Molecular Dynamics Simulations about Adsorption and Displacement of Methane in Carbon Nanochannels

Abstract

Adsorption and displacement are two important issues in the exploitation of shale gas. In this study, molecular dynamics (MD) simulations are employed to study the mechanisms about adsorption and displacement of methane in carbon nanochannels. Here, the nanochannel is modeled as the slit pore. Because of the attractive potentials of the walls, more methane molecules can be stored in the slit pore compared to the bulk phase, and part of them are in the adsorption state. As the width of slit pore increases, the structure of adsorbed methane transforms from single adsorption layer to four adsorption layers. Moreover, it is found that the small slit pore fills up quicker and can store more methane than the larger one under relatively low pressure due to its deeper potential well. To displace the adsorbed methane and enhance the gas recovery, injection gases such as carbon dioxide and nitrogen are simulated and investigated. The displacement mechanisms of the two gases are found to be different: carbon dioxide can replace the adsorbed methane directly while nitrogen works by decreasing the partial pressure of methane. The simulation results show that injection of carbon dioxide gives slow breakthrough time, sharp front, while injection of nitrogen gives fast breakthrough time, wide front. Our work can be of great significance for revealing the mechanisms of adsorption and displacement and guiding the exploitation of shale gas.

Channel-width dependent pressure-driven flow characteristics of shale gas in nanopores

“…At this nanometer scale, MD simulation implemented in LAMMPS package is a reliable and efficient method. 29,30 Fig. 2 shows the schematic representation of our simulation model.…”

“…Because of the adsorption effect, methane content in shale gas reservoir usually exceeds the conventional estimation. 29,30 For methane molecules far away from the walls, they are stored in the bulk state and the density nearly keeps constant. The value of bulk density obtained from our EMD simulations agrees well with that obtained from the National Institute of Standards and Technology (NIST), which can validate the accuracy of our simulation methods.…”

“…In the flow process, adsorption and desorption can reach a dynamic equilibrium. 30 When a pressure gradient is applied, methane molecules in bulk-like layer form a stable viscous flow. Meanwhile, slip flow may occur due to the molecule-wall collisions and surface diffusion (which is also known as hopping mechanism) may occur due to gradients in adsorbed layer.…”

Using graphene to simplify the adsorption of methane on shale in MD simulations

“…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 (CH 4 ) as the same of real pressure (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) in shale. And most pressure condition in experiment was under 20 MPa [20].…”

“…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 CH 4 tigated [29][30][31][32]. Graphene is widely used in the researches of shale and coal.…”