A Molecular Simulation Study on Methane Adsorption Capacity and

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A Molecular Simulation Study on Methane Adsorption Capacity and Mechanism in clay minerals: Effect of Clay Type, Pressure and Water Saturation in Shales Wei Li, Xiongqi Pang, Colin Snape, Bo Zhang, dingye zheng, and Xue Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03462 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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A Molecular Simulation Study on Methane Adsorption Capacity and Mechanism in Clay Minerals: Effect of Clay Type, Pressure and Water Saturation in Shales Wei Li†, *, Xiongqi Pang‡, Colin Snape†, Bo Zhang§, Dingye Zheng‡, Xue Zhang‡ †

Faculty of Engineering, University of Nottingham, Nottingham, NG7 2TU, UK

‡

State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum,

Beijing 102249, China §

Schlumberger Technologies (Beijing)Ltd, Beijing 100000, China

KEYWORDS: Clay minerals; Methane adsorption; Molecular Simulation; Water saturation

ABSTRACT: The methane adsorption behaviors and mechanism in clay minerals are investigated by molecular dynamics and grand canonical Monte Carlo simulation methods. Simulation models of montmorillonite, chlorite and illite with the pore size of 6nm are built based on the characteristics of Yanchang Formation shale samples. The results show that the

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methane adsorption of clay is physical adsorption, for adsorption isosteric heat of methane ranging from 3.62 kJ/mol to 5.77 kJ/mol and less than 42 kJ/mol. The types of clay minerals, pressure and water saturation affect the methane adsorption capacity. The diffusion coefficients of methane in montmorillonite, chlorite and illite are 1.521, 1.635 and 1.693 respectively. The methane adsorption capacity in different clay minerals decreases as the following order: Montmorillonite > Chlorite > Illite. The adsorption area and intermolecular interaction forces are key factors affecting methane adsorption capacity, and they are controlled by crystal structure, chemical composition and physical properties of minerals. The methane adsorption capacity increases as pressure increases and decreases with increasing water saturation. When water saturation increases to 45%, clay minerals almost show no methane adsorption capacity. The water molecule is polar molecule and it has greater van der Waals force and electrostatic force with clay minerals than the methane molecule, the non-polar molecule. Moreover, there are hydrogen bonds between water molecules and no hydrogen bonds between methane molecules. These stronger interaction forces make water molecules occupy more limited adsorption sites and have better adsorption capacity than methane.

1. INTRODUCTION Shale gas is natural gas trapped in fine-grained sedimentary rocks such as mudstone or shale. The main components of shale gas are methane (CH4), ethane (C2H6), carbon dioxide (CO2), hydrogen sulfide (H2S), nitrogen (N2), helium (He) and so on. Methane is the most abundant gas, accounting for 75% ~ 89% of shale gas, and methane is used to simulate shale gas in studies.1, 2 Shale gas are stored as free, adsorbed, and dissolved state.1, 3 The Energy Information Administration (EIA) reports that the global reserve of shale gas is approximately 623 × 1012 m3,


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with a recoverable reserve of 187.6 ×1012 m3. The EIA estimates China’s recoverable shale-gas resource is 31.6 × 1012 m3, which has a recoverable reserve of 24.41 × 101012 m3. 4 Based on the researches on characteristics of shale gas reservoirs in the U.S.A., Curtis and Montgomery estimates that about 20% ~ 85% of the shale gas is stored as adsorbed gas. The adsorbed gas is mainly in nano-pores of the shale.1, 5 Mavor points out that adsorbed shale gas accounts for 61% of the shale-gas in Barnett Shale. 6 The proportion of the adsorbed gas ranges from 63% to 85% in the Yanchang Formation, indicating the adsorbed gas makes up the majority of the total shale gas.7, 8 The total organic carbon content of Yanchang shale is in the range of 0.5% ~ 2.0%, the Ro ranges from 0.6% to 1.3%, the gas adsorption content of most samples exceeds 2.0 ml/g, and the gas adsorption content of shale ranges from 2.2 ml/g to 3.35 ml/g, which means the shale has good adsorption capacity.9, 10 Therefore, investigating the methane adsorption capacity of Yanchang Formation shale is significant for the resource evaluation of Ordos shale gas. The methane adsorption capacity of shale is influenced by multiple factors, such as the total carbon content, kerogen type and maturity, mineral composition, temperature, pressure and water saturation, because of the complexity and heterogeneity of shale reservoirs.3, 5, 11 Component of shale is complex. Organic matter and clay minerals as adsorption carriers, together determine the methane adsorption capacity of shale.12, 13 Organic matter is the main factor affecting the adsorbed gas content in shale, for nano-pores are well developed in organic matter. The content, pore structure, type, maturity and other characteristics of organic matter have been extensively studied.2, 14-16 Clay minerals are another important adsorbent carrier of methane. Clay minerals, whose content can be as high as 50% of the shale, have special crystal structure and develop a large number of micro-mesoporous (< 50 nm), which can provide a large specific surface area and pore volume for the methane storage, especially for the storage of adsorbed methane.13, 17, 18


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The effect of shale clay minerals on methane adsorption is investigated in the Western Canadian Sedimentary Basin. The results show that the clay materials influence adsorption characteristics of shale gas, and clay minerals are capable of adsorbing gas to their surface area. The amount of the surface area is controlled by clay types, and the adsorption behaviors of methane of illite and montmorillonite are related to their clay crystal structures.2, 19, 20 Some studies suggest that clay has a strong adsorption capability, even higher than kerogen, and these studies believe the clay minerals play a vital role in adsorbing gas in the low-organic shale.21, 22 Ji liming et al. indicates that the types of clay minerals greatly affect methane sorption capacity under the experimental conditions. In terms of relative methane sorption capacity: montmorillonite>illite/smectite mixed layer> kaolinite >chlorite >illite.18 Water in clay minerals pores can affect the gas absorption. Clay minerals in shale is hydrophile, and the hydrophilicity of clay minerals makes it easier for water molecules to occupy the adsorption sites and to reduce the pore space for gas adsorption.23 Methane adsorption capacity decreases significantly with the increase of water saturation in shale and coal, mainly because the water molecules compete the adsorption sites with methane molecules.13, 18 But the microscopic mechanism of how these factors affect methane adsorption in clay minerals is not clear. Previous studies2, 5, 18 on methane adsorption are mainly based on macroscopic experiments and theoretical methods. Isothermal adsorption experiments and diffusion experiments are often used to study the adsorption and diffusion characteristics of shale gas respectively. However, only using the experimental methods cannot explain the adsorption behaviors and mechanism from the micro-level.2 The development of molecular simulation provides an effective tool for predicting adsorption performances of gas molecules in complex systems at molecular scale.24-28


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Molecular simulation is a computer simulation method for studying the physical chemistry characteristics of atoms and molecules. It can investigate atomic-scale adsorption phenomena and properties between porous materials and fluid molecules.27, 29 Molecular simulation as a microcosmic theoretical research approach for investigating the adsorption, can reveal the adsorption behaviors and explain the adsorption mechanism based on the simulation results. Mahnaz Firouzi et al. 26 created a relatively realistic three-dimensional pore network by molecular modelling to describe the carbon-based porous structure of coals and analyzed the characteristics of gas adsorption and transport. CH4 and CO2 adsorption in micropores and mesopores of shale and coal are studied by Liu et al. and Mosher et al.25, 29 The pore size can affect the methane adsorption in coal and shale systems, and this impact is characterized and quantified to improve the understanding of methane adsorption in microporous and mesoporous carbon systems by molecular simulation.26, 29 Xiong et al. used molecular simulation method to study the adsorption of methane in organic-rich shale nanopores, and he found the proportion of adsorbed gas decreases as the pressure increases under the same pore size.30 Sharma et al. and Fan et al. investigated the adsorption behaviors of methane and ethane in montmorillonite and illite pores, respectively, using the grand canonical Monte Carlo (GCMC) method.13, 27 Collell et al. and Huang et al. studied the competitive adsorption of methane and ethane on kerogen, using the molecular dynamics and Monte Carlo.31, 32 The calculated results are well fit with the Langmuir model at low pressure. The result shows that molecular simulation is an effective way to study the adsorption capacity of methane. However, the main molecular models built by previous studies mostly are the Single-walled Carbon Nanotube (SWCNT) and Graphite Slit Model, and they are not enough to show the adsorption behaviors of methane on the clay minerals considering the actual conditions.27, 29, 30 Considering actual reservoir conditions,


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including the temperature, pressure, water content, and shale pore structure, and building reasonable molecular simulation models are essential for analyzing the methane adsorption capacity and mechanism in clay minerals. The aim of this article is to build all-atom cylindrical pore simulation models of different clay minerals based on the target area’s sample characteristics, to simulate the methane adsorption behaviors under different simulation conditions and to analyze the methane adsorption capacity and mechanism in clay minerals considering the actual reservoir conditions. The results from molecular dynamics and grand canonical Monte Carlo simulation are used to investigate how the different pressure, types of clay minerals, and water content can influence the adsorption of methane. 2. SAMPLE CHARACTERISTIC Shale samples are collected from the Yanchang Formation in the Ordos Basin, Central China (Figure 1). The Basin is a typical continental oil-bearing basin in the basement of the Mesozoic Craton, which covers an area exceeding 25 × 104 km2 in the western part of Central China (Figure 1A).33 The target area is shown in Figure 1B, and the strata corresponds to Upper Triassic Chang 7 and Chang 9 members of the Yanchang Formation.


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Figure 1. A) A simplified tectonic map of China with the Ordos Basin. B) Location and regional tectonic profile of the Ordos Basin, Central China (modified from Yang et al., 2005) 33 Forty shale samples from the Chang 7 and Chang 9 members of the Upper Triassic Yanchang Formation were collected from seven wells, these samples were taken from a depth of 1130.62m~1796.33m. The average temperature is 328K, the average pressure is 9MPa. Water saturation is in the range of 17%~43%, with an average water saturation of 25.7%. This study chooses 45% as the maximum water saturation of the simulation. X-ray diffraction (XRD) was performed on shale samples to obtain experimental data of mineral composition, and the XRD patterns of the illite-rich, chlorite-rich samples can be seen in figure 2a. The dominant inorganic minerals in the shale are quartz and clay minerals, the quartz content is in the range of 19%~30% (the average is 23.8%), and the clay minerals contents are between 39% and 60% (the average is 53.5%), which mainly includes illite, montmorillonite, chlorite and only a small quantity of kaolinite. Basically, the results of this experiment are consistent with previous studies on Yanchang shale. Jiang et al. found the average content of quartz in Yanchang shale is 27.75%,


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and feldspar and clay minerals are relatively high, with the average content of 26.28% and 42.11%, respectively.10, 34 Studies also showed that the quartz of Chang 7 shale was more than 20%, and clay minerals accounted for 41% to 55%.19, 20, 34 The quartz content of the Yanchang shale is lower than that of North American shale, whose quartz content is bigger than 40%. And the clay minerals content is almost in the same range as that of North American shale, whose content is usually about 40%~ 50%.2, 34 Based on the characteristics of clay minerals and the experimental results, this study chooses illite, montmorillonite and chlorite, as the modelling clay minerals. The pore size and pore structure of the shale samples are analyzed by low-pressure N2 adsorption experiment and Scanning Electron Microscopy (Figure 2).35, 36 The pore size is in the range of 2nm~50nm, and mainly concentrates in the range of 2nm~10nm, the average pore size is 6.38nm, and it is slightly smaller than that of North American shale, whose pore size mainly distributes in the range of 8nm~100nm.37 According to the IUPAC (International Union of Pure and Applied Chemistry) classification standard of shale pores, the pores can be divided into micropores (50nm), and the pores of the Yanchang shales are mainly mesopores and micropores.


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a

b

c

Clay mineral

Y2 1451.53m

intragranular pores

Y5，1387.57m

Figure 2. The components and pores of Yanchang shale. a. XRD patterns of the illite-rich, chlorite-rich samples; b. the nano pores of Yanchang shale; c. the intragranular pores of clay minerals; As shown in Figure 2, the types of shale pores are various. Curtis et al. proposed the shale pores including the layered silicate pores and organic matter pores when studying Fayetteville shale.38 Loucks et al. divided shale pores into intergranular pores, intergranular pores and organic pores.39 According to previous studies of Yanchang shale, the shale pores of Yanchang Formation are mainly divided into intergranular pores, intragranular pores, organic pores and micro cracks, and 77% of the pores are nano pores. The clay minerals in Yanchang shale are


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mainly develop intragranular pores, the quartz and other brittle minerals mainly develop intergranular pores, and seldom micro cracks are developed.34, 40 Base on this, a clay simulation model with a cylindrical hole (diameter=60 Å =6nm) is built to do the simulation. 3. MOLECULAR SIMULATION 3.1 Simulation methods 3.1.1 Grand Canonical Monte Carlo (GCMC) and Molecular Dynamics (MD) Molecular simulation is a method that studies the structure and properties of molecules or molecular system by computer simulations, including Molecular Mechanics, grand canonical Monte Carlo, and Molecular Dynamics simulation and so on.24, 41 In this work, the GCMC and MD are used to study the adsorption behaviors of methane in the pores of clay minerals, the simulation results are used to analyze methane adsorption capacity and mechanism. GCMC is an effective method to study the adsorption behaviors of single component or mixed components gas in clay minerals and organic matter. In the GCMC, the chemical potential, volume and temperature are independent variables. The simulation software used in this study is Materials Studio software42, 43. This study mainly uses MS-Visualizer, MS-Amorphous Cell, MS-Forcite, and MS-Sorption modules. 3.1.2 Force field The molecular force field is a set of potential functions for describing the interaction of molecular systems.24 The common molecular force fields including the polymer consistent forcefield (pcff), consistent-valence forcefield (cvff), Clay forcefield, Dreiding force fields, Universal forcefield, Condensed-phase Optimized Molecular Potentials for Atomistic Simulation


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Studies (COMPASS) force fields and COMPASSⅡ force field.44, 45 COMPASSⅡ force field is adopted in this simulation. COMPASS force field is the first molecular force field which unifies organic molecule system with inorganic molecule system and it is suitable for covalent molecule system, including most organic, inorganic and metal materials. This force field makes rigorous parameterization, where parameters are derived from high level first principle calculation to address the compatibility for the considered model.44, 46 COMPASSII improves the parameters of supercritical methane based on COMPASS. The improved covalent bond model and metal ion model in COMPASSII are suitable for the simulation in this study. Besides COMPASSⅡ, Clayff and Dreiding force field are also widely used in adsorption simulation of shale.47, 48 49, 50 The potential functions of COMPASSII force field mainly include bonded and non-bonded interactions.44, 51 The bond energy is shown as formula (1). 𝐸𝐸𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 = 𝐸𝐸𝑏𝑏 + 𝐸𝐸𝑥𝑥 + 𝐸𝐸𝑡𝑡 + 𝐸𝐸𝑐𝑐 + 𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 (1)

Where, 𝐸𝐸𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏 is the bond energy, 𝐸𝐸𝑏𝑏 is the bond angles bend energy, 𝐸𝐸𝑥𝑥 is the out-of-bond angle plane bend energy, 𝐸𝐸𝑡𝑡 is the bond torsion energy, 𝐸𝐸𝑐𝑐 is the bond compression energy, and 𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 is the cross coupling energy.

The non-bond energy (𝐸𝐸𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 ) include Van der Waals energy (𝐸𝐸Van ), Coulomb energy

(𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 ) and hydrogen bond energy (𝐸𝐸𝐻𝐻 ). The Lennard-Jones (9/6) potential function is used to show Van der Waals energy (𝐸𝐸𝑣𝑣𝑣𝑣𝑣𝑣 ). They are shown as formula (2), (3), (4). 𝐸𝐸𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛𝑛 = 𝐸𝐸𝑣𝑣𝑣𝑣𝑣𝑣 + 𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 + 𝐸𝐸𝐻𝐻 (2) 𝐸𝐸𝑣𝑣𝑣𝑣𝑣𝑣−𝑖𝑖𝑖𝑖 = ε𝑖𝑖𝑖𝑖 ��

𝜎𝜎𝑖𝑖𝑖𝑖 𝑟𝑟𝑖𝑖𝑖𝑖

9

𝜎𝜎𝑖𝑖𝑖𝑖

6

� − � 𝑟𝑟 � � (3) 𝑖𝑖𝑖𝑖


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𝐸𝐸𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = ∑ 𝑖𝑖𝑖𝑖

𝑞𝑞𝑖𝑖 𝑞𝑞𝑗𝑗 𝑟𝑟𝑖𝑖𝑖𝑖

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(4)

Where, 𝑟𝑟𝑖𝑖𝑖𝑖 is the distance between the atoms i and j, nm; 𝜀𝜀𝑖𝑖𝑖𝑖 are the L-J size; 𝜎𝜎𝑖𝑖𝑖𝑖 is the L-J well depth; 𝑞𝑞𝑖𝑖 , 𝑞𝑞𝑗𝑗 are the charges of the atoms in the system.

The L-J parameters are calculated by the standard Lorentz–Berthelot combining rules: as formula (5), (6): ε𝑖𝑖𝑖𝑖 = �ε𝑖𝑖𝑖𝑖 × ε𝑗𝑗𝑗𝑗 (5) 𝜎𝜎𝑖𝑖𝑖𝑖 =

𝜎𝜎𝑖𝑖𝑖𝑖 +𝜎𝜎𝑗𝑗𝑗𝑗 2

(6)

3.1.3 Statistical ensembles Ensembles are used to describe the statistical regularity of thermodynamic systems. In the MD simulation, ensembles are set to control the equilibrium conditions of particles.52 The most common ensembles are micro-canonical ensemble (NVE), canonical ensemble (NVT), macrocanonical ensemble (mVT), isothermal isobaric ensemble (NPT), isobaric isoenthalpy ensemble (NPH).24 In this study, the MD simulation uses NVE and NVT, and the GCMC simulation uses the mVT. The heat exchange and particle exchange can be carried out in mVT, which is an open system. The chemical potential (m), volume (V) and temperature (T) of each sample in the system remain unchanged, while the energy (E) and particle number (N) are variable. 3.2 Construction of molecular simulation models Building the rational models based on the structural characteristics of methane, water and clay minerals, is the most basic and important step in molecular simulation. This paper chooses all-


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atoms model, which can express all atoms and bonds of adsorbents and adsorbates equally. The adsorbates include methane and water; the adsorbents are the porous clay minerals. The molecular models of adsorbates are established based on molecular structure and element composition of methane and water molecular (Figure 3). The structural parameters of methane and water molecules are shown in the Table 1.

C

H

O

Figure 3. The molecular models of methane and water Table1. Parameters of methane and water Adsorbate

Bond length/ Å

Bond angle/°

Methane

1.099

109.471

Water

0.957

104.563

The crystal cells of the clay minerals are derived from inorganic crystallography database (ICSD2009). The detailed cell parameters of montmorillonite (Al4Ca2[Si8O22] (OH)2), chlorite (Mg4FeAl [AlSi3O10] (OH)8) and illite (K[Al4Si2O9] (OH)3) can be found in the work of Viani et al., Zanazzi et al. and Gualtieri et al. respectively53-55. The crystal structures and the cell parameters of the three minerals are shown in Figure 4 and Table 2.


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8Å

8Å

8Å

a. Montmorillonite

C

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H

c. Illite

b. Chlorite O

Al

Si

Ca

K

Mg

Figure 4. Crystal structures of Montmorillonite, Chlorite and Illite. Color scheme: dark gray is the carbon atom; light gray is the hydrogen atom; red is the oxygen atom; purple is the aluminum atom; yellow is the silicon atom or the silicon-aluminum atom in chlorite; green is the calcium atom; dark purple is the potassium atom and yellow green is the magnesium or the magnesiumiron atom in chlorite. The simulation supercell models of adsorbents are rectangular boxes who have periodic boundaries in the X, Y and Z directions, and these models are built based on the crystal cells of adsorbents. The size of the supercell is about 80Å×80Å×30Å in three spatial directions (origin is O, trilateral lengths are OA, OB, OC, corresponding to X, Y, Z direction). The length of the supercell of three minerals is shown in Table 2. A cylindrical pore with a diameter of 60 Å is excavated by deleting the atoms in a cylindrical shape as the pore of this simulation, the bonds and the atoms are exposed equally, and the charge balance is achieved by calculating the residual stoichiometric ratio. This model is built based on the pore characteristics of the samples from Yanchang Formation. The models of adsorbents are shown in Figure 5. Table 2. The parameters of the crystal cell and the simulation models


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Parameters

Montmorillonite

Chlorite

a/Å

5.18

5.327

5.223

b/Å

8.98

9.233

9.018

c/Å

15

14.38

20.14

number of atoms

38

72

76

OA/Å

82.88

79.91

78.34

simulation

OB/Å

80.82

83.1

81.17

model

OC/Å

30

28.76

40.29

Pore Radius/Å

30

30

30

crystal cell

Illite

Figure 5. The molecular models of clay minerals with the cylindrical pores. Color scheme, blue lining is the pore surface; dark gray is the carbon atom; light gray is the hydrogen atom; red is the oxygen atom; purple is the aluminum atom; yellow is the silicon atom or the silicon-


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aluminum atom in chlorite; green is the calcium atom; dark purple is the potassium atom and yellow green is the magnesium or the magnesium-iron atom in chlorite. 3.3 Simulation details Geometry Optimization, Dynamics simulation and GCMC are performed by the Materials Studio software. In this study, the Steepest Descent method is used to optimize the model. Calculation details: Minimization algorithm, Steepest descent; Max iterations, 50000; Summation method, Ewald; Force field, COMPASSII; Cutoff distance: 12.5 Å. Calculation details of Molecular Dynamics simulation: Initial temperature, 300 K; Mid-cycle temperature, 350 K; Simulated temperature, 328k, which is close to the reservoir temperature. Dynamics steps per ramp, 300; Annealing cycles, 5; Ensemble, NVT; Total simulation time, 30 ps (Dynamics simulation has reached equilibrium, shown as Figure 6). The simulation details of GCMC are listed here: Maximum pressure, 40 MPa; Sample pressure, 9Mpa; Temperature, 328K; Equilibration steps,500000; Production steps, 5000000; Force field, COMPASSII; Summation method, Ewald. In the GCMC simulation, the chemical potential is a function of the fugacity rather than the pressure. Therefore, the fugacity is converted to pressure with the fugacity coefficient, which can be calculated by the Peng-Robinson equation.56


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3000

Forecite Dynamics Temperature

2500

Temperature（K）

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2000

1500

1000

equilibrium temperature

500

0

0

5

10

15

20

25

30

Time（ps）

Figure 6. The equilibrium dynamics temperature of montmorillonite

4．RESULT AND DISCUSSION

4.1 Evaluation parameters of methane adsorption capacity Parameters including relative concentration, diffusion coefficient and adsorption isotherm are used to evaluate methane adsorption capacity. They are obtained from MD and GCMC simulation. Relative concentration can characterize the distribution of molecules on each parallel plane in a three-dimensional simulation system. Relative concentration is a dimensionless value. When relative concentration is 2, the number of target molecules on this plane is twice as much as the that of even distribution. When the target molecules distributed evenly in a cylindrical pore space as free state, the relative concentration is shown in Figure 7. There are several molecules near the pore wall and more molecules are distributing in the central part of the pore, showing as a semi-circular arc shape.


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Page 18 of 48

Figure 7. The relative concentration of molecules in evenly distribution state When the target molecules are adsorbed in the pore wall, the concentration is shown as Figure 8. The Figure 8a shows the single-layer adsorption curve, while Figure 8b shows the double-layer adsorption curve. The more molecules are adsorbed, the bigger relative concentration near the wall, and the molecules are lesser near the center of the pore. The curve overall presents a concave shape with relatively high on both sides and relatively low in the middle.

Figure 8. The relative concentration of molecules in adsorbed state


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Energy & Fuels

Diffusion coefficient, as an important index to measure the intensity of micro-particles movement, can quantitatively characterize the degree of molecular diffusion.57 The smaller the diffusion coefficient, the more molecules are existing as the adsorbed state, and the stronger the adsorption capacity.58 The diffusion coefficient can obtain from the plot of the Mean square displacement (MSD) with time, and the coefficient is 1/6 of the MSD-time slope. Isosteric heat and isothermal adsorption curves are also used to evaluate the methane adsorption capacity. Adsorption isosteric heat refers to the thermal effect during the adsorption process. The greater the adsorption isosteric heat, the stronger the adsorption. Adsorption isosteric heat is an important thermodynamic parameter to distinguish chemical adsorption from physical adsorption. It is difficult to detect isosteric heat accurately by experiments, but it is convenient to measure that by molecular simulation method. When the adsorption isosteric heat is greater than 42 kJ/mol, the adsorption is chemical adsorption, and when the adsorption isosteric heat is less than 42 kJ/mol, it is physical adsorption.59 Adsorption isotherms refer to the amount of adsorbed methane under different pressures at the same temperature. The adsorption isotherms of methane can obtain by molecular dynamics simulation. Through isothermal adsorption experiments, many adsorption models have summarized, among which Langmuir isothermal adsorption model is the most widely used model. It can express as:

V = 𝑉𝑉𝐿𝐿

𝑃𝑃 𝑃𝑃 + 𝑃𝑃𝐿𝐿

Where, V is the content of adsorption at a certain pressure; 𝑉𝑉𝐿𝐿 is Langmuir volume, indicating the maximum theoretical content of adsorption; 𝑃𝑃 is the experimental pressure; 𝑃𝑃𝐿𝐿 is Langmuir pressure


19

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Page 20 of 48

4.2 The validation and the effect of pressure on methane adsorption The clay models are validated by comparing the simulation results with the experiment results. Previous studies have shown that the methane adsorption isosteric heat of different minerals is different.30, 47, 60 Feng et al. calculated the thermodynamic adsorption isosteric heat of methane in shale was in the range of 15.5 kJ/mol ~ 17.65 kJ/mol.60 Xiong et al. tested the adsorption isosteric heat of methane in kerogen І and kerogenⅡ were 10.3 kJ/mol and 21.9 kJ/mol respectively by molecular dynamics simulation method, which was much higher than that of clay minerals.30 Huang et al. and others analyzed that the adsorption isosteric heat of clay minerals mainly distributed in range of 6.301 kJ/mol ~15.178 kJ/mol by simulation.28 Ji et al. calculated the adsorption isosteric heat of methane in montmorillonite, illite, kaolinite and chlorite based on the isothermal adsorption experiments and they were 16.6, 10.3, 9.6 and 9.4 kJ/mol, respectively.18 The methane adsorption isosteric heat of montmorillonite, chlorite and illite measured in this study mainly distributes in the range of 3.62 kJ/mol ~5.77 kJ/mol (Table 3.), which is less than 42 kJ/mol, indicating methane adsorption of clay minerals is the physical adsorption,59 and this simulation result is consistent with the previous simulation results and isothermal adsorption experiment results.18, 30, 60 This consistency means the models are validated. These studies show the adsorption isosteric heat of the same mineral is different under different experimental conditions. Although the isosteric heat changes with the conditions, the adsorption isosteric heat of methane in kerogen is always higher than that of clay minerals. The deviations of clay minerals between simulation results and the experimental results, mainly caused by the difference of experimental conditions, samples and the simulation models. What’s more, the pore structure of the clay minerals constructed in the simulation are single cylindrical pore, while the experimental results show the pore sizes of montmorillonite, kaolinite, illite and chlorite samples


20

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Energy & Fuels

are bigger than 6.0 nm with different shapes, and the pores are continuous distributed in a wide range.18 Table 3. shows the methane adsorption isosteric heat increases with the increased pressure, indicating there is a positive correlation between the methane adsorption capacity and pressure. The data also shows that adsorption isosteric heat of montmorillonite is higher than that of illite and chlorite under the same pressure condition, and the adsorption isosteric heat of chlorite is bigger than that of illite in most cases, except when pressure is in the low range the heat of illite is bigger than that of chlorite. Table 3. The adsorption isosteric heat of methane in clay minerals under different pressure Pressure/

adsorption isosteric heat at 328 K/ kJ/mol

Kpa

Illite

Chlorite

Montmorillonite

4000

3.92

3.62

5.05

8000

4.26

3.90

4.99

12000

4.17

4.17

5.08

16000

4.12

4.31

5.18

20000

4.32

4.54

5.24

24000

4.39

4.70

5.27

28000

4.47

4.83

5.41

32000

4.53

4.94

5.54

36000

4.56

5.16

5.64

40000

4.59

5.29

5.77

The simulation results of adsorption isotherms show the methane adsorption capacity of clay minerals. We further validate the models by comparing our simulation results of adsorption isotherms with the experimental data. In Figure 9, the experiment results of isothermal adsorption of different clay minerals show that, although there are slight differences of the methane adsorption capacity of these minerals, the curves are still suitable for Langmuir model.


21

Energy & Fuels

The parameters VL of the models are quite different, however, the overall trend of change is almost the same, that is, with the increase of pressure, the adsorption capacity is increasing until to the maximum adsorption.

CH4-isothermal adsorption (T=323.5K)

0.3 Montmorillonite

Adsorbed CH4 mmol /g rock

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 48

Chlorite

0.25

Illite

0.2 0.15 0.1 0.05 0 0

2000

4000

6000

8000

10000

12000

14000

CH4 Pressure/Kpa

Figure 9. CH4 adsorption isotherms on clay-mineral dominated rocks at 323.5K (the data is collected from Ji).18 All the absolute adsorption isotherms in this work are fitted well with the Langmuir model (Figure 10). The models are validated. As shown in the Figure 10, at 328K, the number of adsorbed methane molecules is increasing with the increasing pressure. At the low-pressure stage, the methane adsorption capacity of the three minerals increases rapidly with the increase of pressure and increases slowly when the pressure is at a relatively high state, then it approaches a constant value at last. The rate of adsorption on different minerals are different, the pressure can influence the adsorption capacity at a certain pressure range and pressure has the greatest impact on montmorillonite, followed by chlorite and illite. The reason is that when the temperature is


22

Page 23 of 48

constant, the increased pressure directly leads to the increase of the number of methane molecules in the same volume, providing more adsorbates for adsorbents. The results are coincided with Langmuir's law, indicating that the adsorption simulation model established in this study is reasonable.

CH4-isothermal adsorption (Sw=0%, T=328K)

1200

CH4 average loading /per cell

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Montmorillonite

1000

Chlorite 800

Illite

600 400 200 0 0

5000

10000

15000

20000 25000 Pressure/Kpa

30000

35000

40000

Figure 10. Adsorption isotherms of methane in clay minerals by simulation 4.3 Methane adsorption capacity of the clay minerals The types of clay minerals affect the methane adsorption. Different clay minerals have different chemical composition, structural morphology, crystal structure and particle size.61 In order to investigate the methane adsorption capacity of the montmorillonite, illite and chlorite in Yanchang shale, the MD simulation is conducted considering the reservoir conditions and the sample characteristics (temperature, 328K; pressure, 9Mpa; pore diameter, 6nm). The relative concentration and diffusion coefficient of methane on different clay minerals are obtained.


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As shown in Figure 11, the relative concentration of the three minerals under reservoir conditions (328K, 9Mpa), shows a downward trend when compared with the even distribution curve near the pore center, and has a relatively high value near the pore wall, which means there are more methane molecules adsorbed on the pore wall. All the three minerals can adsorb methane and the methane adsorption ability are different. Figure 11 shows that the relative concentration of montmorillonite is higher near the pore wall, and the highest is 2.25. The relative concentration is much lower at the center of the pore, and the lowest part is 0.5. The relative concentration of montmorillonite shows a more obvious concave shape than that of illite and chlorite, which means montmorillonite has a relatively stronger adsorption capacity. The relative concentration of illite and chlorite near the pore wall is lower than that of montmorillonite, and the concentration value of illite and chlorite near the pore center is about 1.5, less than the value of concentration of 2 when methane is evenly distributed. The concave shape of illite and chlorite is not obvious, which means although the chlorite and illite have the adsorption capacity, they are not as strong as that of montmorillonite.


24

Page 25 of 48

2.5

Montmorillonite (CH4 Sw=0%)

a

montmorillonite-CH4

Relative concentration

2 evenly distributed ideally

1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 2.5

Chlorite (CH4 Sw=0%)

b

chlorite-CH4



1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 2.5

Illite (CH4 Sw=0%)

c

illite-CH4

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

evenly distributed ideally

1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å

Figure 11. The relative concentration of methane on the clay minerals


25

Energy & Fuels

We can calculate the diffusion coefficient from the correlation of MSD and time (Figure 12). The diffusion coefficients of methane in the pore of montmorillonite, chlorite and illite are 1.521, 1.635 and 1.693 respectively. The results show that methane molecules are more easily diffused in illite pores, followed by chlorite and montmorillonite, which also indicate that montmorillonite has the strongest methane adsorption capacity, then chlorite and illite.

90

CH4-MSD (Sw=0%)

Montmorillonite

80

Mean square displacement/Å2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 48

Chlorite

70

y = 10.156x + 1.996

Illite

60

y = 9.8117x + 1.4545

50 40 y = 9.1287x + 1.0023

30 20 10 0 0

1

2

3

4 Time/ps

5

6

7

8

Figure 12. The correlation of MSD and time This conclusion is in line with some previous works17, 18, but is inconsistent with other studies2, 13. Ji et al. found the type of minerals significantly affected the methane adsorption capacity; the content of methane adsorption of montmorillonite is the largest, 8.5 mL/g, followed by chlorite and illite, 2.3 mL/g and 1.9 mL/g, respectively.18 Liu et al. found that the methane adsorption capacity on clay minerals decreased in the following order: montmorillonite > kaolinite > illite. 17

Ross et al. found that the methane adsorption capacity on clay minerals decreased in the

following order: illite > kaolinite > montmorillonite.2 Fan et al. reported that the methane


26

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Energy & Fuels

adsorption capacity on clay minerals decreased in the following order: montmorillonite > kaolinite > illite > chlorite.13 All these studies indicate the clay minerals have the capacity to adsorb methane molecules, but the order of the methane adsorption capacity of the different clay minerals has not yet been definitively determined. The difference may be related to the experimental samples and the experimental conditions.17, 18, 62, 63 The methane adsorption on clay minerals is physical adsorption, without any chemical reaction.30, 62 Therefore, under the same conditions of pore size, temperature, pressure and water saturation in this study, the difference of methane adsorption capacity of the montmorillonite, illite and chlorite is mainly caused by the adsorption area provided by the adsorbents and the interaction forces between the adsorbates and the adsorbents. The surface area and the interaction forces are determined by crystal structure, chemical composition and physical properties of adsorbents. Clay minerals can provide additional surface area for the adsorption of the gas molecules and the surface area are influenced by their structure.64 The interaction forces, including the Van der Waals, electrostatic force, and hydrogen bonds, are also different because the difference of the crystal structure and chemical composition.30 As shown in Figure 13a, 13b, 13c, the crystal structure of the three minerals are different. Most atoms in montmorillonite are concentrated on one side of the crystal cell, while the atoms in chlorite and illite distribute throughout the cell. The difference in crystal structure makes montmorillonite not only has an external surface area like illite and chlorite, but also has a large internal surface area (Figure 13c), some methane molecules can adsorb on the internal surface (Figure 13d,13e). In fact, the internal surface area is much larger than the external surface.20 The interaction force between methane molecules and clay minerals is another important factor


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Page 28 of 48

affecting the methane adsorption of clay minerals. Van der Waals force and electrostatic force are the main interaction forces, and Dai et al. found the contribution of electrostatic force was 36.72%~46.38%, and the contribution of Van der Waals force was 53.62%~63.28%. What’s more, montmorillonite also has higher cation exchange capacity, so its adsorption capacity is higher than that of illite and chlorite.65, 66


28

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a. Supercell of chlorite

external surface

external surface

external surface

internal surface

30Å

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

internal surface

c. Supercell of montmorillonite

b. Supercell of illite

e

d 80Å CH4

H2 O

20Å Pore surface

C

H

O

Al

Si

Ca

K

Mg

Figure 13. The snapshot of crystal structure of minerals and the methane adsorption of montmorillonite. Color scheme: dark gray is the carbon atom; light gray is the hydrogen atom; red is the oxygen atom; purple is the aluminum atom; yellow is the silicon atom or the silicon-


29

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Page 30 of 48

aluminum atom in chlorite; green is the calcium atom; dark purple is the potassium atom and yellow green is the magnesium or the magnesium-iron atom in chlorite. 4.4 Effect of water saturation on methane adsorption Pores and specific surface area are well developed in clay minerals, so that clay minerals can not only adsorb natural gas, but also adsorb other liquids or solids.62, 67, 68 In this section, the results of MD simulations reveal how water influence methane adsorption of clay minerals. The relative concentration of water and methane under different water saturation is calculated, and the results are shown in Figure 14, 15, 16. The simulation results show that water molecules are easier adsorbed on clay minerals than methane molecules, and water saturation can affect methane adsorption capacity of clay minerals. As shown in Figure 14, Figure 15 and Figure 16, when water and methane exist at the same time (Sw=15%, Sw=30%, Sw=45%), more water molecules distribute near the pore wall and less distribute in the middle of the pore, and relative concentration of water is closer to the adsorption state than that of methane, which show water molecules are easily adsorbed on the pore surface than methane. We can find that with the increase of water saturation, the relative concentration (CH4) of montmorillonite, chlorite and illite is changing from adsorption state to evenly distribution state gradually. The methane relative concentration near the pore wall at low water saturation (Sw=0%, Sw=15%) is relative higher than that at high water saturation (Sw=30%, Sw=45%). With the increase of water saturation, the relative concentration of methane near the pore wall becomes smaller and the peak value gradually disappears. The methane molecules are gathered at the center of the pore, and the curve of relative concentration gets closer to the even distribution curve. When the water saturation is 45%, the relative concentration of methane is close to the even distribution curve, which means few methane molecules are absorbed on the surface of clay minerals.


30

Page 31 of 48

3.5



a

3

montmorillonite-CH4

2.5


2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 3.5



b

3

montmorillonite-CH4 evenly distributed ideally montmorillonite-H2O

2.5 2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 3.5



c

3

montmorillonite-CH4 evenly distributed ideally montmorillonite-H2O

2.5 2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 3.5

Montmorillonite (CH4 Sw=45%) montmorillonite-CH4 evenly distributed ideally montmorillonite-H2O

d

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

2.5 2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å

Figure 14. the relative concentration (CH4, H2O) of montmorillonite under different water saturation


31

Energy & Fuels

3.5


a

chlorite-CH4


3 2.5


2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 3.5



chlorite-CH4 evenly distributed ideally chlorite-H2O

b

3 2.5 2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 3.5


c

chlorite-CH4 evenly distributed ideally chlorite-H2O


3 2.5 2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å 3.5

Chlorite (CH4 Sw=45%) chlorite-CH4 evenly distributed ideally chlorite-H2O

d

3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 48

2.5 2 1.5 1 0.5 0

0

10

20

30

40

50

60

70

80

90

Distance/Å

Figure 15. the relative concentration (CH4, H2O) of chlorite under different water saturation


32

Page 33 of 48

2.5

a

Illite (CH4 Sw=0%) illite-CH4



1.5

1

0.5

0

0

10

20

30

40

50

60

70

80

90

Distance/Å 2.5

Illite (CH4 Sw=15%)

b

illite-CH4 evenly distributed ideally illite-H2O


2

1.5

1

0.5

0

0

10

2.5

20

30

40

50

60

70


80

90

Distance/Å Illite (CH4 Sw=30%)

c


2

1.5

1

0.5

0

0

10

20

30

40

50

60

70

80

90

Distance/Å 2.5

Illite (CH4 Sw=45%)

d


2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1.5

1

0.5

0

0

10

20

30

40

50

60

70

80

90

Distance/Å

Figure 16. the relative concentration (CH4, H2O) of illite under different water saturation


33

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Page 34 of 48

The MSD (Figure 17a, 17b, 17c) and diffusion coefficient of methane (Figure 17d) show the same result. Figure 17d shows the average diffusion coefficients of the clay minerals in different water saturation (Sw=0%, Sw=15%, Sw=30%, Sw=45%). The diffusion coefficients of montmorillonite are 1.52, 2.42, 3.09 and 3.59; the coefficients of chlorite are 1.64, 2.45, 2.69 and 5.91; the coefficients of illite are 1.69, 2.83, 3.4 and 6.51respectively. Figure 17d also shows the diffusion coefficients increase with the increase of water saturation in the montmorillonite, chlorite and illite minerals. With the increase of water saturation, the methane adsorption capacity of montmorillonite, chlorite and illite decreases gradually, and the influence of same water saturation on different clay minerals is different. Compared with Sw= 0%, when water saturation is 15%, 30% and 45%, the adsorption capacity of montmorillonite decreases by 38%, 52%, 58%, the chlorite decreases by 33%, 40%, 72% and the illite decreases by 41%, 51%, 75%, respectively. Which means when water saturation increases from 15% to 45%, the adsorption capacity of clay is reduced mainly in the range of 30%~80%, and water has the biggest influence on the adsorption capacity of illite and this result is closed to the experimental value. Feng Dong69 found that when the water saturation was 10%, 20%, 30% and 40%, the adsorption capacity of illite decreased by 7.63%, 34.35%, 86.26% and 87.79%, and adsorption capacity of kaolinite decreased by 7.01%, 12.90%, 22.57% and 38.56%, respectively. The influence of water saturation on the adsorption capacity of illite is more significant.69 Tests on the adsorption capacity of shale in North America2, 3 also show that when water reaches the equilibrium condition, the methane adsorption capacity of organic-rich shale decreases by 20%-60%, and the methane adsorption capacity of clay-rich shale decreases by 65%-90%. Compared with organic-rich shale, the adsorption capacity of clay-rich shale is influenced by water significantly. Under dry conditions, organic matter and clay minerals


34

Page 35 of 48

together determine the methane adsorption capacity of shale, but under water-bearing conditions, methane adsorption capacity of clay (illite, kaolinite, montmorillonite) decreases by 80-95%.2 High clay content in shale is generally considered to be the main reason for the decline of adsorption capacity.12, 63, 70

300

Montmorillonite-MSD (CH4)

160

Sw=0% Sw=15% Sw=30% Sw=45%

140 120



180

y = 21.527x + 2.1969

y = 18.523x + 2.0751

100 80

y = 14.498x + 2.2855

60 40

y = 9.1287x + 1.0023

Chlorite-MSD (CH4) Sw=0% Sw=15% Sw=30% Sw=45%

250 200

y = 35.465x - 0.735

150 y = 16.13x + 2.3291

100

y = 14.695x + 0.7966

50

y = 9.8117x + 1.4545

20 0

0 0

1

2

3

4

5

6

7

8

0

1

2

3

a 350

6

7

8

b 6

Diffusion coefficient/(Å2/ps)

250

5

7

Illite-MSD (CH4) Sw=0% Sw=15% Sw=30% Sw=45%

300

4

Time/ps

Time/ps


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

y = 39.077x + 2.4104

200 y = 20.371x + 0.1744

150

y = 16.966x + 0.796

100 50

y = 10.156x + 1.996

5

6.51 5.91

montmorillonite chlorite illite

4 3 2

3.40 3.09 2.69

2.83 2.422.45

3.59

1.521.641.69

1 0

0 0

1

2

3

4

5

6

7

8

0

Time/ps

c

15

30

45

Water saturation/(%)

d

Figure 17. The MSD and diffusion coefficient of methane. a. the MSD of methane in montmorillonite under different water saturation; b. the MSD of methane in chlorite under different water saturation; c. the MSD of methane in illite under different water saturation; d. the diffusion coefficients of the clay minerals in different water saturation Clay minerals are usually highly hydrophilic, for most clay minerals have polar surface and there are electric charges on the surface. Water molecules can closely combine with clay particles with


35

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Page 36 of 48

hydrogen bonds, electrostatic forces and intermolecular forces.71 Rosset al. believed that with the increase of water, the adsorption gas content was gradually decreasing, and he argued that the presence of water on the clay surface could block the access of gas molecules to the adsorption sites, so the contribution of clay minerals to the adsorption capacity of shales would decrease.2 The dynamic simulation results show that under the same condition, with the increase of water, there are more water molecules adsorbed on the pore surface and occupy more adsorption sites, which can make water molecules form a close-packed water layer, reducing the adsorption sites for methane. In Figure 18, when water saturation is 30%, before the dynamic simulation the water and methane molecules are evenly distributed in montmorillonite pore. During the dynamic simulation, the water molecules are gathering to the pore wall and taking the adsorbed sites, while after the dynamic simulation, most water molecules are adsorbed on the pore wall, forming the water layer and few methane molecules adsorbed on the pore wall. The results show that the adsorption capacity of water is better than that of methane, and the increasing water content reduces the methane adsorption capacity by seizing more adsorption sites.

80Å

80Å

a. before the dynamic simulation CH4

H2O

80Å

b. during the dynamic simulation Pore surface

C

H

O

Al

c. after the dynamic simulation Si

Ca

K

Mg

Figure 18． Water molecules adsorbed on the pore surface of montmorillonite (Sw=30%)


36

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From the microscopic aspect, the difference of interaction forces between adsorbates and adsorbents directly leads to the difference of adsorption capacity. Chalmers et al. believed that the main reason for the influence of water on the adsorption capacity of shale was that water occupies the surface of inorganic hydrophilic minerals, resulting in the reduction of the surface area for methane adsorption.35 Studies72, 73 show that when the surface has charges, there will be Coulomb force between the surfaces and particles, and the clay surface is usually charged. Although water molecules are not charged, water molecules are polar molecules and can be polarized by the charge on the clay surface. Therefore, there is an electrostatic force between the clay surface and water molecules. Water, as a polar molecule, has stronger electrostatic force and Van der Waals force with the clay minerals than those of the methane molecule. Moreover, water molecules have hydrogen bonds and methane molecules do not. These stronger interaction forces make water occupy more adsorption sites and have stronger adsorption capacity than methane. Therefore, the methane molecules can adsorb and accumulate on the pore wall when the water saturation (Sw < 15%) is low, and this is because there is relatively enough surface area for both water and methane molecules to adsorb. When the water saturation increases (bigger than 45%), the adsorption sites are gradually covered mainly by water molecules, and the methane molecules break away from the adsorption potential and present a free state, thus the methane adsorption capacity of clay minerals can be ignored. Therefore, searching for low watersaturation shale reservoirs in this study area is very important for shale exploration and development.

5. CONCLUSION


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Page 38 of 48

In this work, three simulation models of montmorillonite, chlorite and illite are constructed based on Chang 7 shale. The adsorption behaviors and mechanism of methane in the clay minerals models with various water saturation are investigated using MD and GCMC simulations. After analyzing the effects of pressure, clay types and water saturation on methane adsorption capacity of clay minerals, the following conclusions based on molecular simulation of Chang 7 shale can be reached: The adsorption isosteric heat of methane on clay minerals in this study ranges from 3.62 kJ/mol to 5.77 kJ/mol, less than 42 kJ/mol, indicating the adsorption of methane in clay minerals is physical adsorption. The number of the adsorbed gas in different clay minerals increases as the pressure increases under the same condition, and the adsorption isotherms in this work are fitted well with the Langmuir model, which indicate that the adsorption simulation models established in this study are reasonable. Among the clay minerals of Chang 7 shale, the methane adsorption capacity decreases in the order of Montmorillonite > Chlorite > Illite. The methane adsorption capacity of different clay minerals is determined by adsorption area and interaction forces, including the Van der Waals and electrostatic force, and these forces are controlled by the crystal structure, chemical composition and physical properties of adsorbents and sorbents. Water molecules are easily to be adsorbed and occupy the limited adsorption sites than methane molecules in this study, for water molecules have greater interaction forces. The methane adsorption capacity in the clay minerals decreases with the increase of water saturation. When water saturation increases to 45%, clay minerals of Chang7 shale almost show no methane adsorption capacity.


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ACKNOWLEDGMENT We wish to thank the reviewers for their constructive comments and suggestions, and we are also grateful for China University of Petroleum, Beijing, for access to experimental apparatus and data. We also thank China University of Petroleum, Beijing, and the University of Nottingham, UK for software support, financing support and the writing guidance. REFERENCES 1.

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A Molecular Simulation Study on Methane Adsorption Capacity and

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