Mechanism of Methane Adsorption/Desorption in Low-Rank Vitrain and Durain Coal Affected by Pore Structure and Wettability: A Case Study in Dafosi Area, South Ordos Basin, China

: Water content and water–coal interface wettability are always the difﬁcult issues of coalbed methane adsorption/desorption. In order to study the effects of the pore structure and wettability of different macro coal components on methane adsorption and desorption, we compared and analyzed the wettability difference between vitrain and durain, and revealed the inﬂuence of wettability on methane adsorption and desorption through a pore structure analysis, wettability measurements, an adsorption–desorption experiment and adsorption heat calculations under different conditions, taking the No. 4 coal in Dafosi Coal Mine of the Huanglong coalﬁeld as the research object. The results show that both vitrain and durain are relatively hydrophilic substances. However, vitrain has a low ash content, high volatility, and less oxygen, and the pores are mainly semi-closed pores compared with dark coal. Vitrain also has poor connectivity, poor sorting, a small pore diameter, and a coarser surface, resulting in poor surface wettability. The large speciﬁc surface area (SSA) and relatively poor wettability of vitrain leads to more adsorption sites in methane, which makes the adsorption capacity of vitrain greater than that of durain, but the good pore connectivity of durain causes the strong desorption capacity of durain. The isosteric adsorption heat of the adsorption process is greater than that of the desorption process, indicating that there is a desorption hysteresis phenomenon which is essentially due to the lack of energy in desorption. Surfactants change the wettability of the coal surface, and different surfactants have different effects on methane adsorption and desorption. Relatively speaking, the methane desorption of coal samples treated with G502 and 6501 are better. The research results provide scientiﬁc reference for the study of gas–water transport in the desorption process of low-rank CBM, and provide evidence for the methane desorption model of vitrain and durain.


Introduction
The coalbed methane (CBM) resources of low-rank coal reservoirs reach about 16 × 10 12 m 3 in China, accounting for 40% of the total CBM resources, and they have gradually become a focus of the exploration and development of CBM in China [1][2][3]. However, the average gas production of CBM wells is still low, and it is urgent to improve the development efficiency of CBM wells. Compared to the high-and medium-rank coals, the characteristics of low-rank coals are a relatively high permeability and a low

Samples and Experiments
The coal samples for the study were collected from the No. 4 coal seam of Yan'an Formation in Middle Jurassic, on the working face 40,112 of Dafosi in Binchang Mining Area, southern Ordos Basin. Vitrain and durain were stripped by hand for the experiments.

Coal Composition
The proximate analysis and ultimate analysis of the coal samples were conducted based on Proximate analysis of Coal (GB/T 212-2008) and Method of Determining Microlithotype Composition (GB/T 6948-2008) of the State Standard of the People's Republic of China. The instrument for the ultimate analysis was the CHN Analyzer SDCHN435, whose testing ranges are C (0.02-100%), H (0.02-50%) and N (0.01-50%).

Characterization of Pore Structure
The size and shape of the pores in coal have a significant influence on the adsorption/desorption and migration of coalbed methane [24][25][26][27][28][29]. Mercury porosimetry analysis has its particular advantage in measuring macropores and mesopores, covering a pore radius range of 3.75-750 nm. The pore structure type of coal can be judged according to the curve characters of mercury intrusion and extrusion. The method has an obvious limitation in measuring nanoscale pores, which is unable to reflect the information about closed pores [30]. Liquid nitrogen adsorption analysis is very useful for measuring the pore diameter with a lower limit of 0.6 nm, to calculate the pore distribution, pore volume, and specific surface area based on different pore interpretation models, and to determine the pore shape and the degree of openness based on the form of the hysteresis loop [31]. The micropore structure analysis of the coal samples was mainly conducted through a mercury porosimetry experiment and a liquid nitrogen adsorption experiment. The former experiment was conducted at the Petroleum and Geology Experiment Center of Exploration and Production Research Institute of Zhongyuan Oilfield Company. A Mercury Intrusion Apparatus Auto pore 9505 was adopted for parametric measurement, with a pore throat ranging from 0.004 μm~63 μm and a resolution ratio of 0.0001 MPa. The experimental process was based on Part 1: Mercury Porosimetry of Pore Size Distribution and Porosity of Solid Materials by Mercury Porosimetry and Gas Adsorption (GB/T 21650.  of the State Standard of the People's Republic of China. Based on the decimalism pore throat classification system of Hodot (1961), pore throat classification and parameter calculation were performed on the test data. A liquid nitrogen adsorption experiment was conducted at the Coalbed Methane Lab of the Xi'an University of Science and Technology, with a Surface Area and Pore Size Analyzer ASAP2020 from Beckman Coulter, Inc. All the samples were tested in a vacuum drying oven at 80 °C and a nitrogen

Samples and Experiments
The coal samples for the study were collected from the No. 4 coal seam of Yan'an Formation in Middle Jurassic, on the working face 40,112 of Dafosi in Binchang Mining Area, southern Ordos Basin. Vitrain and durain were stripped by hand for the experiments.

Coal Composition
The proximate analysis and ultimate analysis of the coal samples were conducted based on Proximate analysis of Coal (GB/T 212-2008) and Method of Determining Microlithotype Composition (GB/T 6948-2008) of the State Standard of the People's Republic of China. The instrument for the ultimate analysis was the CHN Analyzer SDCHN435, whose testing ranges are C (0.02-100%), H (0.02-50%) and N (0.01-50%).

Characterization of Pore Structure
The size and shape of the pores in coal have a significant influence on the adsorption/desorption and migration of coalbed methane [24][25][26][27][28][29]. Mercury porosimetry analysis has its particular advantage in measuring macropores and mesopores, covering a pore radius range of 3.75-750 nm. The pore structure type of coal can be judged according to the curve characters of mercury intrusion and extrusion. The method has an obvious limitation in measuring nanoscale pores, which is unable to reflect the information about closed pores [30]. Liquid nitrogen adsorption analysis is very useful for measuring the pore diameter with a lower limit of 0.6 nm, to calculate the pore distribution, pore volume, and specific surface area based on different pore interpretation models, and to determine the pore shape and the degree of openness based on the form of the hysteresis loop [31]. The micropore structure analysis of the coal samples was mainly conducted through a mercury porosimetry experiment and a liquid nitrogen adsorption experiment. The former experiment was conducted at the Petroleum and Geology Experiment Center of Exploration and Production Research Institute of Zhongyuan Oilfield Company. A Mercury Intrusion Apparatus Auto pore 9505 was adopted for parametric measurement, with a pore throat ranging from 0.004 µm~63 µm and a resolution ratio of 0.0001 MPa. The experimental process was based on Part 1: Mercury Porosimetry of Pore Size Distribution and Porosity of Solid Materials by Mercury Porosimetry and Gas Adsorption (GB/T 21650.  of the State Standard of the People's Republic of China. Based on the decimalism pore throat classification system of Hodot (1961), pore throat classification and parameter calculation were performed on the test data. A liquid nitrogen adsorption experiment was conducted at the Coalbed Methane Lab of the Xi'an University of Science and Technology, with a Surface Area and Pore Size Analyzer ASAP2020 from Beckman Coulter, Inc. All the samples were tested in a vacuum drying oven at 80 • C and a nitrogen purging station after 24 hours' drying and degassing. The experimental process was based on Part 2: Analysis of

Wettability Test
Contact Angle is an important index reflecting wettability. A wetting test was conducted at the coalbed methane lab of the Xi'an University of Science and Technology. A video optical contact angle measuring device OCA20 from dataphysics in Germany was adopted to measure the contact angle of durain and vitrain ( Figure 2). First, raw coal sample a selected, which was cut into the same specification of 3 × 3 × 2 cm cubes of coal, and then a PG type 2 metallographic sample polishing machine (1000 mesh sandpaper) was used for coal surface grinding in order to make it smooth while considering the original state; the state coal reservoir water then cut the dish specifications samples that were saturated after immersion in a dry area, eventually resulting in a balanced water sample.

Wettability Test
Contact Angle is an important index reflecting wettability. A wetting test was conducted at the coalbed methane lab of the Xi'an University of Science and Technology. A video optical contact angle measuring device OCA20 from dataphysics in Germany was adopted to measure the contact angle of durain and vitrain ( Figure 2). First, raw coal sample a selected, which was cut into the same specification of 3 × 3 × 2 cm cubes of coal, and then a PG type 2 metallographic sample polishing machine (1000 mesh sandpaper) was used for coal surface grinding in order to make it smooth while considering the original state; the state coal reservoir water then cut the dish specifications samples that were saturated after immersion in a dry area, eventually resulting in a balanced water sample. Surfactants can significantly reduce the surface tension of the solution and improve the hydrophilicity of coal. The corresponding surfactants selected in this paper are shown in Table 1. LAS, JFC, and 6501 can be prepared with distilled water as a solvent for contact angle measurement experiments, while a G502 wetting agent is difficult to prepare with aqueous solution because of its strong hydrophobic properties. Therefore, anhydrous ethanol was used as the solvent to prepare the G502 solution, and then the coal was placed in it for soaking for 12 h. The coal was then dried in a drying oven at 50 °C for 1-1.5 h to evaporate the ethanol completely, in order to exclude its influence on the wettability of the coal.  Surfactants can significantly reduce the surface tension of the solution and improve the hydrophilicity of coal. The corresponding surfactants selected in this paper are shown in Table 1. LAS, JFC, and 6501 can be prepared with distilled water as a solvent for contact angle measurement experiments, while a G502 wetting agent is difficult to prepare with aqueous solution because of its strong hydrophobic properties. Therefore, anhydrous ethanol was used as the solvent to prepare the G502 solution, and then the coal was placed in it for soaking for 12 h. The coal was then dried in a drying oven at 50 • C for 1-1.5 h to evaporate the ethanol completely, in order to exclude its influence on the wettability of the coal. The Langmuir Equation (1) was adopted to describe the methane adsorptive process of vitrain and durain [32][33][34]. Due to the hysteresis of desorption, the desorptive Equation (2) proposed by [35][36][37][38] was adopted.
where, V a stands for adsorption capacity with coal reservoir pressure p, cm 3 /g; a a for the utmost adsorption capacity of the coal sample, cm 3 /g; and b a for the integrated parameter of adsorption/desorption speed and adsorption heat.
where, V d stands for the adsorption capacity when the pressure is p during CBM desorption, cm 3 /g; a d for the utmost adsorption capacity of the coal sample, cm 3 /g; b d for the integrated parameter of adsorption/desorption speed and adsorption heat; and c for constant or residual adsorption, cm 3 /g.

2.
The process of methane adsorption and desorption involves a change in energy. The interaction between adsorbate and adsorbent can be measured by studying the same amount of adsorption heat in the process of methane adsorption and desorption (when the adsorption capacity on the adsorbent surface is constant, there is still an infinitesimal amount of heat released after adsorption, which is the change in enthalpy value at the moment of the adsorption process). In the process of methane adsorption by coal, the stronger the property of the coal adsorption performance is, the greater the enhancement in the interaction between the surface molecules of coal and methane molecules, and an increase in the adsorption heat can be detected, irrespectively. The isosteric heat of methane adsorption and desorption can be calculated indirectly using the Clausius Clapeyron Equation (3) [39]: where q st is equal to adsorption heat, kJ/mol; f is fugacity, Pa; T is temperature, K; R is gas constant, taking 8.314 J/(mol·K). By transforming Equation (3), it can be concluded that: The relationship between fugacity f and pressure p is obtained as: where p is the pressure and Z is the compression factor. According to the above relations: where R is the gas constant, 8.314 J/(mo1·K), and C is the slope of the linear fitting relationship. According to the one-to-one corresponding relationship between different temperature and pressure lnf and adsorption capacity n, the data relationship between them was fitted and the lnf -n relationship was obtained.

Coal Composition
A proximate analysis and an ultimate analysis were conducted in turn for ten groups of stripped vitrain and durain samples ( Table 2). According to the results of the proximate analysis, moisture and fixed carbon were roughly equal between vitrain and durain, and the ash content of durain was greater than that of vitrain; however, the volatility of durain was lower than that of vitrain. According to the results of the ultimate analysis, the variation ranges of C, H, O, and N for both vitrain and durain were small, at 82.4-83.6%, 4.68-4.80%, 0.86-0.89%, and 10.73-12.03%, respectively. With the calculation of the element content average of the 10 samples, the C/H of vitrain was 17.42, which was lower than the 17.61 for durain. The C/O of vitrain was 7.79, higher than the 6.85 for durain. The data show that in terms of macromolecular structure, the cyclic saturated hydrocarbon fragments in vitrain were numerous than those in durain, and the oxygen-containing functional groups in durain were more numerous than those in vitrain [40]. Vitrain is generated through gelatinization in a reducing environment, and it is pure in nature, simple in maceral, and without exogenous impurities or mineral substances. Its macromolecules have abundant functional groups (such as -O-, -COOH, C-O-C-, -OH, and -OCH 3 ), with a relatively greater number of aromatic and hydrocarbon molecules. Therefore, it has more denaturalizing and volatile components at high temperatures. Its contents of moisture, hydrogen, oxygen, and nitrogen are relatively high. Durain is generated in an oxidation environment, and a large number of coal molecule branches are lost, leaving the main carbon skeletons. Moreover, exogenous impurities and mineral substances often mix themselves into durain, making the components more complicated, and making the air-dried ash and carbon contents higher.

Pore Structure Characteristics
Through mercury porosimetry, the maximum displacement pressure (P d ) of vitrain and durain was found to be similar, with both being low. In terms of the maximum saturation of mercury (S HG ), that of vitrain was around 2/3 that of durain. The mercury intrusion volume was lower in vitrain pores. The mercury extrusion efficiency (W E ) of durain was obviously lower than that of vitrain. Therefore, compared with durain, vitrain has lower mercury intrusion volume and is easier to use for mercury extrusion. Hence, there are differences in the pore openness between vitrain and durain. There were almost no differences found in pore parameters or sorting, but durain had a stronger flexure ( Figure 3 and Table 3). The migration of the fluid (mercury) in the coal pores shows that vitrain has poorer connectivity (dominated by semi-closed pores) and that durain has a better connectivity (dominated by open pores). − R is the average pore throat radius; S p is the sorting coefficient; S kp is skewness; P d is the displacement pressure; S HG is mercury saturation; W E is mercury extrusion efficiency.  According to the experimental data obtained from the liquid nitrogen adsorption experiment (Table 4) for the No. 4 coal seam of Dafosi Mine, the specific surface area (SSA) and total pore volume (TPV) of vitrain were larger than those of durain, but the average pore size (APZ) of the former was smaller than that of the latter. Through micropore analysis, the total micropore volume (TMV) and the most probable pore size (MPPZ) of vitrain were found to be smaller than those of durain. The results from the liquid nitrogen adsorption were accordance with those from mercury porosimetry. Note: SSA is the specific surface area; TPV is the total pore volume; APZ is the average pore size; MTV is the micropore total volume; MPPZ is the most probable pore size.

Wettability
The contact angle is generally used to characterize wettability, and the smaller the contact angle, the better the wettability [41][42][43]. In this study, the contact angle was for water and the test solvent was distilled water to prevent the interference of mineral impurities. For the test analysis, the average contact angle (three times) of vitrain was 66.9°; for durain samples it was 61.2° ( Table 5). The results indicate that both durain and vitrain were hydrophilic and the former was more easily wetted. The material composition and pore structure of coal have a significant influence on its wettability. Inorganic minerals (ash) with hydrophilic properties can enhance the wettability of coal, and since volatile substances are usually hydrophobic, the wettability of coal becomes poor. The influence of C and O on the wettability is complex and depends on the type of oxygen-containing functional groups. Compared with durain, the pores of vitrain were found to be mainly According to the experimental data obtained from the liquid nitrogen adsorption experiment (Table 4) for the No. 4 coal seam of Dafosi Mine, the specific surface area (SSA) and total pore volume (TPV) of vitrain were larger than those of durain, but the average pore size (APZ) of the former was smaller than that of the latter. Through micropore analysis, the total micropore volume (TMV) and the most probable pore size (MPPZ) of vitrain were found to be smaller than those of durain. The results from the liquid nitrogen adsorption were accordance with those from mercury porosimetry. Note: SSA is the specific surface area; TPV is the total pore volume; APZ is the average pore size; MTV is the micropore total volume; MPPZ is the most probable pore size.

Wettability
The contact angle is generally used to characterize wettability, and the smaller the contact angle, the better the wettability [41][42][43]. In this study, the contact angle was for water and the test solvent was distilled water to prevent the interference of mineral impurities. For the test analysis, the average contact angle (three times) of vitrain was 66.9 • ; for durain samples it was 61.2 • ( Table 5). The results indicate that both durain and vitrain were hydrophilic and the former was more easily wetted. The material composition and pore structure of coal have a significant influence on its wettability. Inorganic minerals (ash) with hydrophilic properties can enhance the wettability of coal, and since volatile substances are usually hydrophobic, the wettability of coal becomes poor. The influence of C and O on the wettability is complex and depends on the type of oxygen-containing functional groups. Compared with durain, the pores of vitrain were found to be mainly semi-closed pores with poor connectivity, poor sorting, a small pore size, a large specific surface area, and a rougher surface, resulting in worse wettability. agent solution (G502). After adding different surfactants, the contact angle of vitrain was still larger than that of durain, indicating that the wettability of durain was better than that of vitrain. At the same time, it was found that the size of the contact angle after treatment with different surfactants was G502 > production water > distilled water > 6501 > LAS > JFC ( Table 5). The hydrophobic wetting agent was G502, and the hydrophilic wetting agent was JFC, LAS, and 6501.
The JFC, LAS, and 6501 were able to effectively reduce the contact angle of the coalwater interface, improve the wettability of coal, and reduce the contact angle of vitrain and durain coal to 33.8 • , 39.9 • , and 49.9 • , and to 32.5 • , 36.7 • , and 42.1 • , respectively. Compared with the original contact angle of coal (measured with distilled water), the contact angle of the coal samples after surfactant modification decreased by 49.5%, 47.7%, and 40.4%, and by 46.9%, 47.5%, and 40.0%, respectively, indicating that the three kinds of surfactants have basically the same effect on the wettability modification of vitrain and durain, that the change rate of contact angle is roughly the same, and that the wettability of modified durain is still better than that of vitrain. The contact angles of vitrain and durain increased to 98.8 • and 90.3 • , respectively, after the action of G502, and the change rates of the contact angles were 25.4% and 31.2%, respectively. The coal changed from hydrophilicity to hydrophobicity, and the wettability was reversed. Therefore, G502 is a hydrophobic agent which can effectively promote the hydrophobicity of coal.

Methane Adsorption and Desorption
The experimental data include saturated adsorption Langmuir volume V L , Langmuir pressure P L , and residual adsorption c. V L can reflect the maximum adsorption capacity of vitrain and durain coal to methane. P L can reflect the growth rate of the adsorption capacity when the pressure is low. When the figure for the P L value increased, as the growth rate of pressure decreased, a fall in both the increasing rate of the adsorption capacity at the low-pressure stage and the adsorption rate of methane were detected, respectively. The residual adsorption amount c could only indicate the size of the residual adsorption. The results of the methane adsorption and desorption of the vitrain and durain coal samples at five temperature points are shown in Figure 4.

Methane Adsorption Characteristics
The fitting results of methane adsorption/desorption are shown in Table 6. The fitting degree R 2 in the process of adsorption and desorption was greater than 0.99, and the

Methane Adsorption Characteristics
The fitting results of methane adsorption/desorption are shown in Table 6. The fitting degree R 2 in the process of adsorption and desorption was greater than 0.99, and the fitting effect was good. The saturated adsorption capacity of vitrain was 11.87~16.04 cm 3 /g in the process of the pressure-up adsorption of methane. The saturated adsorption capacity of durain was 9.76~12.18 cm 3 /g. When the temperature was the same, the saturated adsorption capacity of durain was smaller than that of vitrain, which shows that the adsorption capacity of vitrain is stronger than that of durain, and the adsorption capacities of vitrain and durain are the same under different temperatures. Secondly, the Langmuir pressure P L of vitrain was 2.37~2.73 MPa, and that of durain was 2.33~2.70 MPa. When the temperature was the same, the Langmuir pressure P L of durain was larger than that of vitrain. The adsorption rate of methane by durain was slower than that of vitrain, so that it was more difficult to reach the saturated adsorption state. The above characteristics were present at different temperatures. Table 6. Fitting results of the methane adsorption/desorption of vitrain and durain.

Sample
T/ • C

Methane Desorption Characteristics
The desorption efficiency of methane desorption characteristics can be measured using theoretical desorption or recovery. The higher the theoretical desorption ratio and recovery ratio, the easier the desorption and production of methane. The theoretical desorption ratio of a coal sample is the ratio of the desorption and saturated adsorption when the pressure equals zero in the desorption process; namely, In fact, the productivity of coalbed methane is usually measured by the recovery factor of coalbed methane. The recovery factor of coalbed methane refers to the ratio between the desorption capacity and the saturated adsorption capacity when the pressure drops to abandonment pressure (generally 0.7 MPa).
where c des refers to the residual adsorption capacity corresponding to the desorption curve at waste pressure, mL/g. According to the above formula, the desorption parameters and desorption efficiency of coal samples can be fitted and calculated. The theoretical desorption efficiency of vitrain and durain was 75.91% and 80.78%, respectively, and the recovery efficiency was 66.02% and 71.01%, respectively. The theoretical desorption efficiency and recovery efficiency of durain were higher than those of vitrain, indicating that methane desorption and production are easier in durain than in vitrain. Under the same pressure, the adsorption capacity of vitrain and durain coal in the desorption process was greater than that in the adsorption process, and the amount of methane adsorbed in the coal was not fully desorbed; that is, there was a desorption hysteresis phenomenon, which further indicates that the adsorption and desorption process of methane in the coal is not completely reversible. At the same temperature of the same coal sample, the Langmuir pressure P L of the desorption process was significantly higher than that of the adsorption process, which indicates that the desorption ratio of methane is relatively fast and methane is relatively easy to desorb at the initial stage of the desorption process; however, with the further development of the desorption process, the desorption difficulty increases and the desorption effect gradually weakens. The pore size of durain was larger and the connectivity was good, while semi-closed pores such as ink bottle pores were more numerous and the connectivity was relatively poor in vitrain. Therefore, after the adsorption expansion of the coal matrix of vitrain, the pore channels with poor connectivity become narrower or even closed pores. Finally, during the depressurization desorption, the methane molecules that can be desorbed through the narrow pore channels are less numerous (partially enclosed in water). Therefore, the desorption ratio of durain is higher than that of vitrain.

Adsorption Heat Analysis of Methane
Fixed n values were used to calculate the lnf at the corresponding temperature; according to the obtained lnf, the lnf −1/T was made different under different apparent adsorption capacities (0.1~0.5 mmol/g), and the isosteric adsorption heat was calculated from its slope. The adsorption/desorption data of at least three temperature points are needed to calculate the adsorption/desorption heat in the process of adsorption/desorption. Based on the experimental data of methane isothermal adsorption/desorption at the 20 • C, 25 • C, 30 • C, 35 • C, and 40 • C five temperature points of vitrain and durain in low-rank No. 4 coal seam of the Dafosi minefield, the isosteric adsorption heat of the adsorption and desorption process of vitrain and durain at different temperatures was calculated. The energy variation characteristics of the adsorption and desorption process were obtained.

Adsorption Heat Calculation
(1) Adsorption process Based on the isothermal adsorption experimental data of vitrain and durain at different temperatures, the relationship between the natural logarithm of fugacity lnf (Kpa) and the adsorption capacity n (mmol/g) ( Figure 5) was established, and then the lnf -n data points were linearly fitted. The fitting formula was lnf = An + B, and the fitting results are shown in Table 7. (1) Adsorption process Based on the isothermal adsorption experimental data of vitrain and durain at different temperatures, the relationship between the natural logarithm of fugacity lnf (Kpa) and the adsorption capacity n (mmol/g) ( Figure 5) was established, and then the lnf-n data points were linearly fitted. The fitting formula was lnf = An + B, and the fitting results are shown in Table 7.  According to the fitting formula above, given different adsorption capacities, the lnf values at different temperatures were calculated under different fixed adsorption capacities, and linear fitting was carried out for the relationship between lnf and T −1 . The fitting equation is shown in Figure 6.  According to the fitting formula above, given different adsorption capacities, the lnf values at different temperatures were calculated under different fixed adsorption capacities, and linear fitting was carried out for the relationship between lnf and T −1 . The fitting equation is shown in Figure 6. Using experimental data from the methane isothermal adsorption of vitrain and durain samples in the Dafosi area, the calculation results of isosteric adsorption heat of methane in the adsorption process are shown in Table 8. Different adsorption capacities resulted in different methane isosteric adsorption heat values. In the range of an adsorption capacity of 0.1~0.5 mmol·g −1 , the isosteric adsorption heat of vitrain was 14.88~58.57 kJ·mol −1 , and that of durain coal was 25.79~40.23 kJ·mol −1 . In the process of methane adsorption, the isosteric adsorption heat was dynamically changing. In the adsorption process, the isosteric adsorption heat was proportional to the methane adsorption range, the increased rate of the isosteric adsorption heat of vitrain was greater than that of durain coal, and the isosteric adsorption heat of vitrain increased faster (Figure 7). In the initial stage of adsorption (when the adsorption capacity was less than 0.3 mmol·g −1 ), the pressure was lower, the ability of methane to adsorb durain coal was stronger, and its isosteric adsorption heat was greater. In the later stage of adsorption (when the adsorption capacity was greater than 0.3 mmol·g −1 ), with the increase in pressure, methane was mainly adsorbed into the micropores, which are difficult to be adsorbed in. The developed micropores of vitrain caused the isosteric adsorption heat of vitrain to become greater than that of durain coal in the middle and later stages of the adsorption process, and the molecular force between coal and methane was strong, which made methane adsorption easier. This was consistent with the results of the iso- Using experimental data from the methane isothermal adsorption of vitrain and durain samples in the Dafosi area, the calculation results of isosteric adsorption heat of methane in the adsorption process are shown in Table 8. Different adsorption capacities resulted in different methane isosteric adsorption heat values. In the range of an adsorption capacity of 0.1~0.5 mmol·g −1 , the isosteric adsorption heat of vitrain was 14.88~58.57 kJ·mol −1 , and that of durain coal was 25.79~40.23 kJ·mol −1 . In the process of methane adsorption, the isosteric adsorption heat was dynamically changing. In the adsorption process, the isosteric adsorption heat was proportional to the methane adsorption range, the increased rate of the isosteric adsorption heat of vitrain was greater than that of durain coal, and the isosteric adsorption heat of vitrain increased faster (Figure 7). In the initial stage of adsorption (when the adsorption capacity was less than 0.3 mmol·g −1 ), the pressure was lower, the ability of methane to adsorb durain coal was stronger, and its isosteric adsorption heat was greater. In the later stage of adsorption (when the adsorption capacity was greater than 0.3 mmol·g −1 ), with the increase in pressure, methane was mainly adsorbed into the micropores, which are difficult to be adsorbed in. The developed micropores of vitrain caused the isosteric adsorption heat of vitrain to become greater than that of durain coal in the middle and later stages of the adsorption process, and the molecular force between coal and methane was strong, which made methane adsorption easier. This was consistent with the results of the isothermal adsorption experiment.
faster (Figure 7). In the initial stage of adsorption (when the adsorption capacity was less than 0.3 mmol·g −1 ), the pressure was lower, the ability of methane to adsorb durain coal was stronger, and its isosteric adsorption heat was greater. In the later stage of adsorption (when the adsorption capacity was greater than 0.3 mmol·g −1 ), with the increase in pressure, methane was mainly adsorbed into the micropores, which are difficult to be adsorbed in. The developed micropores of vitrain caused the isosteric adsorption heat of vitrain to become greater than that of durain coal in the middle and later stages of the adsorption process, and the molecular force between coal and methane was strong, which made methane adsorption easier. This was consistent with the results of the isothermal adsorption experiment. (2) Desorption process According to the same method mentioned above, the desorption experimental data of the vitrain and durain samples were processed. The relationship between the logarithm of fugacity lnf (Kpa) and the adsorption capacity n (mmol/g) of the methane desorption process at different temperatures is shown in Figure 8. The results of the fitting relationship lnf = An + B are shown in Table 9. (2) Desorption process According to the same method mentioned above, the desorption experimental data of the vitrain and durain samples were processed. The relationship between the logarithm of fugacity lnf (Kpa) and the adsorption capacity n (mmol/g) of the methane desorption process at different temperatures is shown in Figure 8. The results of the fitting relationship lnf = An + B are shown in Table 9.  According to the above fitting formula, given different adsorption capacities, the lnf value at different temperatures was calculated; under different fixed adsorption capacities, the relationship data between lnf and T −1 were linearly fitted (Figure 9), and the slope C was obtained. The final results of the isosteric adsorption heat of the methane desorption process of vitrain and durain are shown in Table 10.
Methane in coal seam was desorbed by absorbing heat, and the adsorption heat corresponding to the desorption process represents the absorbed heat during desorption. In the desorption process, within the range of an adsorption capacity of 0.1~0.5 mmol/g, the equivalent adsorption heat of vitrain was 19.27~84.46 kJ·mol −1 , and that of durain was 46.73~51.87 kJ·mol −1 (Table 10).  According to the above fitting formula, given different adsorption capacities, the lnf value at different temperatures was calculated; under different fixed adsorption capacities, the relationship data between lnf and T −1 were linearly fitted (Figure 9), and the slope C was obtained. The final results of the isosteric adsorption heat of the methane desorption process of vitrain and durain are shown in Table 10.  The adsorption heat of the desorption process reflects the heat to be absorbed during methane desorption. Within the desorption process, the desorption amount increases, the adsorption amount decreases, and the isosteric adsorption heat decreases ( Figure 10). In the process of desorption, the change trend in the adsorption heat of vitrain and durain coal was the same. In the initial stage of desorption (when the adsorption capacity was more than 0.3 mmol/g), methane was first desorbed in large and medium pores. The pores of durain coal are more evenly developed and have better connectivity. There was a greater number of micropores and poorer connectivity in vitrain. Compared with durain coal, it was more difficult for vitrain to desorb methane. Therefore, the desorption of methane in vitrain needed to absorb more heat than durain coal in the middle and late stages of the desorption process (when the adsorption capacity was less than 0.3 mmol/g), methane desorption in micropores was dominant, and vitrain developed more micropores, which is an advantage in desorption at this stage. Therefore, vitrain required less heat absorption, and the isosteric adsorption heat was less than that of durain.  Methane in coal seam was desorbed by absorbing heat, and the adsorption heat corresponding to the desorption process represents the absorbed heat during desorption. In the desorption process, within the range of an adsorption capacity of 0.1~0.5 mmol/g, the equivalent adsorption heat of vitrain was 19.27~84.46 kJ·mol −1 , and that of durain was 46.73~51.87 kJ·mol −1 ( Table 10).
The adsorption heat of the desorption process reflects the heat to be absorbed during methane desorption. Within the desorption process, the desorption amount increases, the adsorption amount decreases, and the isosteric adsorption heat decreases ( Figure 10). In the process of desorption, the change trend in the adsorption heat of vitrain and durain coal was the same. In the initial stage of desorption (when the adsorption capacity was more than 0.3 mmol/g), methane was first desorbed in large and medium pores. The pores of durain coal are more evenly developed and have better connectivity. There was a greater number of micropores and poorer connectivity in vitrain. Compared with durain coal, it was more difficult for vitrain to desorb methane. Therefore, the desorption of methane in vitrain needed to absorb more heat than durain coal in the middle and late stages of the desorption process (when the adsorption capacity was less than 0.3 mmol/g), methane desorption in micropores was dominant, and vitrain developed more micropores, which is an advantage in desorption at this stage. Therefore, vitrain required less heat absorption, and the isosteric adsorption heat was less than that of durain.

Adsorption Heat Variation
Under the same adsorption capacity, the adsorption heat of the desorption process of vitrain and durain was greater than the corresponding adsorption heat of the adsorption process (Table 11 and Figure 11). The heat released by the adsorption of unit amount of methane molecules cannot make the same amount of methane molecules desorb, indicating that under the same conditions, the desorption of methane needs to absorb additional heat, and that is the essence of desorption hysteresis. Desorption heat supplement is equivalent to methane desorption needing to absorb additional heat, that is, the heat supplement of the external environment during methane desorption. a greater number of micropores and poorer connectivity in vitrain. Compared with durain coal, it was more difficult for vitrain to desorb methane. Therefore, the desorption of methane in vitrain needed to absorb more heat than durain coal in the middle and late stages of the desorption process (when the adsorption capacity was less than 0.3 mmol/g), methane desorption in micropores was dominant, and vitrain developed more micropores, which is an advantage in desorption at this stage. Therefore, vitrain required less heat absorption, and the isosteric adsorption heat was less than that of durain.

Adsorption Heat Variation
Under the same adsorption capacity, the adsorption heat of the desorption process of vitrain and durain was greater than the corresponding adsorption heat of the adsorption process (Table 11 and Figure 11). The heat released by the adsorption of unit amount of methane molecules cannot make the same amount of methane molecules desorb, indicating that under the same conditions, the desorption of methane needs to absorb additional heat, and that is the essence of desorption hysteresis. Desorption heat supplement is equivalent to methane desorption needing to absorb additional heat, that is, the heat supplement of the external environment during methane desorption.  In addition, under different adsorption capacities, that is, in different pores, the adsorption heat difference of the methane adsorption and desorption process was dynamically affected by the corresponding pore structure. Due to the obvious difference in pore characteristics between vitrain and durain, the difference in adsorption heat between adsorption and desorption under different adsorption capacities was also very great, which can be regarded as the energy absorbed from the coal methane system when the corresponding amount of methane was desorbed. When the adsorption capacity was low (corresponding to the larger pores in the coal), the required heat of methane desorption within the vitrain was less than that in the durain; when the adsorption capacity was high (corresponding to the smaller pores in the coal), the needed heat of methane desorption in the vitrinite was more than that in the durain (Figure 12). In addition, under different adsorption capacities, that is, in different pores, the adsorption heat difference of the methane adsorption and desorption process was dynamically affected by the corresponding pore structure. Due to the obvious difference in pore characteristics between vitrain and durain, the difference in adsorption heat between adsorption and desorption under different adsorption capacities was also very great, which can be regarded as the energy absorbed from the coal methane system when the corresponding amount of methane was desorbed. When the adsorption capacity was low (corresponding to the larger pores in the coal), the required heat of methane desorption within the vitrain was less than that in the durain; when the adsorption capacity was high (corresponding to the smaller pores in the coal), the needed heat of methane desorption in the vitrinite was more than that in the durain (Figure 12).

Characteristics of Wettability on Adsorption and Desorption
The stronger the methane adsorption capacity and the faster the efficiency of coal samples with different surfactants, the higher the adsorption curve will be ( Figure 13). The saturated adsorption capacity of methane was G502 > air dry basis sample > equilibrium water sample > 6501 > JFC > LAS. The coal samples treated with G502 had the greatest methane adsorption capacity and the fastest efficiency. However, the better the wettability of the coal sample treated with the surfactants (positive wetting agent), the stronger its attraction to water molecules, occupying the adsorption space of methane molecules and resulting in a reduction in methane adsorption. The saturated adsorption capacity of LAS with good wettability was only 5.626 cm 3 ·g −1 (Table 12), while that of G502 with strong hydrophobicity was 26.984 cm 3 ·g −1 .
The coal sample with poor wettability had the strongest adsorption capacity for methane; that is, it had the weakest desorption capacity for methane, and the desorption curve was at the top among other agents' curves, such as that of the G502 coal sample ( Figure 13). As the wettability worsened, the adsorption speed was enhanced, the desorption speed was decreased, and the figure for Langmuir pressure PL was decreased, irrespective of other conditions. For example, the Langmuir pressure PL (4.444 Mpa) of the G502 coal sample was much larger than that of the other samples.

Characteristics of Wettability on Adsorption and Desorption
The stronger the methane adsorption capacity and the faster the efficiency of coal samples with different surfactants, the higher the adsorption curve will be ( Figure 13). The saturated adsorption capacity of methane was G502 > air dry basis sample > equilibrium water sample > 6501 > JFC > LAS. The coal samples treated with G502 had the greatest methane adsorption capacity and the fastest efficiency. However, the better the wettability of the coal sample treated with the surfactants (positive wetting agent), the stronger its attraction to water molecules, occupying the adsorption space of methane molecules and resulting in a reduction in methane adsorption. The saturated adsorption capacity of LAS with good wettability was only 5.626 cm 3 ·g −1 (  The interaction between water and coal surface (hydrogen bond) is greater than that between coal and methane (dispersion force). Water molecules affect the adsorption of methane by occupying the adsorption sites of methane. Therefore, the mechanism of the  The coal sample with poor wettability had the strongest adsorption capacity for methane; that is, it had the weakest desorption capacity for methane, and the desorption curve was at the top among other agents' curves, such as that of the G502 coal sample ( Figure 13). As the wettability worsened, the adsorption speed was enhanced, the desorption speed was decreased, and the figure for Langmuir pressure P L was decreased, irrespective of other conditions. For example, the Langmuir pressure P L (4.444 Mpa) of the G502 coal sample was much larger than that of the other samples.
The interaction between water and coal surface (hydrogen bond) is greater than that between coal and methane (dispersion force). Water molecules affect the adsorption of methane by occupying the adsorption sites of methane. Therefore, the mechanism of the influence of wettability on methane adsorption can be regarded as the substitution of water for methane. The effect of water on methane adsorption is almost nonexistent beyond the critical value of water. The displacement of methane by water can be measured by the displacement capacity and displacement ratio.
For coal samples treated with different surfactants, the displacement capacity and displacement ratio increased with the increase in pressure. At the same temperature and pressure, the absolute value of displacement capacity (from large to small) can be ranked as LAS, JFC, G502, 6501, and equilibrium water (Figure 14), and the absolute value of displacement ratio (from large to small) can be ranked as LAS, G502, JFC, and 6501 ( Figure 14). It can be seen from the above that water is more easily adsorbed on the coal surface; the coal samples treated with the LAS, JFC, and 6501 surfactants with better wettability strengthen the attraction of coal to water, making water molecules occupy the adsorption position on the coal surface more effectively. With the further increase in pressure, the coal with better wettability absorbs more water, displaces more methane, and has a higher displacement ratio, and it further promotes the desorption of methane. G502 has the effect of gas wetting reversal, which is stronger than equilibrium water and 6501 in methane adsorption. Generally speaking, the positive wetting agent can promote the displacement of methane by water, the negative wetting agent is conducive to the adsorption of methane, and the increase in pressure can promote the displacement.
The effect of wettability on the methane adsorption and desorption is essentially determined by changing the wettability of water and the coal surface. The increase in moisture content in coal will lead to the increase in wettability, and the increase in moisture content and wettability will significantly reduce the adsorption capacity of methane. The adsorption of methane and coal can be changed by changing the adsorption degree of coal and water. At present, surfactant can be added to change the wettability of coal surface and further change the influence of coal on methane adsorption and desorption.

Effects of Wettability on Desorption
The effects of surfactants on methane desorption can be studied according to desorption capacity, desorption efficiency, and desorption hysteresis.
(1) Methane desorption capacity Methane desorption capacity is the methane capacity desorbed from the surface of the coal matrix caused by depressurization, as shown in Equation (9). (9) where, Q d is the depressurization desorption amount of methane isothermal desorption, cm 3 , m is the mass of the corresponding coal sample in the methane adsorption/desorption experiment, g; V d and V a , respectively, correspond to the adsorption capacity in the process of depressurization and pressurization, cm 3 /g. The effect of wettability on the methane adsorption and desorption is essentially determined by changing the wettability of water and the coal surface. The increase in moisture content in coal will lead to the increase in wettability, and the increase in moisture content and wettability will significantly reduce the adsorption capacity of methane. The adsorption of methane and coal can be changed by changing the adsorption degree of coal and water. At present, surfactant can be added to change the wettability of coal surface and further change the influence of coal on methane adsorption and desorption.

Effects of Wettability on Desorption
The effects of surfactants on methane desorption can be studied according to desorption capacity, desorption efficiency, and desorption hysteresis.
(1) Methane desorption capacity Methane desorption capacity is the methane capacity desorbed from the surface of the coal matrix caused by depressurization, as shown in Equation (9).
where, Qd is the depressurization desorption amount of methane isothermal desorption, As shown in Figure 15, the desorption curves of coal samples treated with different surfactants showed that the desorption capacity decreased with the increase in pressure; that is, the depressurized desorption capacity decreased. When the pressure was less than 2 MPa, the curve gradually slowed down with the increase in pressure; that is, the amount of depressurized desorption decreases, indicating that a pressure less than 2 MPa is not conducive to methane desorption. The desorption curve is obviously the steepest in the whole pressure range, and the desorption efficiency is the fastest. However, the desorption curve of the high-pressure stage (5~8 MPa) slows down with the increase in pressure, and the desorption ratio was slow, indicating that the desorption of methane is not utilized at a higher pressure. Therefore, on the whole, the medium pressure stage (2~5 MPa) is most favorable for methane desorption. At the same time, for the depressurization desorption capacity of coal samples treated with different surfactants was G502 (strong G502 gas wetting effect) > 6501 > equilibrium water > air dry basis > JFC > LAS, and the depressurization desorption of coal samples treated with different surfactants increased with the increase in temperature. pressure, and the desorption ratio was slow, indicating that the desorption of methane is not utilized at a higher pressure. Therefore, on the whole, the medium pressure stage (2~5 MPa) is most favorable for methane desorption. At the same time, for the depressurization desorption capacity of coal samples treated with different surfactants was G502 (strong G502 gas wetting effect) > 6501 > equilibrium water > air dry basis > JFC > LAS, and the depressurization desorption of coal samples treated with different surfactants increased with the increase in temperature. (2) Methane desorption efficiency Desorption efficiency refers to the rate of desorption of methane gas from coal samples with unit pressure drop in unit time, cm 3 /(g·min). The equilibrium time of methane isothermal adsorption/desorption experiment is 12 h, as shown in Equation (10).
where V is the desorption efficiency of methane in the depressurization desorption process, cm 3 /(g·min). The desorption efficiency of the coal sample decreased with the increase in pressure ( Figure 16). Under the same temperature and pressure, the desorption efficiency was G502 > 6501 > JFC > LAS. Excepting 6501, the desorption efficiency of the coal sample treated with other surfactants was relatively small; the change trend was relatively close, and the desorption efficiency of wetting reversal agent G502 was the highest. (2) Methane desorption efficiency Desorption efficiency refers to the rate of desorption of methane gas from coal samples with unit pressure drop in unit time, cm 3 /(g·min). The equilibrium time of methane isothermal adsorption/desorption experiment is 12 h, as shown in Equation (10).
where V is the desorption efficiency of methane in the depressurization desorption process, cm 3 /(g·min). The desorption efficiency of the coal sample decreased with the increase in pressure ( Figure 16). Under the same temperature and pressure, the desorption efficiency was G502 > 6501 > JFC > LAS. Excepting 6501, the desorption efficiency of the coal sample treated with other surfactants was relatively small; the change trend was relatively close, and the desorption efficiency of wetting reversal agent G502 was the highest. (3) Desorption hysteresis ratio Desorption hysteresis ratio is an important index to measure the desorption hysteresis phenomenon. When coal adsorbs methane, the adsorption curve does not coincide with the desorption curve, and the desorption curve is generally above the adsorption curve. This phenomenon is called desorption hysteresis. The desorption hysteresis ratio (δ) can be obtained according to the aforementioned Equation (1) and Equation (11), namely, Equation (5)  (3) Desorption hysteresis ratio Desorption hysteresis ratio is an important index to measure the desorption hysteresis phenomenon. When coal adsorbs methane, the adsorption curve does not coincide with the desorption curve, and the desorption curve is generally above the adsorption curve. This phenomenon is called desorption hysteresis. The desorption hysteresis ratio (δ) can be obtained according to the aforementioned Equation (1) and Equation (11), namely, Equation (5): where δ is desorption hysteresis ratio of methane during depressurization, %. The desorption hysteresis ratio of the coal samples showed a logarithmic downward trend with the increase in pressure (Figure 17), the desorption curve changed from steep to slow, and the desorption hysteresis ratio changed from large to small. The desorption hysteresis ratio curves at different temperatures were consistent, and the desorption hysteresis degree decreased with the increase in temperature. In the low-pressure stage, methane was adsorbed in coal for a period of time, and then gradually desorbed out. The hysteresis degree of the relatively high-pressure stage was very large. Different coal samples had different hysteresis ratios. Under the same temperature and pressure, the desorption hysteresis ratios were 6501 > G502 > LAS > JFC.
(3) Desorption hysteresis ratio Desorption hysteresis ratio is an important index to measure the desorption hysteresis phenomenon. When coal adsorbs methane, the adsorption curve does not coincide with the desorption curve, and the desorption curve is generally above the adsorption curve. This phenomenon is called desorption hysteresis. The desorption hysteresis ratio (δ) can be obtained according to the aforementioned Equation (1) and Equation (11), namely, Equation (5): where δ is desorption hysteresis ratio of methane during depressurization, %. The desorption hysteresis ratio of the coal samples showed a logarithmic downward trend with the increase in pressure (Figure 17), the desorption curve changed from steep to slow, and the desorption hysteresis ratio changed from large to small. The desorption hysteresis ratio curves at different temperatures were consistent, and the desorption hysteresis degree decreased with the increase in temperature. In the low-pressure stage, methane was adsorbed in coal for a period of time, and then gradually desorbed out. The hysteresis degree of the relatively high-pressure stage was very large. Different coal samples had different hysteresis ratios. Under the same temperature and pressure, the desorption hysteresis ratios were 6501 > G502 > LAS > JFC.

Mechanism of CBM Desorption in Coal Covered with Water
As we know, drainage and depressurization can exploit coalbed methane in water-rich coal seams, and for water-deficient coal seams, it can also promote the normal production of CBM under sufficient differential pressures. The present production mechanisms of CBM mainly include depressurizing desorption [44,45], temperature-rising desorption [46], displacement desorption [47,48], diffusion desorption, and induced desorption [41,[49][50][51][52], but the means of depressurizing desorption is still the most common method of CBM production. Therefore, in this study, the mechanism of coalbed methane desorption in coal covered with water is discussed in detail.

Vapor Displacement
In the coal reservoir, coalbed methane adsorption and desorption are accompanied by vapor desorption and adsorption, influencing each other [53,54]. During CBM exploitation, for the coal seam covered with water, the energy mainly comes from the water turbulence resulting from water pumps. The phenomenon of water turbulence in wellbore caused by drainage has been pointed out by some researchers [43,[55][56][57]. Then, the migration pathways of particles through pore throats and cavitation provide conditions for water vaporization, whose adsorption on the pore surface offers energy for methane desorption (Figure 18). by vapor desorption and adsorption, influencing each other [53,54]. During CBM exploitation, for the coal seam covered with water, the energy mainly comes from the water turbulence resulting from water pumps. The phenomenon of water turbulence in wellbore caused by drainage has been pointed out by some researchers [43,[55][56][57]. Then, the migration pathways of particles through pore throats and cavitation provide conditions for water vaporization, whose adsorption on the pore surface offers energy for methane desorption ( Figure 18). At standard atmospheric pressure (101.325 KPa), the heat of water evaporation is 40.8 kJ/mol, which is far higher than the adsorption heat of either vitrain or durain, and far higher than the heat required by the desorption of CH4 from vitrain or durain. The saturated vapor pressure increases with temperature increasing and the evaporation heat decreases with temperature increasing (Figure 19). Thus, competitive adsorption takes place between water vapor (H2O (g)) and CH4 in coal, urging the desorption of CH4. Regarding the adsorption on the coal surface, the adsorption heat of H2O (g) and VDW force is far higher than that of CH4, which is also proved by [58]. At standard atmospheric pressure (101.325 KPa), the heat of water evaporation is 40.8 kJ/mol, which is far higher than the adsorption heat of either vitrain or durain, and far higher than the heat required by the desorption of CH 4 from vitrain or durain. The saturated vapor pressure increases with temperature increasing and the evaporation heat decreases with temperature increasing (Figure 19). Thus, competitive adsorption takes place between water vapor (H 2 O (g)) and CH 4 in coal, urging the desorption of CH 4 . Regarding the adsorption on the coal surface, the adsorption heat of H 2 O (g) and VDW force is far higher than that of CH 4 , which is also proved by [58]. In the coal reservoir covered with water ( Figure 20I), while air is pumped out, the fluid pressure in pores becomes P-ΔP (ΔP being the decrease in saturated vapor pressure above water surface, equivalent to HΔP-the fall of the water surface). The capillary water column in the pore throat rises (Ⅱ→Ⅲ), and the pore volume increases by V', whereas the pressure decreases. The liquid water vaporizes at B, and H2O (g) effuses in the closed pores. The adsorption heat of H2O (g) on the pore surface is higher than 40.8 kJ/mol, while the adsorption heat of CH4 is no more than 31 kJ/mol. In other words, the heat released by H2O (g) adsorption on the pore surface is enough for CH4 to shift from an absorbed state to a free state, which can be seen as the displacement of H2O (g) for CH4. With sustained depressurization, the displacement continues. With the increase in free methane in the pores, the partial pressure of H2O (g) decreases and that of CH4 increases, urging CH4 to break through the water column at the pore throat. Thus, a two-phase flow of liquid and gas comes into being, moving toward the direction of depressurization, and CH4 is produced. In the coal reservoir covered with water ( Figure 20I), while air is pumped out, the fluid pressure in pores becomes P-∆P (∆P being the decrease in saturated vapor pressure above water surface, equivalent to H∆P-the fall of the water surface). The capillary water column in the pore throat rises (II→III), and the pore volume increases by V', whereas the pressure decreases. The liquid water vaporizes at B, and H 2 O (g) effuses in the closed pores. The adsorption heat of H 2 O (g) on the pore surface is higher than 40.8 kJ/mol, while the adsorption heat of CH 4 is no more than 31 kJ/mol. In other words, the heat released by H 2 O (g) adsorption on the pore surface is enough for CH4 to shift from an absorbed state to a free state, which can be seen as the displacement of H 2 O (g) for CH 4 . With sustained depressurization, the displacement continues. With the increase in free methane in the pores, the partial pressure of H 2 O (g) decreases and that of CH 4 increases, urging CH 4 to break through the water column at the pore throat. Thus, a two-phase flow of liquid and gas comes into being, moving toward the direction of depressurization, and CH 4 is produced.
an absorbed state to a free state, which can be seen as the displacement of H2O (g) for CH4. With sustained depressurization, the displacement continues. With the increase in free methane in the pores, the partial pressure of H2O (g) decreases and that of CH4 increases, urging CH4 to break through the water column at the pore throat. Thus, a two-phase flow of liquid and gas comes into being, moving toward the direction of depressurization, and CH4 is produced. Figure 20. Desorption mechanism of CH4 in vitrain (semi-open pores) covered with water. Ⅰ: The coal reservoir covered with water; Ⅱ: Initial state; Ⅲ : Air is pumped out, the capillary water column in the pore throat rises; Ⅳ: The pore volume increases by V'; V: CH4 to break through the water column at the pore throat.

CBM Desorption in Interconnected Pores
Durain is wetter than vitrain, and the pore throats are more interconnected. In a water covering environment, the static pressure P continues to decrease (Figure 21), ΔP1→(ΔP1 + ΔP2)→(ΔP1 + ΔP2 + ΔP3). The water column from the up interface of the durain stripe and that from the down interface are labeled as HP up and HP down, respectively. Close to the upright pore throat, the solute methane (II, B) forms cavities, Figure 20. Desorption mechanism of CH 4 in vitrain (semi-open pores) covered with water. I: The coal reservoir covered with water; II: Initial state; III: Air is pumped out, the capillary water column in the pore throat rises; IV: The pore volume increases by V'; V: CH 4 to break through the water column at the pore throat.

CBM Desorption in Interconnected Pores
Durain is wetter than vitrain, and the pore throats are more interconnected. In a water covering environment, the static pressure P continues to decrease (Figure 21), ∆P1→(∆P1 + ∆P2)→(∆P1 + ∆P2 + ∆P3). The water column from the up interface of the durain stripe and that from the down interface are labeled as HP up and HP down, respectively. Close to the upright pore throat, the solute methane (II, B) forms cavities, moving upward and downward (to the direction of depressurization). It ultimately cavitates and breaks at the water surface, becoming free CH 4 , and enters the top of the vacuum vessel, and discharges under negative pressure. Meanwhile, a small amount of water particles (II, A) cavitate. H 2 O (g) strengthens the desorption of the methane absorbed on the pore surface.  Close to the horizontal pore throat (II, C, D), no matter whether methane cavitates or water vaporizes, whether pressure drops toward pores or vessels, there are no other than two types of phenomena. One is that pressure drops toward the pores, and the capillary force of the water columns in the pore throats is lower than the pressure drop, so that water spreads out on the pore surface (IV, C, D). The liquid-solid interface is more likely to cavitate at the pressure drop. The other one is that in the condition of water covering, capillary pore throats (V, E) are refilled with water under the dual function of Close to the horizontal pore throat (II, C, D), no matter whether methane cavitates or water vaporizes, whether pressure drops toward pores or vessels, there are no other than two types of phenomena. One is that pressure drops toward the pores, and the capillary force of the water columns in the pore throats is lower than the pressure drop, so that water spreads out on the pore surface (IV, C, D). The liquid-solid interface is more likely to cavitate at the pressure drop. The other one is that in the condition of water covering, capillary pore throats (V, E) are refilled with water under the dual function of capillary force and pressure drop. This kind of dynamic process circulates on the condition of depressurization. The dynamic equilibrium of adsorption and desorption is shown as the desorption of methane.

Conclusions
(1) Vitrain has more developed micropores than durain, and the specific surface area (SSA) and total pore volume (TPV) of vitrain are slightly larger than those of durain, with most of them being semi-closed and ink-bottle-shaped pores. However, the average pore size and pore connectivity of durain are better than those of vitrain.
(2) Coal-water contact angles are all less than 90 • , showing hydrophilicity. The contact angle of durain (61.2 • ) is smaller than that of vitrain (66.9 • ). The wettability of the coal samples treated with different surfactants from poor to good is, in order, G502 (hydrophobic), 6501, LAS, and JFC. G502 causes the wettability of coal to reverse, and the contact angle increases to 98.8 • . The other three surfactants can improve the wettability of coal, and the contact angle decreases to 32.5 • .
(3) The wettability of coal is negatively correlated with methane adsorption capacity. The better the wettability is, the weaker the methane adsorption capacity is. The order for the adsorption capacity of different wettability coal samples from strong to weak is G502 sample, air dry sample, balanced water sample, 6501, JFC, and LAS. The desorption capacity was positively correlated with the wettability. The better the wettability, the higher the desorption ratio and the lower the desorption hysteresis ratio. The desorption capacity was, in descending order, G502, 6501, JFC, and LAS.
(4) The effect of coal wettability on methane adsorption and desorption is in essence a comprehensive reflection of coal-water and coal-methane molecular interactions. The better the wettability of coal, the stronger the molecular interaction of the coal-water interface, and the surface of the coal matrix is easily occupied by a large number of water molecules. The competitive adsorption of water and methane molecules weakens the methane adsorption capacity of coal, but it also promotes methane desorption.  Data Availability Statement: All data, models, and code generated or used during the study appear in the paper.

Conflicts of Interest:
The authors declare no potential conflict of interest with respect to the research, authorship, and publication of this paper.