Restricted CO2/CH4 diffusion in nanopores: A quantitative framework to characterize nanoconfinement effect of shale organic pore

The utilization of carbon dioxide (CO 2 ) to enhance depleted unconventional resources can reduce greenhouse gas emission and increase the recovery ratio. Self-diffusion coefficient of nanoconfined CO 2 /CH 4 is a crucial factor in CO 2 -enhanced shale gas recovery and CO 2 geological sequestration. A workflow was proposed which included the reconstruction of amorphous shale organic nanopores, detection of effective pore size, and integration of grand canonical Monte Carlo (GCMC) method with molecular dynamics (MD) simulations to calculate self-diffusion coefficient. Challenges such as complex pore surface and extreme reservoir condition were considered, and the self-diffusion coefficients of nanoconfined fluid were obtained. It was found that the self-diffusion coefficient of nanoconfined CO 2 /CH 4 is much smaller than that of bulk CO 2 /CH 4 , as a concequence of nanoconfinement effect. The effects of temperature, pressure and pore size were considered, and a nondimensional self-diffusion coefficient was proposed to derive a concise relation with Knudsen number. Additionally, a two-segment correlation formula was proposed to describe the concise relation. Owing to the strong CO 2 -pore surface interaction, CO 2 was adsorbed on the pore surface and CH 4 was restricted in a smaller pore space. Therefore, this fluid-pore surface interaction reduced the self-diffusion coefficient of binary mixture CO 2 /CH 4 . The workflow, nondimensional self-diffusion coefficient, and correlation formula derived in this study can be used for extensive applications, such as catalyst, fuel cell and sewage treatment.

Automated summary

In this study, a workflow was proposed to reconstruct amorphous shale organic nanopores, detect effective pore size, and calculate the self-diffusion coefficient of nanoconfined CO 2 /CH 4 using the grand canonical Monte Carlo (GCMC) method and molecular dynamics (MD) simulations. The effects of temperature, pressure, and pore size were considered, and a nondimensional self-diffusion coefficient and correlation formula were derived. The strong interaction between CO 2 and the pore surface reduced the self-diffusion coefficient of the binary mixture CO 2 /CH 4 . The derived workflow, nondimensional self-diffusion coefficient, and correlation formula can be applied to catalyst, fuel cell, and sewage treatment systems.