CO₂ sorption-induced coal swelling negatively correlates with CO₂ self-diffusivity.

CO₂ – coal interaction controls the behaviors of coal swelling/CO₂ self-diffusivity.

Swelling/self-diffusivity are separated into two opposing stages at around 8 MPa.

Transport diffusivity exceeds self-diffusivity as thermodynamic factor >1.

CO₂ permeability exponentially increases with decreasing reservoir gas pressure.