Using graphene to simplify the adsorption of methane on shale in MD simulations
Graphical abstract
Introduction
As a unconventional gas resource, shale gas plays an increasingly important role in renewable energy, because of its great reserves throughout the world and its clean when burned [1], [2]. Most shale gas production in North America attributes to the development of horizontal drilling and fracking technology [3], [4]. Also shale gas has been extensively explored in Australia, Europe and China in recent years [5]. Even though thousands of shale gas wells are in production around the world, there are still many difficulties in shale gas exploitation technology [6], [7], [8], [9], [10], and the thermodynamic properties of shale gas are still poorly understood. Hence, researchers have increasingly tried to explore the properties of shale gas and find out an efficient way in enhancing shale gas recovery (ESGR) [11], [12], [13], [14]. Experiment is a direct way to explore the properties of shale gas in shale [15], [16], [17], [18], [19], but is difficult to achieve high-pressure conditions of methane (CH4) as the same of real pressure (20–30 MPa) in shale. And most pressure condition in experiment was under 20 MPa [20]. In addition, earlier studies show that shale gas adsorbs in the nanopores [21], [22], [23]. Therefore, it is difficult to experimentally explore the microscopic mechanisms of shale gas adsorption in nanopores. Simulation provides an efficient and inexpensive way to investigate the microscopic properties of CH4 in shale. However, the components and microstructure of the shale are very complex. For convenience of calculations, we need to find a simple geometry yet similar properties material as a model of shale in the shale gas recovery simulations.
Molecular dynamics (MD) simulations have been extensively conducted to investigate shale gas adsorption. Firoozabadi made a great contribution to the research of hydrocarbons energy production and illuminated the use of thermodynamics in reservoir and transportation of shale gas, and MD simulations were performed in his research [24]. In previous simulations, CH4 was considered under thermodynamic equilibrium in three-dimensional periodic orthorhombic pore geometry consisting of upper and lower pore-walls made of graphene [25]. Coal and shale both require a thorough understanding of the CO2 adsorption properties in microporous carbon-based materials. Wilcox et al. investigated the adsorption of CO2 in microporous carbons. They explored CH4 adsorption properties on graphitic surfaces as an initial model of coal and kerogen of gas shale [26], [27]. The adsorption behavior of oil within nanoscale carbonaceous slits of shale systems was studied by using MD simulation, with graphene sheets as the model of shale [28]. Recently, the atomic mechanisms and adsorption properties of CH4 on graphene (or carbon nanopores) have been investigated [29], [30], [31], [32]. Graphene is widely used in the researches of shale and coal. Even though many researchers chose graphene as a model of shale or coal, the reasons for this choice are not clear or the evidence remains insufficient. Hence, material similarity between graphene and shale for CH4 adsorption need to be explored in detail.
In this paper, thermodynamic properties of the CH4 adsorption on graphene are investigated and the results are compared with the experiment. Firstly, we used theoretical analysis to investigate the interaction potential between CH4 and graphene, the effect of the number of graphene layers on the interaction potential was explored. Secondly, we provide a way to calculate the limiting heat of adsorption of CH4 on graphene, and compared the limiting heat with experimental results. Thirdly, the grand canonical Monte Carlo (GCMC) simulations were performed to predict the CH4 adsorption isotherm at 300 K, 320 K, 340 K and 360 K, and the fugacity from 1 MPa to 40 MPa, with 2 nm, 3 nm, 5 nm, 7 nm, 9 nm and 11 nm slit pore sizes. And we provide a way to transform fugacity and absolute adsorption into pressure and excess adsorption. Finally, the isoteric adsorption heats of CH4 in multiple graphene slit pore sizes at different temperatures were investigated. Our results and related analyses may help to understand the CH4 adsorption on graphene. More importantly, the results provide a strong evidence of using graphene in modeling shale in simulating the shale gas adsorption/desorption. This study aims to quantify whether graphene may be used to represent the wall boundaries of nanopores in shale gas adsorption measurements in MD simulations. It presents future researchers to simplify the description of shale nanostructure with an equivalent slit pore consisting of graphene.
Section snippets
Methods
MD simulations implemented in LAMMPS [33] have been carried out. We simulated the limiting adsorption heat of CH4 on graphene adopting consistent valence force-field (CVFF) [34], which is based on the ab initio calculations and experiments. The total potential energy consists of the bond energy and nonbond energy . is the sum of bond and angle energy. is the interaction potential between two atoms i and j, which is Lennard-Jones (LJ) energies:where is
Limiting heat of adsorption of CH4 on graphene
When the amount adsorbed approaches zero, the isosteric heat of adsorption is called the limiting heat of adsorption. It is a representative thermal effect of the adsorption. The limiting heat of adsorption can be evaluated from Henry constants if they are available for several temperatures, and the Henry constant varies with temperature following the van’t Hoff equation [35]:where is the Henry constant, is the absolute temperature, is the difference of molar enthalpy
Conclusions
For the first time, the interaction potential between multilayer graphene and CH4 were investigated. The results indicate that the first two layers of graphene play the main role of the interaction potential between multilayer graphene and CH4. We can just consider investigating the interaction between CH4 with monolayer graphene (85.5%) or bilayer graphene (96.6%). The limiting heat of CH4 adsorption on monolayer graphene is 5.97 kcal/mol, and that of CH4 adsorption on bilayer graphene becomes
Acknowledgments
This research was supported in part by the National Natural Science Foundation of China (NSFC, Grant No. U1562105), and by the Chinese Academy of Sciences (CAS) through CAS Interdisciplinary Innovation Team Project, the CAS Key Research Program of Frontier Sciences (Grant No. QYZDJ-SSW-JSC019) and the CAS Strategic Priority Research Program (Grant No. XDB22040401).
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