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.

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This paper presents a review on the numerical manifold method (NMM), which covers the basic theories of the NMM, such as NMM components, NMM displacement approximation, formulations of the discrete system of equations, integration scheme, imposition of the boundary conditions, treatment of contact problems involved in the NMM, and also the recent developments and applications of the NMM. Modeling the strong discontinuities within the framework of the NMM is specially emphasized. Several examples demonstrating the capability of the NMM in modeling discrete block system, strong discontinuities, as well as weak discontinuities are given. The similarities and distinctions of the NMM with various other numerical methods such as the finite element method (FEM), the extended finite element method (XFEM), the generalized finite element method (GFEM), the discontinuous deformation analysis (DDA), and the distinct element method (DEM) are investigated. Further developments on the NMM are suggested.

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