(189e) Molecular Dynamics Simulations of Natural Gas Transport in Carbon Nano-Pore Structures

Shale gas is attracting increasing attention as a rising energy supply and numerous successful production plays have been achieved. However, accurate reserve estimation and production projection are still challenging and require a sound understanding about mechanisms of natural gas transport. In contrast to conventional reservoirs, more than 80% of pores in shale formation are micro- or meso-pores, where the length scale of pores is smaller than 30 nm. Gas permeability through nano-porous media is ultra-low and the transport behavior remains poorly understood because continuum theory breaks down at these length scales. Moreover, the majority of shale gas is stored as adsorbed gas in organic matter. Consequently, the interactions between natural gas and the organic matter molecules may affect the flow of natural gas through nano-pore structures. Modeling the transport of gas through shale requires fundamental understanding of the interaction between molecules, for which molecular dynamics simulation provides the theoretical foundation.

There are two main challenges for modeling natural gas transport through nano-pore structure. First, organic matter molecules found in shale formation exhibit complex molecular structures. As one alternative, the molecular structure can be replaced by a simpler existing carbon structure. Although carbon structures cannot reproduce the detailed atomic-scale interactions, such structures are computationally inexpensive and are easily implemented. Therefore, we model the organic matter molecules using carbon structure. Second, shale exhibits a complex pore network structure. Using digital rock reconstruction techniques, we statistically model 3-D nano-pore structure in organic matter from scanning electron micrographs of shale rock sample. Subsequently, the complex pore structure is discretized into orthogonal solid and pore grids whose length is determined by the resolution of the SEM figure. By inserting a particular carbon structure into solid grid, we generate a different complex 3-D carbon nano-pore structure for each simulation.

To study gas transport behavior through shale nano-porous media, we employ methane to model natural gas. Two geometries are investigated to model the organic nano-slit pore: two parallel plates with FCC (001) carbon structure and a carbon nanotube with zigzag structure. Lennard-Jones intermolecular potential is applied to model the methane-methane and carbon-carbon interaction. Methane-carbon interaction is modelled by taking the average of the two interactions based on Lorentz-Berthelot mixing rule. All simulations are conducted using LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). We first use equilibrium molecular dynamics simulations to determine the mechanism of storage of methane. Density profiles of methane confined between two parallel plates and inside a nanotube reveal that the density of methane near the walls is much higher than that in the center. It confirms that the methane adsorbs on the carbon. We measure the amount of methane molecules in the adsorbed layer as a function of the free gas pressure to obtain the adsorption isotherm, which obeys the Langmuir isotherm and is in good agreement with experimental data from literatures; this finding indicates that adsorption is single-layer adsorption and adsorbed sites on the surface are energetically homogeneous. To characterize the flow behavior, the velocity profile and the relation between the average velocity and external body force are simulated using non-equilibrium molecular dynamics simulation based on the intermolecular interaction from equilibrium molecular dynamics simulation. Different velocity profiles through two geometries are observed. The velocity profile through the two parallel plates is nearly uniform, whereas the velocity profile in carbon nanotube depends on the external body force. If the external body force is small, gas transport near the boundary is much slower than gas transport in the center and the velocity profile is nearly parabolic. If the external body force is large, the velocity profile in the nanotube is again nearly constant. A linear relationship between average velocity and external body force is observed for flow between parallel plates, whereas a nonlinear relationship is observed for flow in a carbon nanotube. This deviation between two geometries indicates that surface interaction affects gas transport in  nano-scale systems because the specific surface areas of two geometries are different. Different numbers of carbon atoms exposed in two geometries dramatically change the intermolecular force between methane and carbon molecule, so that the methane-carbon interactions dominate the flow behavior. Finally, we model flow in a 3-D FCC (001) carbon nanopore structure and find a linear relationship between the average flux and the external body force. These results represent the flow dynamics in realistic nano-porous media and yield new insights into gas transport in carbon nano-porous media.