Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Effect of Wettability on Adsorption and Desorption of Coalbed Methane: A Case Study from Low-Rank Coals in the Southwestern Ordos Basin, China Pei Li,*,†,‡,§ Dongmin Ma,∥ Jinchuan Zhang,†,‡,§ Xuan Tang,†,‡,§ Zhipeng Huo,†,‡,§ Zhen Li,†,‡,§ and Junlan Liu†,‡,§ †
School of Energy Resources, China University of Geosciences, Beijing 100083, China Key Laboratory of Strategy Evaluation for Shale Gas, Ministry of Land and Resources, Beijing 100083, China § Beijing Key Laboratory of Unconventional Natural Gas Geological Evaluation and Development Engineering, Beijing 100083, China ∥ School of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, China
Ind. Eng. Chem. Res. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/25/18. For personal use only.
‡
ABSTRACT: It is important to quantitatively study the effect of wettability on low-rank coals and further analyze the mechanism of moisture on methane adsorption/desorption for the production technology of coalbed methane (CBM) on low-rank coals and its reservoir evaluation. In this work, a typical long-flame coal sample was chosen to perform methane adsorption/desorption experiments and water wettability tests. The results indicated that the application of surfactant can effectively enhance the water wettability of a coal matrix, and then promote methane desorption; it can improve the critical desorption pressure and theoretical recovery efficiency of CBM. The wettability reversal agent modified coal properties, which is conducive to gas (methane) wettability. Depressurized desorption capacity, desorption efficiency, and desorption hysteresis showed a logarithmic decrease with the increasing pressure. Compared with the relatively low pressure stage, the specific pressure drop in the relatively high pressure stage has no significant effect on methane desorption. High pressure restrained the growth of desorption hysteresis; however, low pressure did the opposite. Different coal samples had different desorption hystereses due to the intrinsic differences in wettability. Wettability characteristics and porefracture development affected moisture distribution in coal matrix. The wetting migration as liquid water and the diffusion of gaseous water (vaporization) were the two main forms of water migration. Additionally, the mechanism of depressurized desorption was only the external consequence for replacement desorption; inversely, the wettability effect and phase transformation of water may have been the essence of the replacement desorption of CBM.
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recovery efficiency of CBM. Research12−16 has shown that moisture content in low-rank coals is generally higher than in medium- and high-rank coals. The statistical data reveal that moisture content in China’s long-flame coal is in the range 3− 12%, and the lignite in particular has a high moisture content.17,18 The process of adsorption/desorption and the migration of CBM is closely related to the water form. Moreover, the coal wettability directly affects the distribution and content of moisture in a coal matrix and ultimately determines the recovery efficiency.4 Therefore, the influence of moisture, especially coal wettability, must be considered when adsorption/desorption characteristics of low-rank coals are studied.
INTRODUCTION In China, low-rank coals (with vitrinite reflectance Ro,max < 0.70%) are widely distributed and can form coalbed methane (CBM) accumulation with favorable conditions of generation, storage, sealing, and preservation. According to the CBM resource assessment in China in 2016, the in-place CBM resources shallower than 2000 m were approximately 30 × 1012 m3, of which CBM resources in low-rank coals account for approximately 34%.1 However, compared to that of highmedium rank coals, the current CBM exploration and development of low-rank coals in China has been left behind.2 The mechanism of methane storage in low-rank coals and the complex interaction among methane, water, and coal is poorly understood.3−8 Generally, CBM is approximately 88−98% in the form of an adsorbed state, besides free and deliquescent states.9−11 The desorption rate of CBM on the surface of coal matrixes and migration efficiency in pore fracture jointly restrict the © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
May 3, 2018 July 7, 2018 August 9, 2018 August 9, 2018 DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 1. Quality Analysis of Low-Rank Coal Samples Used in This Work proximate analysis element ash moisture volatile matter fixed carbon
ultimate analysis
content (wt %) a
8.54 4.32a 31.93c 59.33a
maceral composition
element
content (wt %, ada)
element
content (vol %, mmfb)
carbon hydrogen oxygen nitrogen sulfur+d
83.55 4.19 10.14 0.83 1.29
vitrinite liptinite inertinite mineral Ro,maxe(%)
22.8 2.2 68.1 6.9 0.43
a
Air-dried basis. bMineral matter free basis. cDry ash-free basis. dSulfur and other elements. eMaximum vitrinite reflectance coefficient.
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EXPERIMENTAL SECTION Experiment on Coal Wettability. Sample Preparation. To discuss the characteristics of the material composition of low-rank coals, a coal sample was collected from the Yanan Formation in the middle Jurassic in the southern Ordos Basin, China. Specifically, seven samples of the No. 4 coal seam were taken directly from the working face of 40105 in the Dafosi coal mine. In this work, vitrinite maximum reflectance values (Ro,max) were determined by reflected light optical microscopy using oil immersion. The reflectance measurements and maceral analyses were performed in accordance with Chinese National Standard GB/T 8899-1998, and the proximate analysis was performed in accordance with GB/T 212-2008. The elemental compositions of samples were determined in accordance with Standard GB/T 476-2001. As shown in Table 1, the results of physical components indicated that the coal sample was a typical low-metamorphic long-flame coal, which showed the characteristics of high inertinite content (68.1%) and low vitrinite content (22.8%), and the average exinite (liptinite) content was only 2.2%. The moisture content on an air-dried basis was 4.32%, the volatile yield on a dry ash-free basis and the fixed carbon content were high (31.9% and 59.3%, respectively), and the average ash content was low (8.54%). The ultimate analysis showed that the average carbon content of samples was the highest (83.55%), the average contents of oxygen, hydrogen, and nitrogen were 10.14, 4.19, and 0.83%, respectively, and the sulfur and other elements were only 1.29%. Wettability can be evaluated through various methods, of which the CA measurement can be used to quantitatively assess coal wettability.46−48 Meanwhile, there are many approaches49−52 to measure the CA. In this experiment, cube lump coal samples with a size of 3 × 3 × 2 cm were prepared to ensure that samples had the same stratification direction and similar surface properties. To compare wettability and adsorption/desorption characteristics of coal samples treated with different surfactants, three kinds of wetting agents were selected in this work (Table 2).
There are many studies on the relationship between wettability and moisture content. Wei and co-workers19 pointed out that the higher the moisture content of wood, the smaller the initial contact angle. Liu and colleagues20 deemed that soil wettability is strongly affected by the soil water content. In turn, wettability influences water distribution in soil pores and thereby soil water retention characteristics. As for coal wettability, generally, increased moisture content is believed to improve coal dust wettability,21,22 but Hu23 believed that the moisture content has little effect on the wettability. Methane adsorbability of low-rank coal is mainly controlled by the moisture content, and the strong hydrophilicity leads to a high moisture content ranging from 2.41 to 13.51%, with an average of 6.81% in low-rank coal.24 Additionally, our previous research4 showed that the moisture content is influenced by the coal wettability, and the relationships among wettability, moisture content, desorption rate, and recovery efficiency are complicated, which is restricted by the critical water content and the critical temperature. Previous studies have mainly focused on the effect of moisture on the coal matrix, and water molecules were adsorbed primarily onto the surface of coal matrix by hydrogen bonds and van der Waals interactions; therefore, the methane adsorption of coal was physical.14,25−28 The influence of moisture content on methane adsorption of coal is significant when moisture content is low, but the influence decreases as moisture content exceeds a certain critical level.29−35 One explanation is that liquid water can lead to the blockage of pore-fracture channels by forming water film or droplets and then inhibiting methane desorption.36−38 However, in the past, it had been found that liquid-phase water may promote methane adsorption because water film can provide effective adsorption sites for methane.39,40 Additionally, reported studies41−43 suggest that the physical composition of coal, types and quantity of oxygen-containing functional groups, and surfactants greatly affect coal wettability by influencing the contact angle (CA). To efficiently ensure fracturing fluid flows back in the development of CBM, some methods,41,44,45 including screening surfactants and wettability modification, are applied for increasing the hydrophobicity of the coal matrix. In short, few scholars have determined the essential effect of wettability on the adsorption/desorption of CBM, judging from experimental analysis. This work selected typical low-rank coal samples treated with surfactants, and CBM adsorption/desorption experiments and wettability tests on coal−water were conducted. Then, several adsorption/desorption parameters and replacement parameters of CBM were defined and calculated. Furthermore, the behavior and mechanism of CBM desorption were analyzed comprehensively and quantitatively. Finally, the effect of wettability on adsorption/desorption of CBM was revealed.
Table 2. Category of Surfactants Selected in the Test of CA category fluoroalkyl silane nonionic surfactants anionic surfactants distilled water production water B
code name G502 6501 LAS DW PW
full name 12 fluoroalkyl trimethoxysilane coconut fatty acid diethanolamide sodium dodecyl benzenesulfonate − −
principal component C14F12H2OSiO3 C16H33O3N C18H29NaO3S H2O H2O and minerals
DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Anionic and nonionic surfactants can effectively reduce the surface tension of a solution and enhance hydrophilicity.53 The former is named sodium dodecyl benzenesulfonate (LAS), and the latter is coconut fatty acid diethanolamide (6501). Anionic surfactants generally dissociate hydrophilic anions in solution, and anions have surfactivity partially. Although LAS has a long carbon chain and a benzene ring with unfavorable hydrophilicity, it has a strong water-soluble acid group named sulfonate. Moreover, LAS is easily soluble in water, which can effectively reduce the surface tension of water and increase coal hydrophilicity. On the contrary, nonionic surfactants (6501) do not produce ions in aqueous solution, but 6501 has two hydrophilic structures of hydroxyl group (−OH) and ether bond (R−O−R′). Cun41 considered that the alkanolamide surfactants can effectively improve the coal wettability. The 12 fluoroalkyl trimethoxysilane (G502), a wettability reversal agent made by Harbin Xuejia fluorine silicon Chemical Co., Ltd., is often used as a modifier for superhydrophobic materials, and it combines the advantages of organosilicon and organic fluoride, with low surface energy and good hydrophobicity. Additionally, distilled water and the production water of a CBM well in Dafosi minefield were selected as the experimental blank references. For preparing block coal samples containing equilibrated moisture, a metallographic specimen polishing machine was used to make the surface of a block coal sample smooth and uniform. Considering the original state of the coal reservoir, the polished block samples saturated with distilled water were put into a dry dish with saturated K2SO4 solution, and finally, block samples containing equilibrated moisture were finished. However, G502 was difficult to prepare as an aqueous solution because of strong hydrophobicity. Therefore, absolute ethyl alcohol was selected as a solvent, and then block coal samples were immersed in the solution for 2 h, after which samples were dried at 50 °C for 1−1.5 h until the ethanol completely evaporated to eliminate its influence on coal wettability. Finally, the hydrophobic block samples were finished. Apparatus and Principle. In 1805, Thomas Young54 was the first to describe the concept of CA and wettability. Conventionally, it describes the behavior of a liquid droplet on a solid surface in the air and is defined as the angle between tangents at the three-phase point and solid surface. The wetting process of coal refers to the fact that the liquid wets the surface of a coal matrix and replaces the adsorbed site occupied by a gas and then spreads on the coal surface. Briefly, the wetting process is when a solid−gas interface is replaced by a solid−liquid interface and a liquid−gas interface. Generally, solid surfaces with CAs < 90° are deemed hydrophilic, while surfaces with CAs > 90° are hydrophobic (Figure 1). The CA value (θ) can be calculated by the Young equation.55
The OCA 20 type of optical contact angle measuring device produced by German Dataphysics Co. was employed in this experiment, and the static CA measurement method was applied.56 The droplet solution was set up to 4 μL, and the sample table was raised when measuring to touch the suspended droplets to prevent gravity errors. When a droplet dropped on the coal surface, it stayed for 12 s to measure the CA.57,58 The CA of each coal sample was measured at different surface positions, and finally, the average value was taken as the final one. Experiment of Adsorption/Desorption of CBM. Sample Preparation. Coal samples of isothermal adsorption/desorption experiments were divided into three types: airdried, moisture-equilibrated, and different surfactants. The raw and block coal samples were made into a standard powder with a grain size of 0.23−0.18 mm according to the national standards of China.59 The air-dried and moisture-equilibrated samples were prepared in accordance with relevant standards.59−61 The test of equilibrated moisture content could only partially recover the corresponding decomposition water, combined water, and adsorbed water, while free water could not be recovered under the condition of 30 °C and 96% relative humidity.62 The equilibrated moisture content tested in this work was 11.92%. Prior to the preparation of coal samples treated with surfactants, pulverized coal samples needed air-drying treatment. Because different concentrations of surfactants have different wetting effects, different surfactants adopted a critical micelle concentration (cmc) to control variability and reduce error. The cmc creates a dramatic change in several physical properties of a surfactant system including osmotic pressure, light scattering, molar conductivity, and interfacial (surface) tension. The 6501 and LAS used distilled water as the solvent, and the volume concentration was adjusted to final concentrations of 0.61 and 0.63%, respectively. In addition, it was ensured that the moisture content of coal samples treated with surfactants was the same as previous moistureequilibrated samples. For coal samples treated with wettability reversal agents, G502 was first dissolved in anhydrous ethanol, and the cmc was 1%. The solution was sprayed into the pulverized coal samples, after fully stirring and wetting, and then samples were properly dried to guarantee that the ethanol completely evaporated in order to prevent any influence on coal wettability. Additionally, the He, CH4, and N2 for experiments were supplied by Xi’an Changte Gas Co., Ltd., with purities of 99.999, 99.99, and 99.999%, respectively. Apparatus and Principle. At present, a volumetric method (static adsorption method) is used to study adsorption characteristics of methane in the CBM industry. As shown in Figure 2, the isothermal adsorption/desorption experiments of CBM were accomplished by employing an AST-2000 simulated experimental apparatus, which is mainly composed of a host control system, thermostat system, measurement system, high-pressure gas supply system, and vacuum system. The maximum pressure of the instrument reached 25 MPa, the temperature could be adjusted in the range 0−80 °C, and the precision of the constant temperature was ±0.2−0.5 °C. The experimental data of CBM isothermal adsorption can be analyzed by the adsorption theory of a single molecular layer,
Figure 1. CA measurement equipment based on droplet image analyzing method. (Adapted from ref 48. Copyright 1964 American Chemical Society.) (a) θ < 90°; (b) θ> 90°. C
DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 2. Experimental setup of AST-2000 for CH4 adsorption/ desorption.
Figure 3. CA measurement images of the block coal sample. Samples: (a) DW; (b) PW; (c) 6501; (d) LAS; (e) G502. The volume concentrations of 6501, LAS, and G502 were 0.61, 0.63, and 1%, respectively.
and adsorption characteristic parameters were calculated by the Langmuir model (eq 1).28,63,64 Va =
VLP P + PL
G502. The specific chemical mechanism would be revealed in our follow-up study. Through the comparison between 6501 and LAS (Figure 4c), it was found that increasing the concentration of surfactant solution could make CA decrease, and this exhibited a trend of rapid decline followed by a gradual decline. At the beginning, the CA value decreased rapidly with the increase of surfactant concentration, and the variation was very large. However, when the surface tension of a surfactant stopped changing, the concentration of the surfactant increased to a constant.67−69 However, the solute (surfactant) in the solution still acted on the coal matrix. The higher the concentration of the solution, the stronger the interaction force between the surfactant and the coal macromolecule, and ultimately, the interfacial tension of solid−liquid decreased. As the CA value continued to decrease, it eventually became stable near a fixed value. Combined with the overall data on droplet shape and CA value, 6501 and LAS can effectively reduce surface tension and improve water wettability, and LAS is better. Compared with the distilled water, LAS and 6501 reduce the CA by an average of 10−20%, which also means that the coal hydrophilicity increases by 10−20%. However, G502 has good hydrophobicity and increases the CA by more than 1-fold, which also means a 1-fold increase in hydrophobicity. Adsorption/Desorption Characteristics. The fitting results of the CBM adsorption/desorption of coal samples treated with surfactants at the actual coal reservoir temperature of 23 °C are shown in Table 3 and Figure 5, which reveal that coal samples treated with surfactants represent the irreversible characteristics of the adsorption/desorption process. The maximum adsorption capacity of the desorption process is greater than that of the adsorption process, which can be called desorption hysteresis because this phenomenon is related to the physical and chemical effects of temperature, pressure, moisture, and capillary condensation during the desorption process. In the range of 0−8 MPa pressure, the methane adsorption capacity increased with the increasing pressure, and methane did not absorb saturation at the high-pressure stage. Overall, the adsorption capacity from large to small can be ordered as follows: G502, air-dried, 6501, moisture-equilibrated, LAS. The wettability reversal agent (like G502) was conducive to methane adsorption (gas wettability), while LAS was the opposite, and it might be beneficial for methane desorption from the adsorbed state to the free state. The adsorption
(1)
where Va represents the adsorption capacity at a methane pressure P in cm3/g; VL is the maximum adsorption capacity in cm3/g; PL is the Langmuir pressure constant that represents the methane pressure at the half-maximum of the total adsorbed capacity in MPa. In addition, the desorption experiment based on the adsorption experiment was a cyclical process of depressurization−equilibrium−depressurization−equilibrium. Because of the desorption hysteresis, a desorption model (Langmuir+C) proposed by Ma and co-workers65,66 (shown in eq 2) was used to calculate the related desorption parameters. Vd =
VL′P +c P + PL′
(2) 3
where C represents the residual adsorption capacity in cm /g; as for the desorption process, Vd, VL′, and PL′ have similar meanings in comparison to the adsorption process. The experimental equilibrium pressure was set at approximately 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, and 8.0 MPa. The temperature was set at 23 °C (the actual coal reservoir average temperature), and the adsorption/desorption equilibrium time was 12 h.
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RESULTS AND DISCUSSION Wettability Characteristics. The images of the CA in the test can be seen in Figure 3. Figure 4a shows that the average CA of distilled water (49.6°) was significantly smaller than the average CA of production water (77.8°); this is because production water contains many minerals, and the hydrochemistry of the Dafosi minefield is dominated by Cl−Na+, which results in the high salinity and high surface tension of groundwater. As a wettability reversal agent, the CA of G502 did not change much over with time, which showed good stability and hydrophobicity. With the increase of the G502 concentration, the CA value changed sharply in two stages: first, when the volume concentration was below 5%, the CA value increased followed by a rapid decrease with the increasing concentration. Second, the CA increased rapidly when the volume concentration exceeded 5% as shown in Figure 4b. The reason why the G502 shows such a change trend is due to the addition of ethanol, we think; the amount and concentration of ethanol may affect the hydrophobicity of D
DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. Comparison of CAs of coal samples treated with surfactants. Samples: (a) DW and PW; (b) G502; (c) 6501 and LAS.
Table 3. Comparison of Adsorption/Desorption Parameters adsorption process 3
desorption process 2
VL′ (cm /g)
PL′ (MPa)
C (cm3/g)
R2
13.835 13.832 22.745 8.905 4.273
5.376 2.101 6.944 2.604 1.761
1.014 0.138 5.569 2.790 1.204
0.992 0.997 0.997 0.984 0.983
3
samples
VL (cm /g)
PL (MPa)
R
moisture-equilibrated air-dried G502 6501 LAS
16.511 15.462 26.984 12.459 5.627
6.536 3.344 4.444 2.841 2.016
0.999 0.996 0.994 0.999 0.985
moisture (or hydrophilicity) inhibited methane adsorption, and the higher hydrophilicity, the more unfavorable methane adsorption was to that sample, which is in agreement with previous research.4 Analysis of the data in Table 3 shows that the maximum adsorption capacity from large to small can be ordered as follows: G502, moisture-equilibrated, air-dried, 6501, LAS. Compared with previous experimental conditions, the adsorption capacity of moisture-equilibrated samples under the limiting conditions was strong, even larger than that of airdried samples. This is probably because higher pressure makes combined water or capillary water easily form a water film on the surface of a coal matrix, which could also provide certain effective adsorption sites for methane adsorption. Li and coworkers70 found the adsorbed water molecules in the moistureequilibrated samples can interact with the hydrophilic functional groups of the coal matrix to form a layer of hydration film, which increases the coal wettability and benefits the adsorption of methane molecules. Furthermore, when the pressure increases, the solubility in water and diffusion speed of methane will increase significantly, and water molecules could envelop methane and benefit their adsorption. The
Figure 5. Comparison of isothermal adsorption/desorption curves. Samples: (1) G502; (2) air-dried; (3) 6501; (4) moistureequilibrated; (5) LAS.
capacity of air-dried samples was stronger than that of the moisture-containing samples. This indicates the existence of E
DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. Changes of different desorption parameters under different pressures: (a) depressurized desorption capacity; (b) desorption hysteresis; (c) desorption efficiency.
experiments of CBM in grams. The meanings of Vd and Va are the same as those in eqs 1 and 2. Theoretical desorption rate refers to the ratio of desorption capacity and saturated adsorption capacity when pressure drops to an exhausted pressure during depressurization (eq 4).
Langmuir volume (VL) of the moisture-equilibrated samples is equivalent to or slightly higher than that of the air-dried samples under the higher pressure stage,39,40 which is consistent with the results in this work (shown in Table 3). Therefore, although the ability of water molecules to adsorb a coal matrix is stronger than that of a methane molecule, the increase in pressure and the existence of hydrated membrane cause the saturated adsorption capacity of a moistureequilibrated sample to be more than that of an air-dried sample. Difference of Methane Desorption. Concept and Definition. In part, the inhibitory effect of moisture on CBM adsorption can be viewed as a relative promotion of methane desorption. The desorption rate, critical desorption pressure, recovery efficiency, desorption hysteresis, and the defined depressurized desorption capacity and desorption efficiency were selected to study the influence of moisture and different surfactants on methane desorption. Depressurized desorption capacity can be defined as the value of the difference between the adsorption and desorption processes of CBM for a pressure point. It is also considered to be the volume of methane desorption from the surface of a coal matrix due to simple depressurization. The depressurized desorption capacity can be calculated by eq 3. Q d = (Vd − Va)m
ij V − ζ = jjj L j VL k
ij c yzz c yzz zz × 100% = jjj1 − z × 100% z j VL zz{ { k
(4)
where ζ represents the theoretical desorption rate of CBM in percent. Desorption hysteresis,71−73 defined as the difference between adsorption and desorption isotherms, can be quantitatively characterized by a hysteresis index. Some methods are available to analyze hysteresis behavior.74−77 This work adopted a common method, which can be expressed as eq 5. δ=
Vd − Va × 100% Va
(5)
Theoretical recovery efficiency can be defined as the ratio of desorption capacity and maximum adsorption capacity when pressure is reduced to an exhausted pressure. Only by drainage for depressurization can the pressure value corresponding to methane desorption from the surface of a coal matrix be called the critical desorption pressure. Critical desorption pressure and theoretical recovery efficiency can be obtained according to eqs 6 and 7.
(3)
where Qd represents the depressurized desorption capacity in the process of isothermal desorption of CBM in cm3; m stands for the quality of coal samples in adsorption/desorption F
DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Pcd =
PLVm VL − Vm
η=1−
Pad(PL + Pcd) Pcd(PL + Pad)
desorption hysteresis at low pressure was large, but at high pressure was small. This is because the sensitivity of methane desorption was weak at the high-pressure stage. VL in the desorption process was close to VL′ in the adsorption process, so the hysteresis was not obvious under a specific pressure drop. However, the sensitivity of desorption was strong at the low-pressure stage, desorption hysteresis was evident when the pressure was reduced, and wettability differences led to a different desorption hysteresis. Adsorbed methane of coal samples treated with G502 was the main body resulting from strong gas wettability. The specific pressure drop was insufficient to facilitate fast methane desorption and, conversely, even cause the phenomenon of methane readsorption. Compared with air-dried coal samples, LAS, 6501, and moisture-equilibrated samples with higher moisture content had higher water wettability, which could result in free water, which occupies many adsorption sites, while gaseous water promotes methane desorption through competitive adsorption, and the desorption hysteresis rate is relatively low. That is why they had the lower desorption hysteresis. The effect of a pore on the binding of methane molecules can be excluded because the same original coal sample was used. Therefore, the change of desorption hysteresis for the same coal sample was mainly controlled by pressure, and the difference between different types of coal samples was mainly due to a wettability difference. Analyses of Limit Desorption Parameters. The limit desorption parameters can be calculated by eqs 4, 6, 7 based on the value of the Langmuir parameters PL and VL. Figure 7
(6)
(7)
where η represents the theoretical recovery efficiency of CBM in percent; Pcd and PL stand for critical desorption pressure and Langmuir pressure in MPa, respectively. Additionally, Pad is exhausted pressure; generally its value is 0.70 MPa according to the experience of CBM production in the United States. VL is the Langmuir volume (maximum adsorption capacity). Vm is the practical gas content of CBM, and its mean value is 4 cm3/ g for the No. 4 coal seam in the Dafosi minefield. Desorption efficiency is the rate of methane desorption at a specific pressure drop and time during depressurization. The adsorption/desorption equilibrium time of CBM in this work was measured over 12 h (720 min). Desorption efficiency can be expressed as eq 8. ν=
Qd 720m
=
Vd − Va 720
(8)
where ν represents the desorption efficiency of CBM in cm3/ (g·min). Analyses of Desorption Parameters. Figure 6a,c shows that the values of depressurized desorption capacity and desorption efficiency of different coal samples showed a logarithmic decrease with the increasing pressure. Compared with the relatively low pressure stage (less than the critical pressure of methane), the specific pressure drop in the relatively high pressure stage (more than the critical pressure of methane) had no significant effect on methane desorption. Because the depressurized desorption capacity involved only adsorbed gas in a coal matrix, coal samples treated with G502 best facilitated methane adsorption. The more methane adsorbed the more methane desorbed, relatively, and the maximum desorption efficiency of G502 corroborated this view (Figure 6b). In addition, for changing pressure, the desorption efficiency was almost the same as the depressurized desorption capacity because higher desorption efficiency showed that the depressurized desorption capacity and desorption rate, which are controlled by desorption time and quality of coal samples, were relatively high under the action of specific pressure. This was also reflected by desorption efficiency and depressurized desorption capacity were positively correlated, but negatively correlated with desorption time. Additionally, there was a positive correlation between desorption rate and desorption efficiency, in theory. The adsorption water ratio and moisture content of low-rank coals are more than those of high-rank coals because of the greater number of polar functional groups, good wettability, high porosity, development of capillaries, and the large internal surface area of low-rank coals. In the process of CBM desorption, various forms of water combine with oxygen on the coal surface, which will block pore throats and reduce the effective desorption capacity and diffusivity ability and lead to desorption hysteresis. This phenomenon (hysteresis behaviors) can be attributed to (I) the tensile strength effect of the adsorbed phase (the instability of meniscus condensation inside pores), (II) the interconnected pore features, and (III) the potential existence of an “ink-bottle” pore.78 Figure 6b shows that the value of desorption hysteresis showed a logarithmic decrease with the increasing pressure, and
Figure 7. Comparison of limit desorption parameters of different coal samples.
shows that critical desorption pressure and theoretical recovery efficiency of coal samples treated with LAS, in comparison to air-dried coal samples, were increased by nearly 3-fold and 1fold (325.8 and 92.8%), respectively, and the values were 4.97 MPa and 63.8%, respectively. The effect of samples treated with 6501 was not as obvious as that of samples treated with LAS, while samples treated with G502 were the opposite, as their corresponding critical desorption pressure and theoretical recovery efficiency decreased by 33.7 and 75.2%, and their values were 0.77 MPa and 8.2%, respectively. In addition, the theoretical desorption rate of air-dried samples reached 99.1%, but the theoretical desorption rates of coal samples treated with 6501 and LAS were lower than 80%, which is because the desorption G
DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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promoting methane desorption partially. Conversely, coal samples treated with G502 played the role of a negative effect of replacement, which showed that the wettability reversal agent is not conductive to moisture adsorption but is beneficial to methane adsorption or gas wettability.79 Figure 8 shows that the replacement capacity and replacement rate of different samples are increased with
hysteresis of air-dried samples was smallest, while these of coal samples treated with 6501 and LAS were largest when the corresponding critical desorption pressure was stable at a relatively low pressure stage. Effect of Replacement. Concept and Definition. Previous studies have shown that moisture is more easily adsorbed on the surface of a coal matrix than methane. Therefore, moisture often occupies more effective adsorption sites and hinders methane adsorption, a phenomenon called the replacement effect, which means that moisture makes methane tend to desorption or transform from an adsorption state to a free state. The replacement effect can be described by replacement capacity (or replacement intensity) and replacement rate in this work. Replacement capacity, also known as replacement intensity, mainly refers to the amount of free gas derived from replacement of adsorbed gas due to the entry of an external gas or liquid substances, which is also equivalent to the amount of methane displacement per unit of coal mass. Replacement capacity can be expressed as eq 9. Q=
q − q2 Δq = 1 |m1 − m2| |Δm|
(9) 3
where Q represents the replacement capacity in cm ; q1 represents the adsorption capacity of moisture-equilibrated samples or other different types of wetting in cm3/g; q2 represents the adsorption capacity of air-dried samples in cm3/g; m1 is the coal mass of moisture-equilibrated samples or other different types of wetting in grams; m2 is the coal mass of air-dried samples in grams. The larger the replacement capacity was, the higher the replacement intensity was, and the larger the negative absolute value was, the more adsorbed methane molecules were dissociated into a free state. The plus−minus of the index had no numerical significance. The negative replacement capacity represents the positive effect of replacement, which means adsorbed methane molecules could effectively desorb into a free state. Conversely, the positive replacement capacity represents the negative effect of replacement, which is only fit for the G502 (wettability reversal agent) in this work. That means more free methane molecules were transformed into a desorbed state. Replacement rate is the efficiency of methane replaced by moisture, and it includes the replacement rate of methane for moisture-equilibrated samples and different types of wetting samples in comparison to air-dried samples. The plus−minus of the replacement rate also has no numerical significance, and physical meaning is analogous to replacement capacity. The replacement rate can be expressed as eq 10. q − q2 ξ= 1 × 100% q2 (10)
Figure 8. Changes of replacement capacity under different pressures.
increasing pressure, of which samples treated with LAS is the largest, and its lowest replacement rate is more than 45%. Therefore, samples treated with LAS have the best methane replacement effect. Except for moisture-equilibrated samples, the absolute value of replacement capacity and replacement ratio under the conditions of the same temperature and pressure, from large to small, can be sorted as follows: LAS, G502, 6501. The increasing pressure facilitates methane adsorption, and at the same time, it is more conducive to the adsorption of water molecules, which makes water molecules occupy adsorption sites more efficiently. Furthermore, the induction effect of hydrophilic surfactants strengthens the ability of water molecules to combine with the coal matrix, and ultimately, under the condition of increasing pressure, leads to the fact that the better water wettability is, the larger the replacement capacity and replacement rate are. Figure 9 shows that moisture-equilibrated samples are different from other types of coal samples: replacement rate decreases with the increase of pressure, which indicates that with the pressure increase, the effect of methane replaced by moisture without surfactants gradually increased in the context of overall weakening. It is proved again that the induction function of surfactants has a significant effect on the adsorption of water molecules and has a certain effect on the co-acting force of methane−water. Effect of Wettability on CBM. Migration Form of Water. Many researchers80−84 believe that water molecules are more easily adsorbed on the surface of a coal matrix than methane molecules because many oxygen-containing functional groups of low-rank coals can form hydrogen bonds with water molecules, which have a resistance to methane adsorption. However, they have paid little attention to the influence of the water phase state on adsorption/desorption of CBM, and the mechanism of depressurized desorption is only the external consequence for replacement desorption; inversely, the
where ξ represents the replacement rate in percent; the meanings of q1 and q2 are the same as in eq 9. Analyses of Replacement Parameters. The negative replacement capacity and replacement rate, as for different coal samples including the LAS, 6501, and moistureequilibrated, were confirmed as the positive effect of replacement, which showed that moisture or moisture affected by surfactants had a significant influence on methane adsorption, and moisture inhibited methane adsorption while H
DOI: 10.1021/acs.iecr.8b01932 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
and wetting area of liquid water. Coal is a heterogeneous and porous medium; main forms of existing liquid water in coal are free water (gravity water), capillary water, and bound water (surface attached water film). In the process of wetting coal, water is transported mainly by a phase change mass transfer (i.e., liquid water vaporized into gaseous state).86 Free water does not easily enter a micropore without the aid of surfactants, but the evaporation of liquid water on the surface of a coal matrix resulting from water steam pressure will produce a difference in humidity inside and outside the coal matrix, which could cause that internal moisture of the coal matrix to migrate from high vapor pressure to low water vapor pressure (the outer surface of matrix). When the evaporation rate of water on the outer surface is greater than the migration rate of internal moisture, then the evaporation gradually goes deep into the interior of the coal matrix, and finally forms two ways of the wetting migration of liquid water and the diffusion of gaseous water (vaporization). The evaporation of water in a coal matrix is mainly manifested in water vaporization and migration in capillary pores. According to the research of Pu et al.,87 when the pore radius of a capillary was more than 100 nm, pore water happened to evaporate and detach within the range of conventional temperatures in the process of evaporation, but pore water with a pore radius of less than 100 nm was reserved in liquid form or wetted migration. Mercury intrusion analyses and liquid nitrogen adsorption method were used to measure pore size distribution of coal samples. Specifically, MIP measurements were performed using a Micromeritics Autopore 9505 porosimeter, following the Chinese National Standard GB/T 21650.2-2008 test method. The low-temperature N2 isotherm adsorption/desorption analysis was performed with a Micromeritics ASAP-2020 automated surface area analyzer, complying with the same standard (GB/T 21650.2-2008). Analysis results showed that the main pores of low-rank coal in the Dafosi minefield were mainly micropores (
