Role of Adsorption-Induced Deformation on Gas Self-Diffusivity in a Flexible Microporous Coal Matrix

Abstract

Adsorption-induced deformation has long been underappreciated in gas transport studies of microporous coal, yet it strongly influences pore configurations and diffusive pathways. Here, a hybrid grand canonical Monte Carlo (GCMC)/molecular dynamics (MD) approach and equilibrium MD (EMD) simulations are employed to investigate how matrix flexibility reshapes pore structures and, in turn, impacts CH₄ and CO₂ self-diffusion in connected pore networks under various gas loadings. The results show that coal matrix deformation enhances adsorption, with CO₂ exhibiting greater uptake and volumetric strain than CH₄. A universal linear relationship emerges among gas loading, free volume ratio, and self-diffusion coefficients for both rigid and flexible matrices. In flexible matrices, this linearity features a gentler slope, indicating reduced diffusion sensitivity to diminishing free volume with loadings. By comparing geometrical and effective tortuosity, it is revealed that strongly adsorbing CO₂ induces significant swelling and complex local rearrangements at elevated loadings, pushing geometrical tortuosity far beyond rigid-matrix levels, whereas CH₄─with weaker adsorption─drives smaller, more uniform structural adjustments that only mildly increase geometrical tortuosity. These differences in tortuosity directly reflect changes in path complexity, which in turn governs self-diffusion behavior. Collectively, the findings clarify the dynamic coupling between gas adsorption, matrix deformation, and self-diffusivity in microporous coal, offering critical guidance for enhanced gas recovery and CO₂ sequestration strategies that rely on accurate modeling of gas transport in deformable media.