovery ratio than durain. Ref. [39] studied the effects of lithofacies composition on gas adsorption and found that bright coal has much higher adsorption than dark coal at the same coal rank, consistent with this study.
The relationship between microscopic pore structure and methane adsorption and desorption parameters is shown in Figure 16. There is a negative correlation between SSA and methane adsorption capacity, desorption rate, and recovery ratio of durain. The correlation coefficient (R 2 ) between SSA and methane adsorption is greater than 0.36. Vitrain has a relatively complicated relationship between SSA and methane adsorption capacity, desorption rate, and recovery ratio, i.e., the SSA of micropore is positively correlated with methane adsorption capacity, desorption rate, and recovery ratio (Figures 16(a), 16(d), and 16(h)), while the SSA of mesopore and macropore is weakly negatively correlated with methane adsorption capacity, desorption rate, and recovery ratio. The methane adsorption capacity, desorption rate, and recovery ratio are mainly controlled by the micropore of vitrain and by the mesopore and macropore of durain. The weaker closure of the micropores of the coal reservoir leads to higher transmission efficiency of pressure difference, and thereby higher gas desorption volume and higher desorption efficiency on the inner surfaces of the micropores [40][41][42]. This is why vitrain, in which the micropores develop, desorbs methane more easily than durain. durain are quite different in chemical composition. Vitrain has higher moisture content, volatile yield, and vitrinite group content than durain. Vitrain and durain mainly contain vitrinite group and inertinite, respectively

Conclusions
(2) The pores of coal samples are mainly micropores (<2 nm), followed by mesopores (2-50 nm). The SSA of coal mainly distributes in micropores, and the PV mainly distributes in micropores and macropores (>50 nm). Vitrain has larger SSA and PV and smaller SSA and PV of mesopore and macropore than durain. The SSA and PV have a strong positive correlation (3) The influence of ash content on the amount of SSA and PV is rather complicated. The ash content of vitrain and durain is negatively correlated with the SSA and PV. Vitrain has a stronger correlation between ash content and SSA of micropore and weaker correlation between ash content and PV of micropore than durain. For durain, the ash content influences the SSA and PV of mesopore and macropore more greatly than those of micropore (4) The maceral composition is closely related to SSA, especially the SSA of micropores. The SSA has a positive and negative correlation with the vitrinite content of durain and vitrain, respectively. For durain, the inertinite and exinite contents are negatively correlated with the SSA. For vitrain, the SSA is negatively and positively correlated with exinite content and inertinite content, respectively (5) Vitrain has better methane adsorption capacity, desorption rate, and recovery ratio than durain, and these parameters are significantly affected by the micropore characteristics   Figure 16: Correlation between (a) adsorption capacity and SSA, (b) between adsorption rate and SSA, and (c) between recovery ratio and SSA.

Data Availability
The experimental data used to support the findings of this study are included within the manuscript and the supplementary materials.