Multiscale transport mechanism of shale gas in micro/nano-pores

Abstract

A better understanding on the multiscale transport of shale gas will further the advancement of shale gas extraction. Traditional lattice Boltzmann method (LBM) simulations usually underestimate the shale gas transport capacity due to the missed consideration of gas adsorption in nanometer-sized slit pores. Here, we proposed a multiscale LB model considering the adsorption effect to simulate the shale gas transport in micro/nano-pores. The adsorption parameters between organic wall and gas molecules were determined by molecular dynamics (MD) simulations. Multiscale transport mechanism of shale gas in micro/nano slit pores ranging from nanometer to millimeter was studied by, including three different flow characteristics, namely, viscous flow, slippage, and surface diffusion. In microscale pores (pore width H>1μm), viscous flow is the dominated transport mechanism, where the velocity profile displays a typical parabolic shape without slip velocity. In larger nanoscale pores (10nm<H<1μm), slip velocity is no longer zero and increases as pore width decreases. The gas transport is dominated by viscous flow and slippage. In smaller nanometer-scale pores (H<10nm), relatively more gas molecules are adsorbed on organic pore wall, resulting in non-negligible surface diffusion which can increase the mass flux by as much as one order of magnitude. Moreover, apparent permeability obtained from our LBM simulations was compared with previous theoretical models, demonstrating the accuracy of the second-order relationship between the correlation factor and the Knudsen number.