Effect of water occurrences on methane adsorption capacity of coal: A comparison between bituminous coal and anthracite coal

Fuel 266 (2020) 117102

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Effect of water occurrences on methane adsorption capacity of coal: A comparison between bituminous coal and anthracite coal

T

Feng Wanga,b, Yanbin Yaoa,b, , Zhiang Wena,b, Qinping Sunc, Xuehao Yuana,b ⁎

a

School of Energy Resource, China University of Geosciences, Beijing 100083, China Coal Reservoir Laboratory of National Engineering Research Center of CBM Development & Utilization, China University of Geosciences, Beijing 100083, China c Research Institute of Petroleum Exploration and Development, Langfang, Hebei 065007, China b

ARTICLE INFO

ABSTRACT

Keywords: Adsorption capacity Nuclear magnetic resonance (NMR) Water occurrence states Coal Coalbed methane

In this study, an NMR fluid typing method was applied to two typical anthracite and high volatile bituminous coals to identify different occurrence states of water (adsorbed and non-adsorbed water), and multiphase methane (adsorbed, porous medium confined, and bulk methane) and investigate the effect of the water occurrence states on methane adsorption capacity. First, results of isothermal adsorption analysis conducted using the NMR method was compared to those from conventional volumetric method to verify the precision of the NMR method. Then the influence of adsorbed water and non-adsorbed water on methane adsorption was examined based on two parallel treatments. For bituminous coal, the adsorbed water remarkably reduced the Langmuir adsorption volume, whereas adding non-adsorbed water had no effect on gas adsorption. In contrast, for anthracite coal, the methane adsorption capacity revealed a downward trend with increase in both adsorbed water and non-adsorbed water. The explanation is that adsorption competition between water molecules and methane gas molecules can constrain further adsorption of methane on the coal matrix. Thus, adsorbed water negatively impacts gas adsorption for both anthracite and bituminous coal. In terms of non-adsorbed water, due to the superior pore conductivity and hydrophilia of bituminous coal, methane gas can easily pass through the pores in the presence of non-adsorbed water, compared to anthracite, where water blocking and the Jamin effect tend to occur. Even water droplets formed because of anthracite’s hydrophobicity can utterly block the pore throats, resulting in a continuous decrease in methane adsorption capacity. The implications of this study are important for better understanding the influence of various occurrence states of water on the adsorption/desorption processes in unconventional reservoirs.

1. Introduction The methane adsorption capacity of coal is a key parameter for evaluating gas content, gas-in-place resources, and the gas production potential of coalbed methane (CBM) reservoirs [1]. Even though pioneering research on gas adsorption of coal can be traced back to half a century ago, most of these studies have not yet considered the coexistence of water and methane in the pore space of in-situ reservoirs, in which the occurrence and accumulation of gas/water is closely related to the pore structure and pore surface properties of coals. Previous investigations suggested that in a multi-scale coal pore system, both gas and water can appear in different states, including multiphase gas, such as adsorbed methane, free methane, and solution methane [2–5], and multiphase water, such as chemical bonded water, adsorbed water, capillary-bounded water, and free water [3,6–11]. Unlike free methane

⁎

stored in the free space between coal particles, and solution methane that exists in formation water in a dissolved state, adsorbed gas is adhered to the adsorption sites of micro-pores under intermolecular force, playing a dominant role in the total amount of CBM (80%–90%). With regards to multiphase water, chemical-bounded water is quite unique, because it is organically associated within the molecular structure of coal and is released during coalification. Adsorbed water is physically adsorbed in the micro-pores and is characterized by a lower vapor pressure, while free water is retained in a free status in the macro-pores and cleats and has a normal vapor pressure. Capillary-bounded water refers to the water confined in capillaries of the coal matrix with properties falling in-between that of adsorbed water and free water [12]. However, the mechanisms by which various occurrence states of water can influence the mechanisms of adsorption, desorption, diffusion, and migration of methane in coal is of vital importance in the

Corresponding author at: School of Energy Resource, China University of Geosciences, Beijing 100083, China. E-mail address: [email protected] (Y. Yao).

https://doi.org/10.1016/j.fuel.2020.117102 Received 30 September 2019; Received in revised form 13 December 2019; Accepted 13 January 2020 Available online 23 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.

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Nomenclature

M m mw n p r S/V T T2 W

Ai' (i = 1,2…6) T2 amplitude for free gas of sample cell at different pressure, p.u. Cadsorbed coefficient of adsorbed methane, mol/p.u. NC methane adsorption capacity, mol Si adsorbed peak area at ith pressure point, p.u. Vi adsorbed capacity at ith pressure point, cm3/g nfm free methane content, mol surface relaxation rate, μm/ms 2 Ai (i = 1,2…6) T2 amplitude for free gas of reference cell at different pressure, p.u. Fs geometric factor, dimensionless k T2 amplitude, p.u. quantification of CBM productivity, but this has not yet received sufficient attention. Since the 1960s, many coal adsorption studies have been performed based on experimental, mathematical, and numerical methods. Most of these studies involved isothermal gas adsorption experiments, the results of which concluded that there are two main aspects affecting the methane adsorption capacity of coal. One aspect is related to coal characteristics, such as, coal rank, inorganic and organic composition, maceral composition, water content, pore shape and connectivity, ash yield, wettability, particle size, etc. [13–22]. More recently, Busch et al. [22] established a database to discuss the key factors influencing methane adsorption capacity, and they found that the primary factors include the surface area of micropore, oxygen contents in coals, and the aromaticity at comparable maturity, while the secondary factors are the water content and liquid hydrocarbons content in coals. Another aspect relates to external factors including in-situ pressure and temperature, depth and structural stress, and fault sealing and hydrodynamic conditions, among others [1,23–25]. Although lots of investigations have been made, due to variations in coal samples, experimental conditions, data selection, and the limitations of selected methods, some conclusions remain controversial and demand further investigation. One of the most representative examples lies in revealing the effect of water on methane adsorption capacity in both coal and shale reservoirs. For coal reservoirs, most experimental investigations suggested that increasing water content of coal can induce a decrease of adsorption capacity [23,26–28]. Gensterblum et al. [27] indicated that the reduction becomes smaller with coal rank increasing. However, some other scholars reached the opposite conclusion, namely, that the adsorption capacity marginally increases or maintains a constant value with increasing liquid water content [26,29]. Sang et al. [29] indicated that the adsorption of water/gas on the coal matrix surface occurs through multilayer adsorption. Thus, the adsorption sites on the surface of the first layer of water molecules can be used for continuous adsorption of outer layer methane. Moreover, the relationship between water content and adsorption capacity previous were also investigated for shale reservoirs [30–34]. The results showed that the methane adsorption capacity of shales decrease rapidly with increasing water content until the “critical moisture content” was reached and subsequently decrease slowly or remain unchanged [31,35]. In addition to this, occurrences of water also have negative impact on methane adsorption capacity of shales, which effect decreases with increasing TOC content [36]. Great progress has also been made in numerical simulations and mathematical studies of methane gas adsorption in coal. Some Molecular Dynamic Simulation (MDA) and Grand Canonical Monte Carlo (GCMC) simulation results [37–41] confirmed that increasing water content can reduce adsorption capacity. The reason is that the free energy barrier of water molecules is higher than that of methane molecules, making the substitution of water molecules by adsorbed methane molecules unfeasible [42]. Although these simulation methods

calibration coefficient of free methane gas, mol/p.u. the mass of coal powder, g water content, g adsorption capacity of methane, mol pressure, MPa pore radius, µm ratio of pore surface area to pore volume, μm−1 temperature, °C transverse relaxation time, ms calibration coefficient of water, g/p.u. contact angle, degree density, g/cm3 porosity, fraction

can provide more information about microscopic methane adsorption, they are limited in the research on methane at the molecular level or at a nanometer scale. In terms of the mathematic methods, relying on quantum chemical calculations of the second-order Moller-Plesset perturbational theory (MP2), Jiang et al. [43] pointed out that the adsorption potential of water molecules on the coal surface is 5–8 times that of methane molecules, so the coal surface shows preferential adsorption of water molecules when methane and water coexist in coal. However, these mathematical approaches fail to explain some complex adsorption processes on an experimental scale, because in-situ reservoir conditions are hard to replicate due to limited data-handling capacity. Although previous studies have helped us gain a profound theoretical understanding on the subject, problems remain. Considering the inadequacy of conventional experimental, mathematical, and numerical methods in the characterization of complex multiphase fluid interaction processes, particularly with regards to the quantification of the influence of different water occurrence states on gas adsorption, we introduced low-filed nuclear magnetic resonance (NMR) technology to address this problem. The change of water content in the two separated treatments was monitored with regards to the change to adsorption capacity in anthracite and high volatile bituminous coal. A physical model was finally presented to reveal the microscopic mechanism of water occurrence states on gas adsorption in bituminous coal and anthracite coal. 2. Low-field nuclear magnetic resonance The low-field NMR method has been widely used to identify fluid types, and to characterize pore-fracture systems in coal reservoirs for decades [3,44]. For the NMR measurement of rocks, the transverse relaxation time (T2) measurement is generally used because of its fast and nondestructive detection and abundant information regarding fluids in porous rocks. A typical T2 distribution is controlled by three relaxation mechanisms, namely, bulk, surface, and diffusion relaxation of fluids in porous media [45]. When a homogenous magnetic field and Carr-Purcell-Meiboom-Gill measurement sequence (CPMG) is applied to T2, bulk relaxation and diffusion relaxation are insignificant, so only surface relaxation should be considered [44,45]. Finally, there is a linear relationship between T2 and fluids in porous media, as is shown in Eq. (1)

1 T2

2

S = FS 2 V r

(1)

where 2 is the surface relaxation rate (μm/ms), and S/V is the ratio of the pore surface area to the pore volume (μm−1); r is the pore radius (µm) , and Fs is the geometric factor. For spherical pores, Fs is 3; and it is 2 for tube pores. The occurrence states of methane and water in the coal were detected by the response of the proton in the methane and water 2

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molecules in the magnetic field. As seen from Eq. (1), methane adsorbed in the micro-pores, which are characterized by a large specific surface area, exhibits a much faster relaxation than that in larger pores. Thus in a typical T2 spectrum, the peak of adsorbed methane is located at the leftmost side (T2 < 10 ms), followed by a porous medium confined methane in the middle (10 ms < T2 < 250 ms), whereas the peak of free methane gas is sited at the rightmost side (250 ms < T2) [5,46]. In this study, we followed the above standard of division to classify the occurrence states of methane gases. Similarly, based on their different interactions with the solid surface of coal pores, the relaxation response of water in different occurrence states can be divided into a left adsorbed water peak, which produces signal amplitude peaks at low T2 values (0.1 ms < T2 < 10 ms), followed by a capillary water in the middle (10 ms < T2 < 700 ms), and a right free water peak, which relaxes much more slowly and signal amplitude peaks consequently appear at higher T2 values (700 ms < T2) [3]. Considering adsorbed water is distinctly different from both the capillary water and bulk water, to simplify the study, we defined both the capillary confined water and free water as non-adsorbed water (more details will be discussed in Section 4.2).

For both the volumetric and the NMR method, procedures of isothermal adsorption experiments generally included four steps: 1) check the airtightness of the device; 2) vacuum the cells; 3) conduct isothermal adsorption tests; 4) repeat the above steps several times to test the data reliability (detailed procedures see [5]). A significant difference between the volumetric and NMR methods, is whether it is necessary to measure the volume of free gas before calculating the volume of adsorbed gas. In terms of the volumetric method, detailed theory and experimental procedures of gas isothermal adsorption experiments on coal have been discussed in previous papers [17,23]. The method for adsorption measurements uses a reference cell and a sample cell. It is crucial to measure the free volume, which determined by helium expansion. The steps are outlined below: 1) helium gas was injected into a reference cell at a predetermined pressure, 2) the valve was opened to let the gas naturally expand into the sample cell and then record the pressure in the above process. And the methane adsorption capacity can be acquired according to Boyle’s law and ideal gas state equation after the measurement of free volume. In contrast, with regards to the NMR method, the adsorption capacity is directly determined by the T2 spectrum and calibration equation, thus the accurate measurement of the T2 spectrum for two cells requires more attention. The most prominent advantage of the NMR method is that it can quantitatively detect multiphase fluids stored in different pore spaces, which is beyond the capability of the traditional volumetric method [46].

3. Experiments and methods 3.1. Properties of samples Two types of coal samples were taken from the southern Qinshui Basin (anthracite coal) and the southern Junggar Basin (high volatile bituminous coal). The anthracite coal has a mean maximum vitrinite reflectance (Ro,max) of 3.32%, and a maceral composition of 88.52% vitrinite and 11.48% inertinite. The high volatile bituminous coal (Ro,max of 0.5%) has coal maceral composition of 20.7% vitrinite, 76.7% inertinite, and 0.3% exinite (Table. 1). The optical contact angle measurements were carried out by an automatic contact angle meter equipped with 3 M-pixel cameras and an LED light source. The measurement results on the high-pressure compressed disc’s artificial surface, which is made from powdered coal, show that sample A is hydrophobic coal with a contact angle of 107.01°, while sample HVB shows a preference for water wetting with a contact angle of 67.47°. On the basis of Darcy’s law, the measured helium gas absolute permeability of anthracite coal and bituminous coal were 0.228 mD and 1.447 mD respectively. Information regarding the physical properties of the coal is also listed in Table 1.

3.3. Experimental design and procedures In this study, a collected coal block was crushed into a powder sample with a 60–80 mesh size. The experimental procedure consists of three parts (Fig. 2). (1) S1: dry the samples in a drying oven at 105 °C for 48 h, then conduct the isothermal adsorption experiment on a dry sample to obtain the maximum adsorption capacity (Langmuir volume) of the bituminous and anthracite coals; (2) S2-S3: examine the water distribution and the decrease of adsorption capacity under water diffusion conditions. To be more specific, detailed operation procedures mainly include: i) a beaker holding 15 g of 60–80 mesh coal powder that was put in a sealed canister with 100 mL oversaturated K2SO4 solution. Evaporated water molecules diffused into the coal pore system and then adsorbed onto the pore wall. ii) Every 12 h, the coal sample was taken out and weighed immediately, and then the T2 spectrum was measured. iii) We repeated ii) until the change in the T2 amplitude of two adjacent NMR measurements was negligible. S2 was an intermediate state of water diffusion equilibrium, and then the diffused water reached a state of equilibrium in S3; (3) S4-S8: for the same weight in dry coal, a dropper was used to add water on the surface of coal particles. The variation of water distribution and adsorption capacity were not detected until 24 h later when the added water had completely migrated into the coal matrix. In this process, five sets of data were collected in total, lasting over one month. The whole experiment was performed at room temperature (25 °C). It is worth noting that during the detection of the adsorbed methane volume in watered coal samples by means of the conventional NMR method, the proton in H2O can also cause interference to the NMR amplitude. Thus, in order to obtain accurate NMR amplitude of methane, deuterated water (D2O) was chosen to filter the signal noise from water in this study. In short, for each run, H2O was used to obtain the

3.2. Experimental apparatus Fig. 1 illustrates a schematic diagram of the experimental apparatus, consisting of an NMR analysis system and an isothermal adsorption setup. The NMR spectrometer adopted a MiniMR-60 NMR spectrometer with a relatively low magnetic field of 0.55 T, a frequency of 23.402 MHz, and a magnet coil diameter of 60 mm. The isothermal adsorption setup consists of four components: 1) a temperature control system, which allowed for a constant source of electrical heating, was used to maintain certain experimental temperatures; 2) a gas supply system and a pressure boosting device, including a booster pump and a vacuum pump; 3) a pair of nonmagnetic and nonmetallic cells (polymers and composites); and 4) an exhaust system. Table 1 Petrological composition and physical properties of coal samples. Sample ID

A HVB

Basin

Qinshui Junggar

Microscopic composition Vitrinite

Inertinite

Exinite

88.52 20.7

11.48 76.7

0 0.3

3

Ro,max

Contact angle (°)

Porosity (%)

Gas permeability (mD)

3.32 0.50

107.01 67.47

3.65 8.63

0.228 1.447

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Fig. 1. Schematic diagram of the experimental apparatus (modified after [5]).

whenever H2O or D2O was used (Table. 2). Moreover, according to Fu et al. (2004) [47], the solution of methane gas in deionized water is negligible low at the experimental conditions of 20 ℃ and 5 MPa, thus we did not consider the dissolved methane in this study.

information on water distribution in a multi-scale pores system, whereas D2O was used to evaluate the influence of water on the methane adsorption capacity. In order to enhance the reliability of our experimental results, the mass of water was kept almost the same

Fig. 2. Schematic diagram of the experimental procedure. 4

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m w is water content (g); and k is T2 amplitude (p.u.).

Table 2 Water content of experimental coal samples. Procedure

S1: S2: S3: S4: S5: S6: S7: S8:

no water diffused water diffused water added water added water added water added water added water

A

3.4.2. NMR quantification of free methane gases The experimental device was preheated for 4 h at 25 °C, after which, methane gas was injected into the reference cell at a series of elevated pressures (1.57 MPa, 2.54 MPa, 3.65 MPa, 4.60 MPa, 5.51 MPa). During this process, the NMR measurements were carried out and a positive linear correlation between the NMR spectrum amplitude and the mass/volume of free methane was noted (Fig. 3b). The amount of methane gas can be obtained by:

HVB

H2O (g)

D2O (g)

H2O (g)

D2O (g)

/ 0.3341 0.5989 3.2764 5.3155 6.2205 7.5320 10.1591

/ 0.2351 0.5499 3.3671 5.3675 6.2405 7.5280 10.8911

/ 1.1265 1.4205 1.7340 4.1565 4.7550 5.9744 6.6795

/ 1.0110 1.4205 1.6825 4.1415 4.8055 6.0840 6.5990

nfm = M × k (R2 = 0.9993)

where M is calibration coefficient (mol/p.u.), and here M equals 3 × 10 6; nfm is free methane content (mol); and k is T2 amplitude (p.u.).

3.4. Calculations

3.4.3. NMR quantification of adsorbed methane The calibration experiments aim at establishing the relationship between T2 amplitude and methane adsorption capacity of anthracite and bituminous coal for subsequent calculation. The essential idea is that in this isolated system (two cells), the variation of the T2 peak area for free gas is equal to that of adsorbed gas in the process of desorption. A detailed procedure is listed as follows: 1) clean the cells to prevent noise generation; 2) load the sample (anthracite and bituminous coal) into the sample cell and open the upstream valve to let methane gas flow into the sample cell and reference cell respectively at determined pressures; 3) link two cells until the system reaches equilibrium, then

3.4.1. NMR quantification model for water To analyze the occurrence states of water in coal, it is necessary to provide a water mass quantification model based on NMR measurements. The NMR T2 spectra of bulk water with different masses were measured, and a linear relationship between the NMR spectrum amplitude and water mass was presented in Fig. 3a. The quantification model for bulk water can be expressed as

m w = W× k (R2 = 0.9987)

(3)

(2)

where W is calibration coefficient (g/p.u.), and here W equals 0.0002;

Fig. 3. Relationship between T2 amplitude and bulk water content (a); Relationship between T2 amplitude and free methane content (b); Relationship between T2 amplitude and adsorbed methane content of anthracite coal (c) and high volatile bituminous coal (d). 5

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measure the T2 amplitude for free gas of reference cell ( A1) and sample cell ( A1'); and 4) reduce pressure and measure the T2 amplitude for free gas of reference cell ( A2 ) and sample cell ( A2' ) when the isolated system reaches equilibrium again, and repeat the above steps until 5 data sets are obtained. The calculation formula is expressed as:

n = [(A2 + A2 ')

that calculated using the conventional volumetric method. The results are listed in Table 3. Note, S1 is dry sample, S2-S3 are diffused water samples, S4-S8 are added water samples, and total water content increases with the subscript numbers increasing. As is illustrated in Fig. 4, the adsorption isotherms of dry samples from the NMR and volumetric methods are nearly identical, indicating that the NMR method is capable of calculating the amount of adsorbed methane. The comparative results of the other seven sets of experiments indicate that the average absolute deviations of the experimental data points of the two methods fall within ± 2 cm3/g, and the Langmuir volumes calculated with the NMR method have relative deviations < 5% (Table 3). These results suggest that the Langmuir volumes estimated by means of the NMR method and volumetric method are all within the experimental error. Thus, the error from the two methods virtually does not affect the subsequent experimental analysis results when using the NMR method.

(4)

(A1 + A1' )] × M

where n is adsorption capacity of methane (mol); A1 and A2 are the T2 amplitude for free gas of reference cell at different pressure condition (p.u.); A1' and A2' are the T2 amplitude for free gas of sample cell at different pressure condition (p.u.); M is calibration coefficient of free methane gas (mol/p.u.), and here M equals 3 × 10 6. More details can be found in [48]. A good linear correlation between T2 amplitude and methane adsorption capacity of anthracite and bituminous coal was shown in the Fig. 3c and d separately. The calibration equation is expressed as Eq. (5).

4.2. Characteristics of water occurrence in coal

(5)

NC = C × k

where C is calibration coefficient (mol/p.u.), and C equals 2 × 10 6 and 3 × 10 6 for anthracite coal and high volatile bituminous coal, respectively; NC is the methane adsorption capacity (mol); and k is T2 amplitude (p.u.). On the premise of the calibration of adsorbed methane, the NMR gas isothermal adsorption experiment was conducted, and the methane volume can be derived by:

Vi =

Si × Cadsorbed × 22.4 × 1000 m

The T2 relaxation spectra of the coal samples after diffused water and added water treatments were used to study the water content and its occurrence states. As shown in Fig. 5, for anthracite and bituminous coal, the relaxation response of water presents a triple and dual-peak distribution respectively. Note that in this situation, the pore space is occupied by water, thus the different spectra characteristics (i.e. NMR signal form water) reflect the difference in pore characteristics. It is noticeable that in Fig. 5, through S2 to S3, only one peak (P1) appears on the T2 spectrum, within the range of 0.1 ms to 10 ms. This is attributed to the diffusion of evaporated water molecules into the inner coal matrix and adsorbed on the micro-pores. In contrast, through S4 to S8 (adding water stage), a new T2 peak (P2), larger than 10 ms, was encountered, increasing continuously with the increase of the mass of added water, and accompanied by a marginal increase of the P1. This phenomenon reflects that the added water initially appears in the interspace of coal particles during the process of adding water, followed by a gradual penetration to the smaller pores. The water migration behavior through S2 to S8 confirmed that the processes of S2-S3 and S4-S8 are resulted by the adsorbed water and nonadsorbed water, respectively. Note that a tiny T2 peak occurs in the middle of the anthracite NMR spectrum (10 ms < T2 < 50 ms), since it experiences the same change of the right-most peak, it is classified as non-adsorbed water. Compared to the previous classification standards of water in coal [3,7–9], the non-adsorbed water in our classification criteria includes capillary water and free water. In this study, the T2 amplitude of the non-adsorbed water is higher than that of adsorbed water, because the adsorbed water is mainly physically adsorbed in micro-pores, whereas the non-adsorbed water mainly occupies macro-pores and cleats with a greater volume than that of micro-pores. According to the T2 spectra amplitude of the two samples, the bituminous coal has a higher water content (both adsorbed and non-

(6)

Si is where Vi is the adsorbed capacity at the ith pressure spot the adsorbed peak area at the ith pressure spot (p.u.); Cadsorbed is the calibration coefficient of adsorbed methane (mol/p.u.); and m is the mass of coal powder (g).

(cm3 /g) ;

4. Results and discussion 4.1. Validity of the NMR adsorption capacity determination In this study, the NMR method was used to analyze the influence of multiphase water on gas adsorption. Hence, the first requirement is the validity of the NMR method. As we know, the conventional volumetric method is widely applied in calculating the methane adsorption capacity of coal due to its convenience and repeatability [17,23,49,50]. However, the ideal pressure equilibrium state between sample cell and reference cell is difficult to achieve. In addition, the volumetric method is less effective in characterizing occurrence states of water in coal, which is the issue that this paper is most concerned with. Therefore, we appeal to fast and non-destructive NMR technology. This exhibits a predominant advantage in the recognition and characterization of multiphase methane in coal reservoirs [5]. In this section, we compared the adsorption capacity calculated by means of the NMR method with Table 3 Water content, relative error of experimental coal samples. Sample ID

S1 S2 S3 S4 S5 S6 S7 S8

A

HVB

NMR adsorption capacity (cm3/g)

Volumetric adsorption capacity (cm3/g)

Relative error (%)

NMR adsorption capacity (cm3/g)

Volumetric adsorption capacity (cm3/g)

Relative error (%)

40.650 38.911 36.364 29.586 25.445 25.773 20.534 9.588

42.553 39.370 37.037 30.960 26.178 26.881 21.570 10.016

4.47 1.16 1.82 4.44 2.80 4.12 4.80 4.27

22.779 16.904 11.416 11.173 11.062 11.587 11.123 11.351

23.866 17.249 11.987 11.358 11.322 11.863 11.547 11.622

4.55 1.77 4.76 1.63 2.30 2.33 3.67 2.33

6

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Fig. 4. Comparison of isothermal adsorption of a dry sample between the NMR method and the volumetric method (a. Anthracite coal; b. High volatile bituminous coal).

adsorbed water) than anthracite coal (Fig. 5). On the one hand, high volatile bituminous coal commonly has higher pore volumes than anthracite coal [51]. On the other hand, the molecular structure of high volatile bituminous coal consists of many polar groups, such as hydroxyl and carboxyl. These hydrophilic compositions are favorable for water occurrence [27,52].

pressure is below approximately 2 MPa. After the pressure reaches 3 MPa, however, the amounts of porous medium confined methane and free methane exceed that of adsorption methane. Moreover, anthracite shows a higher signal amplitude than bituminous coal, indicating that it has a higher adsorption capacity. 4.4. Influence of water on multiphase gas occurrence states

4.3. Occurrence of multiphase gases in coal

The existence of water does not only cause variations in the peak area, but it also influences the morphology of the T2 spectra. This will be the focus of discussion in this section. Compared with dry samples, both the diffused water and added water coal samples experience a similar variation trend in terms of the impact of water on methane peaks, while the only difference is that the variation degree of amplitudes is distinctive for the diffused water and added water coal samples (Fig. 6c–h). In general, with an increase of water content, the adsorbed methane peak reveals a substantial downward tendency, whereas the decrease of porous medium confined methane and free methane are relatively low. More specifically, as for the equilibrium state of sample A (Fig. 6c), compared with the dry state of sample A (Fig. 6a), the adsorption peak drops by 32.4%, while a total of 27.8% peak area is lost for the porous medium confined methane and free methane. However, when the coal was treated by adding water (Fig. 6g), compared with the dry state of sample A (Fig. 6a), the adsorption peak dropped by 76.7%, whereas the decrease in the remaining two peaks was near constant at 29.0%. In summary, although the mass of water of the two treatments

Fig. 6 shows the methane gas relaxation response under different pressures in dry, diffused water, and added water states. As previously mentioned, D2O was extensively used in this section since it does not influence the detection of gas signals. All the T2 spectra appear to have triple-peak distributions, which is attributed to the disparity of methane occurrence phases and the associated relaxation mechanisms. According to Yao et al. [5], these three peaks, located in the regions of 0.1 ms–10 ms, 10 ms–100 ms, and over 100 ms, correspond to adsorbed methane, porous medium confined methane, and free methane, respectively. For dry samples (Fig. 6a and b), with increase in pressure, although the adsorption peak presents an upward trend, the increment declines gradually, reaching a stable state at about 6.0 MPa. This tendency is in accordance with the characteristics of the Langmuir isothermal adsorption curve. Unlike the adsorption peak, the peaks of porous medium confined methane and free methane rise continuously with increase in pressure. In addition, adsorption is the dominant mechanism when the

Fig. 5. T2 spectra of coal samples after diffused water and added water treatments (a. Anthracite coal; b. High volatile bituminous coal). 7

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Fig. 6. T2 spectrum of methane for dry and watered coal samples at different pressures (a, c, e, g: anthracite coal; b, d, f, h: high volatile bituminous coal).

differ greatly, both diffused water and added water have a similar impact on porous medium confined methane and free methane (Fig. 6c and g). As for adsorbed methane, diffused water causes close to 1/3 of the total adsorbed gas decrease, whereas added water, in quantities of 15 times that of the diffused water, only leads to approximately 2/3 of the reduction in gas adsorption capacity. Apart from the variation of the T2 integral amplitude, the shape of

the T2 spectrum also underwent a notable change. It was commonly found that with an increase in pressure, the peaks of porous medium confined methane and free methane shifted consecutively toward a slower relaxation time. This movement of the T2 peak results from the increased gas molecular density and the reduced free molecular path, lead to more frequent molecular interactions and a slower relaxation process during the pressure increase [3]. 8

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4.5. The influence of water occurrence states on methane adsorption capacity The influence of adsorbed water and non-adsorbed water on gas adsorption capacity for two typical coal samples will be explored in this section. Before the discussion, we present the change of total, adsorbed, and non-adsorbed water content during the dry, diffused water, and added water treatments in Table. 4. As can be seen from Table 4, during water diffusion process (S2-S3), only adsorbed water exists in both anthracite and bituminous coal. In the subsequent process of adding water (S4-S8), the adsorbed water content of bituminous almost maintain a stable state, whereas for anthracite coal, adsorbed water content still increases when non-adsorbed water first encountered (S4) and then fluctuated (S5-S8) within a narrow range. There exist two reasonable explanations about the phenomenon in anthracite coal, on the one hand, due to the hydrophobicity of anthracite, the diffusion and adsorption processes of water molecules in micro-pores are relatively inadequate. On the other hand, it is difficult for water molecules to occupy all adsorption sites because of complicated and tortuous pore network of anthracite coal. The above two reasons still lead to the influence of adsorbed water on methane adsorption capacity in the early stage of adding water. Fig. 7 demonstrates the change to methane adsorption capacity resulting from the increase in the total water content. In this figure, the methane adsorption capacity is represented by the Langmuir volume. In general, for anthracite coal, there is a continuous declining momentum with an increase in total water content. In contrast, for high volatile bituminous coal, although the methane adsorption capacity experiences a similar downward tendency during the water diffusion process, a knee point was encountered when external water was added in the coal powder. The added water did not exert any impact on methane adsorption capacity. This feature is more clearly reflected in Fig. 8, where the total water is divided into adsorbed water and non-adsorbed water. For bituminous coal, before it reaches water diffusion equilibrium (S3), there only exists adsorbed water (Fig. 5b), from S1 to S3, the corresponding Langmuir volume decreases from 22.78 cm3/g to 11.35 cm3/g (Fig. 8a). This is because the adsorption sites of methane gas were occupied by water molecules on the coal matrix surface [28]. In the subsequent process of adding water, the Langmuir volume remains almost unchanged even if the non-adsorbed water content still increases (Fig. 8b). This is because non-adsorbed water does not deter the gas adsorption process occurring in the micro-pores, and thus has no contribution to methane adsorption. Conversely, for anthracite coal, although there is a similar rule during the water diffusion stage (Fig. 8a), and the Langmuir volume decreases continuously by 3.42 cm3/g per unit mass of non-adsorbed water in the water adding stage (Fig. 8b).

Fig. 7. The relationship between Langmuir volume and total water content.

4.6. Differences between anthracite coal and bituminous coal The reasons of the change law in Section 4.5 were discussed in this section. For anthracite coal, according to the results in Table 4, Through S3 to S4, the reason for the adsorption capacity decrease is still that adsorbed water will occupy adsorption sites, as for S4 to S8, two reasonable interpretations were put forward as follow: Firstly, wettability has a certain effect on the gas adsorption processes by controlling the water distribution states [20,53]. In this study, the bituminous coal is being wet with the water with a contact angle of 67°. A fluid with a small contact angle is preferred for spreading over the pore walls and eventually displacing the non-wetting fluid, which does not block the gas flow pathway in the pore center. However, the anthracite coal is hydrophobic with a contact angle of 107°, indicating that water tends to exist in the form of droplets. When the water droplets block the throats completely, the gas can hardly penetrate the water barrier, which is unfavorable for gas adsorption. To eliminate errors caused by wettability heterogeneity in contact angle measurements, we chose an artificial surface made from powdered coal instead of chunk coal. However, inaccuracy still cannot be avoided during the pressure boost stage. It has been reported that the contact angle varies with pressure in previous literatures [54,55], in this study the maximum adsorption pressure was set to about 6.3 MPa, compared to the results of a study by Sakurovs and Lavrencic [54] on the effect of high pressure (15 MPa) on contact angle, we are confident that wetting reversal is unlikely to occur in our experimental conditions.

Table 4 The content of total water, adsorbed water and non-adsorbed water in coal samples. Sample ID

S1 S2 S3 S4 S5 S6 S7 S8

A

HVB

Total water content (g)

Adsorbed water content (g)

Non-adsorbed water content (g)

Total water content (g)

Adsorbed water content (g)

Non-adsorbed water content (g)

0 0.3341 0.5989 3.2764 5.3155 6.2205 7.5320 10.1591

0 0.3341 0.5989 0.9946 1.0224 1.0265 0.9836 1.0722

0 0 0 2.2818 4.2931 5.194 6.5484 9.0869

0 1.1265 1.4205 1.7340 4.1565 4.7550 5.9744 6.6795

0 1.1265 1.4205 1.4197 1.4226 1.4162 1.4275 1.4219

0 0 0 0.3143 2.7339 3.3388 4.5469 5.2576

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Fig. 8. Adsorbed water content vs. Langmuir volume (a), and non-adsorbed water content vs. Langmuir volume (b).

Fig. 9. The mechanism diagram of the effect of water on methane adsorption capacity.

Secondly, although non-adsorbed water in macro-pores has no effect on methane adsorption, the water film tends to restrain gas adsorption through the pore network. In particular, water blocking and the Jamin effect caused by water retention are inclined to occur in anthracite coal because of its extremely low original permeability [56]. As shown in Table. 1, the absolute permeability of sample A is only 0.228 mD, while sample HVB shows a much higher permeability of 1.447 mD. Although a cylindrical coal core was used in the permeability tests, whereas the samples in our adsorption experiment were coal powder, the 60 to 80 mesh powder sample (with a particle diameter between 180 and 250 μm) has size sufficient to represent matrix permeability. A mechanism diagram in Fig. 9 shows the effect of water on methane adsorption capacity. Taken together, for dry coal samples, methane gas that continuously adsorbs onto the coal surface is driven by gas concentration difference. When water begins to diffuse into the coal samples, water molecules have priority over methane molecules in occupying adsorption sites, thus resulting in the methane adsorption capacity decreasing remarkably until the system reaches diffusion equilibrium. When nonadsorbed water appears in the coal-gas system, for bituminous coal, the non-adsorbed water would not enter micro-pores that have contributed

to adsorption. While, for anthracite coal, although non-adsorbed water also has no ability to pass into micro-pores, the increase in water film thickness due to water invasion will trigger water blocking and the Jamin effect, seriously reducing or even blocking gas diffusion pathways. Nevertheless, because of the hydrophobicity of anthracite coal, water tends to appear in the form of droplets that choke the pore throat, thus hindering further diffusion and adsorption of methane gas. Therefore, the non-adsorbed water, encountered in almost every stage of CBM production, such as fracturing and workover, requires an equal amount of attention compared to adsorbed water in guiding the process of depressurization and desorption of CBM. 4.7. Potential application of this study The mechanism of water occurrence on methane adsorption capacity of coal can be applied in design of hydro-fracturing of CBM reservoir. For bituminous coal reservoir, water molecules can diffuse sufficiently because of its strong hydrophilicity, thus there is no need for well-soaking during hydro-fracturing, which will significantly improve the gas desorption capacity. That is to say, it can be directly drained and mined after fracturing for low rank coal CBM wells. In 10

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contrast, for anthracite coal reservoir, the weak hydrophilic ability of coal could lead to insufficient diffusion and the helplessness for completely occupying methane adsorption sites. In addition to this, complicated and tortuous pore network of anthracite coal is yet another reason why it is difficult for water molecules wholly occupy the methane adsorption sites. Therefore, it is necessary to soak wells for a period of time after injecting fracturing fluid for improving the desorption capacity, and thus enhancing the production rate of CBM well. However, hydro-fracturing is extremely complicated; water invasion was subject to heterogeneities of seepage pore and fracture [57]. Thus, there are still lots of other factors should be considered during hydrofracturing CBM reservoir.

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2020.117102.

5. Conclusions

References

Acknowledgments We acknowledge financial support from the National Natural Science Foundation of China (41830427; 41872123), the Key Research and Development Project of the Xinjiang Uygur Autonomous Region (2017B03019-1), and the National Major Research Program for Science and Technology in China (2016ZX05043-001). Appendix A. Supplementary data

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In this study, water migration behaviors on anthracite and bituminous coal samples from Qinshui and Junggar basins, coupled with NMR spectrum measurements, were evaluated for different occurrence states of water to investigate the effect of these states on methane adsorption capacity. Methane relaxation properties was measured for different water contents by using D2O to filter the water signal. The following conclusions can be drawn: (1) The water mainly has two occurrence states: adsorbed water and non-adsorbed water. Diffused water molecules gradually migrate and are adsorbed in coal micro-pores, thus turning into adsorbed water. The added water initially appears in the interspace of coal particles (non-adsorbed water), followed by a slow penetration to the micro-pores (non-adsorbed water). (2) Depending on different relaxation mechanisms, the methane gas in the coal can be divided into adsorbed gas, porous medium confined methane, and free methane. With an increase in pressure, the adsorption methane peak presents an upward trend, however, the increment declines gradually, which is in accordance with the characteristics of the Langmuir isothermal adsorption curve. In contrast, porous medium confined methane and free methane continuously increase with an increase in pressure. (3) For bituminous coal, adsorbed water can induce a reduction in the Langmuir volume, whereas the non-adsorbed water has no impact on gas adsorption. Conversely, for anthracite coal, the methane adsorption capacity reveals a downward trend with the increase of both adsorbed water and non-adsorbed water. (4) The explanation for why non-adsorbed water can hinder gas adsorption into the anthracite coal matrix lies in its complex pore structure and wetting characteristics. The increase of water film thickness in pore walls, and the forming of the water droplet near the pore throat can seriously reduce, or even block, gas diffusion and adsorption into the micro-pores. CRediT authorship contribution statement Feng Wang: Validation, Writing - original draft, Investigation. Yanbin Yao: Conceptualization, Methodology, Supervision, Writing review & editing, Project administration, Funding acquisition. Zhiang Wen: Validation, Investigation. Qinping Sun: Resources. Xuehao Yuan: Investigation, Visualization. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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