Lattice Boltzmann Simulation of the Poroelastic Effect on Apparent Permeability in Coal Micro/Nanopores

Abstract

To investigate fully the poroelastic effect on apparent permeability in coal micro/nanopores, a multi-mechanism apparent permeability model coupling the gas slippage effect and the poroelastic effect is hereby constructed on the strength of the lattice Boltzmann method. The contributions of the permeability of gas slippage, surface diffusion, and viscous flow were investigated. The results showed that the gas transport was controlled by surface diffusion in micro/nanopores with initial sizes of less than 10 nm. Under a low pore pressure, the contribution share of gas slippage permeability to the apparent gas permeability decreased exponentially as the pressure rose. When the pore pressure ascended, the dynamic apparent permeability ratio (i.e., the ratio of the apparent permeability affected by the poroelastic effect to the initial apparent permeability) was subjected to the slippage effect initially and dominated by the poroelastic effect later. Additionally, the slippage effect’s contribution to the apparent permeability ratio plunged under a lower pore pressure, but such decrease slackened as the pore pressure grew to a higher value. During coalbed methane (CBM) recovery in low-permeability coal seams, the slippage effect’s contribution to the CBM recovery production surges first, then falls slowly, and finally restores to a slow increase, and its contribution is enhanced in micro/nanopores with smaller average pore sizes.