Super-Diffusive Gas Recovery from Nanopores

TL;DR

This study uses molecular simulations to analyze gas recovery from nanopores, revealing that strong gas-wall interactions cause super-diffusive recovery behavior, which can be modeled with continuum approaches incorporating adsorption effects.

Contribution

It demonstrates the importance of gas-wall interactions in nanopore gas recovery and shows how super-diffusive scaling laws can be accurately modeled with continuum models including adsorption.

Findings

Gas-wall interactions slow down gas diffusion in nanopores.

Gas recovery follows a super-diffusive scaling law, not the classical diffusive law.

Continuum models with Langmuir adsorption accurately capture the observed recovery behavior.

Abstract

Understanding the recovery of gas from reservoirs featuring pervasive nanopores is essential for effective shale gas extraction. Classical theories cannot accurately predict such gas recovery and many experimental observations are not well understood. Here we report molecular simulations of the recovery of gas from single nanopores, explicitly taking into account molecular gas-wall interactions. We show that, in very narrow pores, the strong gas-wall interactions are essential in determining the gas recovery behavior both quantitatively and qualitatively. These interactions cause the total diffusion coefficients of the gas molecules in nanopores to be smaller than those predicted by kinetic theories, hence slowing down the rate of gas recovery. These interactions also lead to significant adsorption of gas molecules on the pore walls. Because of the desorption of these gas molecules during gas recovery, the gas recovery from the nanopore does not exhibit the usual diffusive scaling law (i.e., the accumulative recovery scales as $R \sim t^{1/2}$ but follows a super-diffusive scaling law $R \sim t^n$ ($n>0.5$), which is similar to that observed in some field experiments. For the system studied here, the super-diffusive gas recovery scaling law can be captured well by continuum models in which the gas adsorption and desorption from pore walls are taken into account using the Langmuir model.