Estimation of Transport Diffusivity of Natural Gas in Organic Matter Using Molecular Dynamics Simulation

Abstract

Abstract Due to the uncertainty in shale reservoir simulation, increasing attention has been drawn to the investigation of nano-scale transport behavior in heterogeneous shale rocks. Molecular dynamics simulation has become a widely-used simulation technique to investigate molecular behaviors at nanoscale. In this paper, we combined one of the state-of-art pore characterization techniques with molecular dynamics simulation to study the natural gas transport behavior in complex nano-pore networks in organic matter. Representative kerogen molecule is selected based on thermal maturity and the origin. Methane molecule is chosen to represent natural gas. Equilibrium molecular dynamic simulations have been performed to construct kerogen clusters and the structural properties of the organic matter have been studied. As a building block, the amorphous kerogen clusters were inserted into the digital rock obtained from focused ion beam scanning electron microscopy (FIB-SEM). Then, non-equilibrium molecular dynamics simulations were performed to study the transport behavior of natural gas in the reconstructed nano-scale digital rock. The transport diffusivity coefficients were thus determined by following the Fick's first-order diffusion equation. The proposed method provides a new insight into the understanding of nano-scale transport behavior of natural gas in shales, which is crucial to the prediction of long-term production performance and reserve estimation in larger-scale reservoir simulation.